Induction of spine growth and synapse formation by regulation of the spine actin cytoskeleton

Photomarking Relocalization Technique for Correlated Two-Photon and Electron Microscopy Imaging of Single Stimulated Synapses

Synapses learn and remember by persistent modifications of their internal structures and composition but, due to their small size, it is difficult to observe these changes at the ultrastructural level in real time. Two-photon fluorescence microscopy (2PM) allows time-course live imaging of individual synapses but lacks ultrastructural resolution. Electron microscopy (EM) allows the ultrastructural imaging of subcellular components but cannot detect fluorescence and lacks temporal resolution. Here we describe a combination of procedures designed to achieve the correlated imaging of the same individual synapse under both 2PM and EM. This technique permits the selective stimulation and live imaging of a single dendritic spine and the subsequent localization of the same spine in EM ultrathin serial sections. Landmarks created through a photomarking method based on the 2-photon-induced precipitation of an electrodense compound are used to unequivocally localize the stimulated synapse. This technique was developed to image, for the first time, the ultrastructure of the postsynaptic density in which long-term potentiation was selectively induced just seconds or minutes before, but it can be applied for the study of any biological process that requires the precise relocalization of micron-wide structures for their correlated imaging with 2PM and EM.

The times they are a-changin': a proposal on how brain flexibility goes beyond the obvious to include the concepts of "upward" and "downward" to neuroplasticity

Since the brain was found to be somehow flexible, plastic, researchers worldwide have been trying to comprehend its fundamentals to better understand the brain itself, make predictions, disentangle the neurobiology of brain diseases, and finally propose up-to-date treatments. Neuroplasticity is simple as a concept, but extremely complex when it comes to its mechanisms. This review aims to bring to light an aspect about neuroplasticity that is often not given enough attention as it should, the fact that the brain's ability to change would include its ability to disconnect synapses. So, neuronal shrinkage, decrease in spine density or dendritic complexity should be included within the concept of neuroplasticity as part of its mechanisms, not as an impairment of it. To that end, we extensively describe a variety of studies involving topics such as neurodevelopment, aging, stress, memory and homeostatic plasticity to highlight how the weakening and disconnection of synapses organically permeate the brain in so many ways as a good practice of its intrinsic physiology. Therefore, we propose to break down neuroplasticity into two sub-concepts, "upward neuroplasticity" for changes related to synaptic construction and "downward neuroplasticity" for changes related to synaptic deconstruction. With these sub-concepts, neuroplasticity could be better understood from a bigger landscape as a vector in which both directions could be taken for the brain to flexibly adapt to certain demands. Such a paradigm shift would allow a better understanding of the concept of neuroplasticity to avoid any data interpretation bias, once it makes clear that there is no morality with regard to the organic and physiological changes that involve dynamic biological systems as seen in the brain.

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.

The branching code: A model of actin-driven dendrite arborization

The cytoskeleton is crucial for defining neuronal-type-specific dendrite morphologies. To explore how the complex interplay of actin-modulatory proteins (AMPs) can define neuronal types in vivo, we focused on the class III dendritic arborization (c3da) neuron of Drosophila larvae. Using computational modeling, we reveal that the main branches (MBs) of c3da neurons follow general models based on optimal wiring principles, while the actin-enriched short terminal branches (STBs) require an additional growth program. To clarify the cellular mechanisms that define this second step, we thus concentrated on STBs for an in-depth quantitative description of dendrite morphology and dynamics. Applying these methods systematically to mutants of six known and novel AMPs, we revealed the complementary roles of these individual AMPs in defining STB properties. Our data suggest that diverse dendrite arbors result from a combination of optimal-wiring-related growth and individualized growth programs that are neuron-type specific.

EVAP: A two-photon imaging tool to study conformational changes in endogenous Kv2 channels in live tissues

A primary goal of molecular physiology is to understand how conformational changes of proteins affect the function of cells, tissues, and organisms. Here, we describe an imaging method for measuring the conformational changes of the voltage sensors of endogenous ion channel proteins within live tissue, without genetic modification. We synthesized GxTX-594, a variant of the peptidyl tarantula toxin guangxitoxin-1E, conjugated to a fluorophore optimal for two-photon excitation imaging through light-scattering tissue. We term this tool EVAP (Endogenous Voltage-sensor Activity Probe). GxTX-594 targets the voltage sensors of Kv2 proteins, which form potassium channels and plasma membrane–endoplasmic reticulum junctions. GxTX-594 dynamically labels Kv2 proteins on cell surfaces in response to voltage stimulation. To interpret dynamic changes in fluorescence intensity, we developed a statistical thermodynamic model that relates the conformational changes of Kv2 voltage sensors to degree of labeling. We used two-photon excitation imaging of rat brain slices to image Kv2 proteins in neurons. We found puncta of GxTX-594 on hippocampal CA1 neurons that responded to voltage stimulation and retain a voltage response roughly similar to heterologously expressed Kv2.1 protein. Our findings show that EVAP imaging methods enable the identification of conformational changes of endogenous Kv2 voltage sensors in tissue.

Alteration of twinfilin1 expression underlies opioid withdrawal-induced remodeling of actin cytoskeleton at synapses and formation of aversive memory

Exposure to drugs of abuse induces alterations of dendritic spine morphology and density that has been proposed to be a cellular basis of long-lasting addictive memory and heavily depend on remodeling of its underlying actin cytoskeleton by the actin cytoskeleton regulators. However, the actin cytoskeleton regulators involved and the specific mechanisms whereby drugs of abuse alter their expression or function are largely unknown. Twinfilin (Twf1) is a highly conserved actin-depolymerizing factor that regulates actin dynamics in organisms from yeast to mammals. Despite abundant expression of Twf1 in mammalian brain, little is known about its importance for brain functions such as experience-dependent synaptic and behavioral plasticity. Here we show that conditioned morphine withdrawal (CMW)-induced synaptic structure and behavior plasticity depends on downregulation of Twf1 in the amygdala of rats. Genetically manipulating Twf1 expression in the amygdala bidirectionally regulates CMW-induced changes in actin polymerization, spine density and behavior. We further demonstrate that downregulation of Twf1 is due to upregulation of miR101a expression via a previously unrecognized mechanism involving CMW-induced increases in miR101a nuclear processing via phosphorylation of MeCP₂ at Ser421. Our findings establish the importance of Twf1 in regulating opioid-induced synaptic and behavioral plasticity and demonstrate its value as a potential therapeutic target for the treatment of opioid addiction.

