Glutamate receptors regulate actin-based plasticity in dendritic spines

Protein tyrosine phosphatase epsilon regulates integrin-mediated podosome stability in osteoclasts by activating Src

The nonreceptor isoform of tyrosine phosphatase epsilon (cyt-PTPe) supports osteoclast adhesion and activity in vivo, leading to increased bone mass in female mice lacking PTPe (EKO mice). The structure and organization of the podosomal adhesion structures of EKO osteoclasts are abnormal; the molecular mechanism behind this is unknown. We show here that EKO podosomes are disorganized, unusually stable, and reorganize poorly in response to physical contact. Phosphorylation and activities of Src, Pyk2, and Rac are decreased and Rho activity is increased in EKO osteoclasts, suggesting that integrin signaling is defective in these cells. Integrin activation regulates cyt-PTPe by inducing Src-dependent phosphorylation of cyt-PTPe at Y638. This phosphorylation event is crucial because wild-type-but not Y638F-cyt-PTPe binds and further activates Src and restores normal stability to podosomes in EKO osteoclasts. Increasing Src activity or inhibiting Rho or its downstream effector Rho kinase in EKO osteoclasts rescues their podosomal stability phenotype, indicating that cyt-PTPe affects podosome stability by functioning upstream of these molecules. We conclude that cyt-PTPe participates in a feedback loop that ensures proper Src activation downstream of integrins, thus linking integrin signaling with Src activation and accurate organization and stability of podosomes in osteoclasts.

A model for cortical rewiring following deafferentation and focal stroke

It is still unclear to what extent structural plasticity in terms of synaptic rewiring is the cause for cortical remapping after a lesion. Recent two-photon laser imaging studies demonstrate that synaptic rewiring is persistent in the adult brain and is dramatically increased following brain lesions or after a loss of sensory input (cortical deafferentation). We use a recurrent neural network model to study the time course of synaptic rewiring following a peripheral lesion. For this, we represent axonal and dendritic elements of cortical neurons to model synapse formation, pruning and synaptic rewiring. Neurons increase and decrease the number of axonal and dendritic elements in an activity-dependent fashion in order to maintain their activity in a homeostatic equilibrium. In this study we demonstrate that synaptic rewiring contributes to neuronal homeostasis during normal development as well as following lesions. We show that networks in homeostasis, which can therefore be considered as adult networks, are much less able to compensate for a loss of input. Interestingly, we found that paused stimulation of the networks are much more effective promoting reorganization than continuous stimulation. This can be explained as neurons quickly adapt to this stimulation whereas pauses prevents a saturation of the positive stimulation effect. These findings may suggest strategies for improving therapies in neurologic rehabilitation.

Synaptic Mechanisms of Activity-Dependent Remodeling in Visual Cortex during Monocular Deprivation

It has long been appreciated that in the visual cortex, particularly within a postnatal critical period for experience-dependent plasticity, the closure of one eye results in a shift in the responsiveness of cortical cells toward the experienced eye. While the functional aspects of this ocular dominance shift have been studied for many decades, their cortical substrates and synaptic mechanisms remain elusive. Nonetheless, it is becoming increasingly clear that ocular dominance plasticity is a complex phenomenon that appears to have an early and a late component. Early during monocular deprivation, deprived eye cortical synapses depress, while later during the deprivation open eye synapses potentiate. Here we review current literature on the cortical mechanisms of activity-dependent plasticity in the visual system during the critical period. These studies shed light on the role of activity in shaping neuronal structure and function in general and can lead to insights regarding how learning is acquired and maintained at the neuronal level during normal and pathological brain development.

Regulation of synaptic structure by ubiquitin C-terminal hydrolase L1

Ubiquitin C-terminal hydrolase L1 (UCH-L1) is a deubiquitinating enzyme that is selectively and abundantly expressed in the brain, and its activity is required for normal synaptic function. Here, we show that UCH-L1 functions in maintaining normal synaptic structure in hippocampal neurons. We found that UCH-L1 activity is rapidly upregulated by NMDA receptor activation, which leads to an increase in the levels of free monomeric ubiquitin. Conversely, pharmacological inhibition of UCH-L1 significantly reduces monomeric ubiquitin levels and causes dramatic alterations in synaptic protein distribution and spine morphology. Inhibition of UCH-L1 activity increases spine size while decreasing spine density. Furthermore, there is a concomitant increase in the size of presynaptic and postsynaptic protein clusters. Interestingly, however, ectopic expression of ubiquitin restores normal synaptic structure in UCH-L1-inhibited neurons. These findings point to a significant role of UCH-L1 in synaptic remodeling, most likely by modulating free monomeric ubiquitin levels in an activity-dependent manner.

Involvement of afadin in the formation and remodeling of synapses in the hippocampus

In the hippocampus, synapses are formed between mossy fiber terminals and CA3 pyramidal cell dendrites and comprise highly developed synaptic junctions (SJs) and puncta adherentia junctions (PAJs). Dynamic remodeling of synapses in the hippocampus is implicated in learning and memory. Components of both the nectin-afadin and cadherin-catenin cell adhesion systems exclusively accumulate at PAJs. We investigated the role of afadin at synapses in mice in which the afadin gene was conditionally inactivated in hippocampal neurons. In these mutant mice, the signals for not only nectins, but also N-cadherin and beta-catenin, were hardly detected in the CA3 area, in addition to loss of the signal for afadin, resulting in disruption of PAJs. Ultrastructural analysis revealed an increase in the number of perforated synapses, suggesting the instability of SJs. These results indicate that afadin is involved not only in the assembly of nectins and cadherins at synapses, but also in synaptic remodeling.

