Preparation of gene gun bullets and biolistic transfection of neurons in slice culture

H-Ras induces exuberant de novo dendritic protrusion growth in mature neurons regardless of cell type

Dendritic protrusions, mainly spines and filopodia, correlate with excitatory synapses in pyramidal neurons (PyNs), but this relationship may not apply universally. We found that ectopic H-Ras expression increased protrusions across various cortical cell types, including layer 2/3 PyNs, parvalbumin (PV)-, and vasoactive intestinal peptide (VIP)-positive interneurons (INs) in the primary motor cortex. The probability of detecting protrusions correlated with local H-Ras activity, indicating its role in protrusion formation. H-Ras overexpression led to high turnover rates by adding protrusions. Two-photon photolysis of glutamate induced de novo spine formation in mature H-Ras expressing neurons, suggesting H-Ras's effect is not limited to early development. In PyNs and PV-INs, but not VIP-INs, spine neck lengths shifted to filopodia-like phenotypes. H-Ras primarily induced filopodia in PyNs and spines in PV- and VIP-INs. Increased protrusions in H-Ras-transfected PyNs lacked key excitatory synaptic proteins and did not affect miniature excitatory postsynaptic currents (mEPSCs), suggesting multifaceted roles beyond excitatory synapses.

A synapse-specific refractory period for plasticity at individual dendritic spines

How newly formed memories are preserved while brain plasticity is ongoing has been a source of debate. One idea is that synapses which experienced recent plasticity become resistant to further plasticity, a type of metaplasticity often referred to as saturation. Here, we probe the local dendritic mechanisms that limit plasticity at recently potentiated synapses. We show that recently potentiated individual synapses exhibit a synapse-specific refractory period for further potentiation. We further found that the refractory period is associated with reduced postsynaptic CaMKII signaling; however, stronger synaptic activation only partially restored the ability for further plasticity. Importantly, the refractory period is released after one hour, a timing that coincides with the enrichment of several postsynaptic proteins to pre-plasticity levels. Notably, increasing the level of the postsynaptic scaffolding protein, PSD95, but not of PSD93, overcomes the refractory period. Our results support a model in which potentiation at a single synapse is sufficient to initiate a synapse-specific refractory period that persists until key postsynaptic proteins regain their steady-state synaptic levels.

BDNF signaling requires Matrix Metalloproteinase-9 during structural synaptic plasticity

Synaptic plasticity underlies learning and memory processes as well as contributes, in its aberrant form, to neuropsychiatric disorders. One of its major forms is structural long-term potentiation (sLTP), an activity-dependent growth of dendritic spines that harbor excitatory synapses. The process depends on the release of brain-derived neurotrophic factor (BDNF), and activation of its receptor, TrkB. Matrix metalloproteinase-9 (MMP-9), an extracellular protease is essential for many forms of neuronal plasticity engaged in physiological as well as pathological processes. Here, we utilized two-photon microscopy and two-photon glutamate uncaging to demonstrate that MMP-9 activity is essential for sLTP and is rapidly (~seconds) released from dendritic spines in response to synaptic stimulation. Moreover, we show that either chemical or genetic inhibition of MMP-9 impairs TrkB activation, as measured by fluorescence lifetime imaging microscopy of FRET sensor. Furthermore, we provide evidence for a cell-free cleavage of proBDNF into mature BDNF by MMP-9. Our findings point to the autocrine mechanism of action of MMP-9 through BDNF maturation and TrkB activation during sLTP.

Activity-Dependent Stabilization of Nascent Dendritic Spines Requires Nonenzymatic CaMKIIα Function

The outgrowth and stabilization of nascent dendritic spines are crucial processes underlying learning and memory. Most new spines retract shortly after growth; only a small subset is stabilized and integrated into the new circuit connections that support learning. New spine stabilization has been shown to rely upon activity-dependent molecular mechanisms that also contribute to long-term potentiation (LTP) of synaptic strength. Indeed, disruption of the activity-dependent targeting of the kinase CaMKIIα to the GluN2B subunit of the NMDA-type glutamate receptor disrupts both LTP and activity-dependent stabilization of new spines. Yet it is not known which of CaMKIIα's many enzymatic and structural functions are important for new spine stabilization. Here, we used two-photon imaging and photolysis of caged glutamate to monitor the activity-dependent stabilization of new dendritic spines on hippocampal CA1 neurons from mice of both sexes in conditions where CaMKIIα functional and structural interactions were altered. Surprisingly, we found that inhibiting CaMKIIα kinase activity either genetically or pharmacologically did not impair activity-dependent new spine stabilization. In contrast, shRNA knockdown of CaMKIIα abolished activity-dependent new spine stabilization, which was rescued by co-expressing shRNA-resistant full-length CaMKIIα, but not by a truncated monomeric CaMKIIα. Notably, overexpression of phospho-mimetic CaMKIIα-T286D, which exhibits activity-independent targeting to GluN2B, enhanced basal new spine survivorship in the absence of additional glutamatergic stimulation, even when kinase activity was disrupted. Together, our results support a model in which nascent dendritic spine stabilization requires structural and scaffolding interactions mediated by dodecameric CaMKIIα that are independent of its enzymatic activities.

Exuberant de novo dendritic spine growth in mature neurons

Dendritic spines are structural correlates of excitatory synapses maintaining stable synaptic communications. However, this strong spine-synapse relationship was mainly characterized in excitatory pyramidal neurons (PyNs), raising a possibility that inferring synaptic density from dendritic spine number may not be universally applied to all neuronal types. Here we found that the ectopic expression of H-Ras increased dendritic spine numbers regardless of cortical cell types such as layer 2/3 pyramidal neurons (PyNs), parvalbumin (PV)- and vasoactive intestinal peptide (VIP)-positive interneurons (INs) in the primary motor cortex (M1). The probability of detecting dendritic spines was positively correlated with the magnitude of H-Ras activity, suggesting elevated local H-Ras activity is involved in the process of dendritic spine formation. H-Ras overexpression caused high spine turnover rate via adding more spines rather than eliminating them. Two-photon photolysis of glutamate triggered de novo dendritic spine formation in mature neurons, suggesting H-Ras induced spine formation is not restricted to the early development. In PyNs and PV-INs, but not VIP-INs, we observed a shift in average spine neck length towards longer filopodia-like phenotypes. The portion of dendritic spines lacking key excitatory synaptic proteins were significantly increased in H-Ras transfected neurons, suggesting that these increased spines have other distinct functions. High spine density caused by H-Ras did not result in change in the frequency or the amplitude of miniature excitatory postsynaptic currents (mEPSCs). Thus, our results propose that dendritic spines possess more multifaceted functions beyond the morphological proxy of excitatory synapse.

Techniques to Render Dendritic Spines Visible in the Microscope

A tiny detail visible on certain neurons at the limit of resolution in light microscopy went in 130 years of neuroscience research through a dazzling career from suspicious staining artifact to what we recognize today as a complex postsynaptic molecular machine: the dendritic spine.This chapter deals with techniques to make spines visible. The original technique, Golgi silver staining, is still being used today. Electron microscopy and automated field ion beam scanning electron microscopy are ultrahigh resolution techniques, albeit specialized. Other methods are intracellular injection, uptake of dyes, and recently the exploitation of genetically modified animals in which certain neurons express fluorescent protein in all their processes, including the nooks and crannies of their dendritic spines.

