Dendritic Spine Plasticity Research Articles

Synaptopodin Regulates Plasticity of Dendritic Spines in Hippocampal Neurons

The spine apparatus is an essential component of dendritic spines of cortical and hippocampal neurons, yet its functions are still enigmatic. Synaptopodin (SP), an actin-binding protein, is tightly associated with the spine apparatus and it may play a role in synaptic plasticity, but it has not yet been linked mechanistically to synaptic functions. We studied endogenous and transfected SP in dendritic spines of cultured hippocampal neurons and found that spines containing SP generate larger responses to flash photolysis of caged glutamate than SP-negative ones. An NMDA-receptor-mediated chemical long-term potentiation caused the accumulation of GFP-GluR1 in spine heads of control but not of shRNA-transfected, SP-deficient neurons. SP is linked to calcium stores, because their pharmacological blockade eliminated SP-related enhancement of glutamate responses, and release of calcium from stores produced an SP-dependent increase of GluR1 in spines. Thus, SP plays a crucial role in the calcium store-associated ability of neurons to undergo long-term plasticity.

Drebrin A regulates dendritic spine plasticity and synaptic function in mature cultured hippocampal neurons

Drebrin A, one of the most abundant neuron-specific F-actin-binding proteins, is found exclusively in dendrites and is particularly concentrated in dendritic spines receiving excitatory inputs. We investigated the role of drebrin A in synaptic transmission and found that overexpression of drebrin A augmented the glutamatergic synaptic transmission, probably through an increase of active synaptic site density. Interestingly, overexpression of drebrin A also affected the frequency, amplitude and kinetics of miniature inhibitory postsynaptic currents (mIPSCs), despite the fact that GABAergic synapse density and transmission efficacy were not modified. Downregulation of drebrin A led to a decrease of both glutamatergic and GABAergic synaptic activity. In heterologous cells, drebrin A reorganized and stabilized F-actin and these effects were mediated by its actin-binding domain. Thus, drebrin A might regulate dendritic spine morphology via regulation of actin cytoskeleton remodeling and dynamics. Our data demonstrate for the first time that drebrin A modulates glutamatergic and GABAergic synaptic activities.

Live-cell imaging of dendritic spines by STED microscopy

Time lapse fluorescence imaging has become one of the most important approaches in neurobiological research. In particular, both confocal and two-photon microscopy have been used to study activity-dependent changes in synaptic morphology. However, the diffraction-limited resolution of light microscopy is often inadequate, forcing researchers to complement the live cell imaging strategy by EM. Here, we report on the first use of a far-field optical technique with subdiffraction resolution to noninvasively image activity-dependent morphological plasticity of dendritic spines. Specifically we show that time lapse stimulated emission depletion imaging of dendritic spines of YFP-positive hippocampal neurons in organotypic slices outperforms confocal microscopy in revealing important structural details. The technique substantially improves the quantification of morphological parameters, such as the neck width and the curvature of the heads of spines, which are thought to play critical roles for the function and plasticity of synaptic connections.

Microtubules in Dendritic Spine Development

It is generally believed that only the actin cytoskeleton resides in dendritic spines and controls spine morphology and plasticity. Here, we report that microtubules (MTs) are present in spines and that shRNA knockdown of the MT plus-end-binding protein EB3 significantly reduces spine formation. Furthermore, stabilization and inhibition of MTs by low doses of taxol and nocodazole enhance and impair spine formation elicited by BDNF (brain-derived neurotrophic factor), respectively. Therefore, MTs play an important role in the control and regulation of dendritic spine development and plasticity.

