Signaling between the actin cytoskeleton and the postsynaptic density of dendritic spines

DXplorer: A Unified Visualization Framework for Interactive Dendritic Spine Analysis Using 3D Morphological Features

Dendritic spines are dynamic, submicron-scale protrusions on neuronal dendrites that receive neuronal inputs. Morphological changes in the dendritic spine often reflect alterations in physiological conditions and are indicators of various neuropsychiatric conditions. However, owing to the highly dynamic and heterogeneous nature of spines, accurate measurement and objective analysis of spine morphology are major challenges in neuroscience research. Most conventional approaches for analyzing dendritic spines are based on two-dimensional (2D) images, which barely reflect the actual three-dimensional (3D) shapes. Although some recent studies have attempted to analyze spines with various 3D-based features, it is still difficult to objectively categorize and analyze spines based on 3D morphology. Here, we propose a unified visualization framework for an interactive 3D dendritic spine analysis system, DXplorer, that displays 3D rendering of spines and plots the high-dimensional features extracted from the 3D mesh of spines. With this system, users can perform the clustering of spines interactively and explore and analyze dendritic spines based on high-dimensional features. We propose a series of high-dimensional morphological features extracted from a 3D mesh of dendritic spines. In addition, an interactive machine learning classifier with visual exploration and user feedback using an interactive 3D mesh grid view ensures a more precise classification based on the spine phenotype. A user study and two case studies were conducted to quantitatively verify the performance and usability of the DXplorer. We demonstrate that the system performs the entire analytic process effectively and provides high-quality, accurate, and objective analysis.

ISGylation is induced in neurons by demyelination driving ISG15-dependent microglial activation

The causes of grey matter pathology and diffuse neuron injury in MS remain incompletely understood. Axonal stress signals arising from white matter lesions has been suggested to play a role in initiating this diffuse grey matter pathology. Therefore, to identify the most upstream transcriptional responses in neurons arising from demyelinated axons, we analyzed the transcriptome of actively translating neuronal transcripts in mouse models of demyelinating disease. Among the most upregulated genes, we identified transcripts associated with the ISGylation pathway. ISGylation refers to the covalent attachment of the ubiquitin-like molecule interferon stimulated gene (ISG) 15 to lysine residues on substrates targeted by E1 ISG15-activating enzyme, E2 ISG15-conjugating enzymes and E3 ISG15-protein ligases. We further confirmed that ISG15 expression is increased in MS cortical and deep gray matter. Upon investigating the functional impact of neuronal ISG15 upregulation, we noted that ISG15 expression was associated changes in neuronal extracellular vesicle protein and miRNA cargo. Specifically, extracellular vesicle-associated miRNAs were skewed toward increased frequency of proinflammatory and neurotoxic miRNAs and decreased frequency of anti-inflammatory and neuroprotective miRNAs. Furthermore, we found that ISG15 directly activated microglia in a CD11b-dependent manner and that microglial activation was potentiated by treatment with EVs from neurons expressing ISG15. Further study of the role of ISG15 and ISGylation in neurons in MS and neurodegenerative diseases is warranted.

Electrical recordings from dendritic spines of adult mouse hippocampus and effect of the actin cytoskeleton

Dendritic spines (DS) are tiny protrusions implicated in excitatory postsynaptic responses in the CNS. To achieve their function, DS concentrate a high density of ion channels and dynamic actin networks in a tiny specialized compartment. However, to date there is no direct information on DS ionic conductances. Here, we used several experimental techniques to obtain direct electrical information from DS of the adult mouse hippocampus. First, we optimized a method to isolate DS from the dissected hippocampus. Second, we used the lipid bilayer membrane (BLM) reconstitution and patch clamping techniques and obtained heretofore unavailable electrical phenotypes on ion channels present in the DS membrane. Third, we also patch clamped DS directly in cultured adult mouse hippocampal neurons, to validate the electrical information observed with the isolated preparation. Electron microscopy and immunochemistry of PDS-95 and NMDA receptors and intrinsic actin networks confirmed the enrichment of the isolated DS preparation, showing open and closed DS, and multi-headed DS. The preparation was used to identify single channel activities and “whole-DS” electrical conductance. We identified NMDA and Ca²⁺-dependent intrinsic electrical activity in isolated DS and in situ DS of cultured adult mouse hippocampal neurons. In situ recordings in the presence of local NMDA, showed that individual DS intrinsic electrical activity often back-propagated to the dendrite from which it sprouted. The DS electrical oscillations were modulated by changes in actin cytoskeleton dynamics by addition of the F-actin disrupter agent, cytochalasin D, and exogenous actin-binding proteins. The data indicate that DS are elaborate excitable electrical devices, whose activity is a functional interplay between ion channels and the underlying actin networks. The data argue in favor of the active contribution of individual DS to the electrical activity of neurons at the level of both the membrane conductance and cytoskeletal signaling.

Mechanical Principles Governing the Shapes of Dendritic Spines

Dendritic spines are small, bulbous protrusions along the dendrites of neurons and are sites of excitatory postsynaptic activity. The morphology of spines has been implicated in their function in synaptic plasticity and their shapes have been well-characterized, but the potential mechanics underlying their shape development and maintenance have not yet been fully understood. In this work, we explore the mechanical principles that could underlie specific shapes using a minimal biophysical model of membrane-actin interactions. Using this model, we first identify the possible force regimes that give rise to the classic spine shapes-stubby, filopodia, thin, and mushroom-shaped spines. We also use this model to investigate how the spine neck might be stabilized using periodic rings of actin or associated proteins. Finally, we use this model to predict that the cooperation between force generation and ring structures can regulate the energy landscape of spine shapes across a wide range of tensions. Thus, our study provides insights into how mechanical aspects of actin-mediated force generation and tension can play critical roles in spine shape maintenance.

F-actin-bundling sites are conserved in proteins with villin-type headpiece domains

Villin is a major actin-bundling protein that assembles the brush border of intestinal and renal epithelial cells. The villin "headpiece" domain and the actin-binding residues within it regulate its actin-bundling function. Substantial experimental and theoretical information about the three-dimensional structure of the isolated villin headpiece, including a description of the actin-binding residues within the headpiece, is available. Despite that, the actin-bundling site in the full-length (FL) villin protein remains unidentified. We used this existing villin headpiece nuclear magnetic resonance data and performed mutational analysis and functional assays to identify the actin-bundling site in FL human villin protein. By careful evaluation of these conserved actin-binding residues in human advillin protein, we demonstrate their functional significance in the over 30 proteins that contain a villin-type headpiece domain. Our study is the first that combines the available structural data on villin headpiece with functional assays to identify the actin-binding residues in FL villin that regulate its filament-bundling activity. Our findings could have wider implications for other actin-bundling proteins that contain a villin-type headpiece domain.