Control of the postsynaptic membrane viscosity

The physical properties of the postsynaptic membrane (PSM), including its viscosity, determine its capacity to regulate the net flux of synaptic membrane proteins such as neurotransmitter receptors. To address these properties, we studied the lateral diffusion of glycophosphatidylinositol-anchored green fluorescent protein and cholera toxin bound to the external leaflet of the plasma membrane. Relative to extrasynaptic regions, their mobility was reduced at synapses and even more at inhibitory than at excitatory ones. This indicates a higher density of obstacles and/or higher membrane viscosity at inhibitory contacts. Actin depolymerization reduced the confinement and accelerated a population of fast, mobile molecules. The compaction of obstacles thus depends on actin cytoskeleton integrity. Cholesterol depletion increased the mobility of the slow diffusing molecules, allowing them to diffuse more rapidly through the crowded PSM. Thus, the PSM has lipid-raft properties, and the density of obstacles to diffusion depends on filamentous actin. Therefore, lipid composition and actin-dependent protein compaction regulate viscosity of the PSM and, consequently, the molecular flow in and out of synapses.

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.

AMPA receptor and GEF-H1/Lfc complex regulates dendritic spine development through RhoA signaling cascade

AMPA receptors (AMPA-R) are major mediators of synaptic transmission and plasticity in the developing and adult central nervous system. Activity-dependent structural plasticity mediated by dynamic changes in the morphology of spines and dendrites is also essential for the formation and tuning of neuronal circuits. RhoA and Rac1 are known to play important roles in the regulation of spine and dendrite development in response to neuronal activity. These Rho GTPases are activated by guanine nucleotide exchange factors (GEFs). In this study, we identified GEF-H1/Lfc as a component of the AMPA-R complex in the brain. GEF-H1 is enriched in the postsynaptic density and is colocalized with GluR1 at spines. GEF-H1 activity negatively regulates spine density and length through a RhoA signaling cascade. In addition, AMPA-R-dependent changes in spine development are eliminated by down-regulation of GEF-H1. Altogether, these results strongly suggest that GEF-H1 is an important mediator of AMPA-R activity-dependent structural plasticity in neurons.

AMPA glutamate receptor subunits 1 and 2 regulate dendrite complexity and spine motility in neurons of the developing neocortex

Within neurons of several regions of the CNS, mature dendrite architecture is attained via extensive reorganization of arbor during the developmental period. Since dendrite morphology determines the firing patterns of the neuron, morphological refinement of dendritic arbor may have important implications for mature network activity. In the neocortex, a region of brain that is sensitive to activity-dependent structural rearrangement of dendritic arbor, the proportion of AMPA receptors increases over the developmental period. However, it is unclear whether changes in AMPA receptor expression contribute to maturation of dendritic architecture. To determine the effects of increasing AMPA receptor expression on dendrite morphology and connectivity within the neocortex, and to determine whether these effects are dependent on specific AMPA receptor subunits, we overexpressed the AMPA glutamate receptor subunit 1 (GluR1) and glutamate receptor subunit 2 (GluR2) in cultured rat neocortical neurons at the time that AMPA receptors would normally be incorporated into synapses. Following expression of GluR1 or GluR2 we observed increases in the length and complexity of dendritic arbor of cortical neurons, and a concurrent reduction in motility of spines. In addition, expression of either subunit was associated with an increased density of excitatory postsynaptic puncta. These results suggest that AMPA receptor expression is an important determinant of dendrite morphology and connectivity in neocortical neurons, and further, that contrary to other regions of the CNS, the effects of AMPA receptors on dendrite morphology are not subunit-specific.

N-cadherin, spine dynamics, and synaptic function

Dendritic spines are one-half (the postsynaptic half) of most excitatory synapses. Ever since the direct observation over a decade ago that spines can continually change size and shape, spine dynamics has been of great research interest, especially as a mechanism for structural synaptic plasticity. In concert with this ongoing spine dynamics, the stability of the synapse is also needed to allow continued, reliable synaptic communication. Various cell-adhesion molecules help to structurally stabilize a synapse and its proteins. Here, we review the effects of disrupting N-cadherin, a prominent trans-synaptic adhesion molecule, on spine dynamics, as reported in Mysore et al. (2007). We highlight the novel method adopted therein to reliably detect even subtle changes in fast and slow spine dynamics. We summarize the structural, functional, and molecular consequences of acute N-cadherin disruption, and tie them in, in a working model, with longer-term effects on spines and synapses reported in the literature.

Profilin, a multi-modal regulator of neuronal plasticity

Thirty years after its initial characterization and more than 1000 publications listed in PubMed describing its properties, the small (ca 15 kDa) protein profilin continues to surprise us with new, recently discovered functions. Originally described as an actin-binding protein, profilin has now been shown to interact with more than a dozen proteins in mammalian cells. Some of the more recently described and intriguing interactions are within neurons involving a neuronal profilin family member. Profilin is now regarded as a regulator of various cellular processes such as cytoskeletal dynamics, membrane trafficking and nuclear transport. Profilin is a necessary element in key steps of neuronal differentiation and synaptic plasticity, and embodies properties postulated for a synaptic tag. These findings identify profilin as an important factor linking cellular and behavioural plasticity in neural circuits.

Co-localization of caldesmon and calponin with cortical afferents, metabotropic glutamate and neurotrophic receptors in the lateral and central nuclei of the amygdala

Caldesmon (Cd) and calponin (Cp) are two actin/calmodulin-binding proteins involved in 'actin-linked' regulation of smooth muscle and non-muscle Mg(2+) actin-activated myosin II ATPase activity. However, in the brain, Cd and Cp are associated with the regulation of the neuronal cytoskeleton. In this study we investigated the subcellular distribution of Cd and Cp in the amygdala and their possible relationship to metabotropic glutamate (mGluR1 alpha and 5) and TrkB receptors which interact with inputs from the cortex and are involved in associative learning. Cd and Cp immunoreactivity (IR) was mainly found in dendritic spines, along dendritic microtubules, and in neuronal perikarya but never in axon terminals. Punctate labeling representing spine labeling was restricted to small patches in the lateral nucleus of amygdala, intercalated cell masses (ICM), and the lateral subdivision of central nucleus. This restricted distribution may reflect local afferent activation. In addition, Cd, Cp, mGluR1 alpha and cortical afferents are co-distributed in the ICM distributed in the lateral nucleus and lateral capsular division of the central nucleus, and the lateral division of the central nucleus itself. Consistent with our previous studies, TrkB IR in the central nucleus was associated with Cd and Cp-immunoreactive spines whereas mGluR1 alpha IR and mGluR5 IR were almost exclusively associated with the PSDs of asymmetric synapses, in most cases apposed by cortical terminals. mGluR1 alpha and TrkB immunoreactivities were invariably associated with each other. Overall, these findings suggest that caldesmon and calponin in the amygdala are closely associated with afferents and receptors that have been strongly implicated in associative learning.