BAX regulates dendritic spine development via mitochondrial fusion

BAX is a Bcl-2 family protein acting on apoptosis. It also promotes mitochondrial fusion by interacting with the mitochondrial fusion protein Mitofusin (Mfn1 and Mfn2). Neuronal mitochondria are important for the development and modification of dendritic spines, which are subcellular compartments accommodating excitatory synapses in postsynaptic neurons. The abundance of dendritic mitochondria influences dendritic spine development. Mitochondrial fusion is essential for mitochondrial homeostasis. Here, we show that in the hippocampal neuron of BAX knockout mice, mitochondrial fusion is impaired, leading to decreases in mitochondrial length and total mitochondrial mass in dendrites. Notably, BAX knockout mice also have fewer dendritic spines and less cellular Adenosine 5'triphosphate (ATP) in dendrites. The spine and ATP changes are abolished by restoring mitochondria fusion via overexpressing Mfn1 and Mfn2. These findings indicate that BAX-mediated mitochondrial fusion in neurons is crucial for the development of dendritic spines and the maintenance of cellular ATP levels.

Induction of input-specific spine shrinkage on dendrites of rodent hippocampal CA1 neurons using two-photon glutamate uncaging

Shrinkage and loss of dendritic spines are vital components of the neuronal plasticity that supports learning. To investigate the mechanisms of spine shrinkage and loss, Oh and colleagues established a two-photon glutamate uncaging protocol that reliably induces input-specific spine shrinkage on dendrites of rodent hippocampal CA1 pyramidal neurons. Here, we provide a detailed description of that protocol and also an optimized version that can be used to induce input- and synapse-specific shrinkage of dendritic spines at physiological Ca²⁺ levels. For complete details on the use and execution of this protocol, please refer to Oh et al. (2013), Stein et al. (2015), Stein et al. (2020), and Stein et al. (2021).

EVAP: A two-photon imaging tool to study conformational changes in endogenous Kv2 channels in live tissues

A primary goal of molecular physiology is to understand how conformational changes of proteins affect the function of cells, tissues, and organisms. Here, we describe an imaging method for measuring the conformational changes of the voltage sensors of endogenous ion channel proteins within live tissue, without genetic modification. We synthesized GxTX-594, a variant of the peptidyl tarantula toxin guangxitoxin-1E, conjugated to a fluorophore optimal for two-photon excitation imaging through light-scattering tissue. We term this tool EVAP (Endogenous Voltage-sensor Activity Probe). GxTX-594 targets the voltage sensors of Kv2 proteins, which form potassium channels and plasma membrane–endoplasmic reticulum junctions. GxTX-594 dynamically labels Kv2 proteins on cell surfaces in response to voltage stimulation. To interpret dynamic changes in fluorescence intensity, we developed a statistical thermodynamic model that relates the conformational changes of Kv2 voltage sensors to degree of labeling. We used two-photon excitation imaging of rat brain slices to image Kv2 proteins in neurons. We found puncta of GxTX-594 on hippocampal CA1 neurons that responded to voltage stimulation and retain a voltage response roughly similar to heterologously expressed Kv2.1 protein. Our findings show that EVAP imaging methods enable the identification of conformational changes of endogenous Kv2 voltage sensors in tissue.

Using a Hand-Held Gene Gun for Genetic Transformation of Tetrahymena thermophila

Biolistic bombardment is widely used as a means of delivering vector-coated microparticles into microorganisms, cultured cells, and tissues. The first particle delivery system contained a helium propulsion unit (the gun) mounted in a vacuum-controlled chamber. In contrast, the hand-held gene gun does not operate within a chamber. It is completely hand-held, easy, and efficient to use, and it requires minimal space on the laboratory bench top. This chapter describes protocols for using a hand-held gene gun to deliver transformation vectors for overexpression of genes or gene replacement into the macronucleus of Tetrahymena thermophila. The protocols provide helpful information for preparing Tetrahymena for biolistic bombardment, preparation of vector-coated microcarriers, and basic gene gun operating procedures.

Rapid Ultrastructural Changes in the PSD and Surrounding Membrane after Induction of Structural LTP in Single Dendritic Spines

The structural plasticity of dendritic spines is considered to be an important basis of synaptic plasticity, learning, and memory. Here, we induced input-specific structural LTP (sLTP) in single dendritic spines in organotypic hippocampal slices from mice of either sex and performed ultrastructural analyses of the spines using efficient correlative light and electron microscopy. We observed reorganization of the PSD nanostructure, such as perforation and segmentation, at 2-3, 20, and 120 min after sLTP induction. In addition, PSD and nonsynaptic axon-spine interface (nsASI) membrane expanded unevenly during sLTP. Specifically, the PSD area showed a transient increase at 2-3 min after sLTP induction. The PSD growth was to a degree less than spine volume growth at 2-3 min and 20 min after sLTP induction but became similar at 120 min. On the other hand, the nsASI area showed a profound and lasting expansion, to a degree similar to spine volume growth throughout the process. These rapid ultrastructural changes in PSD and surrounding membrane may contribute to rapid electrophysiological plasticity during sLTP.SIGNIFICANCE STATEMENT To understand the ultrastructural changes during synaptic plasticity, it is desired to efficiently image single dendritic spines that underwent structural plasticity in electron microscopy. We induced structural long-term potentiation (sLTP) in single dendritic spines by two-photon glutamate uncaging. We then identified the same spines at different phases of sLTP and performed ultrastructural analysis by using an efficient correlative light and electron microscopy method. We found that postsynaptic density undergoes dramatic modification in its structural complexity immediately after sLTP induction. Meanwhile, the nonsynaptic axon-spine interface area shows a rapid and sustained increase throughout sLTP. Our results indicate that the uneven modification of synaptic and nonsynaptic postsynaptic membrane might contribute to rapid electrophysiological plasticity during sLTP.

Altered Phosphorylation of the Proteasome Subunit Rpt6 Has Minimal Impact on Synaptic Plasticity and Learning

Dynamic control of protein degradation via the ubiquitin proteasome system (UPS) is thought to play a crucial role in neuronal function and synaptic plasticity. The proteasome subunit Rpt6, an AAA ATPase subunit of the 19S regulatory particle (RP), has emerged as an important site for regulation of 26S proteasome function in neurons. Phosphorylation of Rpt6 on serine 120 (S120) can stimulate the catalytic rate of substrate degradation by the 26S proteasome and this site is targeted by the plasticity-related kinase Ca²⁺/calmodulin-dependent kinase II (CaMKII), making it an attractive candidate for regulation of proteasome function in neurons. Several in vitro studies have shown that altered Rpt6 S120 phosphorylation can affect the structure and function of synapses. To evaluate the importance of Rpt6 S120 phosphorylation in vivo, we created two mouse models which feature mutations at S120 that block or mimic phosphorylation at this site. We find that peptidase and ATPase activities are upregulated in the phospho-mimetic mutant and downregulated in the phospho-dead mutant [S120 mutated to aspartic acid (S120D) or alanine (S120A), respectively]. Surprisingly, these mutations had no effect on basal synaptic transmission, long-term potentiation (LTP), and dendritic spine dynamics and density in the hippocampus. Furthermore, these mutants displayed no deficits in cued and contextual fear memory. Thus, in a mouse model that blocks or mimics phosphorylation at this site, either compensatory mechanisms negate these effects, or small variations in proteasome activity are not enough to induce significant changes in synaptic structure, plasticity, or behavior.

Rapid recycling of glutamate transporters on the astroglial surface

Glutamate uptake by astroglial transporters confines excitatory transmission to the synaptic cleft. The efficiency of this mechanism depends on the transporter dynamics in the astrocyte membrane, which remains poorly understood. Here, we visualise the main glial glutamate transporter GLT1 by generating its pH-sensitive fluorescent analogue, GLT1-SEP. Fluorescence recovery after photobleaching-based imaging shows that 70-75% of GLT1-SEP dwell on the surface of rat brain astroglia, recycling with a lifetime of ~22 s. Genetic deletion of the C-terminus accelerates GLT1-SEP membrane turnover while disrupting its surface pattern, as revealed by single-molecule localisation microscopy. Excitatory activity boosts surface mobility of GLT1-SEP, involving its C-terminus, metabotropic glutamate receptors, intracellular Ca2+, and calcineurin-phosphatase activity, but not the broad-range kinase activity. The results suggest that membrane turnover, rather than lateral diffusion, is the main 'redeployment' route for the immobile fraction (20-30%) of surface-expressed GLT1. This finding reveals an important mechanism helping to control extrasynaptic escape of glutamate.