Cross-talk between the epigenome and neural circuits in drug addiction

Drug addiction is a behavioral disorder characterized by dysregulated learning about drugs and associated cues that result in compulsive drug seeking and relapse. Learning about drug rewards and predictive cues is a complex process controlled by a computational network of neural connections interacting with transcriptional and molecular mechanisms within each cell to precisely guide behavior. The interplay between rapid, temporally specific neuronal activation, and longer-term changes in transcription is of critical importance in the expression of appropriate, or in the case of drug addiction, inappropriate behaviors. Thus, these factors and their interactions must be considered together, especially in the context of treatment. Understanding the complex interplay between epigenetic gene regulation and circuit connectivity will allow us to formulate novel therapies to normalize maladaptive reward behaviors, with a goal of modulating addictive behaviors, while leaving natural reward-associated behavior unaffected.

Automated spatio-temporal analysis of dendritic spines and related protein dynamics

Cofilin and other Actin-regulating proteins are essential in regulating the shape of dendritic spines, which are sites of neuronal communications in the brain, and their malfunctions are implicated in neurodegeneration related to aging. The analysis of cofilin motility in dendritic spines using fluorescence video-microscopy may allow for the discovery of its effects on synaptic functions. To date, the flow of cofilin has not been analyzed by automatic means. This paper presents Dendrite Protein Analysis (DendritePA), a novel automated pattern recognition software to analyze protein trafficking in neurons. Using spatiotemporal information present in multichannel fluorescence videos, the DendritePA generates a temporal maximum intensity projection that enhances the signal-to-noise ratio of important biological structures, segments and tracks dendritic spines, estimates the density of proteins in spines, and analyzes the flux of proteins through the dendrite/spine boundary. The motion of a dendritic spine is used to generate spine energy images, which are used to automatically classify the shape of common dendritic spines such as stubby, mushroom, or thin. By tracking dendritic spines over time and using their intensity profiles, the system can analyze the flux patterns of cofilin and other fluorescently stained proteins. The cofilin flux patterns are found to correlate with the dynamic changes in dendritic spine shapes. Our results also have shown that the activation of cofilin using genetic manipulations leads to immature spines while its inhibition results in an increase in mature spines.

Inositol-1,4,5-trisphosphate 3-kinase-A (ITPKA) is frequently over-expressed and functions as an oncogene in several tumor types

At present targeted tumor therapy is based on inhibition of proteins or protein mutants that are up-regulated in tumor but not in corresponding normal cells. The actin bundling Inositol-trisphosphate 3-kinase A (ITPKA) belongs to such molecular targets. ITPKA is expressed in a broad range of tumor types but shows limited expression in normal cells. In lung and breast cancer expression of ITPKA is stimulated by gene body methylation which increases with increasing malignancy of these tumors but is not detectable in the corresponding normal tissues. Since ITPKA gene body methylation occurs early in tumor development, it could serve as biomarker for early detection of lung cancer. Detailed mechanistic studies revealed that down-regulation of ITPKA in lung adenocarcinoma cancers reduced both, tumor growth and metastasis. It is assumed that tumor growth is stimulated by the InsP₃Kinase activity of ITPKA and metastasis by its actin bundling activity. A selective inhibitor against the InsP₃Kinase activity of ITPKA has been identified but compounds inhibiting the actin bundling activity are not available yet. Since no curative therapy option for metastatic lung or breast tumors exist, therapies that block activities of ITPKA may offer new options for patients with these tumors. Thus, efforts should be made to develop clinical drugs that selectively target InsP₃Kinase activity as well as actin bundling activity of ITPKA.

Quantification of Filamentous Actin (F-actin) Puncta in Rat Cortical Neurons

Filamentous actin protein (F-actin) plays a major role in spinogenesis, synaptic plasticity, and synaptic stability. Changes in dendritic F-actin rich structures suggest alterations in synaptic integrity and connectivity. Here we provide a detailed protocol for culturing primary rat cortical neurons, Phalloidin staining for F-actin puncta, and subsequent quantification techniques. First, the frontal cortex of E18 rat embryos are dissociated into low-density cell culture, then the neurons grown in vitro for at least 12-14 days. Following experimental treatment, the cortical neurons are stained with AlexaFluor 488 Phalloidin (to label the dendritic F-actin puncta) and microtubule-associated protein 2 (MAP2; to validate the neuronal cells and dendritic integrity). Finally, specialized software is used to analyze and quantify randomly selected neuronal dendrites. F-actin rich structures are identified on second order dendritic branches (length range 25-75 µm) with continuous MAP2 immunofluorescence. The protocol presented here will be a useful method for investigating changes in dendritic synapse structures subsequent to experimental treatments.

Vortioxetine promotes maturation of dendritic spines in vitro: A comparative study in hippocampal cultures

Cognitive dysfunction is prevalent in patients with major depressive disorder (MDD), and cognitive impairments can persist after relief of depressive symptoms. The multimodal-acting antidepressant vortioxetine is an antagonist at 5-HT3, 5-HT7, and 5-HT1D receptors, a partial agonist at 5-HT1B receptors, an agonist at 5-HT1A receptors, and an inhibitor of the serotonin (5-HT) transporter (SERT) and has pro-cognitive properties. In preclinical studies, vortioxetine enhances long-term potentiation (LTP), a cellular correlate of neuroplasticity, and enhances memory in various cognitive tasks. However, the molecular mechanisms by which vortioxetine augments LTP and memory remain unknown. Dendritic spines are specialized, actin-rich microdomains on dendritic shafts and are major sites of most excitatory synapses. Since dendritic spine remodeling is implicated in synaptic plasticity and spine size dictates the strength of synaptic transmission, we assessed if vortioxetine, relative to other antidepressants including ketamine, duloxetine, and fluoxetine, plays a role in the maintenance of dendritic spine architecture in vitro. We show that vortioxetine, ketamine, and duloxetine induce spine enlargement. However, only vortioxetine treatment increased the number of spines in contact with presynaptic terminals. In contrast, fluoxetine had no effect on spine remodeling. These findings imply that the various 5-HT receptor mechanisms of vortioxetine may play a role in its effect on spine dynamics and in increasing the proportion of potentially functional synaptic contacts.

NMDA receptor activation and calpain contribute to disruption of dendritic spines by the stress neuropeptide CRH

The complex effects of stress on learning and memory are mediated, in part, by stress-induced changes in the composition and structure of excitatory synapses. In the hippocampus, the effects of stress involve several factors including glucocorticoids and the stress-released neuropeptide corticotropin-releasing hormone (CRH), which influence the integrity of dendritic spines and the structure and function of the excitatory synapses they carry. CRH, at nanomolar, presumed-stress levels, rapidly abolishes short-term synaptic plasticity and destroys dendritic spines, yet the mechanisms for these effects are not fully understood. Here we tested the hypothesis that glutamate receptor-mediated processes, which shape synaptic structure and function, are engaged by CRH and contribute to spine destabilization. In cultured rat hippocampal neurons, CRH application reduced dendritic spine density in a time- and dose-dependent manner, and this action depended on the CRH receptor type 1. CRH-mediated spine loss required network activity and the activation of NMDA, but not of AMPA receptors; indeed GluR1-containing dendritic spines were resistant to CRH. Downstream of NMDA receptors, the calcium-dependent enzyme, calpain, was recruited, resulting in the breakdown of spine actin-interacting proteins including spectrin. Pharmacological approaches demonstrated that calpain recruitment contributed critically to CRH-induced spine loss. In conclusion, the stress hormone CRH co-opts mechanisms that contribute to the plasticity and integrity of excitatory synapses, leading to selective loss of dendritic spines. This spine loss might function as an adaptive mechanism preventing the consequences of adverse memories associated with severe stress.

