Rapid actin-based plasticity in dendritic spines

Excitatory amino acid involvement in dendritic spine formation, maintenance and remodelling

In the central nervous system, most excitatory synapses occur on dendritic spines, which are small protrusions from the dendritic tree. In the mature cortex and hippocampus, dendritic spines are heterogeneous in shape. It has been shown that the shapes of the spine can affect synapse stability and synaptic function. Dendritic spines are highly motile structures that can undergo actin-dependent shape changes, which occur over a time scale ranging from seconds to tens of minutes or even days. The formation, remodelling and elimination of excitatory synapses on dendritic spines represent ways of refining the microcircuitry in the brain. Here I review the current knowledge on the effects of modulation of AMPA and NMDA ionotropic glutamate receptors on dendritic spine formation, motility and remodelling.

Investigating sub-spine actin dynamics in rat hippocampal neurons with super-resolution optical imaging

Morphological changes in dendritic spines represent an important mechanism for synaptic plasticity which is postulated to underlie the vital cognitive phenomena of learning and memory. These morphological changes are driven by the dynamic actin cytoskeleton that is present in dendritic spines. The study of actin dynamics in these spines traditionally has been hindered by the small size of the spine. In this study, we utilize a photo-activation localization microscopy (PALM)–based single-molecule tracking technique to analyze F-actin movements with ∼30-nm resolution in cultured hippocampal neurons. We were able to observe the kinematic (physical motion of actin filaments, i.e., retrograde flow) and kinetic (F-actin turn-over) dynamics of F-actin at the single-filament level in dendritic spines. We found that F-actin in dendritic spines exhibits highly heterogeneous kinematic dynamics at the individual filament level, with simultaneous actin flows in both retrograde and anterograde directions. At the ensemble level, movements of filaments integrate into a net retrograde flow of ∼138 nm/min. These results suggest a weakly polarized F-actin network that consists of mostly short filaments in dendritic spines.

Interfering with the actin network and its effect on long-term potentiation and synaptic tagging in hippocampal CA1 neurons in slices in vitro

Long-term potentiation (LTP) is a cellular correlate for memory formation, which requires the dynamic changes of the actin cytoskeleton. As shown by others, the polymerization of the actin network is important for early stages of LTP. Here, we investigated the role of actin dynamics in synaptic tagging and particularly in the induction of protein synthesis-dependent late-LTP in the CA1 region in hippocampal slices in vitro. We found that the inhibition of actin polymerization affects protein synthesis-independent early-LTP, prevents late-LTP, and interferes with synaptic tagging in apical dendrites of hippocampal CA1. The transformation of early-LTP into late-LTP was blocked by the application of the structurally different actin polymerization inhibitors latrunculin A or cytochalasin D. We suggest that the actin network is required for early "housekeeping" processes to induce and maintain early-LTP. Furthermore, inhibition of actin dynamics negatively interacts with the setting of the synaptic tagging complex. We propose actin as a further tag-specific molecule in apical CA1 dendrites where it is directly involved in the tagging/capturing machinery. Consequently, inhibition of the actin network prevents the interaction of tagging complexes with plasticity-related proteins. This results in the prevention of late-LTP by inhibition of the actin network during LTP induction.

Dendritic spine dynamics

Dendritic spines are the postsynaptic components of most excitatory synapses in the mammalian brain. Spines accumulate rapidly during early postnatal development and undergo a substantial loss as animals mature into adulthood. In past decades, studies have revealed that the number and size of dendritic spines are regulated by a variety of gene products and environmental factors, underscoring the dynamic nature of spines and their importance to brain plasticity. Recently, in vivo time-lapse imaging of dendritic spines in the cerebral cortex suggests that, although spines are highly plastic during development, they are remarkably stable in adulthood, and most of them last throughout life. Therefore, dendritic spines may provide a structural basis for lifelong information storage, in addition to their well-established role in brain plasticity. Because dendritic spines are the key elements for information acquisition and retention, understanding how spines are formed and maintained, particularly in the intact brain, will likely provide fundamental insights into how the brain possesses the extraordinary capacity to learn and to remember.

Experience-dependent structural synaptic plasticity in the mammalian brain

Synaptic plasticity in adult neural circuits may involve the strengthening or weakening of existing synapses as well as structural plasticity, including synapse formation and elimination. Indeed, long-term in vivo imaging studies are beginning to reveal the structural dynamics of neocortical neurons in the normal and injured adult brain. Although the overall cell-specific morphology of axons and dendrites, as well as of a subpopulation of small synaptic structures, are remarkably stable, there is increasing evidence that experience-dependent plasticity of specific circuits in the somatosensory and visual cortex involves cell type-specific structural plasticity: some boutons and dendritic spines appear and disappear, accompanied by synapse formation and elimination, respectively. This Review focuses on recent evidence for such structural forms of synaptic plasticity in the mammalian cortex and outlines open questions.