Non-ionotropic NMDA receptor signaling gates bidirectional structural plasticity of dendritic spines

Experience-dependent refinement of neuronal connections is critically important for brain development and learning. Here, we show that ion-flow-independent NMDA receptor (NMDAR) signaling is required for the long-term dendritic spine growth that is a vital component of brain circuit plasticity. We find that inhibition of p38 mitogen-activated protein kinase (p38 MAPK), which is downstream of non-ionotropic NMDAR signaling in long-term depression (LTD) and spine shrinkage, blocks long-term potentiation (LTP)-induced spine growth but not LTP. We hypothesize that non-ionotropic NMDAR signaling drives the cytoskeletal changes that support bidirectional spine structural plasticity. Indeed, we find that key signaling components downstream of non-ionotropic NMDAR function in LTD-induced spine shrinkage are also necessary for LTP-induced spine growth. Furthermore, NMDAR conformational signaling with coincident Ca²⁺ influx is sufficient to drive CaMKII-dependent long-term spine growth, even when Ca²⁺ is artificially driven through voltage-gated Ca²⁺ channels. Our results support a model in which non-ionotropic NMDAR signaling gates the bidirectional spine structural changes vital for brain plasticity.

Structural plasticity of dendritic spines is a critical step in the remodeling of brain circuits during learning. Stein et al. demonstrate a vital role for ion-flux-independent NMDAR signaling in plasticity-associated dendritic spine growth, supporting a model in which non-ionotropic NMDAR signaling primes the spine actin cytoskeleton for bidirectional structural plasticity.

A Single-Neuron: Current Trends and Future Prospects

The brain is an intricate network with complex organizational principles facilitating a concerted communication between single-neurons, distinct neuron populations, and remote brain areas. The communication, technically referred to as connectivity, between single-neurons, is the center of many investigations aimed at elucidating pathophysiology, anatomical differences, and structural and functional features. In comparison with bulk analysis, single-neuron analysis can provide precise information about neurons or even sub-neuron level electrophysiology, anatomical differences, pathophysiology, structural and functional features, in addition to their communications with other neurons, and can promote essential information to understand the brain and its activity. This review highlights various single-neuron models and their behaviors, followed by different analysis methods. Again, to elucidate cellular dynamics in terms of electrophysiology at the single-neuron level, we emphasize in detail the role of single-neuron mapping and electrophysiological recording. We also elaborate on the recent development of single-neuron isolation, manipulation, and therapeutic progress using advanced micro/nanofluidic devices, as well as microinjection, electroporation, microelectrode array, optical transfection, optogenetic techniques. Further, the development in the field of artificial intelligence in relation to single-neurons is highlighted. The review concludes with between limitations and future prospects of single-neuron analyses.

Molecular Mechanisms of Non-ionotropic NMDA Receptor Signaling in Dendritic Spine Shrinkage

Structural plasticity of dendritic spines is a key component of the refinement of synaptic connections during learning. Recent studies highlight a novel role for the NMDA receptor (NMDAR), independent of ion flow, in driving spine shrinkage and LTD. Yet little is known about the molecular mechanisms that link conformational changes in the NMDAR to changes in spine size and synaptic strength. Here, using two-photon glutamate uncaging to induce plasticity at individual dendritic spines on hippocampal CA1 neurons from mice and rats of both sexes, we demonstrate that p38 MAPK is generally required downstream of non-ionotropic NMDAR signaling to drive both spine shrinkage and LTD. In a series of pharmacological and molecular genetic experiments, we identify key components of the non-ionotropic NMDAR signaling pathway driving dendritic spine shrinkage, including the interaction between NOS1AP (nitric oxide synthase 1 adaptor protein) and neuronal nitric oxide synthase (nNOS), nNOS enzymatic activity, activation of MK2 (MAPK-activated protein kinase 2) and cofilin, and signaling through CaMKII. Our results represent a large step forward in delineating the molecular mechanisms of non-ionotropic NMDAR signaling that can drive shrinkage and elimination of dendritic spines during synaptic plasticity.SIGNIFICANCE STATEMENT Signaling through the NMDA receptor (NMDAR) is vitally important for the synaptic plasticity that underlies learning. Recent studies highlight a novel role for the NMDAR, independent of ion flow, in driving synaptic weakening and dendritic spine shrinkage during synaptic plasticity. Here, we delineate several key components of the molecular pathway that links conformational signaling through the NMDAR to dendritic spine shrinkage during synaptic plasticity.

Rapid exchange of synaptic and extrasynaptic NMDA receptors in hippocampal CA1 neurons

N-methyl-d-aspartate receptors (NMDARs) are fundamental coincidence detectors of synaptic activity necessary for the induction of synaptic plasticity and synapse stability. Adjusting NMDAR synaptic content, whether by receptor insertion or lateral diffusion between extrasynaptic and synaptic compartments, could play a substantial role defining the characteristics of the NMDAR-mediated excitatory postsynaptic current (EPSC), which in turn would mediate the ability of the synapse to undergo plasticity. Lateral NMDAR movement has been observed in dissociated neurons; however, it is currently unclear whether NMDARs are capable of lateral surface diffusion in hippocampal slices, a more physiologically relevant environment. To test for lateral mobility in rat hippocampal slices, we rapidly blocked synaptic NMDARs using MK-801, a use-dependent and irreversible NMDAR blocker. Following a 5-min washout period, we observed a strong recovery of NMDAR-mediated responses. The degree of the observed recovery was proportional to the amount of induced blockade, independent of levels of intracellular calcium, and mediated primarily by GluN2B-containing NMDA receptors. These results indicate that lateral diffusion of NMDARs could be a mechanism by which synapses rapidly adjust parameters to fine-tune synaptic plasticity.NEW & NOTEWORTHY N-methyl-d-aspartate-type glutamate receptors (NMDARs) have always been considered stable components of synapses. We show that in rat hippocampal slices synaptic NMDARs are in constant exchange with extrasynaptic receptors. This exchange of receptors is mediated primarily by NMDA receptors containing GluN2B, a subunit necessary to undergo synaptic plasticity. Thus this lateral movement of synaptic receptors allows synapses to rapidly regulate the total number of synaptic NMDARs with potential consequences for synaptic plasticity.

Thrombospondin receptor α2δ-1 promotes synaptogenesis and spinogenesis via postsynaptic Rac1

Astrocytes promote synapse formation during development via secreted factors including thrombospondin family proteins, which act through the neuronal calcium channel subunit α2δ-1. Risher et al. demonstrate that this process requires signaling via the Rho GTPase Rac1 to facilitate the maturation of dendritic spine synapses in the cortex.

Astrocytes control excitatory synaptogenesis by secreting thrombospondins (TSPs), which function via their neuronal receptor, the calcium channel subunit α2δ-1. α2δ-1 is a drug target for epilepsy and neuropathic pain; thus the TSP–α2δ-1 interaction is implicated in both synaptic development and disease pathogenesis. However, the mechanism by which this interaction promotes synaptogenesis and the requirement for α2δ-1 for connectivity of the developing mammalian brain are unknown. In this study, we show that global or cell-specific loss of α2δ-1 yields profound deficits in excitatory synapse numbers, ultrastructure, and activity and severely stunts spinogenesis in the mouse cortex. Postsynaptic but not presynaptic α2δ-1 is required and sufficient for TSP-induced synaptogenesis in vitro and spine formation in vivo, but an α2δ-1 mutant linked to autism cannot rescue these synaptogenesis defects. Finally, we reveal that TSP–α2δ-1 interactions control synaptogenesis postsynaptically via Rac1, suggesting potential molecular mechanisms that underlie both synaptic development and pathology.