Shank3 deficiency induces NMDA receptor hypofunction via an actin-dependent mechanism

Shank3, which encodes a scaffolding protein at glutamatergic synapses, is a genetic risk factor for autism. In this study, we examined the impact of Shank3 deficiency on the NMDA-type glutamate receptor, a key player in cognition and mental illnesses. We found that knockdown of Shank3 with a small interfering RNA (siRNA) caused a significant reduction of NMDAR-mediated ionic or synaptic current, as well as the surface expression of NR1 subunits, in rat cortical cultures. The effect of Shank3 siRNA on NMDAR currents was blocked by an actin stabilizer, and was occluded by an actin destabilizer, suggesting the involvement of actin cytoskeleton. Since actin dynamics is regulated by the GTPase Rac1 and downstream effector p21-activated kinase (PAK), we further examined Shank3 regulation of NMDARs when Rac1 or PAK was manipulated. We found that the reducing effect of Shank3 siRNA on NMDAR currents was mimicked and occluded by specific inhibitors for Rac1 or PAK, and was blocked by constitutively active Rac1 or PAK. Immunocytochemical data showed a strong reduction of F-actin clusters after Shank3 knockdown, which was occluded by a PAK inhibitor. Inhibiting cofilin, the primary downstream target of PAK and a major actin depolymerizing factor, prevented Shank3 siRNA from reducing NMDAR currents and F-actin clusters. Together, these results suggest that Shank3 deficiency induces NMDAR hypofunction by interfering with the Rac1/PAK/cofilin/actin signaling, leading to the loss of NMDAR membrane delivery or stability. It provides a potential mechanism for the role of Shank3 in cognitive deficit in autism.

A simple rule for dendritic spine and axonal bouton formation can account for cortical reorganization after focal retinal lesions

Lasting alterations in sensory input trigger massive structural and functional adaptations in cortical networks. The principles governing these experience-dependent changes are, however, poorly understood. Here, we examine whether a simple rule based on the neurons' need for homeostasis in electrical activity may serve as driving force for cortical reorganization. According to this rule, a neuron creates new spines and boutons when its level of electrical activity is below a homeostatic set-point and decreases the number of spines and boutons when its activity exceeds this set-point. In addition, neurons need a minimum level of activity to form spines and boutons. Spine and bouton formation depends solely on the neuron's own activity level, and synapses are formed by merging spines and boutons independently of activity. Using a novel computational model, we show that this simple growth rule produces neuron and network changes as observed in the visual cortex after focal retinal lesions. In the model, as in the cortex, the turnover of dendritic spines was increased strongest in the center of the lesion projection zone, while axonal boutons displayed a marked overshoot followed by pruning. Moreover, the decrease in external input was compensated for by the formation of new horizontal connections, which caused a retinotopic remapping. Homeostatic regulation may provide a unifying framework for understanding cortical reorganization, including network repair in degenerative diseases or following focal stroke.

The adult brain is less hard-wired than traditionally thought. About ten percent of synapses in the mature visual cortex is continually replaced by new ones (structural plasticity). This percentage greatly increases after lasting changes in visual input. Due to the topographically organized nerve connections from the retina in the eye to the primary visual cortex in the brain, a small circumscribed lesion in the retina leads to a defined area in the cortex that is deprived of input. Recent experimental studies have revealed that axonal sprouting and dendritic spine turnover are massively increased in and around the cortical area that is deprived of input. However, the driving forces for this structural plasticity remain unclear. Using a novel computational model, we examine whether the need for activity homeostasis of individual neurons may drive cortical reorganization after lasting changes in input activity. We show that homeostatic growth rules indeed give rise to structural and functional reorganization of neuronal networks similar to the cortical reorganization observed experimentally. Understanding the principles of structural plasticity may eventually lead to novel treatment strategies for stimulating functional reorganization after brain damage and neurodegeneration.

Microdomains in forebrain spines: an ultrastructural perspective

Glutamatergic axons in the mammalian forebrain terminate predominantly onto dendritic spines. Long-term changes in the efficacy of these excitatory synapses are tightly coupled to changes in spine morphology. The reorganization of the actin cytoskeleton underlying this spine "morphing" involves numerous proteins that provide the machinery needed for adaptive cytoskeletal remodeling. Here, we review recent literature addressing the chemical architecture of the spine, focusing mainly on actin-binding proteins (ABPs). Accumulating evidence suggests that ABPs are organized into functionally distinct microdomains within the spine cytoplasm. This functional compartmentalization provides a structural basis for regulation of the spinoskeleton, offering a novel window into mechanisms underlying synaptic plasticity.

Mathematical modeling of sustainable synaptogenesis by repetitive stimuli suggests signaling mechanisms in vivo

The mechanisms of long-term synaptic maintenance are a key component to understanding the mechanism of long-term memory. From biological experiments, a hypothesis arose that repetitive stimuli with appropriate intervals are essential to maintain new synapses for periods of longer than a few days. We successfully reproduce the time-course of relative numbers of synapses with our mathematical model in the same conditions as biological experiments, which used Adenosine-3', 5'-cyclic monophosphorothioate, Sp-isomer (Sp-cAMPS) as external stimuli. We also reproduce synaptic maintenance responsiveness to intervals of Sp-cAMPS treatment accompanied by PKA activation. The model suggests a possible mechanism of sustainable synaptogenesis which consists of two steps. First, the signal transduction from an external stimulus triggers the synthesis of a new signaling protein. Second, the new signaling protein is required for the next signal transduction with the same stimuli. As a result, the network component is modified from the first network, and a different signal is transferred which triggers the synthesis of another new signaling molecule. We refer to this hypothetical mechanism as network succession. We build our model on the basis of two hypotheses: (1) a multi-step network succession induces downregulation of SSH and COFILIN gene expression, which triggers the production of stable F-actin; (2) the formation of a complex of stable F-actin with Drebrin at PSD is the critical mechanism to achieve long-term synaptic maintenance. Our simulation shows that a three-step network succession is sufficient to reproduce sustainable synapses for a period longer than 14 days. When we change the network structure to a single step network, the model fails to follow the exact condition of repetitive signals to reproduce a sufficient number of synapses. Another advantage of the three-step network succession is that this system indicates a greater tolerance of parameter changes than the single step network.