Mechanical processes underlying precise and robust cell matching

During the development of complicated multicellular organisms, the robust formation of specific cell-cell connections (cell matching) is required for the generation of precise tissue structures. Mismatches or misconnections can lead to various diseases. Diverse mechanical cues, including differential adhesion and temporally varying cell contractility, are involved in regulating the process of cell-cell recognition and contact formation. Cells often start the process of cell matching through contact via filopodia protrusions, mediated by specific adhesion interactions at the cell surface. These adhesion interactions give rise to differential mechanical signals that can be further perceived by the cells. In conjunction with contractions generated by the actomyosin networks within the cells, this differentially coded adhesion information can be translated to reposition and sort cells. Here, we review the role of these different cell matching components and suggest how these mechanical factors cooperate with each other to facilitate specificity in cell-cell contact formation.

Synaptosomal Actin Dynamics in the Developmental Visual Cortex Regulate Behavioral Visual Acuity in Rats

Purpose

Synaptosomal actin dynamics are essential for synaptic structural stability. Whether actin dynamics are involved in structural and functional synaptic plasticity within the primary visual cortex (V1) or behavioral visual acuity in rats has still not been thoroughly investigated.

Methods

Synaptosome preparation and western blot analysis were used to analyze synaptosomal actin dynamics. Transmission electron microscopy was used to detect synaptic density and mitochondrial area alterations. A visual water maze task was applied to assess behavioral visual acuity. Microinjection of the actin polymerization inhibitor or stabilizer detected the effect of actin dynamics on visual function.

Results

Actin dynamics, the mitochondrial area, and synaptic density within the area of V1 are increased during the critical period for the development of binocularity. Microinjection of the actin polymerization inhibitor cytochalasin D into the V1 decreased the mitochondrial area, synaptic density, and behavioral visual acuity. Long-term monocular deprivation reduced actin dynamics, the mitochondrial area, and synaptic density within the V1 contralateral to the deprived eye compared with those ipsilateral to the deprived eye and impaired visual acuity in the amblyopic eye. In addition, the mitochondrial area, synaptic density, and behavioral visual acuity were improved by stabilization of actin polymerization by jasplakinolide microinjection.

Conclusions

During the critical period of visual development of binocularity, synaptosomal actin dynamics regulate synaptic structure and function and play roles in behavioral visual acuity in rats.

Stable but not rigid: Chronic in vivo STED nanoscopy reveals extensive remodeling of spines, indicating multiple drivers of plasticity

Excitatory synapses on dendritic spines of pyramidal neurons are considered a central memory locus. To foster both continuous adaption and the storage of long-term information, spines need to be plastic and stable at the same time. Here, we advanced in vivo STED nanoscopy to superresolve distinct features of spines (head size and neck length/width) in mouse neocortex for up to 1 month. While LTP-dependent changes predict highly correlated modifications of spine geometry, we find both, uncorrelated and correlated dynamics, indicating multiple independent drivers of spine remodeling. The magnitude of this remodeling suggests substantial fluctuations in synaptic strength. Despite this high degree of volatility, all spine features exhibit persistent components that are maintained over long periods of time. Furthermore, chronic nanoscopy uncovers structural alterations in the cortex of a mouse model of neurodegeneration. Thus, at the nanoscale, stable dendritic spines exhibit a delicate balance of stability and volatility.

Regulation of Synaptic Transmission and Plasticity by Protein Phosphatase 1

Protein phosphatases, by counteracting protein kinases, regulate the reversible phosphorylation of many substrates involved in synaptic plasticity, a cellular model for learning and memory. A prominent phosphatase regulating synaptic plasticity and neurologic disorders is the serine/threonine protein phosphatase 1 (PP1). PP1 has three isoforms (α, β, and γ, encoded by three different genes), which are regulated by a vast number of interacting subunits that define their enzymatic substrate specificity. In this review, we discuss evidence showing that PP1 regulates synaptic transmission and plasticity, as well as presenting novel models of PP1 regulation suggested by recent experimental evidence. We also outline the required targeting of PP1 by neurabin and spinophilin to achieve substrate specificity at the synapse to regulate AMPAR and NMDAR function. We then highlight the role of inhibitor-2 in regulating PP1 function in plasticity, including its positive regulation of PP1 function in vivo in memory formation. We also discuss the distinct function of the three PP1 isoforms in synaptic plasticity and brain function, as well as briefly discuss the role of inhibitory phosphorylation of PP1, which has received recent emphasis in the regulation of PP1 activity in neurons.

Ube2b-dependent degradation of DNMT3a relieves a transcriptional brake on opiate-induced synaptic and behavioral plasticity

Compelling evidence suggests that synaptic structural plasticity, driven by remodeling of the actin cytoskeleton, underlies addictive drugs-induced long-lasting behavioral plasticity. However, the signaling mechanisms leading to actin cytoskeleton remodeling remain poorly defined. DNA methylation is a critical mechanism used to control activity-dependent gene expression essential for long-lasting synaptic plasticity. Here, we provide evidence that DNA methyltransferase DNMT3a is degraded by the E2 ubiquitin-conjugating enzyme Ube2b-mediated ubiquitination in dorsal hippocampus (DH) of rats that repeatedly self-administrated heroin. DNMT3a degradation leads to demethylation in CaMKK1 gene promotor, thereby facilitating CaMKK1 expression and consequent activation of its downstream target CaMKIα, an essential regulator of spinogenesis. CaMKK1/CaMKIα signaling regulates actin cytoskeleton remodeling in the DH and behavioral plasticity by activation of Rac1 via acting Rac guanine-nucleotide-exchange factor βPIX. These data suggest that Ube2b-dependent degradation of DNMT3a relieves a transcriptional brake on CaMKK1 gene and thus activates CaMKK1/CaMKIα/βPIX/Rac1 cascade, leading to drug use-induced actin polymerization and behavior plasticity.

Spinophilin modulates pain through suppressing dendritic spine morphogenesis via negative control of Rac1-ERK signaling in rat spinal dorsal horn

Both spinophilin (SPN, also known as neurabin 2) and Rac1 (a member of Rho GTPase family) are believed to play key roles in dendritic spine (DS) remodeling and spinal nociception. However, how SPN interacts with Rac1 in the above process is unknown. Here, we first demonstrated natural existence of SPN-protein phosphatase 1-Rac1 complex in the spinal dorsal horn (DH) neurons by both double immunofluorescent labeling and co-immunoprecipitation, then the effects of SPN over-expression and down-regulation on mechanical and thermal pain sensitivity, GTP-bound Rac1-ERK signaling activity, and spinal DS density were studied. Over-expression of SPN in spinal neurons by intra-DH pAAV-CMV-SPN-3FLAG could block both mechanical and thermal pain hypersensitivity induced by intraplantar bee venom injection, however it had no effect on the basal pain sensitivity. Over-expression of SPN also resulted in a significant decrease in GTP-Rac1-ERK activities, relative to naive and irrelevant control (pAAV-MCS). In sharp contrast, knockdown of SPN in spinal neurons by intra-DH pAAV-CAG-eGFP-U6-shRNA[SPN] produced both pain hypersensitivity and dramatic elevation of GTP-Rac1-ERK activities, relative to naive and irrelevant control (pAAV-shRNA [NC]). Moreover, knockdown of SPN resulted in increase in DS density while over-expression of it had no such effect. Collectively, SPN is likely to serve as a regulator of Rac1 signaling to suppress DS morphogenesis via negative control of GTP-bound Rac1-ERK activities at postsynaptic component in rat DH neurons wherein both mechanical and thermal pain sensitivity are controlled.