Mitochondrial GSH Systems in CA1 Pyramidal Cells and Astrocytes React Differently during Oxygen-Glucose Deprivation and Reperfusion

Pyramidal cells and astrocytes have differential susceptibility to oxygen-glucose deprivation and reperfusion (OGD-RP). It is known that excessive reactive oxygen species (ROS) in mitochondria initiates cell death, while glutathione (GSH) is one of the major defenses against ROS. Although it is known that astrocytes contain a higher concentration of GSH than neurons, and that astrocytes can provide neurons with GSH, we are unaware of a detailed and quantitative examination of the dynamic changes in the mitochondrial GSH system in the two cell types during OGD-RP. Here, we determined mitochondrial membrane potential and the degrees of oxidation of the mitochondrially targeted roGFP-based sensors for hydrogen peroxide (OxD_P) and GSH (OxD_G). We also developed a method to estimate the mitochondrial GSH (mGSH) concentration in single cells in the CA1 region of organotypic hippocampal slice cultures at several time-points during OGD-RP. We find that mitochondrial membrane potential drops in pyramidal cells during OGD while it is relatively stable in astrocytes. In both types of cell, the mitochondrial membrane potential decreases during RP. During OGD-RP, mitochondrial peroxide levels are the same. Astrocytic mGSH is more than four times higher than pyramidal cell mGSH (3.2 vs 0.7 mM). Astrocytic mGSH is drained from mitochondria during OGD, whereas in pyramidal cells it remains fairly constant. OxD_GSH prior to and during OGD is lower (less oxidized) in pyramidal cells than in astrocytes, but the two nearly converge during RP. The larger changes of redox status in the GSH system in pyramidal cells than astrocytes is an upstream sign of the higher mortality of the pyramidal cells after facing an insult. The pattern of [mGSH] changes in the two cell types could be recognized as another mechanism by which astrocytes protect neurons from transient, extreme conditions.

Neuronal activity mediated regulation of glutamate transporter GLT-1 surface diffusion in rat astrocytes in dissociated and slice cultures

The astrocytic GLT‐1 (or EAAT2) is the major glutamate transporter for clearing synaptic glutamate. While the diffusion dynamics of neurotransmitter receptors at the neuronal surface are well understood, far less is known regarding the surface trafficking of transporters in subcellular domains of the astrocyte membrane. Here, we have used live‐cell imaging to study the mechanisms regulating GLT‐1 surface diffusion in astrocytes in dissociated and brain slice cultures. Using GFP‐time lapse imaging, we show that GLT‐1 forms stable clusters that are dispersed rapidly and reversibly upon glutamate treatment in a transporter activity‐dependent manner. Fluorescence recovery after photobleaching and single particle tracking using quantum dots revealed that clustered GLT‐1 is more stable than diffuse GLT‐1 and that glutamate increases GLT‐1 surface diffusion in the astrocyte membrane. Interestingly, the two main GLT‐1 isoforms expressed in the brain, GLT‐1a and GLT‐1b, are both found to be stabilized opposed to synapses under basal conditions, with GLT‐1b more so. GLT‐1 surface mobility is increased in proximity to activated synapses and alterations of neuronal activity can bidirectionally modulate the dynamics of both GLT‐1 isoforms. Altogether, these data reveal that astrocytic GLT‐1 surface mobility, via its transport activity, is modulated during neuronal firing, which may be a key process for shaping glutamate clearance and glutamatergic synaptic transmission. GLIA 2016;64:1252–1264

A protocol for preparation and transfection of rat entorhinal cortex organotypic cultures for electrophysiological whole-cell recordings

Understanding how neuromodulators influence synaptic transmission and intrinsic excitability within the entorhinal cortex (EC) is critical to furthering our understanding of the molecular and cellular aspects of this region. Organotypic cultures can provide a cost-effective means to employ selective molecular biological strategies in elucidating cellular mechanisms of neuromodulation in the EC. We therefore adapted our acute slice model for organotypic culture applications and optimized a protocol for the preparation and biolistic transfection of cultured horizontal EC slices. Here, we present our detailed protocol for culturing EC slices. Using an n-methyl-d-glucamine (NMDG)-containing cutting solution, we obtain healthy EC slice cultures for electrophysiological recordings. We also present our protocol for the preparation of "bullets" carrying one or more constructs and demonstrate successful transfection of EC slices. We build upon previous methods and highlight specific aspects in our method that greatly improved the quality of our results. We validate our methods using immunohistochemical, imaging, and electrophysiological techniques. The novelty of this method is that it provides a description of culturing and transfection of EC neurons for specifically addressing their functionality. This method will enable researchers interested in entorhinal function to quickly adopt a similar slice culture transfection system for their own investigations.

Formation and Maintenance of Functional Spines in the Absence of Presynaptic Glutamate Release

Dendritic spines are the major transmitter reception compartments of glutamatergic synapses in most principal neurons of the mammalian brain and play a key role in the function of nerve cell circuits. The formation of functional spine synapses is thought to be critically dependent on presynaptic glutamatergic signaling. By analyzing CA1 pyramidal neurons in mutant hippocampal slice cultures that are essentially devoid of presynaptic transmitter release, we demonstrate that the formation and maintenance of dendrites and functional spines are independent of synaptic glutamate release.

Phosphoinositide-dependent enrichment of actin monomers in dendritic spines regulates synapse development and plasticity

Dendritic spines are small postsynaptic compartments of excitatory synapses in the vertebrate brain that are modified during learning, aging, and neurological disorders. The formation and modification of dendritic spines depend on rapid assembly and dynamic remodeling of the actin cytoskeleton in this highly compartmentalized space, but the precise mechanisms remain to be fully elucidated. In this study, we report that spatiotemporal enrichment of actin monomers (G-actin) in dendritic spines regulates spine development and plasticity. We first show that dendritic spines contain a locally enriched pool of G-actin that can be regulated by synaptic activity. We further find that this G-actin pool functions in spine development and its modification during synaptic plasticity. Mechanistically, the relatively immobile G-actin pool in spines depends on the phosphoinositide PI(3,4,5)P3 and involves the actin monomer-binding protein profilin. Together, our results have revealed a novel mechanism by which dynamic enrichment of G-actin in spines regulates the actin remodeling underlying synapse development and plasticity.

Protracted and asynchronous accumulation of PSD95-family MAGUKs during maturation of nascent dendritic spines

The formation and stabilization of new dendritic spines is a key component of the experience-dependent neural circuit plasticity that supports learning, but the molecular maturation of nascent spines remains largely unexplored. The PSD95-family of membrane-associated guanylate kinases (PSD-MAGUKs), most notably PSD95, has a demonstrated role in promoting spine stability. However, nascent spines contain low levels of PSD95, suggesting that other members of the PSD-MAGUK family might act to stabilize nascent spines in the early stages of spiny synapse formation. Here, we used GFP-fusion constructs to quantitatively define the molecular composition of new spines, focusing on the PSD-MAGUK family. We found that PSD95 levels in new spines were as low as those previously associated with rapid subsequent spine elimination, and new spines did not achieve mature levels of PSD95 until between 12 and 20 h following new spine identification. Surprisingly, we found that the PSD-MAGUKs PSD93, SAP97, and SAP102 were also substantially less enriched in new spines. However, they accumulated in new spines more quickly than PSD95: SAP102 enriched to mature levels within 3 h, SAP97 and PSD93 enriched gradually over the course of 6 h. Intriguingly, when we restricted our analysis to only those new spines that persisted, SAP97 was the only PSD-MAGUK already present at mature levels in persistent new spines when first identified. Our findings uncover a key structural difference between nascent and mature spines, and suggest a mechanism for the stabilization of nascent spines through the sequential arrival of PSD-MAGUKs. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 77: 1161-1174, 2017.

