Single synaptic events evoke NMDA receptor-mediated release of calcium from internal stores in hippocampal dendritic spines

We have used confocal microscopy to monitor synaptically evoked Ca2+ transients in the dendritic spines of hippocampal pyramidal cells. Individual spines respond to single afferent stimuli (<0.1 Hz) with Ca2+ transients or failures, reflecting the probability of transmitter release at the activated synapse. Both AMPA and NMDA glutamate receptor antagonists block the synaptically evoked Ca2+ transients; the block by AMPA antagonists is relieved by low Mg2+. The Ca2+ transients are mainly due to the release of calcium from internal stores, since they are abolished by antagonists of calcium-induced calcium release (CICR); CICR antagonists, however, do not depress spine Ca2+ transients generated by backpropagating action potentials. These results have implications for synaptic plasticity, since they show that synaptic stimulation can activate NMDA receptors, evoking substantial Ca2+ release from the internal stores in spines without inducing long-term potentiation (LTP) or depression (LTD).     

Long Lasting Protein Synthesis- and Activity-Dependent Spine Shrinkage and Elimination after Synaptic Depression

Neuronal circuits modify their response to synaptic inputs in an experience-dependent fashion. Increases in synaptic weights are accompanied by structural modifications, and activity dependent, long lasting growth of dendritic spines requires new protein synthesis. When multiple spines are potentiated within a dendritic domain, they show dynamic structural plasticity changes, indicating that spines can undergo bidirectional physical modifications. However, it is unclear whether protein synthesis dependent synaptic depression leads to long lasting structural changes. Here, we investigate the structural correlates of protein synthesis dependent long-term depression (LTD) mediated by metabotropic glutamate receptors (mGluRs) through two-photon imaging of dendritic spines on hippocampal pyramidal neurons. We find that induction of mGluR-LTD leads to robust and long lasting spine shrinkage and elimination that lasts for up to 24 hours. These effects depend on signaling through group I mGluRs, require protein synthesis, and activity. These data reveal a mechanism for long lasting remodeling of synaptic inputs, and offer potential insights into mental retardation.     

Structural Plasticity Can Produce Metaplasticity

Background

Synaptic plasticity underlies many aspect of learning memory and development. The properties of synaptic plasticity can change as a function of previous plasticity and previous activation of synapses, a phenomenon called metaplasticity. Synaptic plasticity not only changes the functional connectivity between neurons but in some cases produces a structural change in synaptic spines; a change thought to form a basis for this observed plasticity. Here we examine to what extent structural plasticity of spines can be a cause for metaplasticity. This study is motivated by the observation that structural changes in spines are likely to affect the calcium dynamics in spines. Since calcium dynamics determine the sign and magnitude of synaptic plasticity, it is likely that structural plasticity will alter the properties of synaptic plasticity.

Methodology/Principal Findings

In this study we address the question how spine geometry and alterations of N-methyl-D-aspartic acid (NMDA) receptors conductance may affect plasticity. Based on a simplified model of the spine in combination with a calcium-dependent plasticity rule, we demonstrated that after the induction phase of plasticity a shift of the long term potentiation (LTP) or long term depression (LTD) threshold takes place. This induces a refractory period for further LTP induction and promotes depotentiation as observed experimentally. That resembles the BCM metaplasticity rule but specific for the individual synapse. In the second phase, alteration of the NMDA response may bring the synapse to a state such that further synaptic weight alterations are feasible. We show that if the enhancement of the NMDA response is proportional to the area of the post synaptic density (PSD) the plasticity curves most likely return to the initial state.

Conclusions/Significance

Using simulations of calcium dynamics in synaptic spines, coupled with a biophysically motivated calcium-dependent plasticity rule, we find under what conditions structural plasticity can form the basis of synapse specific metaplasticity.     

High speed two-photon imaging of calcium dynamics in dendritic spines: consequences for spine calcium kinetics and buffer capacity