Modulation of dendritic spines by protein phosphatase-1

Protein phosphatase-1 (PP-1), a highly conserved multifunctional serine/threonine phosphatase, is enriched in dendritic spines where it plays a major role in modulating excitatory synaptic activity. In addition to established functions in spine maturation and development, multi-subunit holoenzyme forms of PP-1 modulate higher-order cognitive functions such learning and memory. Mechanisms involved in regulating PP-1 activity and localization in spines include interactions with neurabin and spinophilin, structurally related synaptic scaffolding proteins associated with the actin cytoskeleton. Since PP-1 is a critical element in synaptic development, signaling, and plasticity, alterations in PP-1 signaling in dendritic spines are implicated in various neurological and psychiatric disorders. The effects of PP-1 depend on its isoform-specific association with regulatory proteins and activation of downstream signaling pathways. Here we review the role of PP-1 and its binding proteins neurabin and spinophilin in both developing and established dendritic spines, as well as some of the disorders that result from its dysregulation.

Proline-rich transmembrane protein 2 (PRRT2) regulates the actin cytoskeleton during synaptogenesis

Mutations in proline-rich transmembrane protein 2 (PRRT2) have been recently identified as the leading cause of a clinically heterogeneous group of neurological disorders sharing a paroxysmal nature, including paroxysmal kinesigenic dyskinesia and benign familial infantile seizures. To date, studies aimed at understanding its physiological functions in neurons have mainly focused on its ability to regulate neurotransmitter release and neuronal excitability. Here, we show that PRRT2 expression in non-neuronal cell lines inhibits cell motility and focal adhesion turnover, increases cell aggregation propensity, and promotes the protrusion of filopodia, all processes impinging on the actin cytoskeleton. In primary hippocampal neurons, PRRT2 silencing affects the synaptic content of filamentous actin and perturbs actin dynamics. This is accompanied by defects in the density and maturation of dendritic spines. We identified cofilin, an actin-binding protein abundantly expressed at the synaptic level, as the ultimate effector of PRRT2. Indeed, PRRT2 silencing unbalances cofilin activity leading to the formation of cofilin-actin rods along neurites. The expression of a cofilin phospho-mimetic mutant (cof-S3E) is able to rescue PRRT2-dependent defects in synapse density, spine number and morphology, but not the alterations observed in neurotransmitter release. Our data support a novel function of PRRT2 in the regulation of the synaptic actin cytoskeleton and in the formation of synaptic contacts.

Shedding Light on the Transcriptomic Dark Matter in Biological Psychiatry: Role of Long Noncoding RNAs in D-cycloserine-Induced Fear Extinction in Posttraumatic Stress Disorder

Biological psychiatry scholarship on posttraumatic stress disorder (PTSD) is making strides with new omics technologies. In this context, there is growing recognition that noncoding RNAs are vital for the regulation of gene and protein expression. Long noncoding RNAs (lncRNAs) can modulate splicing, influence RNA editing, messenger RNA (mRNA) stability, translation activation, and microRNA-mRNA interactions, are highly abundant in the brain, and have been implicated in neurodevelopmental disorders. The largest subclass of lncRNAs is long intergenic noncoding RNAs (lincRNAs). We report on lincRNAs and their predicted mRNA targets associated with fear extinction induced by co-administration of D-cycloserine and behavioral fear extinction in a PTSD animal model. Forty-three differentially expressed lincRNAs and 190 differentially expressed mRNAs were found to be associated with fear extinction. Eight lincRNAs were predicted to interact with and regulate 108 of these mRNAs, while seven lincRNAs were predicted to interact with 22 of their pre-mRNA transcripts. Based on the functions of their target mRNAs, we inferred that these lincRNAs bind to nucleotides, ribonucleotides, and proteins; subsequently influence nervous system development, morphology, and immune system functioning; and could be associated with nervous system and mental health disorders. We found the quantitative trait loci that overlapped with fear extinction-related lincRNAs included traits such as serum corticosterone level, neuroinflammation, anxiety, stress, and despair-related responses. To the best of our knowledge, this is the first study to identify lincRNAs and their RNA targets with a putative role in transcriptional regulation during fear extinction in the context of an animal model of PTSD.

MicroRNA134 of Ventral Hippocampus Is Involved in Cocaine Extinction-Induced Anxiety-like and Depression-like Behaviors in Mice

We previously found that cocaine abuse could increase microRNA134 (miR134) levels in the hippocampus; yet the roles of miR134 in cocaine-related abnormal psychiatric outcomes remain unknown. In this study, using the cocaine-induced conditioned place preference (CPP) mice model, we found that mice exhibit enhanced anxiety-like and depression-like behaviors during the cocaine extinction (CE) period of CPP, accompanied by obviously increased miR134 levels and decreased levels of 19 genes that are associated with synaptic plasticity, glia activity, and neurochemical microenvironments, in the ventral hippocampus (vHP). Knockdown of miR134 in vHP in vivo reversed the changes in 15 of 19 potential gene targets of miR134 and rescued the abnormal anxiety-like and depression-like behavioral outcomes in CE mice. In parallel, knockdown of miR134 reversed CE-induced changes in dendritic spines and synaptic proteins and increased the field excitatory postsynaptic potential (fEPSP) of CA1 pyramidal neurons in the vHP of CE mice. In addition, knockdown of miR134 suppressed the CE-enhanced microglia activity, inflammatory, apoptotic, and oxidative stress statuses in the vHP. With the data taken together, miR134 may be involved in cocaine-associated psychiatric problems, potentially via regulating the expressions of its gene targets that are related to synaptic plasticity and neurochemical microenvironments.