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.

Expression of the eight AMPA receptor subunit genes in the developing central nervous system and sensory organs of zebrafish

The AMPA type glutamate receptors mediate the majority of fast synaptic transmission in the vertebrate nervous system. Whereas mammals have four subunit genes, Gria1-4, zebrafish has retained a duplicated set of eight genes named gria1-4a and b. We give here a detailed overview of the expression patterns of all eight zebrafish subunits within the developing central nervous system and sensory organs at 24, 48, and 72 hr after fertilization. Expression domains include distinct neuronal subsets in the developing forebrain, midbrain, hindbrain, and spinal cord, as well as in the ganglion- and inner nuclear layers of the retina. As a general rule, each pair of duplicated gria genes is differentially expressed, indicating subfunctionalization of AMPA receptor subunit expression in the teleost lineage. Our findings suggest that zebrafish can serve as a useful model system to investigate the role of AMPA receptors and their differential expression in the vertebrate nervous system.

Long-range axonal calcium sweep induces axon retraction

Axon guidance molecules trigger a cascade of local signal in growth cones and evoke various morphologic responses, including axon attraction, repulsion, elongation, and retraction. However, little is known about whether subcellular compartments, other than axonal growth cones, control axon outgrowth. We found that in isolated dentate granule cells, local application of glutamate to the somatodendritic areas, but not the axon itself, induced rapid axon retraction, during which a calcium wave propagated from the somata to the axon terminals. The calcium wave and axon retraction were both inhibited by blockade of voltage-sensitive calcium channels and intracellular calcium dynamics. A combination of perisomatic application of calcium ionophore and depolarizing current injection induced axonal calcium sweep and axon retraction. Thus, perisomatic environments can modulate axon behavior through long-range intracellular communication.

Dynamics underlying synaptic gain between pairs of cortical pyramidal neurons

Changes in connectivity between pairs of neurons can serve as a substrate for information storage and for experience-dependent changes in neuronal circuitry. Early in development, synaptic contacts form and break, but how these dynamics influence the connectivity between pairs of neurons is not known. Here we used time-lapse imaging to examine the synaptic interactions between pairs of cultured cortical pyramidal neurons, and found that the axon-dendrite contacts between each neuronal pair were composed of both a relatively stable and a more labile population. Under basal conditions, loss and gain of contacts within this labile population was well balanced and there was little net change in connectivity. Selectively increasing the levels of activated CaMKII in the postsynaptic neuron increased connectivity between pairs of neurons by increasing the rate of gain of new contacts without affecting the probability of contact loss, or the proportion of stable and labile contacts, and this increase required Calcium/calmodulin binding to CaMKII. Our data suggest that activating CaMKII can increase synaptic connectivity through a CaM-dependent increase in contact formation, followed by stabilization of a constant fraction of new contacts.

Differential regulation of dendrite complexity by AMPA receptor subunits GluR1 and GluR2 in motor neurons

Activity-dependent developmental mechanisms in many regions of the central nervous system are thought to be responsible for shaping dendritic architecture and connectivity, although the molecular mechanisms underlying these events remain obscure. Since AMPA glutamate receptors are developmentally regulated in spinal motor neurons, we have investigated the role of activation of AMPA receptors in dendritic outgrowth of spinal motor neurons by overexpression of two subunits, GluR1 and GluR2, and find that dendrite outgrowth is differentially controlled by expression of these subunits. Overexpression of GluR1 was associated with greater numbers of filopodia, and an increase in the length and complexity of dendritic arbor. In contrast, GluR2 expression did not alter dendritic complexity, but was associated with a moderate increase in length of arbor, and decreased numbers of filopodia. Neither GluR1 nor GluR2 had any effect on the motility of filopodia. In addition, GluR1 but not GluR2 expression increased the density of dendritic puncta incorporating a GFP-labeled PSD95, suggesting that GluR1 may mediate its effect in part by augmenting the number of excitatory synapses within motor neuron dendrites. Together these results suggest that in spinal motor neurons, AMPA receptors composed of GluR1 subunits may facilitate neurotrophic mechanisms in these neurons, permitting sustained dendrite outgrowth and synaptogenesis, whereas expression of AMPA receptors containing GluR2 acts to preserve existing dendritic arbor. Thus, the observed downregulation of GluR1 in motor neurons during postnatal development may limit the formation of new dendrite segments and synapses, promoting stabilized synaptic connectivity.

Synaptic function for the Nogo-66 receptor NgR1: regulation of dendritic spine morphology and activity-dependent synaptic strength