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.

Focal adhesion kinase acts downstream of EphB receptors to maintain mature dendritic spines by regulating cofilin activity

Dendritic spines are the postsynaptic sites of most excitatory synapses in the brain and are highly enriched in polymerized F-actin, which drives the formation and maintenance of mature dendritic spines and synapses. We propose that suppressing the activity of the actin-severing protein cofilin plays an important role in the stabilization of mature dendritic spines, and is accomplished through an EphB receptor-focal adhesion kinase (FAK) pathway. Our studies revealed that Cre-mediated knock-out of loxP-flanked fak prompted the reversion of mature dendritic spines to an immature filopodial-like phenotype in primary hippocampal cultures. The effects of FAK depletion on dendritic spine number, length, and morphology were rescued by the overexpression of the constitutively active FAK(Y397E), but not FAK(Y397F), indicating the significance of FAK activation by phosphorylation on tyrosine 397. Our studies demonstrate that FAK acts downstream of EphB receptors in hippocampal neurons and EphB2-FAK signaling controls the stability of mature dendritic spines by promoting cofilin phosphorylation, thereby inhibiting cofilin activity. While constitutively active nonphosphorylatable cofilin(S3A) induced an immature spine profile, phosphomimetic cofilin(S3D) restored mature spine morphology in neurons with disrupted EphB activity or lacking FAK. Further, we found that EphB-mediated regulation of cofilin activity at least partially depends on the activation of Rho-associated kinase (ROCK) and LIMK-1. These findings indicate that EphB2-mediated dendritic spine stabilization relies, in part, on the ability of FAK to activate the RhoA-ROCK-LIMK-1 pathway, which functions to suppress cofilin activity and inhibit cofilin-mediated dendritic spine remodeling.

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.

Characterization of dendritic spines in the Drosophila central nervous system

Dendritic spines are a characteristic feature of a number of neurons in the vertebrate nervous system and have been implicated in processes that include learning and memory. In spite of this, there has been no comprehensive analysis of the presence of spines in a classical genetic system, such as Drosophila, so far. Here, we demonstrate that a subset of processes along the dendrites of visual system interneurons in the adult fly central nervous system, called LPTCs, closely resemble vertebrate spines, based on a number of criteria. First, the morphology, size, and density of these processes are very similar to those of vertebrate spines. Second, they are enriched in actin and devoid of tubulin. Third, they are sites of synaptic connections based on confocal and electron microscopy. Importantly, they represent a preferential site of localization of an acetylcholine receptor subunit, suggesting that they are sites of excitatory synaptic input. Finally, their number is modulated by the level of the small GTPase dRac1. Our results provide a basis to dissect the genetics of dendritic spine formation and maintenance and the functional role of spines.

Molecular mechanisms of psychostimulant-induced structural plasticity

Drug addiction is characterized by persistent behavioral and cellular plasticity throughout the brain's reward regions. Among the many neuroadaptations that occur following repeated drug administration are alterations in cell morphology including changes in dendritic spines. While this phenomenon has been well documented, the underlying molecular mechanisms are poorly understood. Here, within the context of drug abuse, we review and integrate several of the established pathways known to regulate synaptic remodeling, and discuss the contributions of neurotrophic and dopamine signaling in mediating this structural plasticity. Finally, we discuss how such upstream mechanisms could regulate actin dynamics, the common endpoint involved in structural remodeling in neurons.

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.

Regulation of dendritic spine morphology by SPIN90, a novel Shank binding partner

Dendritic spines are highly specialized actin-rich structures on which the majority of excitatory synapses are formed in the mammalian CNS. SPIN90 is an actin-binding protein known to be highly enriched in postsynaptic densities (PSDs), though little is known about its function there. Here, we show that SPIN90 is a novel binding partner for Shank proteins in the PSD. SPIN90 and Shank co-immunoprecipitate from brain lysates and co-localize in postsynaptic dendrites and act synergistically to mediate spine maturation and spine head enlargement. At the same time, SPIN90 causes accumulation of Shank and PSD-95 within dendritic spines. In addition, we found that the protein composition of PSDs in SPIN90 knockout mice is altered as is the actin cytoskeleton of cultured hippocampal SPIN90 knockout neurons. Taken together, these findings demonstrate that SPIN90 is a Shank1b binding partner and a key contributor to the regulation of dendritic spine morphogenesis and brain function.