Temporally precise labeling and control of neuromodulatory circuits in the mammalian brain

Few tools exist to visualize and manipulate neurons that are targets of neuromodulators. We present iTango, a light- and ligand-gated gene expression system based on a light-inducible split tobacco etch virus protease. Cells expressing the iTango system exhibit increased expression of a marker gene in the presence of dopamine and blue-light exposure, both in vitro and in vivo. We demonstrated the iTango system in a behaviorally relevant context, by inducing expression of optogenetic tools in neurons under dopaminergic control during a behavior of interest. We thereby gained optogenetic control of these behaviorally relevant neurons. We applied the iTango system to decipher the roles of two classes of dopaminergic neurons in the mouse nucleus accumbens in a sensitized locomotor response to cocaine. Thus, the iTango platform allows for control of neuromodulatory circuits in a genetically and functionally defined manner with spatial and temporal precision.

A dual role for the RhoGEF Ephexin5 in regulation of dendritic spine outgrowth

The outgrowth of new dendritic spines is closely linked to the formation of new synapses, and is thought to be a vital component of the experience-dependent circuit plasticity that supports learning. Here, we examined the role of the RhoGEF Ephexin5 in driving activity-dependent spine outgrowth. We found that reducing Ephexin5 levels increased spine outgrowth, and increasing Ephexin5 levels decreased spine outgrowth in a GEF-dependent manner, suggesting that Ephexin5 acts as an inhibitor of spine outgrowth. Notably, we found that increased neural activity led to a proteasome-dependent reduction in the levels of Ephexin5 in neuronal dendrites, which could facilitate the enhanced spine outgrowth observed following increased neural activity. Surprisingly, we also found that Ephexin5-GFP levels were elevated on the dendrite at sites of future new spines, prior to new spine outgrowth. Moreover, lowering neuronal Ephexin5 levels inhibited new spine outgrowth in response to both global increases in neural activity and local glutamatergic stimulation of the dendrite, suggesting that Ephexin5 is necessary for activity-dependent spine outgrowth. Our data support a model in which Ephexin5 serves a dual role in spinogenesis, acting both as a brake on overall spine outgrowth and as a necessary component in the site-specific formation of new spines.

Optogenetic Control of Synaptic Composition and Function

The molecular composition of the postsynaptic membrane is sculpted by synaptic activity. During synaptic plasticity at excitatory synapses, numerous structural, signaling, and receptor molecules concentrate at the postsynaptic density (PSD) to regulate synaptic strength. We developed an approach that uses light to tune the abundance of specific molecules in the PSD. We used this approach to investigate the relationship between the number of AMPA-type glutamate receptors in the PSD and synaptic strength. Surprisingly, adding more AMPA receptors to excitatory contacts had little effect on synaptic strength. Instead, we observed increased excitatory input through the apparent addition of new functional sites. Our data support a model where adding AMPA receptors is sufficient to activate synapses that had few receptors to begin with, but that additional remodeling events are required to strengthen established synapses. More broadly, this approach introduces the precise spatiotemporal control of optogenetics to the molecular control of synaptic function.

Adenosine A1 receptor antagonist rolofylline alleviates axonopathy caused by human Tau ΔK280

Accumulation of Tau is a characteristic hallmark of several neurodegenerative diseases but the mode of toxic action of Tau is poorly understood. Here, we show that the Tau protein is toxic due to its aggregation propensity, whereas phosphorylation and/or missorting is not sufficient to cause neuronal dysfunction. Aggregate-prone Tau accumulates, when expressed in vitro at near-endogenous levels, in axons as spindle-shaped grains. These axonal grains contain Tau that is folded in a pathological (MC-1) conformation. Proaggregant Tau induces a reduction of neuronal ATP, concomitant with loss of dendritic spines. Counterintuitively, axonal grains of Tau are not targeted for degradation and do not induce a molecular stress response. Proaggregant Tau causes neuronal and astrocytic hypoactivity and presynaptic dysfunction instead. Here, we show that the adenosine A₁ receptor antagonist rolofylline (KW-3902) is alleviating the presynaptic dysfunction and restores neuronal activity as well as dendritic spine levels in vitro. Oral administration of rolofylline for 2-wk to 14-mo-old proaggregant Tau transgenic mice restores the spatial memory deficits and normalizes the basic synaptic transmission. These findings make rolofylline an interesting candidate to combat the hypometabolism and neuronal dysfunction associated with Tau-induced neurodegenerative diseases.

Miro1 Regulates Activity-Driven Positioning of Mitochondria within Astrocytic Processes Apposed to Synapses to Regulate Intracellular Calcium Signaling

It is fast emerging that maintaining mitochondrial function is important for regulating astrocyte function, although the specific mechanisms that govern astrocyte mitochondrial trafficking and positioning remain poorly understood. The mitochondrial Rho-GTPase 1 protein (Miro1) regulates mitochondrial trafficking and detachment from the microtubule transport network to control activity-dependent mitochondrial positioning in neurons. However, whether Miro proteins are important for regulating signaling-dependent mitochondrial dynamics in astrocytic processes remains unclear. Using live-cell confocal microscopy of rat organotypic hippocampal slices, we find that enhancing neuronal activity induces transient mitochondrial remodeling in astrocytes, with a concomitant, transient reduction in mitochondrial trafficking, mediated by elevations in intracellular Ca(2+). Stimulating neuronal activity also induced mitochondrial confinement within astrocytic processes in close proximity to synapses. Furthermore, we show that the Ca(2+)-sensing EF-hand domains of Miro1 are important for regulating mitochondrial trafficking in astrocytes and required for activity-driven mitochondrial confinement near synapses. Additionally, activity-dependent mitochondrial positioning by Miro1 reciprocally regulates the levels of intracellular Ca(2+) in astrocytic processes. Thus, the regulation of intracellular Ca(2+) signaling, dependent on Miro1-mediated mitochondrial positioning, could have important consequences for astrocyte Ca(2+) wave propagation, gliotransmission, and ultimately neuronal function.

Inhibitory Synapses Are Repeatedly Assembled and Removed at Persistent Sites In Vivo

Older concepts of a hard-wired adult brain have been overturned in recent years by in vivo imaging studies revealing synaptic remodeling, now thought to mediate rearrangements in microcircuit connectivity. Using three-color labeling and spectrally resolved two-photon microscopy, we monitor in parallel the daily structural dynamics (assembly or removal) of excitatory and inhibitory postsynaptic sites on the same neurons in mouse visual cortex in vivo. We find that dynamic inhibitory synapses often disappear and reappear again in the same location. The starkest contrast between excitatory and inhibitory synapse dynamics is on dually innervated spines, where inhibitory synapses frequently recur while excitatory synapses are stable. Monocular deprivation, a model of sensory input-dependent plasticity, shortens inhibitory synapse lifetimes and lengthens intervals to recurrence, resulting in a new dynamic state with reduced inhibitory synaptic presence. Reversible structural dynamics indicate a fundamentally new role for inhibitory synaptic remodeling--flexible, input-specific modulation of stable excitatory connections.

