Rapid actin-based plasticity in dendritic spines

Induction of spine growth and synapse formation by regulation of the spine actin cytoskeleton

We explored the relationship between regulation of the spine actin cytoskeleton, spine morphogenesis, and synapse formation by manipulating expression of the actin binding protein NrbI and its deletion mutants. In pyramidal neurons of cultured rat hippocampal slices, NrbI is concentrated in dendritic spines by binding to the actin cytoskeleton. Expression of one NrbI deletion mutant, containing the actin binding domain, dramatically increased the density and length of dendritic spines with synapses. This hyperspinogenesis was accompanied by enhanced actin polymerization and spine motility. Synaptic strengths were reduced to compensate for extra synapses, keeping total synaptic input per neuron constant. Our data support a model in which synapse formation is promoted by actin-powered motility.

Glutamate transporter cluster formation in astrocytic processes regulates glutamate uptake activity

Glutamate is the predominant excitatory neurotransmitter in the CNS, and it is removed from the synaptic cleft by sodium-dependent glutamate transport activity. Glutamate transporter-1 (GLT-1) is expressed predominantly in astroglial cells and is responsible for the largest proportion of glutamate transport in the adult forebrain. In the present study, we demonstrate the ability of endogenous and recombinant GLT-1 to form clusters in astrocytic processes and characterize the mobility and physiological importance of these clusters in the regulation of GLT-1 activity in the presence or absence of neurons. At the distal end of C6 glioma cell processes, GLT-1 clusters undergo rapid morphological changes in both shape and size, and these changes are inhibited by cytochalasin D treatment, suggesting that the morphogenesis of GLT-1 clusters is highly dependent on the actin network. Treatment of astrocytes with phorbol 12-myristate 13-acetate (PMA) quickly and preferentially decreases GLT-1 localization on the process membrane, leading to de novo generation of GLT-1 clusters along the process shaft. Pretreatment with the PKC inhibitor bisindolylmaleimide II (Bis II), with sucrose (0.4 m), or through the expression of a dominant-negative form of dynamin prevents PMA-induced GLT-1 internalization and cluster formation. In terms of glutamate transporter function, PMA treatment elicits a significant decrease in GLT-1 activity that is prevented by preexposure to either Bis II or hypertonic treatment. Together, these data indicate that GLT-1 trafficking and cluster formation in glial cell processes are dynamic events that play important roles in regulating glutamate uptake in astrocytes and glioma cells.

Cadherin activity is required for activity-induced spine remodeling

Neural activity induces the remodeling of pre- and postsynaptic membranes, which maintain their apposition through cell adhesion molecules. Among them, N-cadherin is redistributed, undergoes activity-dependent conformational changes, and is required for synaptic plasticity. Here, we show that depolarization induces the enlargement of the width of spine head, and that cadherin activity is essential for this synaptic rearrangement. Dendritic spines visualized with green fluorescent protein in hippocampal neurons showed an expansion by the activation of AMPA receptor, so that the synaptic apposition zone may be expanded. N-cadherin-venus fusion protein laterally dispersed along the expanding spine head. Overexpression of dominant-negative forms of N-cadherin resulted in the abrogation of the spine expansion. Inhibition of actin polymerization with cytochalasin D abolished the spine expansion. Together, our data suggest that cadherin-based adhesion machinery coupled with the actin-cytoskeleton is critical for the remodeling of synaptic apposition zone.

Transient expansion of synaptically connected dendritic spines upon induction of hippocampal long-term potentiation

Dendritic spines are small protrusions from dendritic shafts that contain the postsynaptic sites of glutamatergic synapses in the brain. Spines undergo dramatic activity-dependent structural changes that are particularly prominent during neuronal development. Although changes in spine shape or number have been proposed to contribute to forms of synaptic plasticity that underlie learning and memory, the extent to which spines remain plastic in the adult brain is unclear. We find that induction of long-term potentiation (LTP) of synaptic transmission in acute hippocampal slices of adult mice evokes a reliable, transient expansion in spines that are synaptically activated, as determined with calcium imaging. Similar to LTP, transient spine expansion requires N-methyl-D-aspartate (NMDA) receptor-mediated Ca2+ influx and actin polymerization. Moreover, like the early phase of LTP induced by the stimulation protocol, spine expansion does not require Ca2+ influx through L-type voltage-gated Ca2+ channels nor does it require protein synthesis. Thus, transient spine expansion is a characteristic feature of the initial phases of plasticity at mature synapses and so may contribute to synapse remodeling important for LTP.