Rapid calcium concentration changes in postsynaptic structures are crucial for synaptic plasticity. Thus far, the determinants of postsynaptic calcium dynamics have been studied predominantly based on the decay kinetics of calcium transients. Calcium rise times in spines in response to single action potentials (AP) are almost never measured due to technical limitations, but they could be crucial for synaptic plasticity. With high-speed, precisely-targeted, two-photon point imaging we measured both calcium rise and decay kinetics in spines and secondary dendrites in neocortical pyramidal neurons. We found that both rise and decay kinetics of changes in calcium-indicator fluorescence are about twice as fast in spines. During AP trains, spine calcium changes follow each AP, but not in dendrites. Apart from the higher surface-to-volume ratio (SVR), we observed that neocortical dendritic spines have a markedly smaller endogenous buffer capacity with respect to their parental dendrites. Calcium influx time course and calcium extrusion rate were both in the same range for spines and dendrites when fitted with a dynamic multi-compartment model that included calcium binding kinetics and diffusion. In a subsequent analysis we used this model to investigate which parameters are critical determinants in spine calcium dynamics. The model confirmed the experimental findings: a higher SVR is not sufficient by itself to explain the faster rise time kinetics in spines, but only when paired with a lower buffer capacity in spines. Simulations at zero calcium-dye conditions show that calmodulin is more efficiently activated in spines, which indicates that spine morphology and buffering conditions in neocortical spines favor synaptic plasticity.     

A Compartmental Model for Activity-Dependent Dendritic Spine Branching

Dendritic spines are small, mushroom-like protrusions from the arbor of a neuron in the central nervous system. Interdependent changes in the morphology, biochemistry, and activity of spines have been associated with learning and memory. Moreover, post-mortem cortices from patients with Alzheimer’s or Parkinson’s disease exhibit biochemical and physical alterations within their dendritic arbors and a reduction in the number of dendritic spines. For over a decade, experimentalists have observed perforations in postsynaptic densities on dendritic spines after induction of long-term potentiation, a sustained enhancement of response to a brief electrical or chemical stimulus, associated with learning and memory. In more recent work, some suggest that activity-dependent intraspine calcium may regulate the surface area of the spine head, and reorganization of postsynaptic densities on the surface. In this paper, we develop a model of a dendritic spine with the ability to partition its transmission and receptor zones, as well as the entire spine head. Simulations are initially performed with fixed parameters for morphology to study electrical properties and identify parameters that increase efficacy of the synaptic connection. Equations are then introduced to incorporate calcium as a second messenger in regulating continuous changes in morphology. In the model, activity affects compartmental calcium, which regulates spine head morphology. Conversely, spine head morphology affects the level of local activity, whether the spines are modeled with passive membrane properties, or excitable membrane using Hodgkin–Huxley kinetics. Results indicate that merely separating the postsynaptic receptors on the surface of the spine may add to the diversity of circuitry, but does not change the efficacy of the synapse. However, when the surface area of the spine is a dynamic variable, efficacy of the synapse may vary continuously over time.     

Modeling the role of lateral membrane diffusion in AMPA receptor trafficking along a spiny dendrite

AMPA receptor trafficking in dendritic spines is emerging as a major postsynaptic mechanism for the expression of plasticity at glutamatergic synapses. AMPA receptors within a spine are in a continuous state of flux, being exchanged with local intracellular pools via exo/endocytosis and with the surrounding dendrite via lateral membrane diffusion. This suggests that one cannot treat a single spine in isolation. Here we present a model of AMPA receptor trafficking between multiple dendritic spines distributed along the surface of a dendrite. Receptors undergo lateral diffusion within the dendritic membrane, with each spine acting as a spatially localized trap where receptors can bind to scaffolding proteins or be internalized through endocytosis. Exocytosis of receptors occurs either at the soma or at sites local to dendritic spines via constitutive recycling from intracellular pools. We derive a reaction–diffusion equation for receptor trafficking that takes into account these various processes. Solutions of this equation allow us to calculate the distribution of synaptic receptor numbers across the population of spines, and hence determine how lateral diffusion contributes to the strength of a synapse. A number of specific results follow from our modeling and analysis. (1) Lateral membrane diffusion alone is insufficient as a mechanism for delivering AMPA receptors from the soma to distal dendrites. (2) A source of surface receptors at the soma tends to generate an exponential-like distribution of receptors along the dendrite, which has implications for synaptic democracy. (3) Diffusion mediates a heterosynaptic interaction between spines so that local changes in the constitutive recycling of AMPA receptors induce nonlocal changes in synaptic strength. On the other hand, structural changes in a spine following long term potentiation or depression have a purely local effect on synaptic strength. (4) A global change in the rates of AMPA receptor exo/endocytosis is unlikely to be the sole mechanism for homeostatic synaptic scaling. (5) The dynamics of AMPA receptor trafficking occurs on multiple timescales and varies according to spatial location along the dendrite. Understanding such dynamics is important when interpreting data from inactivation experiments that are used to infer the rate of relaxation to steady-state.     