Using a Handheld Gene Gun for Genetic Transformation of Tetrahymena thermophila

This chapter describes protocols for using a handheld gene gun to deliver transformation vectors for overexpression of genes or gene replacement in the macronucleus of Tetrahymena thermophila. The protocols provide helpful information for preparing Tetrahymena for biolistic bombardment, preparation of vector-coated microcarriers, and basic gene gun operating procedures.

Non-Ionotropic NMDA Receptor Signaling Drives Activity-Induced Dendritic Spine Shrinkage

The elimination of dendritic spine synapses is a critical step in the refinement of neuronal circuits during development of the cerebral cortex. Several studies have shown that activity-induced shrinkage and retraction of dendritic spines depend on activation of the NMDA-type glutamate receptor (NMDAR), which leads to influx of extracellular calcium ions and activation of calcium-dependent phosphatases that modify regulators of the spine cytoskeleton, suggesting that influx of extracellular calcium ions drives spine shrinkage. Intriguingly, a recent report revealed a novel non-ionotropic function of the NMDAR in the regulation of synaptic strength, which relies on glutamate binding but is independent of ion flux through the receptor (Nabavi et al., 2013). Here, we tested whether non-ionotropic NMDAR signaling could also play a role in driving structural plasticity of dendritic spines. Using two-photon glutamate uncaging and time-lapse imaging of rat hippocampal CA1 neurons, we show that low-frequency glutamatergic stimulation results in shrinkage of dendritic spines even in the presence of the NMDAR d-serine/glycine binding site antagonist 7-chlorokynurenic acid (7CK), which fully blocks NMDAR-mediated currents and Ca(2+) transients. Notably, application of 7CK or MK-801 also converts spine enlargement resulting from a high-frequency uncaging stimulus into spine shrinkage, demonstrating that strong Ca(2+) influx through the NMDAR normally overcomes a non-ionotropic shrinkage signal to drive spine growth. Our results support a model in which NMDAR signaling, independent of ion flux, drives structural shrinkage at spiny synapses.

SIGNIFICANCE STATEMENT

Dendritic spine elimination is vital for the refinement of neural circuits during development and has been linked to improvements in behavioral performance in the adult. Spine shrinkage and elimination have been widely accepted to depend on Ca(2+) influx through NMDA-type glutamate receptors (NMDARs) in conjunction with long-term depression (LTD) of synaptic strength. Here, we use two-photon glutamate uncaging and time-lapse imaging to show that non-ionotropic NMDAR signaling can drive shrinkage of dendritic spines, independent of NMDAR-mediated Ca(2+) influx. Signaling through p38 MAPK was required for this activity-dependent spine shrinkage. Our results provide fundamental new insights into the signaling mechanisms that support experience-dependent changes in brain structure.

Optimized real-time monitoring of glutathione redox status in single pyramidal neurons in organotypic hippocampal slices during oxygen-glucose deprivation and reperfusion

A redox-sensitive Grx1-roGFP2 fusion protein was introduced by transfection into single pyramidal neurons in the CA1 subfield of organotypic hippocampal slice cultures (OHSCs). We assessed changes in the GSH system in neuronal cytoplasm and mitochondria during oxygen-glucose deprivation and reperfusion (OGD/RP), an in vitro model of stroke. Pyramidal cells in a narrow range of depths below the surface of the OHSC were transfected by gene gun or single-cell electroporation with cyto- or mito-Grx1-roGFP2. To mimic the conditions of acute stroke, we developed an optimized superfusion system with the capability of rapid and reproducible exchange of the solution bathing the OHSCs. Measurements of pO2 as a function of tissue depth show that in the region containing the transfected cells, the pO2 is well-controlled. We also found that the pO2 changes on the same time scale as changes in intracranial pressure, cerebral blood flow, and pO2 during acute stroke. Determining the reduction potential, EGSH, from the ratiometric fluorescence signal requires an absolute intensity measurement during calibration of the Grx1-roGFP2. Using the signal from cotransfected tdTomato as an internal standard during calibration improves quantitative measurements of Grx1-roGFP2 redox status and allows EGSH to be determined. EGSH becomes more reducing during OGD and more oxidizing during RP in mitochondria while changes in cytoplasm are not significant compared with controls.

RoR2 functions as a noncanonical Wnt receptor that regulates NMDAR-mediated synaptic transmission

Wnt signaling has a well-established role as a regulator of nervous system development, but its role in the maintenance and regulation of established synapses in the mature brain remains poorly understood. At excitatory glutamatergic synapses, NMDA receptors (NMDARs) have a fundamental role in synaptogenesis, synaptic plasticity, and learning and memory; however, it is not known what controls their number and subunit composition. Here we show that the receptor tyrosine kinase-like orphan receptor 2 (RoR2) functions as a Wnt receptor required to maintain basal NMDAR-mediated synaptic transmission. In addition, RoR2 activation by a noncanonical Wnt ligand activates PKC and JNK and acutely enhances NMDAR synaptic responses. Regulation of a key component of glutamatergic synapses through RoR2 provides a mechanism for Wnt signaling to modulate synaptic transmission, synaptic plasticity, and brain function acutely beyond embryonic development.

Regioselective biolistic targeting in organotypic brain slices using a modified gene gun

Transfection of DNA has been invaluable for biological sciences and with recent advances to organotypic brain slice preparations, the effect of various heterologous genes could thus be investigated easily while maintaining many aspects of in vivo biology. There has been increasing interest to transfect terminally differentiated neurons for which conventional transfection methods have been fraught with difficulties such as low yields and significant losses in viability. Biolistic transfection can circumvent many of these difficulties yet only recently has this technique been modified so that it is amenable for use in mammalian tissues.

New modifications to the accelerator chamber have enhanced the gene gun's firing accuracy and increased its depths of penetration while also allowing the use of lower gas pressure (50 psi) without loss of transfection efficiency as well as permitting a focused regioselective spread of the particles to within 3 mm. In addition, this technique is straight forward and faster to perform than tedious microinjections. Both transient and stable expression are possible with nanoparticle bombardment where episomal expression can be detected within 24 hr and the cell survival was shown to be better than, or at least equal to, conventional methods. This technique has however one crucial advantage: it permits the transfection to be localized within a single restrained radius thus enabling the user to anatomically isolate the heterologous gene's effects. Here we present an in-depth protocol to prepare viable adult organotypic slices and submit them to regioselective transfection using an improved gene gun.

Activity-dependent regulation of dendritic growth and maintenance by glycogen synthase kinase 3β

Activity-dependent dendritic development represents a crucial step in brain development, but its underlying mechanisms remain to be fully elucidated. Here we report that glycogen synthase kinase 3β (GSK3β) regulates dendritic development in an activity-dependent manner. We find that GSK3β in somatodendritic compartments of hippocampal neurons becomes highly phosphorylated at serine-9 upon synaptogenesis. This phosphorylation-dependent GSK3β inhibition is mediated by neurotrophin signalling and is required for dendritic growth and arbourization. Elevation of GSK3β activity leads to marked shrinkage of dendrites, whereas its inhibition enhances dendritic growth. We further show that these effects are mediated by GSK3β regulation of surface GABAA receptor levels via the scaffold protein gephyrin. GSK3β activation leads to gephyrin phosphorylation to reduce surface GABAA receptor clusters, resulting in neuronal hyperexcitability that causes dendrite shrinkage. These findings thus identify GSK3β as a key player in activity-dependent regulation of dendritic development by targeting the excitatory-inhibitory balance of the neuron.