Regulation of Synaptic Connectivity With Chronic Cocaine

In animal models of addiction, repeated exposure to drugs of abuse, such as cocaine, amphetamines, and nicotine, increases the number of dendritic spines (panel A) in the nucleus accumbens (NAc), a key brain region involved in reward, pleasure, and motivation. Dendritic spines are the primary anatomical sites of excitatory synapses. Many scientists deem that this long-lasting increase in synaptic connectivity in the NAc may underlie the similarly persistent behaviors of drug taking and drug seeking associated with drug addiction and relapse. Although this concept is appealing, the functional relationship between increased synaptic connectivity in the NAc and addiction-related behaviors is poorly understood. Our recent findings shed some important light on this issue. We recently found that cocaine regulates myocyte enhancer factor 2 (MEF2) transcription factors in the NAc to control the observed increase in synaptic connectivity. During development, MEF2 sculpts mature brain circuits by promoting activity-dependent synapse elimination. Excitatory synaptic activity and subsequent calcium signaling events stimulate MEF2 activity. However, after chronic cocaine exposure or activation of dopamine receptor signaling, we observed a decrease in MEF2 activity in the striatum and found that decreasing MEF2 activity in the NAc promoted an increase in dendritic spine density. We also found that chronic cocaine and dopamine signaling reduced MEF2 activity in the striatum through novel signaling pathways involving the regulator of calmodulin signaling (RCS), calcineurin (CaN), and Cdk5 (panel B). Reducing MEF2 is required for cocaine-induced dendritic spine plasticity in vivo. On the other hand, we observed that expression of a hyperactive MEF2 in the NAc, the condition that blocked dendritic spine changes, increased behavioral responses to repeated cocaine exposure. This suggests that the increased spine density in the NAc is not required for cocaineinduced behavioral plasticity and that increased synaptic connections during chronic drug use may actually limit behavioral changes associated with drug addiction rather than support them. Together with the previous literature, our findings suggest that the increased density of synapses may be a compensatory response to decreased activity of these brain reward regions. Therefore, a focus on enhancing synaptic contacts and neuronal activity in the NAc may be therapeutic in drug addiction.

Cocaine Regulates MEF2 to Control Synaptic and Behavioral Plasticity

Repeated exposure to cocaine causes sensitized behavioral responses and increased dendritic spines on medium spiny neurons of the nucleus accumbens (NAc). We find that cocaine regulates myocyte enhancer factor 2 (MEF2) transcription factors to control these two processes in vivo. Cocaine suppresses striatal MEF2 activity in part through a mechanism involving cAMP, the regulator of calmodulin signaling (RCS), and calcineurin. We show that reducing MEF2 activity in the NAc in vivo is required for the cocaine-induced increases in dendritic spine density. Surprisingly, we find that increasing MEF2 activity in the NAc, which blocks the cocaine-induced increase in dendritic spine density, enhances sensitized behavioral responses to cocaine. Together, our findings implicate MEF2 as a key regulator of structural synapse plasticity and sensitized responses to cocaine and suggest that reducing MEF2 activity (and increasing spine density) in NAc may be a compensatory mechanism to limit long-lasting maladaptive behavioral responses to cocaine.

Interactions between drebrin and Ras regulate dendritic spine plasticity

Dendritic spines are major sites of morphological plasticity in the CNS, but the molecular mechanisms that regulate their dynamics remain poorly understood. Here we show that the association of drebrin with actin filaments plays a major role in regulating dendritic spine stability and plasticity. Overexpressing drebrin or the internal actin-binding site of drebrin in rat hippocampal neurons destabilized mature dendritic spines so that they lost synaptic contacts and came to resemble immature dendritic filopodia. Drebrin-induced spine destabilization was dependent on Ras activation: expression of constitutively active Ras destabilized spine morphology whereas drebrin-induced spine destabilization was rescued by co-expressing dominant negative Ras. Conversely, RNAi-mediated drebrin knockdown prevented Ras-induced destabilization and promoted spine maturation in developing neurons. Together these data demonstrate a novel mechanism in which the balance between stability and plasticity in dendritic spines depends on binding of drebrin to actin filaments in a manner that is regulated by Ras.

Protein-synthesis and neurotrophin dependent structural plasticity in single dendritic spines

Protein-synthesis and neurotrophin dependent structural plasticity in single dendritic spines

Two‐photon imaging of dendritic spine development in the mouse cortex

Dendritic spines are the postsynaptic sites of most excitatory synapses in the mammalian brain. With the advent of two-photon microscopy and transgenic mice expressing fluorescent proteins, dendritic spines can now be imaged in the living cerebral cortex over time scales ranging from seconds to years. Recent studies with this in vivo imaging approach have begun to provide important insights into the development and plasticity of dendritic spines in the intact brain. Here, we review these studies and discuss technical requirements for image acquisition. We envision that intravital two-photon imaging at the level of individual synapses will greatly expand our current understandings of how neuronal networks are assembled and modified throughout life.

BDNF + Protein Synthesis = Synaptic Plasticity

Protein synthesis is required for consolidation of memory and has been used to characterize memory systems. However, it is not clear whether protein synthesis can regulate synaptic plasticity at the level of single synapses and how protein synthesis affects synaptic structures, such as dendritic spines. Tanaka et al. (see the Perspective by Korte) discovered that repetitive synaptic stimulation in synchrony with postsynaptic spike induces rapid and long-term spine enlargement, which occurs at the level of single spines and is associated with increases in glutamate sensitivity. This associative paradigm specifically produces long-term and protein synthesis-dependent changes. Brain-derived neurotrophic factor (BDNF) mimics the effects of postsynaptic spiking and occludes the effects of associative stimulation. Thus, protein synthesis and BDNF are required for the alterations in spine morphology associated with synaptic plasticity. J.-i. Tanaka, Y. Horiike, M. Matsuzaki, T. Miyazaki, G. C. R. Ellis-Davies, H. Kasai, Protein synthesis and neurotrophin-dependent structural plasticity of single dendritic spines. Science 319 , 1683-1687 (2008). [Abstract] [Full Text] M. Korte, A protoplasmic kiss to remember. Science 319 , 1627-1628 (2008). [Summary] [Full Text]