Proteomic analysis of male rat nucleus accumbens, dorsal hippocampus and amygdala on conditioned place aversion induced by morphine withdrawal

Addiction is characterized by compulsive drug seeking and taking behavior, which is thought to result from persistent neuroadaptations, encoded by changes of gene expression. We previously demonstrated that the changes in synaptic plasticity were required for the formation of aversive memories associated with morphine withdrawal. However, the proteins involved in synaptic plasticity and aversive memory formation have not been well explored. In the present study, we employed a two-dimensional gel electrophoresis (2-DE)-based proteomic technique to detect the changes of protein expression in the nucleus accumbens, amygdala and dorsal hippocampus of the rats that had developed conditioned morphine withdrawal. We found that twenty-three proteins were significantly altered in the amygdala and dorsal hippocampus after conditioned morphine withdrawal. These proteins can be classified into multiple categories, such as energy metabolism, signal transduction, synaptic transmission, cytoskeletal proteins, chaperones, and protein metabolism according to their biological functions. Eight proteins related to synaptic plasticity were further confirmed by western blot analysis. It is very likely that these identified proteins may contribute to conditioned morphine withdrawal-induced neural plasticity and aversive memory formation. Thus, our work will help understand the potential mechanism associated with generation of drug withdrawal memories.

Rapid effect of bisphenol A on glutamate-induced Ca2+ influx in hippocampal neurons of rats

Intracellular Ca²⁺ signaling plays an essential role in synaptic plasticity. This study examined the effect of BPA on concentration of intracellular Ca²⁺ ([Ca²⁺]_i) by measuring fluorescence intensity of Ca²⁺ in hippocampal neurons in vitro. The results showed that BPA for 30 min exerted dose-dependently dual effects on glutamate-elevated [Ca²⁺]_i: BPA at 1-10 μM suppressed but at 1-100 nM enhanced glutamate-raised [Ca²⁺]_i. BPA-potentiated [Ca²⁺]_i was blocked by the antagonist of NMDA receptor and was eliminated by an estrogen-related receptor gamma (ERRγ) antagonist rather than an AR antagonist. Both inhibitors of MAPK/ERKs and MAPK/p38 blocked BPA-enhanced [Ca²⁺]_i. Co-treatment of BPA with 17β-E₂ or DHT eliminated the enhancement of 17β-E₂, DHT, and BPA in glutamate-elevated [Ca²⁺]_i. These results suggest that BPA at nanomole level rapidly enhances Ca²⁺ influx through NMDA receptor by ERRγ-mediated MAPK/ERKs and MAPK/p38 signaling pathways. However, BPA antagonizes both estrogen and androgen enhancing NMDA receptor-mediated Ca²⁺ influx in hippocampal neurons.

Development of the Adverse Outcome Pathway (AOP): Chronic binding of antagonist to N-methyl-d-aspartate receptors (NMDARs) during brain development induces impairment of learning and memory abilities of children

The Adverse Outcome Pathways (AOPs) are designed to provide mechanistic understanding of complex biological systems and pathways of toxicity that result in adverse outcomes (AOs) relevant to regulatory endpoints. AOP concept captures in a structured way the causal relationships resulting from initial chemical interaction with biological target(s) (molecular initiating event) to an AO manifested in individual organisms and/or populations through a sequential series of key events (KEs), which are cellular, anatomical and/or functional changes in biological processes. An AOP provides the mechanistic detail required to support chemical safety assessment, the development of alternative methods and the implementation of an integrated testing strategy. An example of the AOP relevant to developmental neurotoxicity (DNT) is described here following the requirements of information defined by the OECD Users' Handbook Supplement to the Guidance Document for developing and assessing AOPs. In this AOP, the binding of an antagonist to glutamate receptor N-methyl-d-aspartate (NMDAR) receptor is defined as MIE. This MIE triggers a cascade of cellular KEs including reduction of intracellular calcium levels, reduction of brain derived neurotrophic factor release, neuronal cell death, decreased glutamate presynaptic release and aberrant dendritic morphology. At organ level, the above mentioned KEs lead to decreased synaptogenesis and decreased neuronal network formation and function causing learning and memory deficit at organism level, which is defined as the AO. There are in vitro, in vivo and epidemiological data that support the described KEs and their causative relationships rendering this AOP relevant to DNT evaluation in the context of regulatory purposes.

Dendritic Spine Elimination: Molecular Mechanisms and Implications

Dynamic modification of synaptic connectivity in response to sensory experience is a vital step in the refinement of brain circuits as they are established during development and modified during learning. In addition to the well-established role for new spine growth and stabilization in the experience-dependent plasticity of neural circuits, dendritic spine elimination has been linked to improvements in learning, and dysregulation of spine elimination has been associated with intellectual disability and behavioral impairment. Proper brain function requires a tightly regulated balance between spine formation and spine elimination. Although most studies have focused on the mechanisms of spine formation, considerable progress has been made recently in delineating the neural activity patterns and downstream molecular mechanisms that drive dendritic spine elimination. Here, we review the current state of knowledge concerning the signaling pathways that drive dendritic spine shrinkage and elimination in the cerebral cortex and we discuss their implication in neuropsychiatric and neurodegenerative disease.

Low-level Gestational Lead Exposure Alters Dendritic Spine Plasticity in the Hippocampus and Reduces Learning and Memory in Rats

Lead (Pb) is known to impair children's cognitive function. It has been previously shown that developmental Pb exposure alters dendritic spine formation in hippocampal pyramidal neurons. However, the underlying mechanism has not yet been defined. In this study, a low-level gestational Pb exposure (GLE) rat model was employed to investigate the impact of Pb on the spine density of the hippocampal pyramidal neurons and its regulatory mechanism. Pb exposure resulted in impaired performance of the rats in the Morris water maze tasks, and in decreased EPSC amplitudes in hippocampal CA3-CA1 regions. With a 3D reconstruction by the Imaris software, the results from Golgi staining showed that the spine density in the CA1 region was reduced in the Pb-exposed rats in a dose-dependent manner. Decreased spine density was also observed in cultured hippocampal neurons following the Pb treatment. Furthermore, the expression level of NLGN1, a postsynaptic protein that mediates synaptogenesis, was significantly decreased following the Pb exposure both in vivo and in vitro. Up-regulation of NLGN1 in cultured primary neurons partially attenuated the impact of Pb on the spine density. Taken together, our resultssuggest that Pb exposure alters spine plasticity in the developing hippocampus by down-regulating NLGN1 protein levels.