In the mature nervous system, changes in synaptic strength correlate with changes in neuronal structure. Members of the Nogo-66 receptor family have been implicated in regulating neuronal morphology. Nogo-66 receptor 1 (NgR1) supports binding of the myelin inhibitors Nogo-A, MAG (myelin-associated glycoprotein), and OMgp (oligodendrocyte myelin glycoprotein), and is important for growth cone collapse in response to acutely presented inhibitors in vitro. After injury to the corticospinal tract, NgR1 limits axon collateral sprouting but is not important for blocking long-distance regenerative growth in vivo. Here, we report on a novel interaction between NgR1 and select members of the fibroblast growth factor (FGF) family. FGF1 and FGF2 bind directly and with high affinity to NgR1 but not to NgR2 or NgR3. In primary cortical neurons, ectopic NgR1 inhibits FGF2-elicited axonal branching. Loss of NgR1 results in altered spine morphologies along apical dendrites of hippocampal CA1 neurons in vivo. Analysis of synaptosomal fractions revealed that NgR1 is enriched synaptically in the hippocampus. Physiological studies at Schaffer collateral-CA1 synapses uncovered a synaptic function for NgR1. Loss of NgR1 leads to FGF2-dependent enhancement of long-term potentiation (LTP) without altering basal synaptic transmission or short-term plasticity. NgR1 and FGF receptor 1 (FGFR1) are colocalized to synapses, and mechanistic studies revealed that FGFR kinase activity is necessary for FGF2-elicited enhancement of hippocampal LTP in NgR1 mutants. In addition, loss of NgR1 attenuates long-term depression of synaptic transmission at Schaffer collateral-CA1 synapses. Together, our findings establish that physiological NgR1 signaling regulates activity-dependent synaptic strength and uncover neuronal NgR1 as a regulator of synaptic plasticity.

Neurotransmitters and the development of neuronal circuits

In the mature brain, neurotransmitters are used for synaptic communication between neurons. But during nervous system development, neurons often express and release transmitters before their axons establish contacts with their target cells. While much is known about the synaptic effects of neurotransmitters, their extrasynaptic effects are less understood. There is increasing evidence that neurotransmitters in the immature nervous system can act as trophic factors that influence different developmental events such as cell proliferation and differentiation. However, more recent work demonstrates that neurotransmitters can also influence the targeting of migrating neurons and growing axons during the formation of neuronal circuits. This chapter will focus on such guidance effects of neurotransmitters during the development of the nervous system. Elucidating extrasynaptic functions during the nervous system development might also provide insights in their potential roles for plasticity and regeneration in the adult nervous system.

Paralemmin-1, a modulator of filopodia induction is required for spine maturation

Dendritic filopodia are thought to participate in neuronal contact formation and development of dendritic spines; however, molecules that regulate filopodia extension and their maturation to spines remain largely unknown. Here we identify paralemmin-1 as a regulator of filopodia induction and spine maturation. Paralemmin-1 localizes to dendritic membranes, and its ability to induce filopodia and recruit synaptic elements to contact sites requires protein acylation. Effects of paralemmin-1 on synapse maturation are modulated by alternative splicing that regulates spine formation and recruitment of AMPA-type glutamate receptors. Paralemmin-1 enrichment at the plasma membrane is subject to rapid changes in neuronal excitability, and this process controls neuronal activity-driven effects on protrusion expansion. Knockdown of paralemmin-1 in developing neurons reduces the number of filopodia and spines formed and diminishes the effects of Shank1b on the transformation of existing filopodia into spines. Our study identifies a key role for paralemmin-1 in spine maturation through modulation of filopodia induction.

Activity-dependent synaptogenesis: regulation by a CaM-kinase kinase/CaM-kinase I/betaPIX signaling complex

Neuronal activity augments maturation of mushroom-shaped spines to form excitatory synapses, thereby strengthening synaptic transmission. We have delineated a Ca(2+)-signaling pathway downstream of the NMDA receptor that stimulates calmodulin-dependent kinase kinase (CaMKK) and CaMKI to promote formation of spines and synapses in hippocampal neurons. CaMKK and CaMKI form a multiprotein signaling complex with the guanine nucleotide exchange factor (GEF) betaPIX and GIT1 that is localized in spines. CaMKI-mediated phosphorylation of Ser516 in betaPIX enhances its GEF activity, resulting in activation of Rac1, an established enhancer of spinogenesis. Suppression of CaMKK or CaMKI by pharmacological inhibitors, dominant-negative (dn) constructs and siRNAs, as well as expression of the betaPIX Ser516Ala mutant, decreases spine formation and mEPSC frequency. Constitutively-active Pak1, a downstream effector of Rac1, rescues spine inhibition by dnCaMKI or betaPIX S516A. This activity-dependent signaling pathway can promote synapse formation during neuronal development and in structural plasticity.

Balancing structure and function at hippocampal dendritic spines

Dendritic spines are the primary recipients of excitatory input in the central nervous system. They provide biochemical compartments that locally control the signaling mechanisms at individual synapses. Hippocampal spines show structural plasticity as the basis for the physiological changes in synaptic efficacy that underlie learning and memory. Spine structure is regulated by molecular mechanisms that are fine-tuned and adjusted according to developmental age, level and direction of synaptic activity, specific brain region, and exact behavioral or experimental conditions. Reciprocal changes between the structure and function of spines impact both local and global integration of signals within dendrites. Advances in imaging and computing technologies may provide the resources needed to reconstruct entire neural circuits. Key to this endeavor is having sufficient resolution to determine the extrinsic factors (such as perisynaptic astroglia) and the intrinsic factors (such as core subcellular organelles) that are required to build and maintain synapses.

Scaffolding proteins at the postsynaptic density: shank as the architectural framework

Shank proteins are multidomain scaffold proteins of the postsynaptic density, connecting neurotransmitter receptors and other membrane proteins with signaling proteins and the actin cytoskeleton. By virtue of their protein interactions, Shank proteins assemble signaling platforms for G-protein-mediated signaling and the control of calcium homeostasis in dendritic spines. In addition, they participate in morphological changes, leading to maturation of dendritic spines and synapse formation. The importance of the Shank scaffolding function is demonstrated by genetically determined forms of mental retardation, which may be caused by haploinsufficiency for the SHANK3 gene. Consistent with its central function within the postsynaptic density, the availability of Shank is tightly controlled by local synthesis and degradation, as well as actin-dependent dynamic rearrangements within the dendritic spine.