The Subspine Organization of Actin Fibers Regulates the Structure and Plasticity of Dendritic Spines

Synapse function and plasticity depend on the physical structure of dendritic spines as determined by the actin cytoskeleton. We have investigated the organization of filamentous (F-) actin within individual spines on CA1 pyramidal neurons in rat hippocampal slices. Using two-photon photoactivation of green fluorescent protein fused to beta-actin, we found that a dynamic pool of F-actin at the tip of the spine quickly treadmilled to generate an expansive force. The size of a stable F-actin pool at the base of the spine depended on spine volume. Repeated two-photon uncaging of glutamate formed a third pool of F-actin and enlarged the spine. The spine often released this "enlargement pool" into the dendritic shaft, but the pool had to be physically confined by a spine neck for the enlargement to be long-lasting. Ca2+/calmodulin-dependent protein kinase II regulated this confinement. Thus, spines have an elaborate mechanical nature that is regulated by actin fibers.

Protein Synthesis and Neurotrophin-Dependent Structural Plasticity of Single Dendritic Spines

Long-term potentiation (LTP) at glutamatergic synapses is considered to underlie learning and memory and is associated with the enlargement of dendritic spines. Because the consolidation of memory and LTP require protein synthesis, it is important to clarify how protein synthesis affects spine enlargement. In rat brain slices, the repetitive pairing of postsynaptic spikes and two-photon uncaging of glutamate at single spines (a spike-timing protocol) produced both immediate and gradual phases of spine enlargement in CA1 pyramidal neurons. The gradual enlargement was strongly dependent on protein synthesis and brain-derived neurotrophic factor (BDNF) action, often associated with spine twitching, and was induced specifically at the spines that were immediately enlarged by the synaptic stimulation. Thus, this spike-timing protocol is an efficient trigger for BDNF secretion and induces protein synthesis-dependent long-term enlargement at the level of single spines.

An additional human chromosome 21 causes suppression of neural fate of pluripotent mouse embryonic stem cells in a teratoma model.

BackgroundDown syndrome (DS), caused by trisomy of human chromosome 21 (HSA21), is the most common genetic cause of mental retardation in humans. Among complex phenotypes, it displays a number of neural pathologies including smaller brain size, reduced numbers of neurons, reduced dendritic spine density and plasticity, and early Alzheimer-like neurodegeneration. Mouse models for DS show behavioural and cognitive defects, synaptic plasticity defects, and reduced hippocampal and cerebellar neuron numbers. Early postnatal development of both human and mouse-model DS shows the reduced capability of neuronal precursor cells to generate neurons. The exact molecular cause of this reduction, and the role played by increased dosage of individual HSA21 genes, remain unknown.ResultsWe have subcutaneously injected mouse pluripotent ES cells containing a single freely segregating supernumerary human chromosome 21 (HSA21) into syngeneic mice, to generate transchromosomic teratomas. Transchromosomic cells and parental control cells were injected into opposite flanks of thirty mice in three independent experiments. Tumours were grown for 30 days, a time-span equivalent to combined intra-uterine, and early post-natal mouse development. When paired teratomas from the same animals were compared, transchromosomic tumours showed a three-fold lower percentage of neuroectodermal tissue, as well as significantly reduced mRNA levels for neuron specific (Tubb3) and glia specific (Gfap) genes, relative to euploid controls. Two thirds of transchromosomic tumours also showed a lack of PCR amplification with multiple primers specific for HSA21, which were present in the ES cells at the point of injection, thus restricting a commonly retained trisomy to less than a third of HSA21 genes.ConclusionWe demonstrate that a supernumerary chromosome 21 causes Inhibition of Neuroectodermal DIfferentiation (INDI) of pluripotent ES cells. The data suggest that trisomy of less than a third of HSA21 genes, in two chromosomal regions, might be sufficient to cause this effect.