Dendritic spine morphology determines membrane-associated protein exchange between dendritic shafts and spine heads

The purpose of this study was to examine whether variability in the shape of dendritic spines affects protein movement within the plasma membrane. Using a combination of confocal microscopy and the fluorescence loss in photobleaching technique in living hippocampal CA1 pyramidal neurons expressing membrane-linked GFP, we observed a clear correlation between spine shape parameters and the diffusion and compartmentalization of membrane-associated proteins. The kinetics of membrane-linked GFP exchange between the dendritic shaft and the spine head compartment were slower in dendritic spines with long necks and/or large heads than in those with short necks and/or small heads. Furthermore, when the spine area was reduced by eliciting epileptiform activity, the kinetics of protein exchange between the spine compartments exhibited a concomitant decrease. As synaptic plasticity is considered to involve the dynamic flux by lateral diffusion of membrane-bound proteins into and out of the synapse, our data suggest that spine shape represents an important parameter in the susceptibility of synapses to undergo plastic change.

Spine neck geometry determines spino-dendritic cross-talk in the presence of mobile endogenous calcium binding proteins

Dendritic spines are thought to compartmentalize second messengers like Ca2+. The notion of isolated spine signaling, however, was challenged by the recent finding that under certain conditions mobile endogenous Ca(2+)-binding proteins may break the spine limit and lead to activation of Ca(2+)-dependent dendritic signaling cascades. Since the size of spines is variable, the spine neck may be an important regulator of this spino-dendritic crosstalk. We tested this hypothesis by using an experimentally defined, kinetic computer model in which spines of Purkinje neurons were coupled to their parent dendrite by necks of variable geometry. We show that Ca2+ signaling and calmodulin activation in spines with long necks is essentially isolated from the dendrite, while stubby spines show a strong coupling with their dendrite, mediated particularly by calbindin D28k. We conclude that the spine neck geometry, in close interplay with mobile Ca(2+)-binding proteins, regulates the spino-dendritic crosstalk.

Dendrites of mammalian neurons contain specialized P-body-like structures that respond to neuronal activation

Intracellular mRNA transport and local translation play a key role in neuronal physiology. Translationally repressed mRNAs are transported as a part of ribonucleoprotein (RNP) particles to distant dendritic sites, but the properties of different RNP particles and mechanisms of their repression and transport remain largely unknown. Here, we describe a new class of RNP-particles, the dendritic P-body-like structures (dlPbodies), which are present in the soma and dendrites of mammalian neurons and have both similarities and differences to P-bodies of non-neuronal cells. These structures stain positively for a number of P-body and microRNP components, a microRNA-repressed mRNA and some translational repressors. They appear more heterogeneous than P-bodies of HeLa cells, and they rarely contain the exonuclease Xrn1 but are positive for rRNA. These particles show motorized movements along dendrites and relocalize to distant sites in response to synaptic activation. Furthermore, Dcp1a is stably associated with dlP-bodies in unstimulated cells, but exchanges rapidly on neuronal activation, concomitantly with the loss of Ago2 from dlP-bodies. Thus, dlP-bodies may regulate local translation by storing repressed mRNPs in unstimulated cells, and releasing them on synaptic activation.

Dendritic Spines: Similarities with Protrusions and Adhesions in Migrating Cells

Dendritic spines are specialized, micron-sized post-synaptic compartments that support synaptic function. These actin-based protrusions push the post-synaptic membrane, establish contact with the presynaptic membrane and undergo dynamic changes in morphology during development, as well as in response to synaptic neurotransmission. These processes are propelled by active remodeling of the actin cytoskeleton, which includes polymerization, filament disassembly, and organization of the actin in supramolecular arrays, such as branched networks or bundles. Dendritic spines contain a plethora of adhesion and synaptic receptors, signaling, and cytoskeletal proteins that regulate their formation, maturation and removal. Whereas many of the molecules involved in dendritic spine formation have been identified, their actual roles in spine formation, removal and maturation are not well understood. Using parallels between migrating fibroblasts and dendritic spines, we point to potential mechanisms and approaches for understanding spine development and dynamics.

Actin and Actin-Binding Proteins: Masters of Dendritic Spine Formation, Morphology, and Function

Dendritic spines are actin-rich protrusions that comprise the postsynaptic sites of synapses and receive the majority of excitatory synaptic inputs in the central nervous system. These structures are central to cognitive processes, and alterations in their number, size, and morphology are associated with many neurological disorders. Although the actin cytoskeleton is thought to govern spine formation, morphology, and synaptic functions, we are only beginning to understand how modulation of actin reorganization by actin-binding proteins (ABPs) contributes to the function of dendritic spines and synapses. In this review, we discuss what is currently known about the role of ABPs in regulating the formation, morphology, motility, and plasticity of dendritic spines and synapses.

