Spatially restricted actin-regulatory signaling contributes to synapse morphology

The actin cytoskeleton in dendritic spines is organized into microdomains, but how signaling molecules that regulate actin are spatially governed is incompletely understood. Here we examine how the localization of the RacGEF kalirin-7, a well-characterized regulator of actin in spines, varies as a function of post-synaptic density area and spine volume. Using serial section electron microscopy, we find that extrasynaptic, but not synaptic, expression of kalirin-7 varies directly with synapse size and spine volume. Moreover, we find that overall expression levels of kalirin-7 differ in spines bearing perforated and non-perforated synapses, due primarily to extrasynaptic pools of kalirin-7 expression in the former. Overall, our findings indicate that kalirin-7 is differentially compartmentalized in spines as a function of both synapse morphology and spine size.     

Do thin spines learn to be mushroom spines that remember?

Dendritic spines are the primary site of excitatory input on most principal neurons. Long-lasting changes in synaptic activity are accompanied by alterations in spine shape, size and number. The responsiveness of thin spines to increases and decreases in synaptic activity has led to the suggestion that they are 'learning spines', whereas the stability of mushroom spines suggests that they are 'memory spines'. Synaptic enhancement leads to an enlargement of thin spines into mushroom spines and the mobilization of subcellular resources to potentiated synapses. Thin spines also concentrate biochemical signals such as Ca(2+), providing the synaptic specificity required for learning. Determining the mechanisms that regulate spine morphology is essential for understanding the cellular changes that underlie learning and memory.     

Dendritic spine morphogenesis and plasticity

Dendritic spines are small protrusions off the dendrite that receive excitatory synaptic input. Spines vary in size, likely correlating with the strength of the synapses they form. In the developing brain, spines show highly dynamic behavior thought to facilitate the formation of new synaptic contacts. Recent studies have illuminated the numerous molecules regulating spine development, many of which converge on the regulation of actin filaments. In addition, interactions with glial cells are emerging as important regulators of spine morphology. In many cases, spine morphogenesis, plasticity, and maintenance also depend on synaptic activity, as shown by recent studies demonstrating changes in spine dynamics and maintenance with altered sensory experience.     

N-wasp and the arp2/3 complex are critical regulators of actin in the development of dendritic spines and synapses

Changes in the number, size, and shape of dendritic spines are associated with synaptic plasticity, which underlies cognitive functions such as learning and memory. This plasticity is attributed to reorganization of actin, but the molecular signals that regulate this process are poorly understood. In this study, we show neural Wiskott-Aldrich syndrome protein (N-WASP) regulates the formation of dendritic spines and synapses in hippocampal neurons. N-WASP localized to spines and active, functional synapses as shown by loading with FM4-64 dye. Knock down of endogenous N-WASP expression by RNA interference or inhibition of its activity by treatment with a specific inhibitor, wiskostatin, caused a significant decrease in the number of spines and excitatory synapses. Deletion of the C-terminal VCA region of N-WASP, which binds and activates the actin-related protein 2/3 (Arp2/3) complex, dramatically decreased the number of spines and synapses, suggesting activation of the Arp2/3 complex is critical for spine and synapse formation. Consistent with this, Arp3, like N-WASP, was enriched in spines and excitatory synapses and knock down of Arp3 expression impaired spine and synapse formation. A similar defect in spine and synapse formation was observed when expression of an N-WASP activator, Cdc42, was knocked down. Thus, activation of N-WASP and, subsequently, the Arp2/3 complex appears to be an important molecular signal for regulating spines and synapses. Arp2/3-mediated branching of actin could be a mechanism by which dendritic spine heads enlarge and subsequently mature. Collectively, our results point to a critical role for N-WASP and the Arp2/3 complex in spine and synapse formation.     

alpha5 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.     

Molecular regulation of dendritic spine shape and function

Dendritic spines are discrete membrane protrusions from dendritic shafts where the large majority of excitatory synapses are located. Their highly heterogeneous morphology is thought to be the morphological basis for synaptic plasticity. Electron microscopy and time-lapse imaging studies have suggested that the shape and number of spines can change after long-term potentiation (LTP), although there is no evidence that morphological changes are necessary for LTP induction and maintenance. An increasing number of proteins have been found to be morphogens for dendritic spines and provide new insights into the molecular mechanisms regulating spine formation and morphology.     

Time-lapse imaging of dendritic spines in vitro

Dendritic spines are small protrusions present postsynaptically at approximately 90% of excitatory synapses in the brain. Spines undergo rapid spontaneous changes in shape that are thought to be important for alterations in synaptic connectivity underlying learning and memory. Visualization of these dynamic changes in spine morphology are especially challenging because of the small size of spines (approximately 1 microm). Here we describe a microscope system, based on a spinning-disk confocal microscope, suitable for imaging mature dendritic spines in brain slice preparations, with a time resolution of seconds. We discuss two commonly used in vitro brain slice preparations and methods for transfecting them. Preparation and transfection require approximately 1 d, after which slices must be cultured for at least 21 d to obtain spines of mature morphology. We also describe imaging and computer analysis routines for studying spine motility. These procedures require in the order of 2 to 4 h.     