SIRT1 regulates dendritic development in hippocampal neurons

Dendritic arborization is required for proper neuronal connectivity. SIRT1, a NAD+ dependent histone deacetylase, has been associated to ageing and longevity, which in neurons is linked to neuronal differentiation and neuroprotection. In the present study, the role of SIRT1 in dendritic development was evaluated in cultured hippocampal neurons which were transfected at 3 days in vitro with a construct coding for SIRT1 or for the dominant negative SIRT1H363Y, which lacks the catalytic activity. Neurons overexpressing SIRT1 showed an increased dendritic arborization, while neurons overexpressing SIRT1H363Y showed a reduction in dendritic arbor complexity. The effect of SIRT1 was mimicked by treatment with resveratrol, a well known activator of SIRT1, which has no effect in neurons overexpressing SIRT1H363Y indicating that the effect of resveratrol was specifically mediated by SIRT1. Moreover, hippocampal neurons overexpressing SIRT1 were resistant to dendritic dystrophy induced by Aβ aggregates, an effect that was dependent on the deacetylase activity of SIRT1. Our findings indicate that SIRT1 plays a role in the development and maintenance of dendritic branching in hippocampal neurons, and suggest that these effects are mediated by the ROCK signaling pathway.

Retinal pigment epithelium response to oxidant injury in the pathogenesis of early age-related macular degeneration

Age-related macular degeneration (AMD) represents the leading cause of vision loss in the elderly. Accumulation of lipid- and protein-rich deposits under the retinal pigment epithelium (RPE) heralds the onset of early AMD, but the pathogenesis of subretinal deposit formation is poorly understood. Numerous hypothetical models of deposit formation have been proposed, including hypotheses for a genetic basis, choroidal hypoperfusion, abnormal barrier formation, and lysosomal failure. This review explore the RPE injury hypothesis, characterized by three distinct stages (1) Initial RPE oxidant injury, caused by any number of endogenous or exogenous oxidants, results in extrusion of cell membrane "blebs," together with decreased activity of matrix metalloproteinases (MMPs), promoting bleb accumulation under the RPE as basal laminar deposits (BLD). (2) RPE cells are subsequently stimulated to increase synthesis of MMPs and other molecules responsible for extracellular matrix turnover (i.e., producing decreased collagen), affecting both RPE basement membrane and Bruchs membrane (BrM). This process leads to progression of BLD into basal linear deposits (BLinD) and drusen by admixture of blebs into BrM, followed by the formation of new basement membrane under the RPE to trap these deposits within BrM. We postulate that various hormones and other plasma-derived molecules related to systemic health cofactors are implicated in this second stage. (3) Finally, macrophages are recruited to sites of RPE injury and deposit formation. The recruitment of nonactivated or scavenging macrophages may remove deposits without further injury, while the recruitment of activated or reparative macrophages, through the release of inflammatory mediators, growth factors, or other substances, may promote complications and progression to the late forms of the disease.

Potential role of drebrin a, an f-actin binding protein, in reactive synaptic plasticity after pilocarpine-induced seizures: functional implications in epilepsy

Several neurological disorders characterized by cognitive deficits, including Alzheimer's disease, down syndrome, and epilepsy exhibit abnormal spine density and/or morphology. Actin-based cytoskeleton network dynamics is critical for the regulation of spine morphology and synaptic function. In this paper, I consider the functions of drebrin A in cell shaping, spine plasticity, and synaptic function. Developmentally regulated brain protein (drebrin A) is one of the most abundant neuron-specific binding proteins of F-actin and its expression is increased in parallel with synapse formation. Drebrin A is particularly concentrated in dendritic spines receiving excitatory inputs. Our recent findings point to a critical role of DA in dendritic spine structural integrity and stabilization, likely via regulation of actin cytoskeleton dynamics, and glutamatergic synaptic function that underlies the development of spontaneous recurrent seizures in pilocarpine-treated animals. Further research into this area may provide useful insights into the pathology of status epilepticus and epileptogenic mechanisms and ultimately may provide the basis for future treatment options.

Cofilin under control of β-arrestin-2 in NMDA-dependent dendritic spine plasticity, long-term depression (LTD), and learning

Dendritic spines are dynamic, actin-rich structures that form the postsynaptic sites of most excitatory synapses in the brain. The F-actin severing protein cofilin has been implicated in the remodeling of dendritic spines and synapses under normal and pathological conditions, by yet unknown mechanisms. Here we report that β-arrestin-2 plays an important role in NMDA-induced remodeling of dendritic spines and synapses via translocation of active cofilin to dendritic spines. NMDAR activation triggers cofilin activation through calcineurin and phosphatidylinositol 3-kinase (PI3K)-mediated dephosphorylation and promotes cofilin translocation to dendritic spines that is mediated by β-arrestin-2. Hippocampal neurons lacking β-arrestin-2 develop mature spines that fail to remodel in response to NMDA. β-Arrestin-2-deficient mice exhibit normal hippocampal long-term potentiation, but significantly impaired NMDA-dependent long-term depression and spatial learning deficits. Moreover, β-arrestin-2-deficient hippocampal neurons are resistant to Aβ-induced dendritic spine loss. Our studies demonstrate unique functions of β-arrestin-2 in NMDAR-mediated dendritic spine and synapse plasticity through spatial control over cofilin activation.

Actin-binding proteins and signalling pathways associated with the formation and maintenance of dendritic spines

INTRODUCTION

Dendritic spines are the main sites of excitatory synaptic contacts. Moreover, they present plastic responses to different stimuli present in synaptic activity or damage, ranging from an increase or decrease in their total number, to redistribution of progenitor dendritic spines, to variations in their size or shape. However, the spines can remain stable for a long time.

BACKGROUND

The use of experimental models has shown that different molecules of the F-actin binding and signalling pathways are closely related to the development, maintenance and plasticity of excitatory synapses, which could affect the number, size and shape of the dendritic spines; these mechanisms affect and depend on the reorganisation of the actin cytoskeleton.

DEVELOPMENT

It is proposed that the filopodia are precursors of dendritic spines. Drebrin is an F-actin binding protein, and it is responsible for concentrating F-actin and PSD-95 in filopodia that will guide the formation of the new spines.

CONCLUSION

The specific mechanisms of actin regulation are an integral part in the formation, maturing process and plasticity of dendritic spines in association with the various actin cytoskeleton-binding proteins The signalling pathways mediated by small GTPases and the equilibrium between G-actin and F-actin are also involved.

Alzheimer's disease-linked presenilin mutation (PS1M146L) induces filamin expression and γ-secretase independent redistribution

Presenilin mutations are linked to the early onset familial Alzheimer's disease (FAD) and lead to a range of neuronal changes, indicating that presenilins interact with multiple cellular pathways to regulate neuronal functions. In this report, we demonstrate the effects of FAD-linked presenilin 1 mutation (PS1M146L) on the expression and distribution of filamin, an actin cross-linking protein that interacts with PS1 both physically and genetically. By using immunohistochemical methods, we evaluated hippocampal dentate gyrus for alterations of proteins involved in synaptic plasticity. Among many proteins expressed in the hippocampus, calretinin, glutamic acid decarboxylase (GAD67), parvalbumin, and filamin displayed distinct changes in their expression and/or distribution patterns. Striking anti-filamin immunoreactivity was associated with the polymorphic cells of hilar region only in transgenic mice expressing PS1M146L. In over 20% of the PS1M146L mice, the hippocampus of the left hemisphere displayed more pronounced upregulation of filamin than that of the right hemisphere. Anti-filamin labeled the hilar neurons only after the PS1M146L mice reached after four months of age. Double labeling immunohistochemical analyses showed that anti-filamin labeled neurons partially overlapped with cholecystokinin (CCK), somatostatin, GAD67, parvalbumin, and calretinin immunoreactive neurons. In cultured HEK293 cells, PS1 overexpression resulted in filamin redistribution from near cell peripheries to cytoplasm. Treatment of CHO cells stably expressing PS1 with WPE-III-31C or DAPT, selective γ-secretase inhibitors, did not suppress the effects of PS1 overexpression on filamin. These studies support a γ-secretase-independent role of PS1 in modulation of filamin-mediated actin cytoskeleton.