Arcadlin Signals Internalization

The abundance of the cell adhesion molecule N-cadherin is one factor that contributes to dendritic spine plasticity. When N-cadherin activity is compromised, spine number is decreased. Yasuda et al . found that conditions that stimulate excitatory synapses, such as maximal electroconvulsive seizure in mice or application of glutamate or agents that elevate the concentration of adenosine 3′,5′-monophosphate (cAMP) to cultured hippocampal neurons, increased the abundance of arcadlin, a protocadherin protein that was recruited to excitatory dendrites. Furthermore, arcadlin coimmunoprecipitated with N-cadherin from rat hippocampi after stimulation, as well as from transfected cultured cells. Cultured cells transfected with arcadlin and N-cadherin exhibited diminished homophilic adhesiveness. Stimulated hippocampal cultures exhibited decreased abundance of N-cadherin at the cell surface (based on a surface biotinylation assay and immunofluorescent analysis). In neurons cultured from acad –/– mice, increases in cAMP concentration did not decrease N-cadherin surface abundance. A yeast two-hybrid screen identified a kinase that the authors named TAO2β, which appears to be an alternative splice variant of TAO2, as a protein that interacted with arcadlin. This interaction was confirmed by coimmunoprecipitation of the two proteins from hippocampi of mice after maximal electroconvulsive seizure and in colocalization studies in transfected cells. TAO2 is a mitogen-activated protein kinase kinase kinase (MAPKKK), and when transfected cells or primary cultures of rat hippocampal neurons were treated with the arcadlin extracellular domain (Acad-EC) to ligate the arcadlin proteins, phosphorylated mitogen-activated protein kinase kinase 3 (MEK3) and p38 MAPK were detected, suggesting that TAO2β is also a p38 MAPKKK. The abundance of arcadlin at the cell surface, as well as that of N-cadherin, was decreased in response to Acad-EC through a mechanism that required TAO2β and p38 activity. TAO2 could not substitute for TAO2β in triggering arcadlin endocytosis, yet endocytosis required p38, suggesting a possible feedback mechanism depending on TAO2β. Indeed, in vitro phosphorylation assays confirmed that TAO2β was a substrate for p38, and, in cells transfected with mutants that lacked the phosphorylation site, the importance of this phosphorylation for triggering endocytosis of arcadlin was confirmed. Finally, the increase in spine density seen in cultured neurons from acad –/– mice was reversed by knockdown of N-cadherin. Thus, arcadlin appears to link the endocytosis of N-cadherin to excitatory stimuli through a MAPK pathway. S. Yasuda, H. Tanaka, H. Sugiura, K. Okamura, T. Sakaguchi, U. Tran, T. Takemiya, A. Mizoguchi, Y. Yagita, T. Sakurai, E. M. De Robertis, Activity-induced protocadherin arcadlin regulates dendritic spine number by triggering N-cadherin endocytosis via TAO2β and p38 MAP kinases. Neuron 56 , 456-471 (2007). [PubMed]

Dynamic observe that μ and δ opiate receptor modulate the plasticity of dendritic spines

目的 探讨μ阿片受体(MOR)和δ阿片受体(DOR)在海马神经元树突棘可塑性调节中的功能角色.方法 用磷酸钙共沉淀法将编码绿色荧光蛋白EGFP的质粒转染到5-9DIV原代培养海马神经元,EGFP标记的神经元发育到2周时,随机挑选形态健康的神经元作为观察目标,使用倒置荧光显微镜对同一视野活细胞定时定位观察MOR受体促进剂吗啡和DOR受体促进剂DPDPE给药前及给药后3 h、24 h、72 h树突棘形态大小及密度变化.结果 1.吗啡组和吗啡+DPDPE组,从形态学观察,部分树突棘在给药后24～72h内逐渐变小,甚至消失;2.吗啡组加药后3h树突棘的密度无明显变化(P＞0.05),24h后降低了28%,72h后降低了37%(P＜0.01);吗啡+DPDPE组加药后3 h树突棘的密度无明显变化(P＞0.05),24h后降低了27%(P＜0.05),72h后降低了38%(P＜0.01);DPDPE组和对照组在给药后3～72h树突棘的密度无明显变化(P＞0.05);而DPDPE单独给药组和对照组神经元树突棘形态大小、密度在各时间点均无明显变化.结论 本实验表明阿片MOR受体参与了神经元树突棘可塑性的调节,动摇了树突棘的稳定性,而DOR受体无显著作用,同时MOR和DOR受体在对神经元可塑性调节方面也无协同作用。

Impaired Spine Stability Underlies Plaque-Related Spine Loss in an Alzheimer's Disease Mouse Model