Calcium dynamics of spines depend on their dendritic location

Dendritic spines are morphologically and functionally heterogeneous. To understand this diversity, we use two-photon imaging of layer 5 neocortical pyramidal cells and measure action potential-evoked [Ca(2+)]i transients in spines. Spine calcium kinetics are controlled by (i) the diameter of the parent dendrite, (ii) the length of the spine neck, and (iii) the strength of spine calcium pumps. These factors produce different calcium dynamics in spines from basal, proximal apical, and distal apical dendrites, differences that are more pronounced without exogenous buffers. In proximal and distal apical dendrites, different calcium dynamics correlate with different susceptibility to synaptic depression, and modifying calcium kinetics in spines changes the expression of long-term depression. Thus, the spine location apparently determines its calcium dynamics and synaptic plasticity. Our results highlight the precision in design of neocortical neurons.     

Morphological constraints on calcium dependent glutamate receptor trafficking into individual dendritic spine

Glutamate receptor trafficking into dendritic spines is a pivotal step in synaptic plasticity, yet the relevance of plasticity-producing rise of [Ca2+]i and of spine morphology to subsequent delivery of glutamate receptors into dendritic spine heads are still not well understood. Following chemical induction of LTP, an increase in eGFP-GluR1 fluorescence in short but not long dendritic spines of cultured hippocampal neurons was found. Repeated flash photolysis of caged calcium, which produced a transient rise of [Ca2+]i inside spine heads caused a selective, actin and protein synthesis dependent increase of eGFP-GluR1 in these spines. Strikingly, GluR1 increase was correlated with the ability of a calcium transient generated in the spine head to diffuse into the parent dendrite, and inversely correlated with the length of the spine: short spines were more likely to raise GluR1 than long ones. These observations link, for the first time, calcium transients in dendritic spines with spine morphology and its ability to undergo synaptic plasticity.     

Disruption of MET Receptor Tyrosine Kinase,an Autism Risk Factor,Impairs Developmental Synaptic Plasticity in the Hippocampus

As more genes conferring risks to neurodevelopmental disorders are identified, translating these genetic risk factors into biological mechanisms that impact the trajectory of the developing brain is a critical next step. Here, we report that disrupted signaling mediated MET receptor tyrosine kinase (RTK), an established risk factor for autism spectrum disorders, in the developing hippocampus glutamatergic circuit leads to profound deficits in neural development, synaptic transmission, and plasticity. In cultured hippocampus slices prepared from neonatal mice, pharmacological inhibition of MET kinase activity suppresses dendritic arborization and disrupts normal dendritic spine development. In addition, single‐neuron knockdown (RNAi) or overexpression of Met in the developing hippocampal CA1 neurons leads to alterations, opposite in nature, in basal synaptic transmission and long‐term plasticity. In forebrain‐specific Met conditional knockout mice (Met^fx/fx;emx1^cre), an enhanced long‐term potentiation (LTP) and long‐term depression (LTD) were observed at early developmental stages (P12–14) at the Schaffer collateral to CA1 synapses compared with wild‐type littermates. In contrast, LTP and LTD were markedly reduced at young adult stage (P56–70) during which wild‐type mice show robust LTP and LTD. The altered trajectory of synaptic plasticity revealed by this study indicate that temporally regulated MET signaling as an intrinsic, cell autonomous, and pleiotropic mechanism not only critical for neuronal growth and functional maturation, but also for the timing of synaptic plasticity during forebrain glutamatergic circuits development.     

PTEN is recruited to the postsynaptic terminal for NMDA receptor‐dependent long‐term depression

Phosphatase and tensin homolog deleted on chromosome ten (PTEN) is an important regulator of phosphatidylinositol‐(3,4,5,)‐trisphosphate signalling, which controls cell growth and differentiation. However, PTEN is also highly expressed in the adult brain, in which it can be found in dendritic spines in hippocampus and other brain regions. Here, we have investigated specific functions of PTEN in the regulation of synaptic function in excitatory hippocampal synapses. We found that NMDA receptor activation triggers a PDZ‐dependent association between PTEN and the synaptic scaffolding molecule PSD‐95. This association is accompanied by PTEN localization at the postsynaptic density and anchoring within the spine. On the other hand, enhancement of PTEN lipid phosphatase activity is able to drive depression of AMPA receptor‐mediated synaptic responses. This activity is specifically required for NMDA receptor‐dependent long‐term depression (LTD), but not for LTP or metabotropic glutamate receptor‐dependent LTD. Therefore, these results reveal PTEN as a regulated signalling molecule at the synapse, which is recruited to the postsynaptic membrane upon NMDA receptor activation, and is required for the modulation of synaptic activity during plasticity.     