Regulation of the postsynaptic cytoskeleton: roles in development, plasticity, and disorders

The small size of dendritic spines belies the elaborate role they play in excitatory synaptic transmission and ultimately complex behaviors. The cytoskeletal architecture of the spine is predominately composed of actin filaments. These filaments, which at first glance might appear simple, are also surprisingly complex. They dynamically assemble into different structures and serve as a platform for orchestrating the elaborate responses of the spine during experience-dependent plasticity. This mini-symposium review will feature ongoing research into how spines are regulated by actin-signaling pathways during development and plasticity. It will also highlight evolving studies into how disruptions to these pathways might be functionally coupled to congenital disorders such as mental retardation.

Regional and temporal profiles of calpain and caspase-3 activities in postnatal rat brain following repeated propofol administration

Exposure of newborn rats to a variety of anesthetics has been shown to induce apoptotic neurodegeneration in the developing brain. We investigated the effect of the general anesthetic propofol on the brain of 7-day-old (P7) Wistar rats during the peak of synaptic growth. Caspase and calpain protease families most likely participate in neuronal cell death. Our objective was to examine regional and temporal patterns of caspase-3 and calpain activity following repeated propofol administration (20 mg/kg). P7 rats were exposed for 2, 4 or 6 h to propofol and killed 0, 4, 16 and 24 h after exposure. Relative caspase-3 and calpain activities were estimated by Western blot analysis of the proteolytic cleavage products of α-II-spectrin, protein kinase C and poly(ADP-ribose) polymerase 1. Caspase-3 activity and expression displayed a biphasic pattern of activation. Calpain activity changed in a region- and time-specific manner that was distinct from that observed for caspase-3. The time profile of calpain activity exhibited substrate specificity. Fluoro-Jade B staining revealed an immediate neurodegenerative response that was in direct relationship to the duration of anesthesia in the cortex and inversely related to the duration of anesthesia in the thalamus. At later post-treatment intervals, dead neurons were detected only in the thalamus 24 h following the 6-hour propofol exposure. Strong caspase-3 expression that was detected at 24 h was not followed by cell death after 2- and 4-hour exposures to propofol. These results revealed complex patterns of caspase-3 and calpain activities following prolonged propofol anesthesia and suggest that both are a manifestation of propofol neurotoxicity at a critical developmental stage.

Plastic synaptic networks of the amygdala for the acquisition, expression, and extinction of conditioned fear

The last 10 years have witnessed a surge of interest for the mechanisms underlying the acquisition and extinction of classically conditioned fear responses. In part, this results from the realization that abnormalities in fear learning mechanisms likely participate in the development and/or maintenance of human anxiety disorders. The simplicity and robustness of this learning paradigm, coupled with the fact that the underlying circuitry is evolutionarily well conserved, make it an ideal model to study the basic biology of memory and identify genetic factors and neuronal systems that regulate the normal and pathological expressions of learned fear. Critical advances have been made in determining how modified neuronal functions upon fear acquisition become stabilized during fear memory consolidation and how these processes are controlled in the course of fear memory extinction. With these advances came the realization that activity in remote neuronal networks must be coordinated for these events to take place. In this paper, we review these mechanisms of coordinated network activity and the molecular cascades leading to enduring fear memory, and allowing for their extinction. We will focus on Pavlovian fear conditioning as a model and the amygdala as a key component for the acquisition and extinction of fear responses.

Accelerators, Brakes, and Gears of Actin Dynamics in Dendritic Spines

Dendritic spines are actin-rich structures that accommodate the postsynaptic sites of most excitatory synapses in the brain. Although dendritic spines form and mature as synaptic connections develop, they remain plastic even in the adult brain, where they can rapidly grow, change, or collapse in response to normal physiological changes in synaptic activity that underlie learning and memory. Pathological stimuli can adversely affect dendritic spine shape and number, and this is seen in neurodegenerative disorders and some forms of mental retardation and autism as well. Many of the molecular signals that control these changes in dendritic spines act through the regulation of filamentous actin (F-actin), some through direct interaction with actin, and others via downstream effectors. For example, cortactin, cofilin, and gelsolin are actin-binding proteins that directly regulate actin dynamics in dendritic spines. Activities of these proteins are precisely regulated by intracellular signaling events that control their phosphorylation state and localization. In this review, we discuss how actin-regulating proteins maintain the balance between F-actin assembly and disassembly that is needed to stabilize mature dendritic spines, and how changes in their activities may lead to rapid remodeling of dendritic spines.

Molecular architecture of synaptic actin cytoskeleton in hippocampal neurons reveals a mechanism of dendritic spine morphogenesis

Excitatory synapses in the brain play key roles in learning and memory. The formation and functions of postsynaptic mushroom-shaped structures, dendritic spines, and possibly of presynaptic terminals, rely on actin cytoskeleton remodeling. However, the cytoskeletal architecture of synapses remains unknown hindering the understanding of synapse morphogenesis. Using platinum replica electron microscopy, we characterized the cytoskeletal organization and molecular composition of dendritic spines, their precursors, dendritic filopodia, and presynaptic boutons. A branched actin filament network containing Arp2/3 complex and capping protein was a dominant feature of spine heads and presynaptic boutons. Surprisingly, the spine necks and bases, as well as dendritic filopodia, also contained a network, rather than a bundle, of branched and linear actin filaments that was immunopositive for Arp2/3 complex, capping protein, and myosin II, but not fascin. Thus, a tight actin filament bundle is not necessary for structural support of elongated filopodia-like protrusions. Dynamically, dendritic filopodia emerged from densities in the dendritic shaft, which by electron microscopy contained branched actin network associated with dendritic microtubules. We propose that dendritic spine morphogenesis begins from an actin patch elongating into a dendritic filopodium, which tip subsequently expands via Arp2/3 complex-dependent nucleation and which length is modulated by myosin II-dependent contractility.

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.