Dendritic spines, the site of most excitatory synapses in the brain, are lost in Alzheimer's disease and in related mouse models, undoubtedly contributing to cognitive dysfunction. We hypothesized that spine loss results from plaque-associated alterations of spine stability, causing an imbalance in spine formation and elimination. To investigate effects of plaques on spine stability in vivo, we observed cortical neurons using multiphoton microscopy in a mouse model of amyloid pathology before and after extensive plaque deposition. We also observed age-matched nontransgenic mice to study normal effects of aging on spine plasticity. We found that spine density and structural plasticity are maintained during normal aging. Tg2576 mice had normal spine density and plasticity before plaques appeared, but after amyloid pathology is established, severe disruptions were observed. In control animals, spine formation and elimination were equivalent over 1 hour of observation ( approximately 5% of observed spines), resulting in stable spine density. However, in aged Tg2576 mice spine elimination increased, specifically in the immediate vicinity of plaques. Spine formation was unchanged, resulting in spine loss. These data show a small population of rapidly changing spines in adult and even elderly mouse cortex; further, in the vicinity of amyloid plaques, spine stability is markedly impaired leading to loss of synaptic structural integrity.

Anatomical and Physiological Plasticity of Dendritic Spines

In excitatory neurons, most glutamatergic synapses are made on the heads of dendritic spines, each of which houses the postsynaptic terminal of a single glutamatergic synapse. We review recent studies demonstrating in vivo that spines are motile and plastic structures whose morphology and lifespan are influenced, even in adult animals, by changes in sensory input. However, most spines that appear in adult animals are transient, and the addition of stable spines and synapses is rare. In vitro studies have shown that patterns of neuronal activity known to induce synaptic plasticity can also trigger changes in spine morphology. Therefore, it is tempting to speculate that the plastic changes of spine morphology reflect the dynamic state of its associated synapse and are responsible to some extent for neuronal circuitry remodeling. Nevertheless, morphological changes are not required for all forms of synaptic plasticity, and whether changes in the spine shape and size significantly impact synaptic signals is unclear.

Choice of cranial window type for in vivo imaging affects dendritic spine turnover in the cortex

Determining the degree of synapse formation and elimination is essential for understanding the structural basis of brain plasticity and pathology. We show that in vivo imaging of dendritic spine dynamics through an open-skull glass window, but not a thinned-skull window, is associated with high spine turnover and substantial glial activation during the first month after surgery. These findings help to explain existing discrepancies in the degree of dendritic spine plasticity observed in the mature cortex.

α5 Integrin Signaling Regulates the Formation of Spines and Synapses in Hippocampal Neurons

The actin-based dynamics of dendritic spines play a key role in synaptic plasticity, which underlies learning and memory. Although it is becoming increasingly clear that modulation of actin is critical for spine dynamics, the upstream molecular signals that regulate the formation and plasticity of spines are poorly understood. In non-neuronal cells, integrins are critical modulators of the actin cytoskeleton, but their function in the nervous system is not well characterized. Here we show that alpha5 integrin regulates spine morphogenesis and synapse formation in hippocampal neurons. Knockdown of alpha5 integrin expression using small interfering RNA decreased the number of dendritic protrusions, spines, and synapses. Expression of constitutively active or dominant negative alpha5 integrin also resulted in alterations in the number of dendritic protrusions, spines, and synapses. alpha5 integrin signaling regulates spine morphogenesis and synapse formation by a mechanism that is dependent on Src kinase, Rac, and the signaling adaptor GIT1. Alterations in the activity or localization of these molecules result in a significant decrease in the number of spines and synapses. Thus, our results point to a critical role for integrin signaling in regulating the formation of dendritic spines and synapses in hippocampal neurons.

In Vivo Imaging Reveals Dendritic Targeting of Laminated Afferents by Zebrafish Retinal Ganglion Cells

Targeting of axons and dendrites to particular synaptic laminae is an important mechanism by which precise patterns of neuronal connectivity are established. Although axons target specific laminae during development, dendritic lamination has been thought to occur largely by pruning of inappropriately placed arbors. We discovered by in vivo time-lapse imaging that retinal ganglion cell (RGC) dendrites in zebrafish show growth patterns implicating dendritic targeting as a mechanism for contacting appropriate synaptic partners. Populations of RGCs labeled in transgenic animals establish distinct dendritic strata sequentially, predominantly from the inner to outer retina. Imaging individual cells over successive days confirmed that multistratified RGCs generate strata sequentially, each arbor elaborating within a specific lamina. Simultaneous imaging of RGCs and subpopulations of presynaptic amacrine interneurons revealed that RGC dendrites appear to target amacrine plexuses that had already laminated. Dendritic targeting of prepatterned afferents may thus be a novel mechanism for establishing proper synaptic connectivity.