Dendritic spine geometry can localize GTPase signaling in neurons

Dendritic spines are the postsynaptic terminals of most excitatory synapses in the mammalian brain. Learning and memory are associated with long-lasting structural remodeling of dendritic spines through an actin-mediated process regulated by the Rho-family GTPases RhoA, Rac, and Cdc42. These GTPases undergo sustained activation after synaptic stimulation, but whereas Rho activity can spread from the stimulated spine, Cdc42 activity remains localized to the stimulated spine. Because Cdc42 itself diffuses rapidly in and out of the spine, the basis for the retention of Cdc42 activity in the stimulated spine long after synaptic stimulation has ceased is unclear. Here we model the spread of Cdc42 activation at dendritic spines by means of reaction-diffusion equations solved on spine-like geometries. Excitable behavior arising from positive feedback in Cdc42 activation leads to spreading waves of Cdc42 activity. However, because of the very narrow neck of the dendritic spine, wave propagation is halted through a phenomenon we term geometrical wave-pinning. We show that this can account for the localization of Cdc42 activity in the stimulated spine, and, of interest, retention is enhanced by high diffusivity of Cdc42. Our findings are broadly applicable to other instances of signaling in extreme geometries, including filopodia and primary cilia.     

Ca(2+) signaling in dendritic spines

Dendritic spines are cellular microcompartments that are isolated from their parent dendrites and neighboring spines. Recently, imaging studies of spine Ca(2+) dynamics have revealed that Ca(2+) can enter spines through voltage-sensitive and ligand-activated channels, as well as through Ca(2+) release from intracellular stores. Relationships between spine Ca(2+) signals and induction of various forms of synaptic plasticity are beginning to be elucidated. Measurements of spine Ca(2+) concentration are also being used to probe the properties of single synapses and even individual calcium channels in their native environment.     

Learning,AMPA receptor mobility and synaptic plasticity depend on n‐cofilin‐mediated actin dynamics

Neuronal plasticity is an important process for learning, memory and complex behaviour. Rapid remodelling of the actin cytoskeleton in the postsynaptic compartment is thought to have an important function for synaptic plasticity. However, the actin‐binding proteins involved and the molecular mechanisms that in vivo link actin dynamics to postsynaptic physiology are not well understood. Here, we show that the actin filament depolymerizing protein n‐cofilin is controlling dendritic spine morphology and postsynaptic parameters such as late long‐term potentiation and long‐term depression. Loss of n‐cofilin‐mediated synaptic actin dynamics in the forebrain specifically leads to impairment of all types of associative learning, whereas exploratory learning is not affected. We provide evidence for a novel function of n‐cofilin function in synaptic plasticity and in the control of extrasynaptic excitatory AMPA receptors diffusion. These results suggest a critical function of actin dynamics in associative learning and postsynaptic receptor availability.     

Do thin spines learn to be mushroom spines that remember?

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

Calcium influx and postsynaptic proteins coordinate the dendritic filopodium‐spine transition

Dendritic spines are the major locations of excitatory synapses in the mammalian central nervous system. The transformation from dendritic filopodia to dendritic spines has been recognized as one type of spinogenesis. For instance, syndecan‐2 (SDC2), a synaptic heparan sulfate proteoglycan, is highly concentrated at dendritic spines and required for spinogenesis. It induces dendritic filopodia formation, followed by spine formation. However, the molecular regulation of the filopodium‐spine transition induced by SDC2 is still unclear. In this report, we show that calcium is an important signal downstream of SDC2 in regulation of filopodium‐spine transition but not filopodia formation. SDC2 interacted with the postsynaptic proteins calmodulin‐dependent serine kinase (CASK) and LIN7 and further recruited NMDAR to the tips of filopodia induced by SDC2. Calcium influx via NMDAR promoted spine maturation because addition of EGTA or AP5 to the culture medium effectively prevented morphological change from dendritic filopodia to dendritic spines. Our data also indicated that F‐actin rearrangement regulated by calcium influx is involved in the morphological change, because the knockdown of gelsolin, a calcium‐activated F‐actin severing molecule, impaired the filopodium‐spine transition induced by SDC2. In conclusion, our study demonstrates that postsynaptic proteins coordinate to trigger calcium signalling and cytoskeleton rearrangement and consequently control filopodium‐spine transition. © 2014 Wiley Periodicals, Inc. Develop Neurobiol 74: 1011–1029, 2014     