Distinct Roles of Protein Phosphatase 1 Bound on Neurabin and Spinophilin and Its Regulation in AMPA Receptor Trafficking and LTD Induction

Protein phosphatase-1 (PP1) constrains learning and memory formation in part through its effects on the induction threshold of long-term potentiation (LTP) and depression (LTD). LTD induction requires both the enzymatic activity of PP1 and its proper anchoring to synaptic spines. We have shown previously that neurabin, a major synaptic scaffolding protein, targets PP1 to synapses for LTD induction. Here, we show that PP1 bound on spinophilin, a close homolog of neurabin and another major synaptic PP1 anchoring protein, does not play a role in LTD induction, which suggests that neurabin plays a privileged role in nanodomain targeting of PP1 in LTD induction. We found that protein kinase A can significantly weaken the neurabin-PP1 interaction in neurons via phosphorylation of neurabin at serine 461, a phosphorylation site adjacent to the PP1-binding motif that is not conserved in spinophilin. Finally, we found that a neurabin mutation (S461E), which mimics phosphorylation, blocked AMPA receptor endocytosis and LTD induction. The results indicate the critical importance of nanodomain targeting of PP1 within synaptic spines and its regulation in LTD induction.

Altered Development of Synapse Structure and Function in Striatum Caused by Parkinson's Disease-Linked LRRK2-G2019S Mutation

Mutations in the gene encoding leucine-rich repeat kinase 2 (LRRK2) can cause Parkinson's disease (PD), and the most common disease-associated mutation, G2019S, increases kinase activity. Because LRRK2 expression levels rise during synaptogenesis and are highest in dorsal striatal spiny projection neurons (SPNs), we tested the hypothesis that the LRRK2-G2019S mutation would alter development of excitatory synaptic networks in dorsal striatum. To circumvent experimental confounds associated with LRRK2 overexpression, we used mice expressing LRRK2-G2019S or D2017A (kinase-dead) knockin mutations. In whole-cell recordings, G2019S SPNs exhibited a fourfold increase in sEPSC frequency compared with wild-type SPNs in postnatal day 21 mice. Such heightened neural activity was increased similarly in direct- and indirect-pathway SPNs, and action potential-dependent activity was particularly elevated. Excitatory synaptic activity in D2017A SPNs was similar to wild type, indicating a selective effect of G2019S. Acute exposure to LRRK2 kinase inhibitors normalized activity, supporting that excessive neural activity in G2019S SPNs is mediated directly and is kinase dependent. Although dendritic arborization and densities of excitatory presynaptic terminals and postsynaptic dendritic spines in G2019S SPNs were similar to wild type, G2019S SPNs displayed larger spines that were matched functionally by a shift toward larger postsynaptic response amplitudes. Acutely isolating striatum from overlying neocortex normalized sEPSC frequency in G2019S mutants, supporting that abnormal corticostriatal activity is involved. These findings indicate that the G2019S mutation imparts a gain-of-abnormal function to SPN activity and morphology during a stage of development when activity can permanently modify circuit structure and function.

SIGNIFICANCE STATEMENT

Mutations in the kinase domain of leucine-rich repeat kinase 2 (LRRK2) follow Parkinson's disease (PD) heritability. How such mutations affect brain function is poorly understood. LRRK2 expression levels rise after birth at a time when synapses are forming and are highest in dorsal striatum, suggesting that LRRK2 regulates development of striatal circuits. During a period of postnatal development when activity plays a large role in permanently shaping neural circuits, our data show how the most common PD-causing LRRK2 mutation dramatically alters excitatory synaptic activity and the shape of postsynaptic structures in striatum. These findings provide new insight into early functional and structural aberrations in striatal connectivity that may predispose striatal circuitry to both motor and nonmotor dysfunction later in life.

Age-dependent differential regulation of anxiety- and depression-related behaviors by neurabin and spinophilin

Affective disorders impact nearly 10% of the adult population in the United States in a given year. Synaptic dysfunction has recently emerged as a key neurobiological mechanism underlying affective disorders such as anxiety and depression. In this study, we investigate the potential role of two synaptic scaffolding proteins, neurabin and spinophilin, in regulating anxiety- and depression-related behaviors at different ages using genetically deficient mice. Loss of the neurabin gene reduces anxiety-like behavior in the elevated zero maze in young adult mice (3-5 months old), but not in middle aged mice (11-13 months old), whereas loss of spinophilin decreases anxiety in middle-aged mice, but not in young adult mice. Neurabin knockout (KO) mice also show reduced immobility in the repeated force swim test (FST) at 3-5 months, but not 11-3 months, of age, compared to age- and strain-matched wild type (WT) controls. Conversely, spinophilin KO mice display a lower level of this behavioral despair than matched WT controls after repeated FST trials at the middle age (11-13 months) but not the young age (3-5 months). Together, these data indicate that, despite their structural similarities and overlapping function in regulating synaptic cytoskeleton, the two homologs neurabin and spinophilin play important yet distinct roles in the regulation of anxiety- and depression-like behaviors in an age-dependent manner. Our studies provide new insights into the complex neurobiology of affective disorders.

Dendritic mitoflash as a putative signal for stabilizing long-term synaptic plasticity

Mitochondrial flashes (mitoflashes) are recently discovered excitable mitochondrial events in many cell types. Here we investigate their occurrence in the context of structural long-term potentiation (sLTP) at hippocampal synapses. At dendritic spines stimulated by electric pulses, glycine, or targeted glutamate uncaging, induction of sLTP is associated with a phasic occurrence of local, quantized mitochondrial activity in the form of one or a few mitoflashes, over a 30-min window. Low-dose nigericin or photoactivation that elicits mitoflashes stabilizes otherwise short-term spine enlargement into sLTP. Meanwhile, scavengers of reactive oxygen species suppress mitoflashes while blocking sLTP. With targeted photoactivation of mitoflashes, we further show that the stabilization of sLTP is effective within the critical 30-min time-window and a spatial extent of ~2 μm, similar to that of local diffusive reactive oxygen species. These findings indicate a potential signaling role of dendritic mitochondria in synaptic plasticity, and provide new insights into the cellular function of mitoflashes.

Mitoflashes are dynamic events in mitochondria, associated with depolarization and release of reactive oxygen species, and have been associated with several cellular functions. The authors now show that in neurons, dendritic mitoflashes are involved in structural postsynaptic changes during LTP.

Protracted and asynchronous accumulation of PSD95-family MAGUKs during maturation of nascent dendritic spines

The formation and stabilization of new dendritic spines is a key component of the experience-dependent neural circuit plasticity that supports learning, but the molecular maturation of nascent spines remains largely unexplored. The PSD95-family of membrane-associated guanylate kinases (PSD-MAGUKs), most notably PSD95, has a demonstrated role in promoting spine stability. However, nascent spines contain low levels of PSD95, suggesting that other members of the PSD-MAGUK family might act to stabilize nascent spines in the early stages of spiny synapse formation. Here, we used GFP-fusion constructs to quantitatively define the molecular composition of new spines, focusing on the PSD-MAGUK family. We found that PSD95 levels in new spines were as low as those previously associated with rapid subsequent spine elimination, and new spines did not achieve mature levels of PSD95 until between 12 and 20 h following new spine identification. Surprisingly, we found that the PSD-MAGUKs PSD93, SAP97, and SAP102 were also substantially less enriched in new spines. However, they accumulated in new spines more quickly than PSD95: SAP102 enriched to mature levels within 3 h, SAP97 and PSD93 enriched gradually over the course of 6 h. Intriguingly, when we restricted our analysis to only those new spines that persisted, SAP97 was the only PSD-MAGUK already present at mature levels in persistent new spines when first identified. Our findings uncover a key structural difference between nascent and mature spines, and suggest a mechanism for the stabilization of nascent spines through the sequential arrival of PSD-MAGUKs. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 77: 1161-1174, 2017.