Spindle positioning in mouse oocytes relies on a dynamic meshwork of actin filaments

Female meiosis in higher organisms consists of highly asymmetric divisions, which retain most maternal stores in the oocyte for embryo development. Asymmetric partitioning of the cytoplasm results from the spindle's "off-center" positioning, which, in mouse oocytes, depends mainly on actin filaments [1, 2]. This is a unique situation compared to most systems, in which spindle positioning requires interactions between astral microtubules and cortical actin filaments [3]. Formin 2, a straight-actin-filament nucleator, is required for the first meiotic spindle migration to the cortex and cytokinesis in mouse oocytes [4, 5]. Although the requirement for actin filaments in the control of spindle positioning is well established in this model, no one has been able to detect them in the cytoplasm [6]. Through the expression of an F-actin-specific probe and live confocal microscopy, we show the presence of a cytoplasmic actin meshwork, organized by Formin 2, that controls spindle migration. In late meiosis I, these filaments organize into a spindle-like F-actin structure, which is connected to the cortex. At anaphase, global reorganization of this meshwork allows polar-body extrusion. In addition, using actin-YFP, our FRAP analysis confirms the presence of a highly dynamic cytoplasmic actin meshwork that is tightly regulated in time and space.

Neural palmitoyl-proteomics reveals dynamic synaptic palmitoylation

Palmitoylation regulates diverse aspects of neuronal protein trafficking and function. Here a global characterization of rat neural palmitoyl-proteomes identifies most of the known neural palmitoyl proteins-68 in total, plus more than 200 new palmitoyl-protein candidates, with further testing confirming palmitoylation for 21 of these candidates. The new palmitoyl proteins include neurotransmitter receptors, transporters, adhesion molecules, scaffolding proteins, as well as SNAREs and other vesicular trafficking proteins. Of particular interest is the finding of palmitoylation for a brain-specific Cdc42 splice variant. The palmitoylated Cdc42 isoform (Cdc42-palm) differs from the canonical, prenylated form (Cdc42-prenyl), both with regard to localization and function: Cdc42-palm concentrates in dendritic spines and has a special role in inducing these post-synaptic structures. Furthermore, assessing palmitoylation dynamics in drug-induced activity models identifies rapidly induced changes for Cdc42 as well as for other synaptic palmitoyl proteins, suggesting that palmitoylation may participate broadly in the activity-driven changes that shape synapse morphology and function.

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.

SynGAP regulates steady-state and activity-dependent phosphorylation of cofilin

SynGAP, a prominent Ras/Rap GTPase-activating protein in the postsynaptic density, regulates the timing of spine formation and trafficking of glutamate receptors in cultured neurons. However, the molecular mechanisms by which it does this are unknown. Here, we show that synGAP is a key regulator of spine morphology in adult mice. Heterozygous deletion of synGAP was sufficient to cause an excess of mushroom spines in adult brains, indicating that synGAP is involved in steady-state regulation of actin in mature spines. Both Ras- and Rac-GTP levels were elevated in forebrains from adult synGAP(+/-) mice. Rac is a well known regulator of actin polymerization and spine morphology. The steady-state level of phosphorylation of cofilin was also elevated in synGAP(+/-) mice. Cofilin, an F-actin severing protein that is inactivated by phosphorylation, is a downstream target of a pathway regulated by Rac. We show that transient regulation of cofilin by treatment with NMDA is also disrupted in synGAP mutant neurons. Treatment of wild-type neurons with 25 mum NMDA triggered transient dephosphorylation and activation of cofilin within 15 s. In contrast, neurons cultured from mice with a homozygous or heterozygous deletion of synGAP lacked the transient regulation by the NMDA receptor. Depression of EPSPs induced by a similar treatment of hippocampal slices with NMDA was disrupted in slices from synGAP(+/-) mice. Our data show that synGAP mediates a rate-limiting step in steady-state regulation of spine morphology and in transient NMDA-receptor-dependent regulation of the spine cytoskeleton.

Principles of long-term dynamics of dendritic spines

Long-term potentiation of synapse strength requires enlargement of dendritic spines on cerebral pyramidal neurons. Long-term depression is linked to spine shrinkage. Indeed, spines are dynamic structures: they form, change their shapes and volumes, or can disappear in the space of hours. Do all such changes result from synaptic activity, or do some changes result from intrinsic processes? How do enlargement and shrinkage of spines relate to elimination and generation of spines, and how do these processes contribute to the stationary distribution of spine volumes? To answer these questions, we recorded the volumes of many individual spines daily for several days using two-photon imaging of CA1 pyramidal neurons in cultured slices of rat hippocampus between postnatal days 17 and 23. With normal synaptic transmission, spines often changed volume or were created or eliminated, thereby showing activity-dependent plasticity. However, we found that spines changed volume even after we blocked synaptic activity, reflecting a native instability of these small structures over the long term. Such "intrinsic fluctuations" showed unique dependence on spine volume. A mathematical model constructed from these data and the theory of random fluctuations explains population behaviors of spines, such as rates of elimination and generation, stationary distribution of volumes, and the long-term persistence of large spines. Our study finds that generation and elimination of spines are more prevalent than previously believed, and spine volume shows significant correlation with its age and life expectancy. The population dynamics of spines also predict key psychological features of memory.