Effects of N-cadherin disruption on spine morphological dynamics

Structural changes at synapses are thought to be a key mechanism for the encoding of memories in the brain. Recent studies have shown that changes in the dynamic behavior of dendritic spines accompany bidirectional changes in synaptic plasticity, and that the disruption of structural constraints at synapses may play a mechanistic role in spine plasticity. While the prolonged disruption of N-cadherin, a key synaptic adhesion molecule, has been shown to alter spine morphology, little is known about the short-term regulation of spine morphological dynamics by N-cadherin. With time-lapse, confocal imaging in cultured hippocampal neurons, we examined the progression of structural changes in spines following an acute treatment with AHAVD, a peptide known to interfere with the function of N-cadherin. We characterized fast and slow timescale spine dynamics (minutes and hours, respectively) in the same population of spines. We show that N-cadherin disruption leads to enhanced spine motility and reduced length, followed by spine loss. The structural effects are accompanied by a loss of functional connectivity. Further, we demonstrate that early structural changes induced by AHAVD treatment, namely enhanced motility and reduced length, are indicators for later spine fate, i.e., spines with the former changes are more likely to be subsequently lost. Our results thus reveal the short-term regulation of synaptic structure by N-cadherin and suggest that some forms of morphological dynamics may be potential readouts for subsequent, stimulus-induced rewiring in neuronal networks.

Dendritic spine plasticity: Looking beyond development

Most excitatory synapses in the CNS form on dendritic spines, tiny protrusions from the dendrites of excitatory neurons. As such, spines are likely loci of synaptic plasticity. Spines are dynamic structures, but the functional consequences of dynamic changes in these structures in the mature brain are unclear. Changes in spine density, morphology, and motility have been shown to occur with paradigms that induce synaptic plasticity, as well as altered sensory experience and neuronal activity. These changes potentially lead to an alteration in synaptic connectivity and strength between neuronal partners, affecting the efficacy of synaptic communication. Here, we review the formation and modification of excitatory synapses on dendritic spines as it relates to plasticity in the central nervous system after the initial phase of synaptogenesis. We will also discuss some of the molecular links that have been implicated in both synaptic plasticity and the regulation of spine 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.

Activin increases the number of synaptic contacts and the length of dendritic spine necks by modulating spinal actin dynamics

Long-lasting modifications in synaptic transmission depend on de novo gene expression in neurons. The expression of activin, a member of the transforming growth factor beta (TGF-beta) superfamily, is upregulated during hippocampal long-term potentiation (LTP). Here, we show that activin increased the average number of presynaptic contacts on dendritic spines by increasing the population of spines that were contacted by multiple presynaptic terminals in cultured neurons. Activin also induced spine lengthening, primarily by elongating the neck, resulting in longer mushroom-shaped spines. The number of spines and spine head size were not significantly affected by activin treatment. The effects of activin on spinal filamentous actin (F-actin) morphology were independent of protein and RNA synthesis. Inhibition of cytoskeletal actin dynamics or of the mitogen-activated protein (MAP) kinase pathway blocked not only the activin-induced increase in the number of terminals contacting a spine but also the activin-induced lengthening of spines. These results strongly suggest that activin increases the number of synaptic contacts by modulating actin dynamics in spines, a process that might contribute to the establishment of late-phase LTP.

Involvement of the Snk-SPAR pathway in glutamate-induced excitotoxicity in cultured hippocampal neurons

The serum-induced kinase (Snk)-spine-associated Rap GTPase-activating protein (SPAR) signaling pathway is reported as a new molecular mechanism in activity-dependent remodeling of synapses. However, the relationship between Snk-SPAR pathway and glutamate-induced excitotoxicity is not well understood. We report here that in cultured hippocampal neurons, glutamate stimulation induces the activation of Snk-SPAR pathway, and leads to a loss of mature dendritic spines. The time-dependent changes in Snk and SPAR expression after glutamate exposure are also elucidated. Furthermore, the activation of Snk-SPAR pathway induced by glutamate treatment can be blocked by an NMDA receptor antagonist, MK801. These results demonstrate that Snk-SPAR pathway may play a pivotal role in glutamate-induced excitotoxic damage in CNS through regulating the stability of synapse.

Fast regulation of axonal growth cone motility by electrical activity

Axonal growth cones are responsible for the correct guidance of developing axons and the establishment of functional neural networks. They are highly motile because of fast and continuous rearrangements of their actin-rich cytoskeleton. Here we have used live imaging of axonal growth cones of hippocampal neurons in culture and quantified their motility with a temporal resolution of 2 s. Using novel methods of analysis of growth cone dynamics, we show that transient activation of kainate receptors by bath-applied kainate induced a fast and reversible growth cone stalling. This effect depends on electrical activity and can be mimicked by the transient discharge of action potentials elicited in the neuron by intracellular current injections at the somatic level through a patch pipette. Growth cone stalling induced by electrical stimulation is mediated by calcium entry from the extracellular medium as well as by calcium release from intracellular stores that define spatially restricted microdomains directly affecting cytoskeletal dynamics. We propose that growth cone motility is dynamically controlled by transient bursts of spontaneous electrical activity, which constitutes a prominent feature of developing neural networks in vivo.

All-trans-retinoic acid stimulates translation and induces spine formation in hippocampal neurons through a membrane-associated RARalpha

Differentiation and patterning in the developing nervous system require the vitamin A metabolite all-trans-retinoic acid (atRA). Recent data suggest that higher cognitive functions, such as creation of hippocampal memory, also require atRA and its receptors, RAR, through affecting synaptic plasticity. Here we show that within 30 min atRA increased dendritic growth approximately 2-fold, and PSD-95 and synaptophysin puncta intensity approximately 3-fold, in cultured mouse hippocampal neurons, suggesting increased synapse formation. atRA (10 nM) increased ERK1/2 phosphorylation within 10 min. In synaptoneurosomes, atRA rapidly increased phosphorylation of ERK1/2, its target 4E-BP, and p70S6K, and its substrate, ribosome protein S6, indicating activation of MAPK and mammalian target of rapamycin (mTOR). Immunofluorescence revealed intense dendritic expression of RARalpha in the mouse hippocampus and localization of RARalpha on the surfaces of primary cultures of hippocampal neurons, with bright puncta along soma and neurites. Surface biotinylation confirmed the locus of RARalpha expression. Knockdown of RARalpha by shRNA impaired atRA-induced spine formation and abolished dendritic growth. Prolonged atRA stimulation reduced surface/total RARalpha by 43%, suggesting internalization, whereas brain-derived nerve growth factor or bicuculline increased the ratio by approximately 1.8-fold. atRA increased translation in the somatodendritic compartment, similar to brain-derived nerve growth factor. atRA specifically increased dendritic translation and surface expression of the alpha-amino-3-hydroxyl-5-methyl-4-isoxazole propionate receptor (AMPAR) subunit 1 (GluR1), without affecting GluR2. These data provide mechanistic insight into atRA function in the hippocampus and identify a unique membrane-associated RARalpha that mediates rapid induction of neuronal translation by atRA.