Photomarking Relocalization Technique for Correlated Two-Photon and Electron Microcopy Imaging of Single Stimulated Synapses

Synapses learn and remember by persistent modifications of their internal structures and composition but, due to their small size, it is difficult to observe these changes at the ultrastructural level in real time. Two-photon fluorescence microscopy (2PM) allows time-course live imaging of individual synapses but lacks ultrastructural resolution. Electron microscopy (EM) allows the ultrastructural imaging of subcellular components but cannot detect fluorescence and lacks temporal resolution. Here, we describe a combination of procedures designed to achieve the correlated imaging of the same individual synapse under both 2PM and EM. This technique permits the selective stimulation and live imaging of a single dendritic spine and the subsequent localization of the same spine in EM ultrathin serial sections. Landmarks created through a photomarking method based on the 2-photon-induced precipitation of an electrodense compound are used to unequivocally localize the stimulated synapse. This technique was developed to image, for the first time, the ultrastructure of the postsynaptic density in which long-term potentiation was selectively induced just seconds or minutes before, but it can be applied for the study of any biological process that requires the precise relocalization of micron-wide structures for their correlated imaging with 2PM and EM.

Organization and dynamics of the actin cytoskeleton during dendritic spine morphological remodeling

In the central nervous system, most excitatory post-synapses are small subcellular structures called dendritic spines. Their structure and morphological remodeling are tightly coupled to changes in synaptic transmission. The F-actin cytoskeleton is the main driving force of dendritic spine remodeling and sustains synaptic plasticity. It is therefore essential to understand how changes in synaptic transmission can regulate the organization and dynamics of actin binding proteins (ABPs). In this review, we will provide a detailed description of the organization and dynamics of F-actin and ABPs in dendritic spines and will discuss the current models explaining how the actin cytoskeleton sustains both structural and functional synaptic plasticity.

New Perspectives on Genomic Imprinting, an Essential and Multifaceted Mode of Epigenetic Control in the Developing and Adult Brain

Mammalian evolution entailed multiple innovations in gene regulation, including the emergence of genomic imprinting, an epigenetic regulation leading to the preferential expression of a gene from its maternal or paternal allele. Genomic imprinting is highly prevalent in the brain, yet, until recently, its central roles in neural processes have not been fully appreciated. Here, we provide a comprehensive survey of adult and developmental brain functions influenced by imprinted genes, from neural development and wiring to synaptic function and plasticity, energy balance, social behaviors, emotions, and cognition. We further review the widespread identification of parental biases alongside monoallelic expression in brain tissues, discuss their potential roles in dosage regulation of key neural pathways, and suggest possible mechanisms underlying the dynamic regulation of imprinting in the brain. This review should help provide a better understanding of the significance of genomic imprinting in the normal and pathological brain of mammals including humans.

Two-Photon Correlation Spectroscopy in Single Dendritic Spines Reveals Fast Actin Filament Reorganization during Activity-Dependent Growth

Two-photon fluorescence correlation spectroscopy (2P-FCS) within single dendritic spines of living hippocampal pyramidal neurons was used to resolve various subpopulations of mobile F-actin during activity-dependent structural changes such as potentiation induced spine head growth. Two major classes of mobile F-actin were discovered: very dynamic and about a hundred times less dynamic F-actin. Spine head enlargement upon application of Tetraethylammonium (TEA), a protocol previously used for the chemical induction of long-term potentiation (cLTP) strictly correlated to changes in the dynamics and filament numbers in the different actin filament fractions. Our observations suggest that spine enlargement is governed by a mechanism in which longer filaments are first cut into smaller filaments that cooperate with the second, increasingly dynamic shorter actin filament population to quickly reorganize and expand the actin cytoskeleton within the spine head. This process would allow a fast and efficient spine head enlargement using a major fraction of the actin filament population that was already present before spine head growth.

Altered prefrontal cortical MARCKS and PPP1R9A mRNA expression in schizophrenia and bipolar disorder

BACKGROUND

We previously observed dendritic spine loss in the dorsolateral prefrontal cortex (DLPFC) from schizophrenia and bipolar disorder subjects. In the current study, we sought to determine if the mRNA expression of genes known to regulate the actin cytoskeleton and spines correlated with spine loss.

METHODS

Five candidate genes were identified using previously obtained microarray data from the DLPFC from schizophrenia and control subjects. The relative mRNA expression of the genes linked to dendritic spine growth and function, i.e. IGF1R, MARCKS, PPP1R9A, PTPRF, and ARHGEF2, was assessed using quantitative real-time PCR (qRT-PCR) in the DLPFC from a second cohort including schizophrenia, bipolar disorder, and control subjects. Functional pathway analysis was conducted to determine which actin cytoskeleton-regulatory pathways the genes of interest interact with.

RESULTS

MARCKS mRNA expression was increased in both schizophrenia and bipolar disorder subjects. PPP1R9A mRNA expression was increased in bipolar disorder subjects. For IGF1R, mRNA expression did not differ significantly among groups; however, it did show a significant, negative correlation with dendrite length. MARCKS and PPP1R9A mRNA expression did not correlate with spine loss, but they interact with NMDA receptor signaling pathways that regulate the actin cytoskeleton and spines.

CONCLUSIONS

MARCKS and PPP1R9A might contribute to spine loss in schizophrenia and bipolar disorder through their interactions, possibly indirect ones, with NMDA signaling pathways that regulate spine structure and function.

Trip6 promotes dendritic morphogenesis through dephosphorylated GRIP1-dependent myosin VI and F-actin organization

Thyroid receptor-interacting protein 6 (Trip6), a multifunctional protein belonging to the zyxin family of LIM proteins, is involved in various physiological and pathological processes, including cell migration and tumorigenesis. However, the role of Trip6 in neurons remains unknown. Here, we show that Trip6 is expressed in mouse hippocampal neurons and promotes dendritic morphogenesis. Through interaction with the glutamate receptor-interacting protein 1 (GRIP1) and myosin VI, Trip6 is crucial for the total dendritic length and the number of primary dendrites in cultured hippocampal neurons. Trip6 depletion reduces F-actin content and impairs dendritic morphology, and this phenocopies GRIP1 or myosin VI knockdown. Furthermore, phosphorylation of GRIP1(956T) by AKT1 inhibits the interaction between GRIP1 and myosin VI, but facilitates GRIP1 binding to 14-3-3 protein, which is required for regulating F-actin organization and dendritic morphogenesis. Thus, the Trip6-GRIP1-myosin VI interaction and its regulation on F-actin network play a significant role in dendritic morphogenesis.