Preso, a novel PSD-95-interacting FERM and PDZ domain protein that regulates dendritic spine morphogenesis

PSD-95 is an abundant postsynaptic density (PSD) protein involved in the formation and regulation of excitatory synapses and dendritic spines, but the underlying mechanisms are not comprehensively understood. Here we report a novel PSD-95-interacting protein Preso that regulates spine morphogenesis. Preso is mainly expressed in the brain and contains WW (domain with two conserved Trp residues), PDZ (PSD-95/Dlg/ZO-1), FERM (4.1, ezrin, radixin, and moesin), and C-terminal PDZ-binding domains. These domains associate with actin filaments, the Rac1/Cdc42 guanine nucleotide exchange factor betaPix, phosphatidylinositol-4,5-bisphosphate, and the postsynaptic scaffolding protein PSD-95, respectively. Preso overexpression increases the density of dendritic spines in a manner requiring WW, PDZ, FERM, and PDZ-binding domains. Conversely, knockdown or dominant-negative inhibition of Preso decreases spine density, excitatory synaptic transmission, and the spine level of filamentous actin. These results suggest that Preso positively regulates spine density through its interaction with the synaptic plasma membrane, actin filaments, PSD-95, and the betaPix-based Rac1 signaling pathway.

Inhibition of Arp2/3-mediated actin polymerization by PICK1 regulates neuronal morphology and AMPA receptor endocytosis

The dynamic regulation of actin polymerization plays crucial roles in cell morphology and endocytosis. The mechanistic details of these processes and the proteins involved are not fully understood, especially in neurons. PICK1 is a PDZ-BAR-domain protein involved in regulated AMPA receptor (AMPAR) endocytosis in neurons. Here, we demonstrate that PICK1 binds filamentous (F)-actin and the actin-nucleating Arp2/3 complex, and potently inhibits Arp2/3-mediated actin polymerization. RNA interference (RNAi) knockdown of PICK1 in neurons induces a reorganization of the actin cytoskeleton resulting in aberrant cell morphology. Wild-type PICK1 rescues this phenotype, but a mutant PICK1, PICK1(W413A), that does not bind or inhibit Arp2/3 has no effect. Furthermore, this mutant also blocks NMDA-induced AMPAR internalization. This study identifies PICK1 as a negative regulator of Arp2/3-mediated actin polymerization that is critical for a specific form of vesicle trafficking, and also for the development of neuronal architecture.

Ubiquitin proteasome-mediated synaptic reorganization: a novel mechanism underlying rapid ischemic tolerance

Ischemic tolerance is an endogenous neuroprotective mechanism in brain and other organs, whereby prior exposure to brief ischemia produces resilience to subsequent normally injurious ischemia. Although many molecular mechanisms mediate delayed (gene-mediated) ischemic tolerance, the mechanisms underlying rapid (protein synthesis-independent) ischemic tolerance are relatively unknown. Here we describe a novel mechanism for the induction of rapid ischemic tolerance mediated by the ubiquitin-proteasome system. Rapid ischemic tolerance is blocked by multiple proteasome inhibitors [carbobenzoxy-L-leucyl-L-leucyl-L-leucinal (MG132), MG115 (carbobenzoxy-L-leucyl-L-leucyl-L-norvalinal), and clasto-lactacystin-beta-lactone]. A proteomics strategy was used to identify ubiquitinated proteins after preconditioning ischemia. We focused our studies on two actin-binding proteins of the postsynaptic density that were ubiquitinated after rapid preconditioning: myristoylated, alanine-rich C-kinase substrate (MARCKS) and fascin. Immunoblots confirm the degradation of MARCKS and fascin after preconditioning ischemia. The loss of actin-binding proteins promoted actin reorganization in the postsynaptic density and transient retraction of dendritic spines. This rapid and reversible synaptic remodeling reduced NMDA-mediated electrophysiological responses and renders the cells refractory to NMDA receptor-mediated toxicity. The dendritic spine retraction and NMDA neuroprotection after preconditioning ischemia are blocked by actin stabilization with jasplakinolide, as well as proteasome inhibition with MG132. Together these data suggest that rapid tolerance results from changes to the postsynaptic density mediated by the ubiquitin-proteasome system, rendering neurons resistant to excitotoxicity.

Drug addiction as a pathology of staged neuroplasticity

Using addictive drugs can evolve from controlled social use into the compulsive relapsing disorder that characterizes addiction. This transition to addiction results from genetic, developmental, and sociological vulnerabilities, combined with pharmacologically induced plasticity in brain circuitry that strengthens learned drug-associated behaviors at the expense of adaptive responding for natural rewards. Advances over the last decade have identified the brain circuits most vulnerable to drug-induced changes, as well as many associated molecular and morphological underpinnings. This growing knowledge has contributed to an expanded understanding of how drugs usurp normal learning circuitry to create the pathology of addiction, as evidenced by involuntary activation of reward circuits in response to drug-associated cues and simultaneous reports of drug craving. This new understanding provides unprecedented potential opportunities for novel pharmacotherapeutic targets in treating addiction. There appears to be plasticity associated with the addiction phenomenon in general as well as changes produced by addiction to a specific class of addicting drugs. These findings also provide the basis for the current understanding of addiction as a chronic, relapsing disease of the brain with changes that persist long after the last use of the drug. Here, we describe the neuroplasticity in brain circuits and cell function induced by addictive drugs that is thought to underlie the compulsions to resume drug-taking, and discuss how this knowledge is impelling exploration and testing of novel addiction therapies.

Role of actin cytoskeleton in dendritic spine morphogenesis

Dendritic spines are the postsynaptic receptive regions of most excitatory synapses, and their morphological plasticity play a pivotal role in higher brain functions, such as learning and memory. The dynamics of spine morphology is due to the actin cytoskeleton concentrated highly in spines. Filopodia, which are thin and headless protrusions, are thought to be precursors of dendritic spines. Drebrin, a spine-resident side-binding protein of filamentous actin (F-actin), is responsible for recruiting F-actin and PSD-95 into filopodia, and is suggested to govern spine morphogenesis. Interestingly, some recent studies on neurological disorders accompanied by cognitive deficits suggested that the loss of drebrin from dendritic spines is a common pathognomonic feature of synaptic dysfunction. In this review, to understand the importance of actin-binding proteins in spine morphogenesis, we first outline the well-established knowledge pertaining to the actin cytoskeleton in non-neuronal cells, such as the mechanism of regulation by small GTPases, the equilibrium between globular actin (G-actin) and F-actin, and the distinct roles of various actin-binding proteins. Then, we review the dynamic changes in the localization of drebrin during synaptogenesis and in response to glutamate receptor activation. Because side-binding proteins are located upstream of the regulatory pathway for actin organization via other actin-binding proteins, we discuss the significance of drebrin in the regulatory mechanism of spine morphology through the reorganization of the actin cytoskeleton. In addition, we discuss the possible involvement of an actin-myosin interaction in the morphological plasticity of spines.

Polychlorinated biphenyls disrupt the actin cytoskeleton in hippocampal neurons

It is well known that developmental exposure to polychlorinated biphenyls (PCBs) could cause learning and memory deficits, but the underlying mechanisms are not clear. Actin cytoskeleton is directly involved in synaptic plasticity which is considered critical to learning and memory formation by LIM kinase 1 (LIMK-1)/cofilin pathway. To determine whether PCBs could alter actin cytoskeleton, we exposed the cultured hippocampal neurons to PCBs mixture Aroclor 1254 (A 1254). By biochemical measurement, fluorimetric assay and fluorescence microscopy, we found that A 1254 elicited a loss of filamentous actin, which preceded cytotoxicity. Western blots showed that a concentration-dependent decrease in the phosphorylation of cofilin and a decrease in LIMK-1 were induced by A 1254. We concluded that PCBs induced actin depolymerization in hippocampal neurons, probably by inhibiting the LIMK-1/cofilin signaling pathway. The above findings offer new perspectives for the understanding of PCBs-induced learning and memory deficits.