Drebrin与认知功能障碍

树突棘和突触的病理改变在认知功能障碍发病机制中具有十分重要的作用,研究表明大脑发育调节蛋白（developmentregulationbrainprotein,Drebrin）能够调节树突棘和突触的形态和重塑。Drebrin的减少可能通过树突棘内细胞骨架变化,使树突棘的形态结构受到影响,导致突触功能和结构的变化。但目前阿尔茨海默病（Alzheimer’Sdisease,AD）脑内突触病理变化的具体机制及Drebrin和突触之间的关系仍不明确。探讨Drebrin与认知功能的关系及其机制,对临床上早期干预认知功能障碍、寻找AD的有效诊断治疗措施具有重要意义。     

Long‐term perturbation of spine plasticity results in distinct impairments of cognitive function

Dendritic spines serve as the post‐synaptic structural component of synapses. The structure and function of dendritic spines are dynamically regulated by a number of signaling pathways and allow for normal neural processing, whereas aberrant spine changes are thought to contribute to cognitive impairment in neuropsychiatric and neurodegenerative disorders. However, spine changes within different brain regions and their contribution to specific cognitive functions, especially later in adulthood, is not well understood. In this study, we used late‐adult KALRN‐deficient mice as a tool to investigate the vulnerability of different cognitive functions to long‐term perturbations in spine plasticity in different forebrain regions. We found that in these mice, loss of one or both copies of KALRN lead to genotype and brain region‐dependent reductions in spine density. Surprisingly, heterozygote and knockout mice showed differential impairments in cognitive phenotypes, including working memory, social recognition, and social approach. Correlation analysis between the site and magnitude of spine loss and behavioral alterations suggests that the interplay between brain regions is critical for complex cognitive processing and underscores the importance of spine plasticity in normal cognitive function. Long‐term perturbation of spine plasticity results in distinct impairments of cognitive function. Using genetically modified mice deficient in a central regulator of spine plasticity, we investigated the brain region‐specific contribution of spine numbers to various cognitive functions. We found distinct cognitive functions display differential sensitivity to spine loss in the cortex and hippocampus. Our data support spines as neuronal structures important for cognition and suggest interplay between brain regions is critical for complex cognitive processing.     

The effects of NMDA subunit composition on calcium influx and spike timing-dependent plasticity in striatal medium spiny neurons

Calcium through NMDA receptors (NMDARs) is necessary for the long-term potentiation (LTP) of synaptic strength; however, NMDARs differ in several properties that can influence the amount of calcium influx into the spine. These properties, such as sensitivity to magnesium block and conductance decay kinetics, change the receptor's response to spike timing dependent plasticity (STDP) protocols, and thereby shape synaptic integration and information processing. This study investigates the role of GluN2 subunit differences on spine calcium concentration during several STDP protocols in a model of a striatal medium spiny projection neuron (MSPN). The multi-compartment, multi-channel model exhibits firing frequency, spike width, and latency to first spike similar to current clamp data from mouse dorsal striatum MSPN. We find that NMDAR-mediated calcium is dependent on GluN2 subunit type, action potential timing, duration of somatic depolarization, and number of action potentials. Furthermore, the model demonstrates that in MSPNs, GluN2A and GluN2B control which STDP intervals allow for substantial calcium elevation in spines. The model predicts that blocking GluN2B subunits would modulate the range of intervals that cause long term potentiation. We confirmed this prediction experimentally, demonstrating that blocking GluN2B in the striatum, narrows the range of STDP intervals that cause long term potentiation. This ability of the GluN2 subunit to modulate the shape of the STDP curve could underlie the role that GluN2 subunits play in learning and development.     

Dynamics of dendritic spines and their afferent terminals: spines are more motile than presynaptic boutons

Previous work has established that dendritic spines, sites of excitatory input in CNS neurons, can be highly dynamic, in later development as well as in mature brain. Although spine motility has been proposed to facilitate the formation of new synaptic contacts, we have reported that spines continue to be dynamic even if they bear synaptic contacts. An outstanding question related to this finding is whether the presynaptic terminals that contact dendritic spines are as dynamic as their postsynaptic targets. Using multiphoton time-lapse microscopy of GFP-labeled Purkinje cells and DiI-labeled granule cell parallel fiber afferents in cerebellar slices, we monitored the dynamic behavior of both presynaptic terminals and postsynaptic dendritic spines in the same preparation. We report that while spines are dynamic, the presynaptic terminals they contact are quite stable. We confirmed the relatively low levels of presynaptic terminal motility by imaging parallel fibers in vivo. Finally, spine motility can occur when a functional presynaptic terminal is apposed to it. These analyses further call into question the function of spine motility, and to what extent the synapse breaks or maintains its contact during the movement of the spine.     

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

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