Role of Wnt/β-catenin signaling in the protective effect of epigallocatechin-3-gallate on lead-induced impairments of spine formation in the hippocampus of rats

Epigallocatechin-3-gallate (EGCG) may significantly reverse Pb-related spine damage in developing rats by increasing the expression of Wnt7a and the activity of the Wnt/β-catenin pathway.

Developmental lead exposure alters synaptogenesis through inhibiting canonical Wnt pathway in vivo and in vitro

Lead (Pb) exposure has been implicated in the impairment of synaptic plasticity in the developing hippocampus, but the mechanism remains unclear. Here, we investigated whether developmental lead exposure affects the dendritic spine formation through Wnt signaling pathway in vivo and in vitro. Sprague–Dawley rats were exposed to lead throughout the lactation period and Golgi-Cox staining method was used to examine the spine density of pyramidal neurons in the hippocampal CA1 area of rats. We found that lead exposure significantly decreased the spine density in both 14 and 21 days-old pups, accompanied by a significant age-dependent decline of the Wnt7a expression and stability of its downstream protein (β-catenin). Furthermore, in cultured hippocampal neurons, lead (0.1 and 1 µM lead acetate) significantly decreased the spine density in a dose-dependent manner. Exogenous Wnt7a application attenuated the decrease of spine density and increased the stability of the downstream molecules in Wnt signaling pathway. Together, our results suggest that lead has a negative impact on spine outgrowth in the developing hippocampus through altering the canonical Wnt pathway.

Activity-induced Nr4a1 regulates spine density and distribution pattern of excitatory synapses in pyramidal neurons

Excitatory synapses occur mainly on dendritic spines, and spine density is usually correlated with the strength of excitatory synaptic transmission. We report that Nr4a1, an activity-inducible gene encoding a nuclear receptor, regulates the density and distribution of dendritic spines in CA1 pyramidal neurons. Nr4a1 overexpression resulted in elimination of the majority of spines; however, postsynaptic densities were preserved on dendritic shafts, and the strength of excitatory synaptic transmission was unaffected, showing that excitatory synapses can be dissociated from spines. mRNA expression profiling studies suggest that Nr4a1-mediated transcriptional regulation of the actin cytoskeleton contributes to this effect. Under conditions of chronically elevated activity, when Nr4a1 was induced, Nr4a1 knockdown increased the density of spines and PSDs specifically at the distal ends of dendrites. Thus, Nr4a1 is a key component of an activity-induced transcriptional program that regulates the density and distribution of spines and synapses.

Subcellular organization of camkii in rat hippocampal pyramidal neurons

Calcium/calmodulin-dependent protein kinase II (CaMKII) plays a key role in N-methyl-D-aspartate (NMDA) receptor-dependent long-term synaptic plasticity; its location is critical for signal transduction, and may provide clues that further elucidate its function. We therefore examined the subcellular localization of CaMKII in CA1 stratum radiatum of adult rat hippocampus, by using immuno-electron microscopy after chemical fixation. When tissue was fixed quickly, the concentration of CaMKIIα (assessed by pre-embedding immunogold) was significantly higher in dendritic shafts than in spine heads. However, when tissue was fixed 5 minutes after perfusion with normal saline, the density of labeling decreased in dendritic shaft while increasing in spine heads, implying rapid translocation into the spine during brief perimortem stress. Likewise, in quickly fixed tissue, CaMKII within spine heads was found at comparable concentrations in the "proximal" half (adjacent to the spine neck) and the "distal" half (containing the postsynaptic density [PSD]), whereas after delayed fixation, label density increased in the distal side of the spine head, suggesting that CaMKII within the spine head moves toward the PSD during this interval. To estimate its distribution at the synapse in vivo, we performed postembedding immunogold staining for CaMKII in quick-fixed tissue, and found that the enzyme did not concentrate primarily within the central matrix of the PSD. Instead, labeling density peaked ∼40 nm inside the postsynaptic membrane, at the cytoplasmic fringe of the PSD. Labeling within 25 nm of the postsynaptic membrane concentrated at the lateral edge of the synapse. This lateral "PSD core" pool of CaMKII may play a special role in synaptic plasticity.

The relationship between PSD-95 clustering and spine stability in vivo

The appearance and disappearance of dendritic spines, accompanied by synapse formation and elimination may underlie the experience-dependent reorganization of cortical circuits. The exact temporal relationship between spine and synapse formation in vivo remains unclear, as does the extent to which synapse formation enhances the stability of newly formed spines and whether transient spines produce synapses. We used in utero electroporation of DsRedExpress- and eGFP-tagged postsynaptic density protein 95 (PSD-95) to investigate the relationship between spine and PSD stability in mouse neocortical L2/3 pyramidal cells in vivo. Similar to previous studies, spines and synapses appeared and disappeared, even in naive animals. Cytosolic spine volumes and PSD-95-eGFP levels in spines covaried over time, suggesting that the strength of many individual synapses continuously changes in the adult neocortex. The minority of newly formed spines acquired PSD-95-eGFP puncta. Spines that failed to acquire a PSD rarely survived for more than a day. Although PSD-95-eGFP accumulation was associated with increased spine lifetimes, most new spines with a PSD did not convert into persistent spines. This indicates that transient spines may serve to produce short-lived synaptic contacts. Persistent spines that were destined to disappear showed, on average, reduced PSD-95-eGFP levels well before the actual pruning event. Altogether, our data indicate that the PSD size relates to spine stability in vivo.

Local pruning of dendrites and spines by caspase-3-dependent and proteasome-limited mechanisms

Synapse loss occurs normally during development and pathologically during neurodegenerative disease. Long-term depression, a proposed physiological correlate of synapse elimination, requires caspase-3 and the mitochondrial pathway of apoptosis. Here, we show that caspase-3 activity is essential--and can act locally within neurons--for regulation of spine density and dendrite morphology. By photostimulation of Mito-KillerRed, we induced caspase-3 activity in defined dendritic regions of cultured neurons. Within the photostimulated region, local elimination of dendritic spines and dendrite retraction occurred in a caspase-3-dependent manner without inducing cell death. However, pharmacological inhibition of inhibitor of apoptosis proteins or proteasome function led to neuronal death, suggesting that caspase activation is spatially restricted by these "molecular brakes" on apoptosis. Caspase-3 knock-out mice have increased spine density and altered miniature EPSCs, confirming a physiological involvement of caspase-3 in the regulation of spines in vivo.