Dynamic regulation of spine-dendrite coupling in cultured hippocampal neurons

We investigated the role of dendritic spine morphology in spine-dendrite calcium communication using novel experimental and theoretical approaches. A transient rise in [Ca2+]i was produced in individual spine heads of Fluo-4-loaded cultured hippocampal neurons by flash photolysis of caged calcium. Following flash photolysis in the spine head, a delayed [Ca2+]i transient was detected in the parent dendrites of only short, but not long, spines. Delayed elevated fluorescence in the dendrite of the short spines was also seen with a membrane-bound fluorophore and fluorescence recovery from bleaching of a calcium-bound fluorophore had a much slower kinetics, indicating that the dendritic fluorescence change reflects a genuine diffusion of free [Ca2+]i from the spine head to the parent dendrite. Calcium diffusion between spine head and the parent dendrite was regulated by calcium stores as well as by a Na-Ca exchanger. Spine length varied with the recent history of the [Ca2+]i variations in the spine, such that small numbers of calcium transients resulted in elongation of spines whereas large numbers of calcium transients caused shrinkage of the spines. Consequently, spine elongation resulted in a complete isolation of the spine from the dendrite, while shrinkage caused an enhanced coupling with the parent dendrite. These studies highlight a dynamically regulated coupling between a dendritic spine head and its parent dendrite.

Inhibition of Rho-Kinase in the Brainstem Augments Baroreflex Control of Heart Rate in Rats

The Rho/Rho-kinase pathway in the nucleus tractus solitarii (NTS) of the brain stem contributes to blood pressure regulation. Activation of this pathway might be involved in the central nervous system mechanisms of hypertension. The aim of the present study was to determine whether baroreflex control of heart rate is altered by inhibition of Rho-kinase in the NTS. Adenovirus vectors encoding dominant-negative Rho-kinase or beta-galactosidase were transfected into the nucleus tractus solitarii of Wistar Kyoto rats (WKY) and spontaneously hypertensive rats (SHR). Baroreflex control of heart rate was examined by changing arterial pressure with an intravenous infusion of phenylephrine or sodium nitroprusside. The maximum gain of baroreflex control of heart rate was attenuated in SHR compared with WKY before the gene transfer. Transfection of adenovirus vectors encoding dominant-negative Rho-kinase significantly augmented the maximum gain in both WKY and SHR. The extent of this augmentation, however, was greater in SHR than in WKY. After treatment with metoprolol, the maximum gain was significantly decreased in rats transfected with adenovirus vectors encoding dominant-negative Rho-kinase, but not in nontransfected rats. In contrast, after treatment with atropine, the maximum gain was greater in rats transfected with adenovirus vectors encoding dominant-negative Rho-kinase compared with nontransfected rats, although it was decreased in both groups. These results suggest that inhibition of Rho-kinase in the NTS augments baroreflex control of heart rate, in both WKY and SHR, probably because of a cardiac sympathoinhibitory effect.

Chemically controlled formation of a DNA/calcium phosphate coprecipitate: Application for transfection of mature hippocampal neurons

Numerous methods exist for transfecting postmitotic neurons, for example, DNA/calcium phosphate coprecipitation, cationic lipids, viruses, and physical methods such as microinjection, electroporation, and biolistics. Most methods, however, are either toxic to the cell, yield only poor transfection efficiencies, or cells have to be electroporated before plating. In this article, we present a standardized and fast transfection method using DNA/calcium phosphate coprecipitates that efficiently transfer DNA into mature, postmitotic hippocampal neurons. Shifting to CO(2)-independent media with a well-defined pH allows for the tight control of the coprecipitate formation and for adjusting the transfection parameters for the individual DNA plasmid used. The two critical parameters for reproducible and efficient transfections are: the precise pH during crystal formation, and the incubation time of the cells with the coprecipitate. This improved procedure now enables biochemical approaches. By transfecting a dominant-positive Ras mutant, we activate the Erk/MAP kinase signal transduction pathway. Furthermore, using a siRNA plasmid directed against MAP2, the level of an endogenously expressed protein is down-regulated upon transfection. These two approaches demonstrate that the presented transient transfection method can now be used to address questions on a biochemical level in hippocampal neurons.