Quantifying neuronal size: summing up trees and splitting the branch difference

Neurons vary greatly in size, shape, and complexity depending on their underlying function. Overall size of neuronal trees affects connectivity, area of influence, and other biophysical properties. Relative distributions of neuronal extent, such as the difference between subtrees at branch points, are also critically related to function and activity. This review covers neuromorphological research that analyzes shape and size to elucidate their functional role for different neuron types. We also introduce a novel morphometric, "caulescence", capturing the extent to which trees exhibit a main path. Neuronal tree types differ vastly in caulescence, suggesting potential neurocomputational correlates of this property.

Actin depolymerization contributes to ethanol inhibition of NMDA receptors in primary cultured cerebellar granule cells

We have previously reported that a 30s ethanol (10 and 100mM) pre-exposure significantly enhanced EtOH inhibition of N-methyl-d-aspartate (NMDA-induced currents)-induced peak currents in primary cultured cerebellar granule cells (CGCs). The purpose of this study was to determine if intracellular factors play a role in ethanol pre-exposure-enhanced inhibition of NMDA-induced currents and if so, to identify the intracellular target(s) mediating this effect. Ethanol pre-exposure-enhanced inhibition was reduced when ethanol was present intracellularly prior to the initiation of the pretreatment protocol. Similar to results acquired with the whole-cell configuration, ethanol pre-exposure-enhanced inhibition of NMDA-induced currents was also observed in the perforated patch-clamp mode. Collectively, these results suggest an intracellular target not easily dialyzed from the cell. Perturbation of the actin cytoskeleton was responsible for the ethanol pre-exposure-enhanced inhibition of NMDA-induced currents was supported by the observation that the intracellular presence of the actin stabilizer phalloidin prevented ethanol pre-exposure-enhanced inhibition. Similar to the effects of ethanol, the depolymerizing agent latrunculin A inhibited NMDA-induced currents after a 30s pretreatment exposure with full recovery of receptor function after washout of the drug. Furthermore, latrunculin A occluded the enhanced inhibition of NMDA-induced currents by ethanol pre-exposure for both 10 and 100mM ethanol. The microtubule depolymerizing agent taxol had no affect on ethanol pretreatment-enhanced inhibition of NMDA-induced currents. Confocal microscopy with phalloidin-FITC indicated that F-actin filaments in neurites were depolymerized after a 30s treatment of either latrunculin A or 100mM ethanol. Our observations indicate that ethanol inhibition of NMDAR function may involve perturbation of the actin cytoskeleton.

Impaired dendritic development and synaptic formation of postnatal-born dentate gyrus granular neurons in the absence of brain-derived neurotrophic factor signaling

Neurons are continuously added to the hippocampal dentate gyrus throughout life. These neurons must develop dendritic arbors and spines by which they form synapses for making functional connections with existing neurons. The molecular mechanisms that regulate dendritic development and synaptic formation of postnatal-born granular neurons in the dentate gyrus are largely unknown. Hippocampal dentate gyrus (HDG) has been shown to express high level of brain-derived neurotrophic factor (BDNF). Here we reported that when BDNF is conditionally knockout in the postnatal-born granular neurons of the HDG, the mutant neurons exhibit aberrant morphological development with less dendritic branches, shorter dendritic length, and lower density of dendritic spines, while their primary dendrites are not obviously affected. Even though, these BDNF-deficient granular neurons develop immature dendritic spines to initiate synaptic contacts with afferent axons, they fail to develop or maintain mature spine structures. Thus, these postnatal-born neurons have fewer numbers of synapses, particularly mature synaptic spines. These results suggest that BDNF plays an important role during dendritic development, synaptic formation and synaptic maturation in postnatal-born granular neurons of the HDG in vivo.

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.

Morphogenesis and regulation of Bergmann glial processes during Purkinje cell dendritic spine ensheathment and synaptogenesis

Astrocytes have an important role in synaptic formation and function but how astrocytic processes become associated with synaptic structures during development is not well understood. Here we analyzed the pattern of growth of the processes extending off the main Bergmann glial (BG) shafts during synaptogenesis in the cerebellum. We found that during this period, BG process outgrowth was correlated with increased ensheathment of dendritic spines. In addition, two-photon time-lapse imaging revealed that BG processes were highly dynamic, and processes became more stable as the period of spine ensheathment progressed. While process motility was dependent on actin polymerization, activity of cytoskeletal regulators Rac1 and RhoG did not play a role in glial process dynamics or density, but was critical for maintaining process length. We extended this finding to probe the relationship between process morphology and ensheathment, finding that shortened processes result in decreased coverage of the spine. Furthermore, we found that areas in which BG expressed dn-Rac1, and therefore had a lower level of synaptic ensheathment, showed an overall increase in synapse number. These analyses reveal how BG processes grow to surround synaptic structures, elucidate the importance of BG process structure for proper development of synaptic ensheathment, and reveal a role for ensheathment in synapse formation.