Protracted synaptogenesis after activity-dependent spinogenesis in hippocampal neurons

Activity-dependent morphological plasticity of neurons is central to understanding how the synaptic network of the CNS becomes reconfigured in response to experience. In recent years, several studies have shown that synaptic activation that leads to the induction of long-term potentiation also drives the growth of new dendritic spines, raising the possibility that new synapses are made. We examine this directly by correlating time-lapse two-photon microscopy of newly formed spines on CA1 pyramidal neurons in organotypic hippocampal slices with electron microscopy. Our results show that, whereas spines that are only a few hours old rarely form synapses, older spines, ranging from 15 to 19 h, consistently have ultrastructural hallmarks typical of synapses. This is in agreement with a recent in vivo study that showed that, after a few days, new spines consistently form functional synapses. In addition, our study provides a much more detailed understanding of the first few hours after activity-dependent spinogenesis. Within tens of minutes, physical contacts are formed with existing presynaptic boutons, which slowly, over the course of many hours, mature into new synapses.

Activation of NMDA receptors promotes dendritic spine development through MMP-mediated ICAM-5 cleavage

Matrix metalloproteinase (MMP)-2 and -9 are pivotal in remodeling many tissues. However, their functions and candidate substrates for brain development are poorly characterized. Intercellular adhesion molecule-5 (ICAM-5; Telencephalin) is a neuronal adhesion molecule that regulates dendritic elongation and spine maturation. We find that ICAM-5 is cleaved from hippocampal neurons when the cells are treated with N-methyl-d-aspartic acid (NMDA) or α-amino-3-hydroxy-5-methylisoxazole-propionic acid (AMPA). The cleavage is blocked by MMP-2 and -9 inhibitors and small interfering RNAs. Newborn MMP-2– and MMP-9–deficient mice brains contain more full-length ICAM-5 than wild-type mice. NMDA receptor activation disrupts the actin cytoskeletal association of ICAM-5, which promotes its cleavage. ICAM-5 is mainly located in dendritic filopodia and immature thin spines. MMP inhibitors block the NMDA-induced cleavage of ICAM-5 more efficiently in dendritic shafts than in thin spines. ICAM-5 deficiency causes retraction of thin spine heads in response to NMDA stimulation. Soluble ICAM-5 promotes elongation of dendritic filopodia from wild-type neurons, but not from ICAM-5–deficient neurons. Thus, MMPs are important for ICAM-5–mediated dendritic spine development.

Leptin promotes rapid dynamic changes in hippocampal dendritic morphology

Recent studies have implicated the hormone leptin in synaptic plasticity associated with neuronal development and learning and memory. Indeed, leptin facilitates hippocampal long-term potentiation and leptin-insensitive rodents display impaired hippocampal synaptic plasticity suggesting a role for endogenous leptin. Structural changes are also thought to underlie activity-dependent synaptic plasticity and this may be regulated by specific growth factors. As leptin is reported to have neurotrophic actions, we have examined the effects of leptin on the morphology and filopodial outgrowth in hippocampal neurons. Here, we demonstrate that leptin rapidly enhances the motility and density of dendritic filopodia and subsequently increases the density of hippocampal synapses. This process is dependent on the synaptic activation of NR2A-containing NMDA receptors and is mediated by the MAPK (ERK) signaling pathway. As dendritic morphogenesis is associated with activity-dependent changes in synaptic strength, the rapid structural remodeling of dendrites by leptin has important implications for its role in regulating hippocampal synaptic plasticity and neuronal development.

Caloric restriction eliminates the aging-related decline in NMDA and AMPA receptor subunits in the rat hippocampus and induces homeostasis

Caloric restriction (CR) extends life span and ameliorates the aging-related decline in hippocampal-dependent cognitive function. In the present study, we compared subunit levels of NMDA and AMPA types of the glutamate receptor and quantified total synapses and multiple spine bouton (MSB) synapses in hippocampal CA1 from young (10 months), middle-aged (18 months), and old (29 months) Fischer 344xBrown Norway rats that were ad libitum (AL) fed or caloric restricted (CR) from 4 months of age. Each of these parameters has been reported to be a potential contributor to hippocampal function. Western blot analysis revealed that NMDA and AMPA receptor subunits in AL animals decrease between young and middle age to levels that are present at old age. Interestingly, young CR animals have significantly lower levels of glutamate receptor subunits than young AL animals and those lower levels are maintained across life span. In contrast, stereological quantification indicated that total synapses and MSB synapses are stable across life span in both AL and CR rats. These results indicate significant aging-related losses of hippocampal glutamate receptor subunits in AL rats that are consistent with altered synaptic function. CR eliminates that aging-related decline by inducing stable NMDA and AMPA receptor subunit levels.