Transient gene expression in tobacco using Gibson assembly and the Gene Gun

In order to target a single protein to multiple subcellular organelles, plants typically duplicate the relevant genes, and express each gene separately using complex regulatory strategies including differential promoters and/or signal sequences. Metabolic engineers and synthetic biologists interested in targeting enzymes to a particular organelle are faced with a challenge: For a protein that is to be localized to more than one organelle, the engineer must clone the same gene multiple times. This work presents a solution to this strategy: harnessing alternative splicing of mRNA. This technology takes advantage of established chloroplast and peroxisome targeting sequences and combines them into a single mRNA that is alternatively spliced. Some splice variants are sent to the chloroplast, some to the peroxisome, and some to the cytosol. Here the system is designed for multiple-organelle targeting with alternative splicing. In this work, GFP was expected to be expressed in the chloroplast, cytosol, and peroxisome by a series of rationally designed 5' mRNA tags. These tags have the potential to reduce the amount of cloning required when heterologous genes need to be expressed in multiple subcellular organelles. The constructs were designed in previous work(11), and were cloned using Gibson assembly, a ligation independent cloning method that does not require restriction enzymes. The resultant plasmids were introduced into Nicotiana benthamiana epidermal leaf cells with a modified Gene Gun protocol. Finally, transformed leaves were observed with confocal microscopy.

A novel mechanism for nicotinic potentiation of glutamatergic synapses

Selective strengthening of specific glutamatergic synapses in the mammalian hippocampus is critical for encoding new memories. This is most commonly achieved by input-specific Hebbian-type plasticity involving glutamate-dependent coincident presynaptic and postsynaptic depolarization. Our results demonstrate a novel mechanism by which nicotinic signaling, independently of coincident fast glutamatergic transmission, increases synaptic strength in the hippocampus. Electrophysiological recordings from rat hippocampal neurons in culture revealed that 1-3 h of exposure to 1 μm nicotine, even with action potentials being blocked, produced increases in both the frequency and amplitude of miniature EPSCs. Possible mechanisms were analyzed both in mouse organotypic slice culture and in rat cell culture by inducing the cells to express super-ecliptic pHluorin-tagged GluA1-containing AMPA receptors, which fluoresce only on the cell surface. Pharmacological and genetic manipulation of the cells, in combination with fluorescence-recovery-after-photobleaching experiments, revealed that nicotine, acting through α7-containing nicotinic acetylcholine receptors on the postsynaptic neuron, induces the stabilization and accumulation of GluA1-containing AMPA receptors on dendritic spines. The process relies on intracellular calcium signaling, PDZ [postsynaptic density-95 (PSD-95)/Discs large (Dlg)/zona occludens-1 (ZO-1)] interactions with members of the PSD-95 family, and lateral diffusion of the GluA1 receptors on the cell surface. These findings define a new avenue by which nicotinic signaling modulates synaptic mechanisms thought to subserve learning and memory.

Targeting a heterologous protein to multiple plant organelles via rationally designed 5' mRNA tags

Background

Plant bioengineers require simple genetic devices for predictable localization of heterologous proteins to multiple subcellular compartments.

Results

We designed novel hybrid signal sequences for multiple-compartment localization and characterize their function when fused to GFP in Nicotiana benthamiana leaf tissue. TriTag-1 and TriTag-2 use alternative splicing to generate differentially localized GFP isoforms, localizing it to the chloroplasts, peroxisomes and cytosol. TriTag-1 shows a bias for targeting the chloroplast envelope while TriTag-2 preferentially targets the peroxisomes. TriTag-3 embeds a conserved peroxisomal targeting signal within a chloroplast transit peptide, directing GFP to the chloroplasts and peroxisomes.

Conclusions

Our novel signal sequences can reduce the number of cloning steps and the amount of genetic material required to target a heterologous protein to multiple locations in plant cells. This work harnesses alternative splicing and signal embedding for engineering plants to express multi-functional proteins from single genetic constructs.

LTP-induced long-term stabilization of individual nascent dendritic spines

Learning new tasks has been associated with increased growth and stabilization of new dendritic spines. We examined whether long-term potentiation (LTP), a key cellular mechanism thought to underlie learning, plays a role in selective stabilization of individual new spines during circuit plasticity. Using two-photon glutamate uncaging, we stimulated nascent spines on dendrites of rat hippocampal CA1 neurons with patterns that induce LTP and then monitored spine survival rates using time-lapse imaging. Remarkably, we found that LTP-inducing stimuli increased the long-term survivorship (>14 h) of individual new spines. Activity-induced new spine stabilization required NMDA receptor activation and was specific for stimuli that induced LTP. Moreover, abrogating CaMKII binding to the NMDA receptor abolished activity-induced new spine stabilization. Our findings demonstrate for the first time that, in addition to enhancing the efficacy of preexisting synapses, LTP-inducing stimuli promote the transition of nascent spines from a short-lived, transient state to a longer-lived, persistent state.

Synapse-specific and size-dependent mechanisms of spine structural plasticity accompanying synaptic weakening

Refinement of neural circuits in the mammalian cerebral cortex shapes brain function during development and in the adult. However, the signaling mechanisms underlying the synapse-specific shrinkage and loss of spiny synapses when neural circuits are remodeled remain poorly defined. Here, we show that low-frequency glutamatergic activity at individual dendritic spines leads to synapse-specific synaptic weakening and spine shrinkage on CA1 neurons in the hippocampus. We found that shrinkage of individual spines in response to low-frequency glutamate uncaging is saturable, reversible, and requires NMDA receptor activation. Notably, shrinkage of large spines additionally requires signaling through metabotropic glutamate receptors (mGluRs) and inositol 1,4,5-trisphosphate receptors (IP(3)Rs), supported by higher levels of mGluR signaling activity in large spines. Our results support a model in which signaling through both NMDA receptors and mGluRs is required to drive activity-dependent synaptic weakening and spine shrinkage at large, mature dendritic spines when neural circuits undergo experience-dependent modification.

Palmitoylation at two cysteine clusters on the C-terminus of GluN2A and GluN2B differentially control synaptic targeting of NMDA receptors

Palmitoylation of NMDARs occurs at two distinct cysteine clusters in the carboxyl-terminus of GluN2A and GluN2B subunits that differentially regulates retention in the Golgi apparatus and surface expression of NMDARs. Mutations of palmitoylatable cysteine residues in the membrane-proximal cluster to non-palmitoylatable serines leads to a reduction in the surface expression of recombinant NMDARs via enhanced internalization of the receptors. Mutations in a cluster of cysteines in the middle of the carboxyl-terminus of GluN2A and GluN2B, leads to an increase in the surface expression of NMDARs via an increase in post-Golgi trafficking. Using a quantitative electrophysiological assay, we investigated whether palmitoylation of GluN2 subunits and the differential regulation of surface expression affect functional synaptic incorporation of NMDARs. We show that a reduction in surface expression due to mutations in the membrane-proximal cluster translates to a reduction in synaptic expression of NMDARs. However, increased surface expression induced by mutations in the cluster of cysteines that regulates post-Golgi trafficking of NMDARs does not increase the synaptic pool of NMDA receptors, indicating that the number of synaptic receptors is tightly regulated.

Activity-dependent growth of new dendritic spines is regulated by the proteasome

Growth of new dendritic spines contributes to experience-dependent circuit plasticity in the cerebral cortex. Yet the signaling mechanisms leading to new spine outgrowth remain poorly defined. Increasing evidence supports that the proteasome is an important mediator of activity-dependent neuronal signaling. We therefore tested the role of the proteasome in activity-dependent spinogenesis. Using pharmacological manipulations, glutamate uncaging, and two-photon imaging of GFP-transfected hippocampal pyramidal neurons, we demonstrate that acute inhibition of the proteasome blocks activity-induced spine outgrowth. Remarkably, mutation of serine 120 to alanine of the Rpt6 proteasomal subunit in individual neurons was sufficient to block activity-induced spine outgrowth. Signaling through NMDA receptors and CaMKII, but not PKA, is required to facilitate spine outgrowth. Moreover, abrogating CaMKII binding to the NMDA receptor abolished activity-induced spinogenesis. Our data support a model in which neural activity facilitates spine outgrowth via an NMDA receptor- and CaMKII-dependent increase in local proteasomal degradation.