Imaging of experience-dependent structural plasticity in the mouse neocortex in vivo

The functionality of adult neocortical circuits can be altered by novel experiences or learning. This functional plasticity appears to rely on changes in the strength of neuronal connections that were established during development. Here we will describe some of our studies in which we have addressed whether structural changes, including the remodeling of axons and dendrites with synapse formation and elimination, could underlie experience-dependent plasticity in the adult neocortex. Using 2-photon laser-scanning microscopes and transgenic mice expressing GFP in a subset of pyramidal cells, we have observed that a small subset of dendritic spines continuously appear and disappear on a daily basis, whereas the majority of spines persists for months. Axonal boutons from different neuronal classes displayed similar behavior, although the extent of remodeling varied. Under baseline conditions, new spines in the barrel cortex were mostly transient and rarely survived for more than a week. However, when every other whisker was trimmed, the generation and loss of persistent spines was enhanced. Ultrastructural reconstruction of previously imaged spines and boutons showed that new spines slowly form synapses. New spines persisting for a few days always had synapses, whereas very young spines often lacked synapses. New synapses were predominantly found on large, multi-synapse boutons, suggesting that spine growth is followed by synapse formation, preferentially on existing boutons. Altogether our data indicate that novel sensory experience drives the stabilization of new spines on subclasses of cortical neurons and promotes the formation of new synapses. These synaptic changes likely underlie experience-dependent functional remodeling of specific neocortical circuits.

Cre-dependent expression of multiple transgenes in isolated neurons of the adult forebrain

Background

Transgenic mice with mosaic, Golgi-staining-like expression of enhanced green fluorescent protein (EGFP) have been very useful in studying the dynamics of neuronal structure and function. In order to further investigate the molecular events regulating structural plasticity, it would be useful to express multiple proteins in the same sparse neurons, allowing co-expression of functional proteins or co-labeling of subcellular compartments with other fluorescent proteins. However, it has been difficult to obtain reproducible expression in the same subset of neurons for direct comparison of neurons expressing different functional proteins.

Principal Findings

Here we describe a Cre-transgenic line that allows reproducible expression of transgenic proteins of choice in a small number of neurons of the adult cortex, hippocampus, striatum, olfactory bulb, subiculum, hypothalamus, superior colliculus and amygdala. We show that using these Cre-transgenic mice, multiple Cre-dependent transgenes can be expressed together in the same isolated neurons. We also describe a Cre-dependent transgenic line expressing a membrane associated EGFP (EGFP-F). Crossed with the Cre-transgenic line, EGFP-F expression starts in the adolescent forebrain, is present in dendrites, dendritic protrusions, axons and boutons and is strong enough for acute or chronic in vivo imaging.

Significance

This triple transgenic approach will aid the morphological and functional characterization of neurons in various Cre-dependent transgenic mice.

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.

Histone deacetylase inhibition and blockade of the glycolytic pathway synergistically induce glioblastoma cell death

PURPOSE

High-grade gliomas are difficult to treat due to their location behind the blood-brain barrier and to inherent radioresistance and chemoresistance.

EXPERIMENTAL DESIGN

Because tumorigenesis is considered a multistep process of accumulating mutations affecting distinct signaling pathways, combinations of compounds, which inhibit nonoverlapping pathways, are being explored to improve treatment of gliomas. Histone deacetylase inhibitors (HDI) have proven antitumor activity by blocking cell proliferation, promoting differentiation, and inducing tumor cell apoptosis.

RESULTS

In this report, we show that the HDIs trichostatin A, sodium butyrate, and low nanomolar doses of LAQ824 combined with the glycolysis inhibitor 2-deoxy-d-glucose induce strong apoptosis in cancer cell lines of brain, breast, and cervix in a p53-independent manner. HDIs up-regulate p21, which is blocked by concomitant administration of 2-deoxy-d-glucose.

CONCLUSIONS

We propose simultaneous blockade of histone deacetylation and glycolysis as a novel therapeutic strategy for several major cancers.