Eph/ephrin对树突棘的调控与中枢神经系统疾病

树突棘是神经元树突上的功能性突起结构,通常作为突触后成份与投射来的轴突共同构成完整的突触连接。树突棘的形态与结构具有明显的可塑性,其变化通常会引起突触功能的改变。Eph受体酪氨酸激酶家族分子与其配体ephrin都是重要的神经导向因子,同时对树突棘结构也有直接的调控作用。Eph受体的活化可以促进树突棘的发生并影响树突棘的形态及内部结构;而Eph受体的异常也往往会损害正常的突触功能,甚至导致许多与树突棘结构异常相关的神经系统病变的发生。     

Molecular mechanisms of dendritic spine morphogenesis

Excitatory synapses are formed on dendritic spines, postsynaptic structures that change during development and in response to synaptic activity. Once mature, however, spines can remain stable for many months. The molecular mechanisms that control the formation and elimination, motility and stability, and size and shape of dendritic spines are being revealed. Multiple signaling pathways, particularly those involving Rho and Ras family small GTPases, converge on the actin cytoskeleton to regulate spine morphology and dynamics bidirectionally. Numerous cell surface receptors, scaffold proteins and actin binding proteins are concentrated in spines and engaged in spine morphogenesis.     

Matrix metalloproteinase-7 disrupts dendritic spines in hippocampal neurons through NMDA receptor activation

Dendritic spines are protrusions from the dendritic shaft that host most excitatory synapses in the brain. Although they first emerge during neuronal maturation, dendritic spines remain plastic through adulthood, and recent advances in the molecular mechanisms governing spine morphology have shown them to be exquisitely sensitive to changes in the micro-environment. Among the many factors affecting spine morphology are components and regulators of the extracellular matrix (ECM). Modification of the ECM is critical to the repair of injuries throughout the body, including the CNS. Matrix metalloproteinase (MMP)-7/matrilysin is a key regulator of the ECM during pathogen infection, after nerve crush and in encephalitogenic disorders. We have investigated the effects of MMP-7 on dendritic spines in hippocampal neuron cultures and found that it induces the transformation of mature, short mushroom-shaped spines into long, thin filopodia reminiscent of immature spines. These changes were accompanied by a dramatic redistribution of F-actin from spine heads into thick, rope-like structures in the dendritic shaft. Strikingly, MMP-7 effects on dendritic spines were similar to those of NMDA treatment, and both could be blocked by channel-specific antagonists. These findings are the first direct evidence that MMPs can influence the morphology of mature dendritic spines, and hence synaptic stability.     

Vasodilator-stimulated Phosphoprotein (VASP) Induces Actin Assembly in Dendritic Spines to Promote Their Development and Potentiate Synaptic Strength

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

Dendrite and spine modifications in autism and related neurodevelopmental disorders in patients and animal models

Dendrites and spines are the main neuronal structures receiving input from other neurons and glial cells. Dendritic and spine number, size, and morphology are some of the crucial factors determining how signals coming from individual synapses are integrated. Much remains to be understood about the characteristics of neuronal dendrites and dendritic spines in autism and related disorders. Although there have been many studies conducted using autism mouse models, few have been carried out using postmortem human tissue from patients. Available animal models of autism include those generated through genetic modifications and those non‐genetic models of the disease. Here, we review how dendrite and spine morphology and number is affected in autism and related neurodevelopmental diseases, both in human, and genetic and non‐genetic animal models of autism. Overall, data obtained from human and animal models point to a generalized reduction in the size and number, as well as an alteration of the morphology of dendrites; and an increase in spine densities with immature morphology, indicating a general spine immaturity state in autism. Additional human studies on dendrite and spine number and morphology in postmortem tissue are needed to understand the properties of these structures in the cerebral cortex of patients with autism. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 419–437, 2017     

Structural plasticity of dendritic spines

Dendritic spines are small mushroom-like protrusions arising from neurons where most excitatory synapses reside. Their peculiar shape suggests that spines can serve as an autonomous postsynaptic compartment that isolates chemical and electrical signaling. How neuronal activity modifies the morphology of the spine and how these modifications affect synaptic transmission and plasticity are intriguing issues. Indeed, the induction of long-term potentiation (LTP) or depression (LTD) is associated with the enlargement or shrinkage of the spine, respectively. This structural plasticity is mainly controlled by actin filaments, the principal cytoskeletal component of the spine. Here we review the pioneering microscopic studies examining the structural plasticity of spines and propose how changes in actin treadmilling might regulate spine morphology.     

Actin in dendritic spines: connecting dynamics to function

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

Role of Cdk5 in Kalirin7-Mediated Formation of Dendritic Spines

Neurochemical Research - A majority of excitatory synapses in the brain are localized on the dendritic spines. Alterations of spine density and morphology are associated with many neurological...     