Targeted deletion of a single Sca8 ataxia locus allele in mice causes abnormal gait, progressive loss of motor coordination, and Purkinje cell dendritic deficits

Spinocerebellar ataxia type 8 (SCA8) patients typically have a slowly progressive, adult-onset ataxia. SCA8 is dominantly inherited and is caused by large CTG repeat expansions in the untranslated antisense RNA of the Kelch-like 1 gene (KLHL1), but the molecular mechanism through which this expansion leads to disease is still unknown. To more fully characterize the underlying molecular mechanisms involved in SCA8, we developed a mouse model in which Klhl1 is deleted in either all tissues or is deleted specifically in Purkinje cells only. We found that mice that are either homozygous or heterozygous for the Klhl1 deletion have significant gait abnormalities at an early age and develop a significant loss of motor coordination by 24 weeks of age. This loss progresses more rapidly in homozygous knock-outs. Mice with Klhl1 specifically deleted in only Purkinje cells had a loss of motor coordination that was almost identical to the total-tissue deletion mice. Finally, we found significant Purkinje cell dendritic deficits, as measured by the thickness of the molecular layer, in all mice in which Klhl1 was deleted (both total and Purkinje cell-specific deletions) and an intermediate reduction in molecular layer thickness in mice with reduced levels of Klhl1 expression (heterozygous deletions). The results from this mouse model show that even a partial loss of Klhl1 function leads to degeneration of Purkinje cell function and indicates that loss of KLHL1 activity is likely to play a significant part in the underlying pathophysiology of SCA8.

A regulatory role for actin in dendritic spine proliferation

Dendritic spines are small protrusions that receive 90% of excitatory cortical synapses and are critically important to neural function. Each dendritic spine is supported by a dynamic actin cytoskeleton that responds to internal and external cues to allow spine development, elongation, retraction and movement. Multiple proteins have roles in spinogenesis, but until now, a regulatory role for actin itself has not been established. Here, we show that, in the acute slice preparation, actin expression increases during a period of rapid spinogenesis. Furthermore, actin overexpression in organotypic hippocampal cultures leads to a significant increase in spine density on CA1 pyramidal cells. Specifically, the number of filopodia (long, thin protrusions without heads) increases by 38% on secondary apical dendrites and 88% on basal dendrites and the number of elongated spines with heads increases by 162% on secondary apical dendrites and 113% on basal dendrites. Synapsin-I immunostaining demonstrated that the majority of filopodia and elongated spines are apposed by axon terminals. Additionally, we show that overexpressed actin enters both new and established spines within 24 h. These data demonstrate that neurons undertaking spinogenesis upregulate actin expression, that actin overexpression per se increases spine density, and that both new and established spines incorporate exogenous actin.

Phosphorylation of WAVE1 regulates actin polymerization and dendritic spine morphology

WAVE1--the Wiskott-Aldrich syndrome protein (WASP)--family verprolin homologous protein 1--is a key regulator of actin-dependent morphological processes in mammals, through its ability to activate the actin-related protein (Arp2/3) complex. Here we show that WAVE1 is phosphorylated at multiple sites by cyclin-dependent kinase 5 (Cdk5) both in vitro and in intact mouse neurons. Phosphorylation of WAVE1 by Cdk5 inhibits its ability to regulate Arp2/3 complex-dependent actin polymerization. Loss of WAVE1 function in vivo or in cultured neurons results in a decrease in mature dendritic spines. Expression of a dephosphorylation-mimic mutant of WAVE1 reverses this loss of WAVE1 function in spine morphology, but expression of a phosphorylation-mimic mutant does not. Cyclic AMP (cAMP) signalling reduces phosphorylation of the Cdk5 sites in WAVE1, and increases spine density in a WAVE1-dependent manner. Our data suggest that phosphorylation/dephosphorylation of WAVE1 in neurons has an important role in the formation of the filamentous actin cytoskeleton, and thus in the regulation of dendritic spine morphology.

Molecular mechanisms contributing to dendritic spine alterations in the prefrontal cortex of subjects with schizophrenia

Postmortem studies have revealed reduced densities of dendritic spines in the dorsal lateral prefrontal cortex (DLPFC) of subjects with schizophrenia. However, the molecular mechanisms that might contribute to these alterations are unknown. Recent studies of the intracellular signals that regulate spine dynamics have identified members of the RhoGTPase family (e.g., Cdc42, Rac1, RhoA) as critical regulators of spine structure. In addition, Duo and drebrin are spine-specific proteins that are critical for spine maintenance and spine formation, respectively. In order to determine whether the mRNA expression levels of Cdc42, Rac1, RhoA, Duo or drebrin are altered in schizophrenia, tissue sections containing DLPFC area 9 from 15 matched pairs of subjects with schizophrenia and control subjects were processed for in situ hybridization. Expression levels of these mRNAs were also correlated with DLPFC spine density in a subset of the same subjects. In order to assess the potential influence of antipsychotic medications on the expression of these mRNAs, similar studies were conducted in monkeys chronically exposed to haloperidol or olanzapine. The expression of each of these mRNAs was lower in the gray matter of the subjects with schizophrenia compared to the control subjects, although only the reductions in Cdc42 and Duo remained significant after corrections for multiple comparisons. In addition, spine density was strongly correlated with the expression levels of both Duo (r=0.73, P=0.007) and Cdc42 (r=0.71, P=0.009) mRNAs. In contrast, the expression levels of Cdc42 and Duo mRNAs were not altered in monkeys chronically exposed to antipsychotic medications. In conclusion, reduced expression of Cdc42 and Duo mRNAs may represent molecular mechanisms that contribute to the decreased density of dendritic spines in the DLPFC of subjects with schizophrenia.

Cocaine increases actin cycling: effects in the reinstatement model of drug seeking

Addiction to cocaine is associated with persistent changes in synaptic function. The cycling of actin between polymerized [F (for filamentous)] and depolymerized forms contributes to synaptic plasticity, and acute and withdrawal from repeated cocaine administration produced reversible and enduring elevations, respectively, in F-actin in the nucleus accumbens. Increased F-actin after 3 weeks withdrawal from repeated cocaine resulted from changes in the content or phosphorylation state of actin binding proteins (ABPs) that cosediment with F-actin. The profile of altered APBs was consistent with filopodia formation, including increased mammalian Enabled, phosphorylated (p)-cortactin, and p-vasodilator-stimulated phosphoprotein, and increased actin depolymerization [e.g., reduced LIM (Lin11/Isl-1/Mec3)-kinase and p-cofilin]. In contrast to repeated cocaine, the increase in F-actin after acute cocaine administration resulted from reduced depolymerization and actin cycling. The potential involvement of chronic cocaine-induced increases in actin cycling in cocaine addiction was examined using the reinstatement of cocaine seeking in rats previously trained to self-administer cocaine by inhibiting actin polymerization with intra-accumbens latrunculin A or by accelerating actin depolymerization with a LIM-kinase inhibitor. Disrupting actin cycling via either mechanism augmented cocaine-induced reinstatement of drug seeking but did not affect the locomotor response to acute cocaine administration. Thus, withdrawal from repeated cocaine induces a restructuring of actin-ABP complexes, which increases actin cycling and may modulate cocaine-induced reinstatement of drug seeking.