Electron microscopic 3D-reconstruction of dendritic spines in cultured hippocampal neurons undergoing synaptic plasticity

Dendritic spines are assumed to constitute the locus of neuronal plasticity, and considerable effort has been focused on attempts to demonstrate that new memories are associated with the formation of new spines. However, few studies that have documented the appearance of spines after exposure to plasticity-producing paradigms could demonstrate that a new spine is touched by a bona fida presynaptic terminal. Thus, the functional significance of plastic dendritic spine changes is not clearly understood. We have used quantitative time lapse confocal imaging of cultured hippocampal neurons before and after their exposure to a conditioning medium which activates synaptic NMDA receptors. Following the experiment the cultures were prepared for 3D electron microscopic reconstruction of visually identified dendritic spines. We found that a majority of new, 1- to 2-h-old spines was touched by presynaptic terminals. Furthermore, when spines disappeared, the parent dendrites were sometime touched by a presynaptic bouton at the site where the previously identified spine had been located. We conclude that new spines are most likely to be functional and that pruned spines can be transformed into shaft synapses and thus maintain their functionality within the neuronal network.

Heterologous high-level E. coli expression, purification and biophysical characterization of the spine-associated RapGAP (SPAR) PDZ domain

Spine-associated RapGAP (SPAR) is a 1783 residue, multidomain scaffolding protein which is a component of the NMDA receptor/PSD-95 complex in the post-synaptic density (PSD) of dendritic spines. Using a parallel expression screening approach, we identified a strategy to solubly express the SPAR PDZ domain in Escherichia coli. We show that maltose binding protein is required for the production of solubly expressed protein. We also show that small changes in construct length (2-5 residues) result in differential susceptibilities of the expressed proteins to proteolytic digestion, required for the expression tag removal. This has allowed us to identify a large-scale E. coli expression and purification protocol that results in the production of mg quantities of the SPAR PDZ domain. This is the first time that any of the multiple SPAR functional domains have been expressed in E. coli in quantities suitable for biophysical and biochemical studies, allowing us to investigate the role of the PDZ domain in SPAR function within the PSD.

A molecular clutch between the actin flow and N-cadherin adhesions drives growth cone migration

The adhesion molecule N-cadherin plays important roles in the development of the nervous system, in particular by stimulating axon outgrowth, but the molecular mechanisms underlying this effect are mostly unknown. One possibility, the so-called "molecular clutch" model, could involve a direct mechanical linkage between N-cadherin adhesion at the membrane and intracellular actin-based motility within neuronal growth cones. Using live imaging of primary rat hippocampal neurons plated on N-cadherin-coated substrates and optical trapping of N-cadherin-coated microspheres, we demonstrate here a strong correlation between growth cone velocity and the mechanical coupling between ligand-bound N-cadherin receptors and the retrograde actin flow. This relationship holds by varying ligand density and expressing mutated N-cadherin receptors or small interfering RNAs to perturb binding to catenins. By restraining microsphere motion using optical tweezers or a microneedle, we further show slippage of cadherin-cytoskeleton bonds at low forces, and, at higher forces, local actin accumulation, which strengthens nascent N-cadherin contacts. Together, these data support a direct transmission of actin-based traction forces to N-cadherin adhesions, through catenin partners, driving growth cone advance and neurite extension.

Glutamate receptor dynamics in dendritic microdomains

Among diverse factors regulating excitatory synaptic transmission, the abundance of postsynaptic glutamate receptors figures prominently in molecular memory and learning-related synaptic plasticity. To allow for both long-term maintenance of synaptic transmission and acute changes in synaptic strength, the relative rates of glutamate receptor insertion and removal must be tightly regulated. Interactions with scaffolding proteins control the targeting and signaling properties of glutamate receptors within the postsynaptic membrane. In addition, extrasynaptic receptor populations control the equilibrium of receptor exchange at synapses and activate distinct signaling pathways involved in plasticity. Here, we review recent findings that have shaped our current understanding of receptor mobility between synaptic and extrasynaptic compartments at glutamatergic synapses, focusing on AMPA and NMDA receptors. We also examine the cooperative relationship between intracellular trafficking and surface diffusion of glutamate receptors that underlies the expression of learning-related synaptic plasticity.

The substrates of memory: defects, treatments, and enhancement

Recent work has added strong support to the long-standing hypothesis that the stabilization of both long-term potentiation and memory requires rapid reorganization of the spine actin cytoskeleton. This development has led to new insights into the origins of cognitive disorders, and raised the possibility that a diverse array of memory problems, including those associated with diabetes, reflect disturbances to various components of the same mechanism. In accord with this argument, impairments to long-term potentiation in mouse models of Huntington's disease and in middle-aged rats have both been linked to problems with modulatory factors that control actin polymerization in spine heads. Complementary to the common mechanism hypothesis is the idea of a single treatment for addressing seemingly unrelated memory diseases. First tests of the point were positive: Brain-Derived Neurotrophic Factor (BDNF), a potent activator of actin signaling cascades in adult spines, rescued potentiation in Huntington's disease mutant mice, middle-aged rats, and a mouse model of Fragile-X syndrome. A similar reversal of impairments to long-term potentiation was obtained in middle-aged rats by up-regulating BDNF production with brief exposures to ampakines, a class of drugs that positively modulate AMPA-type glutamate receptors. Work now in progress will test if chronic elevation of BDNF enhances memory in normal animals.