Imaging synaptic protein dynamics using photoactivatable green fluorescent protein

Considerable evidence has accumulated that structural changes in dendritic spines and their synapses are associated with adaptive functional changes in cortical circuits, such as during circuit refinement in young animals and in learning and memory in adults. Understanding the mechanisms of circuit plasticity requires detailed investigation of the structural dynamics of dendritic spines and how they are regulated by neural activity and sensory experience. Studying the dynamic localization of synaptic proteins in dendritic spines and how their stabilization and exchange rates influence spine structural plasticity is also important. This protocol describes imaging approaches to study synaptic protein dynamics in dendritic spines of the rodent cerebral cortex. It gives a strategy for generating photoactivatable green fluorescent protein (PA-GFP)-tagged synaptic proteins and in vitro and in vivo transfection methods for coexpression of these proteins with a spectrally separable cell-filling marker (DsRed-Express). Methods for tracking synaptic protein localization using photoactivation and time-lapse imaging of PA-GFP in spiny pyramidal neuron dendrites are given. A discussion of imaging hardware and software preferences is also included. The methods described here can be used to study the dynamic processes underlying spine synapse development during the formation and plasticity of neural circuits in the mammalian brain.

Dissociated cell culture for testing effects of carbon nanotubes on neuronal growth

Cell culture has emerged as an important research method for studying the effects of carbon nanotubes (CNTs) on primary neurons. We describe the procedure for preparation of dissociated mixed cell culture from postnatal rat hippocampi. Based on morphological criteria and specific neuronal cell markers, neurons can be selected within this mixed cell culture and studied. We present the procedure for the assessment of neuronal cell morphology based on intracellular fluorescence of the vital dye calcein that accumulates in live neurons. This procedure encompasses fluorescence imaging and measurement of the following parameters: neurite number, total neurite length, mean neurite length, number of growth cones, number of branches, and number of branches per neurite. These combined cell culture and fluorescence microscopy approaches can be successfully used for assessment of the effects that CNTs, as water-soluble agents, have on neuronal cell growth and neurite outgrowth.

Loss of PSD-95 enrichment is not a prerequisite for spine retraction

Changes in neuronal structure are thought to underlie long-term behavioral modifications associated with learning and memory. In particular, considerable evidence implicates the destabilization and retraction of dendritic spines along with the loss of spine synapses as an important cellular mechanism for refining brain circuits, yet the molecular mechanisms regulating spine elimination remain ill-defined. The postsynaptic density protein, PSD-95, is highly enriched in dendritic spines and has been associated with spine stability. Because spines with low levels of PSD-95 are more dynamic, and the recruitment of PSD-95 to nascent spines has been associated with spine stabilization, we hypothesized that loss of PSD-95 enrichment would be a prerequisite for spine retraction. To test this hypothesis, we used dual-color time-lapse two-photon microscopy to monitor rat hippocampal pyramidal neurons cotransfected with PSD-95-GFP and DsRed-Express, and we analyzed the relationship between PSD-95-GFP enrichment and spine morphological changes. Consistent with our hypothesis, we found that the majority of spines that retracted were relatively unenriched for PSD-95-GFP. However, in the subset of PSD-95-GFP-enriched spines that retracted, spine shrinkage and loss of PSD-95-GFP were tightly coupled, suggesting that loss of PSD-95-GFP enrichment did not precede spine retraction. Moreover, we found that, in some instances, spine retraction resulted in a significant enrichment of PSD-95-GFP on the dendritic shaft. Our data support a model of spine retraction in which loss of PSD-95 enrichment is not required prior to the destabilization of spines.

Regulation of NMDA-receptor synaptic transmission by Wnt signaling

Wnt ligands are secreted glycoproteins controlling gene expression and cytoskeleton reorganization involved in embryonic development of the nervous system. However, their role in later stages of brain development, particularly in the regulation of established synaptic connections, is not known. We found that Wnt-5a acutely and specifically upregulates synaptic NMDAR currents in rat hippocampal slices, facilitating induction of long-term potentiation, a cellular model of learning and memory. This effect requires an increase in postsynaptic Ca(2+) and activation of noncanonical downstream effectors of the Wnt signaling pathway. In contrast, Wnt-7a, an activator of the canonical Wnt signaling pathway, has no effect on NMDAR-mediated synaptic transmission. Moreover, endogenous Wnt ligands are necessary to maintain basal NMDAR synaptic transmission, adjusting the threshold for synaptic potentiation. This novel role for Wnt ligands provides a mechanism for Wnt signaling to acutely modulate synaptic plasticity and brain function in later stages of development and in the mature organism.

Molecular determinants controlling NMDA receptor synaptic incorporation

Synaptic incorporation of NMDA receptors (NMDARs) is regulated by GluN2 subunits with different rules controlling GluN2A- and GluN2B-containing receptors; whereas GluN2B-containing receptors are constitutively incorporated into synapses, GluN2A incorporation is activity-dependent. We expressed electrophysiologically tagged NMDARs in rat hippocampal slices to identify the molecular determinants controlling the mode of synaptic incorporation of NMDARs. Expressing chimeric GluN2 subunits, we identified a putative N-glycosylation site present in GluN2B, but not in GluN2A, as necessary and sufficient to drive NMDARs into synapses in an activity-independent manner. This suggests a novel mechanism for regulating activity-driven changes and trafficking of NMDARs to the synapse.

Whole cell recording from an organotypic slice preparation of neocortex

We have been studying the expression and functional roles of voltage-gated potassium channels in pyramidal neurons from rat neocortex. Because of the lack of specific pharmacological agents for these channels, we have taken a genetic approach to manipulating channel expression. We use an organotypic culture preparation (16) in order to maintain cell morphology and the laminar pattern of cortex. We typically isolate acute neocortical slices at postnatal days 8-10 and maintain the slices in culture for 3-7 days. This allows us to study neurons at a similar age to those in our work with acute slices and minimizes the development of exuberant excitatory connections in the slice. We record from visually-identified pyramidal neurons in layers II/III or V using infrared illumination (IR-) and differential interference contrast microscopy (DIC) with whole cell patch clamp in current- or voltage-clamp. We use biolistic (Gene gun) transfection of wild type or mutant potassium channel DNA to manipulate expression of the channels to study their function. The transfected cells are easily identified by epifluorescence microscopy after co-transfection with cDNA for green fluorescent protein (GFP). We compare recordings of transfected cells to adjacent, untransfected neurons in the same layer from the same slice.

Organotypic hippocampal slice cultures

The hippocampus, a component of the limbic system, plays important roles in long-term memory and spatial navigation. Hippocampal neurons can modify the strength of their connections after brief periods of strong activation. This phenomenon, known as long-term potentiation (LTP) can last for hours or days and has become the best candidate mechanism for learning and memory. In addition, the well defined anatomy and connectivity of the hippocampus has made it a classical model system to study synaptic transmission and synaptic plasticity. As our understanding of the physiology of hippocampal synapses grew and molecular players became identified, a need to manipulate synaptic proteins became imperative. Organotypic hippocampal cultures offer the possibility for easy gene manipulation and precise pharmacological intervention but maintain synaptic organization that is critical to understanding synapse function in a more naturalistic context than routine culture dissociated neurons methods. Here we present a method to prepare and culture hippocampal slices that can be easily adapted to other brain regions. This method allows easy access to the slices for genetic manipulation using different approaches like viral infection or biolistics. In addition, slices can be easily recovered for biochemical assays, or transferred to microscopes for imaging or electrophysiological experiments.