A critical role for myosin IIb in dendritic spine morphology and synaptic function

Dendritic spines show rapid motility and plastic morphology, which may mediate information storage in the brain. It is presently believed that polymerization/depolymerization of actin is the primary determinant of spine motility and morphogenesis. Here, we show that myosin IIB, a molecular motor that binds and contracts actin filaments, is essential for normal spine morphology and dynamics and represents a distinct biophysical pathway to control spine size and shape. Myosin IIB is enriched in the postsynaptic density (PSD) of neurons. Pharmacologic or genetic inhibition of myosin IIB alters protrusive motility of spines, destabilizes their classical mushroom-head morphology, and impairs excitatory synaptic transmission. Thus, the structure and function of spines is regulated by an actin-based motor in addition to the polymerization state of actin.     

Loss of the actin regulator cyclase-associated protein 1 (CAP1) modestly affects dendritic spine remodeling during synaptic plasticity

Dendritic spines form the postsynaptic compartment of most excitatory synapses in the vertebrate brain. Morphological changes of dendritic spines contribute to major forms of synaptic plasticity such as long-term potentiation (LTP) or depression (LTD). Synaptic plasticity underlies learning and memory, and defects in synaptic plasticity contribute to the pathogeneses of human brain disorders. Hence, deciphering the molecules that drive spine remodeling during synaptic plasticity is critical for understanding the neuronal basis of physiological and pathological brain function. Since actin filaments (F-actin) define dendritic spine morphology, actin-binding proteins (ABP) that accelerate dis-/assembly of F-actin moved into the focus as critical regulators of synaptic plasticity. We recently identified cyclase-associated protein 1 (CAP1) as a novel actin regulator in neurons that cooperates with cofilin1, an ABP relevant for synaptic plasticity. We therefore hypothesized a crucial role for CAP1 in structural synaptic plasticity. By exploiting mouse hippocampal neurons, we tested this hypothesis in the present study. We found that induction of both forms of synaptic plasticity oppositely altered concentration of exogenous, myc-tagged CAP1 in dendritic spines, with chemical LTP (cLTP) decreasing and chemical LTD (cLTD) increasing it. cLTP induced spine enlargement in CAP1-deficient neurons. However, it did not increase the density of large spines, different from control neurons. cLTD induced spine retraction and spine size reduction in control neurons, but not in CAP1-KO neurons. Together, we report that postsynaptic myc-CAP1 concentration oppositely changed during cLTP and cTLD and that CAP1 inactivation modestly affected structural plasticity.     

Cell Surface Heparan Sulfate Proteoglycan Syndecan-2 Induces the Maturation of Dendritic Spines in Rat Hippocampal Neurons

Dendritic spines are small protrusions that receive synapses, and changes in spine morphology are thought to be the structural basis for learning and memory. We demonstrate that the cell surface heparan sulfate proteoglycan syndecan-2 plays a critical role in spine development. Syndecan-2 is concentrated at the synapses, specifically on the dendritic spines of cultured hippocampal neurons, and its accumulation occurs concomitant with the morphological maturation of spines from long thin protrusions to stubby and headed shapes. Early introduction of syndecan-2 cDNA into immature hippocampal neurons, by transient transfection, accelerates spine formation from dendritic protrusions. Deletion of the COOH-terminal EFYA motif of syndecan-2, the binding site for PDZ domain proteins, abrogates the spine-promoting activity of syndecan-2. Syndecan-2 clustering on dendritic protrusions does not require the PDZ domain-binding motif, but another portion of the cytoplasmic domain which includes a protein kinase C phosphorylation site. Our results indicate that syndecan-2 plays a direct role in the development of postsynaptic specialization through its interactions with PDZ domain proteins.     

Biphasic Synaptic Ca Influx Arising from Compartmentalized Electrical Signals in Dendritic Spines

Excitatory synapses on mammalian principal neurons are typically formed onto dendritic spines, which consist of a bulbous head separated from the parent dendrite by a thin neck. Although activation of voltage-gated channels in the spine and stimulus-evoked constriction of the spine neck can influence synaptic signals, the contribution of electrical filtering by the spine neck to basal synaptic transmission is largely unknown. Here we use spine and dendrite calcium (Ca) imaging combined with 2-photon laser photolysis of caged glutamate to assess the impact of electrical filtering imposed by the spine morphology on synaptic Ca transients. We find that in apical spines of CA1 hippocampal neurons, the spine neck creates a barrier to the propagation of current, which causes a voltage drop and results in spatially inhomogeneous activation of voltage-gated Ca channels (VGCCs) on a micron length scale. Furthermore, AMPA and NMDA-type glutamate receptors (AMPARs and NMDARs, respectively) that are colocalized on individual spine heads interact to produce two kinetically and mechanistically distinct phases of synaptically evoked Ca influx. Rapid depolarization of the spine triggers a brief and large Ca current whose amplitude is regulated in a graded manner by the number of open AMPARs and whose duration is terminated by the opening of small conductance Ca-activated potassium (SK) channels. A slower phase of Ca influx is independent of AMPAR opening and is determined by the number of open NMDARs and the post-stimulus potential in the spine. Biphasic synaptic Ca influx only occurs when AMPARs and NMDARs are coactive within an individual spine. These results demonstrate that the morphology of dendritic spines endows associated synapses with specialized modes of signaling and permits the graded and independent control of multiple phases of synaptic Ca influx.