Dendritic spines: the locus of structural and functional plasticity

The introduction of high-resolution time lapse imaging and molecular biological tools has changed dramatically the rate of progress towards the understanding of the complex structure-function relations in synapses of central spiny neurons. Standing issues, including the sequence of molecular and structural processes leading to formation, morphological change, and longevity of dendritic spines, as well as the functions of dendritic spines in neurological/psychiatric diseases are being addressed in a growing number of recent studies. There are still unsettled issues with respect to spine formation and plasticity: Are spines formed first, followed by synapse formation, or are synapses formed first, followed by emergence of a spine? What are the immediate and long-lasting changes in spine properties following exposure to plasticity-producing stimulation? Is spine volume/shape indicative of its function? These and other issues are addressed in this review, which highlights the complexity of molecular pathways involved in regulation of spine structure and function, and which contributes to the understanding of central synaptic interactions in health and disease.

Myosin II regulates actin rearrangement-related structural synaptic plasticity during conditioned taste aversion memory extinction

Similar to memory formation, memory extinction is also a new learning process that requires synaptic plasticity. Actin rearrangement is fundamental for synaptic plasticity, however, whether actin rearrangement in the infralimbic cortex (IL) plays a role in memory extinction, as well as the mechanisms underlying it, remains unclear. Here, using a conditioned taste aversion (CTA) paradigm, we demonstrated increased synaptic density and actin rearrangement in the IL during the extinction of CTA. Targeted infusion of an actin rearrangement inhibitor, cytochalasin D, into the IL impaired memory extinction and de novo synapse formation. Notably, we also found increased myosin II phosphorylation in the IL during the extinction of CTA. Microinfusion of a specific inhibitor of the myosin II ATPase, blebbistatin (Blebb), into the IL impaired memory extinction as well as the related actin rearrangement and changes in synaptic density. Moreover, the extinction deficit and the reduction of synaptic density induced by Blebb could be rescued by the actin polymerization stabilizer jasplakinolide (Jasp), suggesting that myosin II acts via actin filament polymerization to stabilize synaptic plasticity during the extinction of CTA. Taken together, we conclude that myosin II may regulate the plasticity of actin-related synaptic structure during memory extinction. Our studies provide a molecular mechanism for understanding the plasticity of actin rearrangement-associated synaptic structure during memory extinction.

Primary cilia and dendritic spines: different but similar signaling compartments

Primary non-motile cilia and dendritic spines are cellular compartments that are specialized to sense and transduce environmental cues and presynaptic signals, respectively. Despite their unique cellular roles, both compartments exhibit remarkable parallels in the general principles, as well as molecular mechanisms, by which their protein composition, membrane domain architecture, cellular interactions, and structural and functional plasticity are regulated. We compare and contrast the pathways required for the generation and function of cilia and dendritic spines, and suggest that insights from the study of one may inform investigations into the other of these critically important signaling structures.

Myosin X and its motorless isoform differentially modulate dendritic spine development by regulating trafficking and retention of vasodilator-stimulated phosphoprotein

Myosin X (Myo10) is an unconventional myosin with two known isoforms: full-length (FL)-Myo10 that has motor activity, and a recently identified brain-expressed isoform, headless (Hdl)-Myo10, which lacks most of the motor domain. FL-Myo10 is involved in the regulation of filopodia formation in non-neuronal cells; however, the biological function of Hdl-Myo10 remains largely unknown. Here, we show that FL- and Hdl-Myo10 have important, but distinct, roles in the development of dendritic spines and synapses in hippocampal neurons. FL-Myo10 induces formation of dendritic filopodia and modulates filopodia dynamics by trafficking the actin-binding protein vasodilator-stimulated phosphoprotein (VASP) to the tips of filopodia. By contrast, Hdl-Myo10 acts on dendritic spines to enhance spine and synaptic density as well as spine head expansion by increasing the retention of VASP in spines. Thus, this study demonstrates a novel biological function for Hdl-Myo10 and an important new role for both Myo10 isoforms in the development of dendritic spines and synapses.

Doublecortin-like kinase enhances dendritic remodelling and negatively regulates synapse maturation

Dendritic morphogenesis and formation of synapses at appropriate dendritic locations are essential for the establishment of proper neuronal connectivity. Recent imaging studies provide evidence for stabilization of dynamic distal branches of dendrites by the addition of new synapses. However, molecules involved in both dendritic growth and suppression of synapse maturation remain to be identified. Here we report two distinct functions of doublecortin-like kinases, chimeric proteins containing both a microtubule-binding domain and a kinase domain in postmitotic neurons. First, doublecortin-like kinases localize to the distal dendrites and promote their growth by enhancing microtubule bundling. Second, doublecortin-like kinases suppress maturation of synapses through multiple pathways, including reduction of PSD-95 by the kinase domain and suppression of spine structural maturation by the microtubule-binding domain. Thus, doublecortin-like kinases are critical regulators of dendritic development by means of their specific targeting to the distal dendrites, and their local control of dendritic growth and synapse maturation.

The small GTPase RhoA, but not Rac1, is essential for conditioned aversive memory formation through regulation of actin rearrangements in rat dorsal hippocampus

AIM

Actin rearrangements are induced in the dorsal hippocampus after conditioned morphine withdrawal, and involved in the formation of conditioned place aversion. In the present study, we investigated the mechanisms underlying the actin rearrangements in rat dorsal hippocampus induced by conditioned morphine withdrawal.

METHODS

The RhoA-ROCK pathway inhibitor Y27632 (8.56 μg/1 μL per side) or the Rac1 inhibitor NSC23766 (25 μg/1 μL per side) was microinjected into the dorsal hippocampus of rats. Conditioned place aversion (CPA) induced by naloxone-precipitated morphine withdrawal was assessed. Crude synaptosomal fraction of hippocampus was prepared, and the amount of F-actin and G-actin was measured with an Actin Polymerization Assay Kit.

RESULTS

Conditioned morphine withdrawal significantly increased actin polymerization in the dorsal hippocampus at 1 h following the naloxone injection. Preconditioning with microinjection of Y27632, but not NSC23766, attenuated CPA, and blocked the increase in actin polymerization in the dorsal hippocampus.

CONCLUSION

Our results suggest that the small GTPase RhoA, but not Rac1, in the dorsal hippocampus is responsible for CPA formation, mainly through its regulation of actin rearrangements.