Activation of kainate receptors controls the number of functional glutamatergic synapses in the area CA1 of rat hippocampus

The expression and functions of kainate-type glutamate receptors (KARs) in the hippocampus are developmentally regulated. In particular, presynaptic KARs depressing glutamate release are tonically activated during early postnatal development, and this activity is down-regulated in parallel with maturation of the synaptic circuitry. In order to understand the physiological relevance of the tonic KAR-mediated signalling, we have here studied the effect of long-term pharmacological activation of KARs on glutamatergic synaptic connectivity in hippocampal slice cultures where presynaptic KARs are expressed but not endogenously activated. Prolonged (16-20 h) activation of the GluR5 subunit-containing KARs using the agonist ATPA (1 microM) caused a specific and enduring increase in the number of glutamatergic synapses in area CA1, evidenced as an increase in the frequency of action potential-independent spontaneous EPSCs (mEPSCs) and in immunostaining against synaptic marker proteins. The long-term ATPA treatment had no detectable effect on GABAergic transmission or on glutamate release probability. Further, the effect of ATPA on synaptic density was independent of action potential firing and dependent on protein kinase C. A critical role of endogenous KAR activity in synaptic development was revealed by chronic treatment of the cultures with the selective GluR5 antagonist LY382884, which caused a significant impairment of glutamatergic transmission to CA1 pyramidal neurons. Together, these data suggest a role for the GluR5 subunit-containing KARs in the formation and/or stabilization of functional glutamatergic synapses in area CA1.

Activity-Regulated N-Cadherin Endocytosis

Enduring forms of synaptic plasticity are thought to require ongoing regulation of adhesion molecules, such as N-cadherin, at synaptic junctions. Little is known about the activity-regulated trafficking of adhesion molecules. Here we demonstrate that surface N-cadherin undergoes a surprisingly high basal rate of internalization. Upon activation of NMDA receptors (NMDAR), the rate of N-cadherin endocytosis is significantly reduced, resulting in an accumulation of N-cadherin in the plasma membrane. Beta-catenin, an N-cadherin binding partner, is a primary regulator of N-cadherin endocytosis. Following NMDAR stimulation, beta-catenin accumulates in spines and exhibits increased binding to N-cadherin. Overexpression of a mutant form of beta-catenin, Y654F, prevents the NMDAR-dependent regulation of N-cadherin internalization, resulting in stabilization of surface N-cadherin molecules. Furthermore, the stabilization of surface N-cadherin blocks NMDAR-dependent synaptic plasticity. These results indicate that NMDAR activity regulates N-cadherin endocytosis, providing a mechanistic link between structural plasticity and persistent changes in synaptic efficacy.

Thinking outside the pyramidal cell: unexplored contributions of interneurons and neuropeptide Y to estrogen-induced synapse formation in the hippocampus

Since the first finding that 17beta-estradiol (E) can regulate CA1 pyramidal cell synapse formation, subsequent studies have explored many potential E-dependent mechanisms occurring within CA1 pyramidal cells. Fewer studies have focused on E-dependent processes outside of the pyramidal cell that may influence events activity of the pyramidal cells. This review considers hippocampal interneurons, which can potently regulate the excitability of simultaneously firing pyramidal cells. In particular, we discuss neuropeptide Y (NPY) expression by these interneurons because our published findings show that NPY expression is increased by E in a subset of interneurons which coincidentally exhibit E-regulated increase in GABA synthesis and are uniquely situated anatomically such that they may regulate synaptic activity. Here we review the role of different phenotypes of CA1 interneurons, and we propose a model in which E-stimulated NPY gene expression and the release of NPY by interneurons inhibits glutamate release presynaptically and alters glutamate-dependent synaptic events in the rat hippocampus during adulthood.

Role of actin cytoskeleton in dendritic spine morphogenesis

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

Brain-derived neurotrophic factor restores synaptic plasticity in a knock-in mouse model of Huntington's disease

Asymptomatic Huntington's disease (HD) patients exhibit memory and cognition deficits that generally worsen with age. Similarly, long-term potentiation (LTP), a form of synaptic plasticity involved in memory encoding, is impaired in HD mouse models well before motor disturbances occur. The reasons why LTP deteriorates are unknown. Here we show that LTP is impaired in hippocampal slices from presymptomatic Hdh(Q92) and Hdh(Q111) knock-in mice, describe two factors contributing to this deficit, and establish that potentiation can be rescued with brain-derived neurotrophic factor (BDNF). Baseline physiological measures were unaffected by the HD mutation, but LTP induction and, to a greater degree, consolidation were both defective. The facilitation of burst responses that normally occurs during a theta stimulation train was reduced in HD knock-in mice, as was theta-induced actin polymerization in dendritic spines. The decrease in actin polymerization and deficits in LTP stabilization were reversed by BDNF, concentrations of which were substantially reduced in hippocampus of both Hdh(Q92) and Hdh(Q111) mice. These results suggest that the HD mutation discretely disrupts processes needed to both induce and stabilize LTP, with the latter effect likely arising from reduced BDNF expression. That BDNF rescues LTP in HD knock-in mice suggests the possibility of treating cognitive deficits in asymptomatic HD gene carriers by upregulating production of the neurotrophin.

NMDA and AMPA receptors: old channels, new tricks

Learning and memory depend on persistent changes in synaptic strength that require neuronal gene expression. An unresolved question concerns the mechanisms by which activity at synapses is transduced into a nuclear transcriptional response. In the prevailing view, N-methyl-D-aspartate (NMDA)- and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors have distinct roles in controlling synaptic strength: AMPA receptors effect short-term changes in synaptic strength, whereas NMDA receptors regulate genes that are required for the long-term maintenance of these changes. Here, we review recent data on the roles of these two types of receptor in activity-dependent gene expression. We discuss evidence that signals from NMDA receptors and AMPA receptors are integrated to specify transcriptional responses for particular plasticity related genes.