A novel forward genetic screen for identifying mutations affecting larval neuronal dendrite development in Drosophila melanogaster

Vertebrate and invertebrate dendrites are information-processing compartments that can be found on both central and peripheral neurons. Elucidating the molecular underpinnings of information processing in the nervous system ultimately requires an understanding of the genetic pathways that regulate dendrite formation and maintenance. Despite the importance of dendrite development, few forward genetic approaches have been used to analyze the latest stages of dendrite development, including the formation of F-actin-rich dendritic filopodia or dendritic spines. We developed a forward genetic screen utilizing transgenic Drosophila second instar larvae expressing an actin, green fluorescent protein (GFP) fusion protein (actin::GFP) in subsets of sensory neurons. Utilizing this fluorescent transgenic reporter, we conducted a forward genetic screen of >4000 mutagenized chromosomes bearing lethal mutations that affected multiple aspects of larval dendrite development. We isolated 13 mutations on the X and second chromosomes composing 11 complementation groups affecting dendrite outgrowth/branching, dendritic filopodia formation, or actin::GFP localization within dendrites in vivo. In a fortuitous observation, we observed that the structure of dendritic arborization (da) neuron dendritic filopodia changes in response to a changing environment.

Targeting of Proteins of the Striatin Family to Dendritic Spines: Role of the Coiled-Coil Domain

Striatin, SG2NA and zinedin, the three mammalian members of the striatin family are multimodular WD-repeat, calmodulin and calveolin-binding proteins. These scaffolding proteins, involved in both signaling and trafficking, are highly expressed in neurons. Using ultrastructural immunolabeling, we showed that, in Purkinje cells and hippocampal neurons, SG2NA is confined to the somatodendritic compartment with the highest density in dendritic spines. In cultured hippocampal neurons, SG2NA is also highly concentrated in dendritic spines. By expressing truncated forms of HA-tagged SG2NAbeta, we demonstrated that the coiled-coil domain plays an essential role in the targeting of SG2NA within spines. Furthermore, co-immunoprecipitation experiments indicate that this coiled-coil domain is also crucial for the homo- and hetero-oligomerization of these proteins. Thus, oligomerization of the striatin family proteins is probably an obligatory step for their routing to the dendritic spines, and hetero-oligomerization explains why all these proteins are often co-expressed in the neurons of the rat brain and spinal cord.

Synaptic activity and F-actin coordinately regulate CaMKIIalpha localization to dendritic postsynaptic sites in developing hippocampal slices

We examined the timing and mechanisms of CaMKIIalpha recruitment to nascent synapses of developing rat hippocampal pyramidal neurons in slice culture. Time-lapse confocal imaging shows that GFP-CaMKIIalpha in transfected neurons accumulates in spines as they are forming, and loss of CaMKIIalpha coincides with spine destabilization. Immunolabeling shows that endogenous CaMKIIalpha is concentrated at postsynaptic sites in spines under ambient slice culture conditions, and this is not disrupted by short-term (3 h) synaptic activity blockade or Latrunculin-induced F-actin depolymerization. However, the combination of activity blockade and F-actin depolymerization significantly reduces synaptic CaMKIIalpha. Conversely, postsynaptic activation induces synaptic recruitment of CaMKIIalpha even in the presence of F-actin depolymerizing drugs. Thus, synaptic-activity-dependent mechanisms and (synaptic activity-independent) F-actin-based mechanisms are individually sufficient and act in parallel to localize CaMKIIalpha to the dendritic spine compartment. Moreover, the timing of CaMKIIalpha recruitment to developing spines suggests a role for CaMKIIalpha in spine assembly and maintenance.

Polarity proteins in axon specification and synaptogenesis

The neuron is a prime example of a highly polarized cell. It is becoming clear that conserved protein complexes, which have been shown to regulate polarity in such diverse systems as the C. elegans zygote and mammalian epithelia, are also required for neuronal polarization. This review considers the role of these polarity proteins in axon specification and synaptogenesis.

Interactions between CAP70 and actinfilin are important for integrity of actin cytoskeleton structures in neurons

The integrity of dynamic actin structures is coupled to a variety of neurological processes. Actin-binding proteins play a critical role in regulating actin structure dynamics. A link between actin-binding proteins and receptor interacting scaffolding proteins may provide a conduit for transmitting signaling events to the cytoskeleton. Actinfilin is a brain-enriched actin-binding protein, though its functions are currently unknown. We report here that actinfilin interacts with the multi-PDZ domain protein CAP70. Recombinant expression of an actin-binding domain of actinfilin progressively causes marked changes of cellular morphology. The effect on cell morphology may be reduced by co-expression with CAP70. Mutation of actinfilin lacking the ability to interact with CAP70 abolished the effect by CAP70. The evidence suggests a role of actinfilin and possible regulation by scaffolding proteins.

Synaptic targeting by Alzheimer's-related amyloid beta oligomers

The cognitive hallmark of early Alzheimer's disease (AD) is an extraordinary inability to form new memories. For many years, this dementia was attributed to nerve-cell death induced by deposits of fibrillar amyloid beta (Abeta). A newer hypothesis has emerged, however, in which early memory loss is considered a synapse failure caused by soluble Abeta oligomers. Such oligomers rapidly block long-term potentiation, a classic experimental paradigm for synaptic plasticity, and they are strikingly elevated in AD brain tissue and transgenic-mouse AD models. The current work characterizes the manner in which Abeta oligomers attack neurons. Antibodies raised against synthetic oligomers applied to AD brain sections were found to give diffuse stain around neuronal cell bodies, suggestive of a dendritic pattern, whereas soluble brain extracts showed robust AD-dependent reactivity in dot immunoblots. Antigens in unfractionated AD extracts attached with specificity to cultured rat hippocampal neurons, binding within dendritic arbors at discrete puncta. Crude fractionation showed ligand size to be between 10 and 100 kDa. Synthetic Abeta oligomers of the same size gave identical punctate binding, which was highly selective for particular neurons. Image analysis by confocal double-label immunofluorescence established that >90% of the punctate oligomer binding sites colocalized with the synaptic marker PSD-95 (postsynaptic density protein 95). Synaptic binding was accompanied by ectopic induction of Arc, a synaptic immediate-early gene, the overexpression of which has been linked to dysfunctional learning. Results suggest the hypothesis that targeting and functional disruption of particular synapses by Abeta oligomers may provide a molecular basis for the specific loss of memory function in early AD.