Synaptic metaplasticity through NMDA receptor lateral diffusion

Lateral diffusion of glutamate receptors was proposed as a mechanism for regulating receptor numbers at synapses and affecting synaptic functions, especially the efficiency of synaptic transmission. However, a direct link between receptor lateral diffusion and change in synaptic function has not yet been established. In the present study, we demonstrated NMDA receptor (NMDAR) lateral diffusion in CA1 neurons in hippocampal slices by detecting considerable recovery of spontaneous or evoked EPSCs from the block of (+)-MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate], an irreversible NMDAR open-channel blocker. We observed changes on both the number and the composition of synaptic NMDAR on recovery. More importantly, after the recovery, long-term potentiation (LTP)-producing protocol induced only LTD (long-term depression) instead of LTP. In contrast, a complete recovery from competitive NMDAR blocker D,L-AP-5 was observed without subsequent changes on synaptic plasticity. Our data suggest a revised model of NMDAR trafficking wherein extrasynaptic NMDARs, mostly NR1/NR2B receptors, move laterally into synaptic sites, resulting in altered rule of synaptic modification. Thus, CA1 synapses exhibit a novel form of metaplasticity in which the direction of synaptic modification can be reverted through subtype-specific lateral diffusion of NMDA receptors.

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.

Pumping up the synapse

Synchronized control of excitatory and inhibitory synapse maturation is crucial for normal brain wiring, while its dysfunction leads to neurodevelopmental disorders, including autism. A paper in this issue of Neuron identified a novel role for the KCC2 pump, also responsible for the GABAergic synapse developmental switch, in regulating spiny excitatory synapse maturation, implicating it in the coordinated maturation of inhibitory and excitatory synapses.

Release of the styryl dyes from single synaptic vesicles in hippocampal neurons

In small presynaptic boutons in brain, synaptic vesicles are thought not to merge with the plasma membrane when they release transmitter, but instead to close their fusion pores and survive intact for future use (kiss-and-run exocytosis). The strongest evidence for this idea is the slow and incomplete release of the fluorescent membrane marker, FM1-43 [N-(3-triethylammoniumpropyl)-4-(4-(dibutylamino)styryl) pyridinium dibromide], from single vesicles. We investigated the release of FM1-43 from sparse cultures of hippocampal neurons grown on coverslips with no glia. This allowed presynaptic boutons to be imaged at favorable signal-to-noise ratio. Sparingly stained boutons were imaged at high time resolution, while high-frequency electrical stimulation caused exocytosis. The release of FM1-43 was quantal and occurred in abrupt steps, each representing a single fusion event. The fluorescence of vesicle clusters traveling along axons had a distribution with the same quantal size, indicating that a vesicle releases all the dye it contains. In most fusion events, the time constant of dye release was <100 ms, and slower release was rarely observed. After exocytosis, no FM1-43 could be detected in the axon to either side of a bouton, indicating that dye was released before it could spread. Our results are consistent with synaptic vesicles fusing fully with the plasma membrane during high-frequency stimulation.

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.

Regulation of dendritic spine morphology by an NMDA receptor-associated Rho GTPase-activating protein, p250GAP

The NMDA receptor regulates spine morphological plasticity by modulating Rho GTPases. However, the molecular mechanisms for NMDA receptor-mediated regulation of Rho GTPases remain elusive. In this study, we show that p250GAP, an NMDA receptor-associated RhoGAP, regulates spine morphogenesis by modulating RhoA activity. Knock-down of p250GAP increased spine width and elevated the endogenous RhoA activity in primary hippocampal neurons. The increased spine width by p250GAP knock-down was suppressed by the expression of a dominant-negative form of RhoA. Furthermore, p250GAP is involved in NMDA receptor-mediated RhoA activation. In response to NMDA receptor activation, exogenously expressed green fluorescent protein (GFP)-tagged p250GAP was redistributed. Thus, these data suggest that p250GAP plays an important role in NMDA receptor-mediated regulation of RhoA activity leading to spine morphological plasticity.

Intracellular machinery for the transport of AMPA receptors

AMPA-type glutamate receptors are one of the most dynamic components of excitatory synapses. Their regulated addition and removal from synapses leads to long-lasting forms of synaptic plasticity, known as long-term potentiation (LTP) and long-term depression (LTD). In addition, AMPA receptors reach their synaptic targets after a complicated journey involving multiple transport steps through different membrane compartments. This review summarizes our current knowledge of the trafficking pathways of AMPARs and their relation to synaptic function and plasticity.

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.