A role for synaptopodin and the spine apparatus in hippocampal synaptic plasticity

Spines are considered sites of synaptic plasticity in the brain and are capable of remodeling their shape and size. A molecule thathas been implicated in spine plasticity is the actin-associated protein synaptopodin. This article will review a series of studies aimed at elucidating the role of synaptopodin in the rodent brain. First, the developmental expression of synaptopodin mRNA and protein were studied; secondly, the subcellular localization of synaptopodin in hippocampal principal neurons was analyzed using confocal microscopy as well as electron microscopy and immunogold labelling; and, finally, the functional role of synaptopodin was investigated using a synaptopodin-deficient mouse. The results of these studies are: (1) synaptopodin expression byhippocampal principal neurons develops during the first postnatal weeks and increases in parallel with the maturation of spines in the hippocampus. (2) Synaptopodin is sorted to the spine compartment, where it is tightly associated with the spine apparatus, an enigmatic organelle believed to be involved in calcium storage or local protein synthesis. (3) Synaptopodin-deficient mice generated by gene targeting are viable but lack the spine apparatus organelle. These mice show deficitsin synaptic plasticity as well as impaired learning and memory. Taken together, these data implicate synaptopodin and the spine apparatus in the regulation of synaptic plasticity in the hippocampus. Future studies will be aimed at finding the molecular link between synaptopodin, the spine apparatus organelle, and synaptic plasticity.

Chronic membrane depolarization-induced morphological alteration of developing neurons

During development of CNS, young neurons experience various stimuli, and thereafter differentiate to mature neurons in an activity-dependent manner. Membrane depolarization acts as an inducer of excitability and various signals in the neurons, which can be used as a model of neuronal activity. However, the mechanisms of the influence of membrane depolarization on neuronal differentiation have not been fully understood. Therefore, we investigated the effect of membrane depolarization on morphology of spines and generation of valid electrical activity. Using rat hippocampal cultures treated from the plating day with or without high KCl (35 mM, termed HK), we directly observed living neurons transfected with green fluorescence protein-expressing plasmid through a two-photon laser scanning confocal microscope and electrophysiological recording using a patch-clamp technique. Compared with controls, the neurons cultured with HK for 3 days in vitro (DIV) showed marked filopodia-like protrusions as well as an increase in the number of spines, but those cultured with HK for 6 DIV profoundly lost these spines, resulting in a small number of fine filopodia-like protrusions proximally and on the cell body, and a smooth surface of distal dendrites. Electrophysiological recordings showed no spontaneous responses in 6 DIV HK-treated neurons. Moreover, addition of an N-methyl-D-aspartate receptor (NMDAR) antagonist to HK-treated neurons blocked the shrinkage and decrease in the number of filopodia-like protrusions significantly. These findings suggest that membrane depolarization of developing neurons induces synaptogenesis in the early stages of development but chronic treatment with HK causes pathological changes through NMDAR, and that there may be alternative mechanisms for the physiological differentiation of neurons in later developmental stages.

Adaptive plasticity of NMDA receptors and dendritic spines: implications for enhanced vulnerability of the adolescent brain to alcohol addiction

It is now known that brain development continues into adolescence and early adulthood and is highly influenced by experience-dependent adaptive plasticity during this time. Behaviorally, this period is also characterized by increased novelty seeking and risk-taking. This heightened plasticity appears to be important in shaping behaviors and cognitive processes that contribute to proper development of an adult phenotype. However, increasing evidence has linked these same experience-dependent learning mechanisms with processes that underlie drug addiction. As such, the adolescent brain appears to be particularly susceptible to experience-dependent learning processes associated with consumption of alcohol and addictive drugs. At the level of the synapse, homeostatic changes during ethanol consumption are invoked to counter the destabilizing effects of ethanol on neural networks. This homeostatic response may be especially pronounced in the adolescent and young adult brain due to its heightened capacity to undergo experience-dependent changes, and appears to involve increased synaptic targeting of NMDA receptors. Interestingly, recent work from our lab also indicates that the enhanced synaptic localization of NMDA receptors promotes increases in the size of dendritic spines. This increase may represent a structural-based mechanism that supports the formation and stabilization of maladapted synaptic connections that, in a sense, "fix" the addictive behavior in the adolescent and young adult brain.

Calcium-triggered exit of F-actin and IP(3) 3-kinase A from dendritic spines is rapid and reversible

The structure of the actin cytoskeleton in dendritic spines is thought to underlie some forms of synaptic plasticity. We have used fixed and live-cell imaging in rat primary hippocampal cultures to characterize the synaptic dynamics of the F-actin binding protein inositol trisphosphate 3-kinase A (IP3K), which is localized in the spines of pyramidal neurons derived from the CA1 region. IP3K was intensely concentrated as puncta in spine heads when Ca(2+) influx was low, but rapidly and reversibly redistributed to a striated morphology in the main dendrite when Ca(2+) influx was high. Glutamate stimulated the exit of IP3K from spines within 10 s, and re-entry following blockage of Ca(2+) influx commenced within a minute; IP3K appeared to remain associated with F-actin throughout this process. Ca(2+)-triggered F-actin relocalization occurred in about 90% of the cells expressing IP3K endogenously, and was modulated by the synaptic activity of the cultures, suggesting that it is a physiological process. F-actin relocalization was blocked by cytochalasins, jasplakinolide and by the over-expression of actin fused to green fluorescent protein. We also used deconvolution microscopy to visualize the relationship between F-actin and endoplasmic reticulum inside dendritic spines, revealing a delicate microorganization of IP3K near the Ca(2+) stores. We conclude that Ca(2+) influx into the spines of CA1 pyramidal neurons triggers the rapid and reversible retraction of F-actin from the dendritic spine head. This process contributes to changes in spine F-actin shape and content during synaptic activity, and might also regulate spine IP3 signals.

Transmitting on actin: synaptic control of dendritic architecture

Excitatory synaptic transmission in the central nervous system mainly takes place at dendritic spines, highly motile protrusions on the dendritic surface. Depending on the stimuli received, dendritic spines undergo rapid actin-based changes in their morphology. This plasticity appears to involve signaling through numerous proteins that control the organization of the actin cytoskeleton (actin regulators). At least in part, recruitment and activation of these depends on neurotransmitter receptors at the post-synapse, which directly link neurotransmission to changes in dendritic spine architecture. However, other, non-neurotransmitter-receptors present at dendritic spines also participate. It is likely that several receptor types can control the activity of a single actin-regulatory pathway and it is the complex integration of numerous signals that determines the overall architecture of a dendritic spine.