Rapid and persistent modulation of actin dynamics regulates postsynaptic reorganization underlying bidirectional plasticity

The synapse is a highly organized cellular specialization whose structure and composition are reorganized, both positively and negatively, depending on the strength of input signals. The mechanisms orchestrating these changes are not well understood. A plausible locus for the reorganization of synapse components and structure is actin, because it serves as both cytoskeleton and scaffold for synapses and exists in a dynamic equilibrium between F-actin and G-actin that is modulated bidirectionally by cellular signaling. Using a new FRET-based imaging technique to monitor F-actin/G-actin equilibrium, we show here that tetanic stimulation causes a rapid, persistent shift of actin equilibrium toward F-actin in the dendritic spines of rat hippocampal neurons. This enlarges the spines and increases postsynaptic binding capacity. In contrast, prolonged low-frequency stimulation shifts the equilibrium toward G-actin, resulting in a loss of postsynaptic actin and of structure. This bidirectional regulation of actin is actively involved in protein assembly and disassembly and provides a substrate for bidirectional synaptic plasticity.

Linking the synapse to the cytoskeleton: a breath-taking role for microfilaments

Cytoskeletal elements, in particular microtubules and microfilaments, are essential players in a large variety of phenomena requiring cellular and intracellular motility. To name but a few, they are intimately involved in determining cell shape and adhesion, establishment and maintenance of polarity, locomotion and organelle transport in all eukaryotic cells, including neurons. Here, we would like to focus on the synapse in the vertebrate central nervous system, proposing a model for a specific dialogue between neuronal microfilaments and other protein components in neurotransmission and synaptic plasticity.

Regulation of dendritic spine motility and stability by Rac1 and Rho kinase: evidence for two forms of spine motility

Dendritic spines are major sites of excitatory synapses in the brain and display rapid motility, which is believed to be important for synapse formation and plasticity. Spine morphology was previously shown to be regulated by the Rho GTPases Rac1 and RhoA. Here, we analyzed the roles of Rac1 and a downstream effector of RhoA, Rho kinase, in controlling spine morphogenesis and their effects on spine motility and stability. Blockade of Rac1 induced long, thin spines and inhibited spine head growth, morphing, and stability. Spine head growth was more severely affected in mature spines. On the other hand, inhibition of Rho kinase induced new, long spines and protrusive motility. These data demonstrate that Rac1 and RhoA/Rho kinase pathways regulate different aspects of spine morphology, motility, and stability and presumably also different aspects of synaptic functions. Moreover, our data show that there are two different types of spine motility: protrusive motility and head morphing, which are differentially regulated by Rac1 and Rho kinase. We propose that these two different types of spine motility serve different functions in synaptogenesis and synapse maturation.

The role of protein phosphatase-1 in the modulation of synaptic and structural plasticity

Synaptic plasticity is a phenomenon contributing to changes in the efficacy of neuronal transmission. These changes are widely believed to be a major cellular basis for learning and memory. Protein phosphorylation is a key biochemical process involved in synaptic plasticity that operates through a tight balance between the action of protein kinases and protein phosphatases (PPs). Although the majority of research in this field has concentrated primarily on protein kinases, the significant role of PPs is becoming increasingly apparent. This review examines one such phosphatase, PP1, and highlights recent advances in the understanding of its intervention in synaptic and structural plasticity and the mechanisms of learning and memory.

F-actin at identified synapses in the mushroom body neuropil of the insect brain

The distribution of f-actin stained by fluorescent phalloidin was investigated in the brain of several insect species, with a special focus on the mushroom body. For localizing f-actin in identified neurons and at synapses, additional staining with fluorescent dextrans and anti-synapsin I immunostaining was employed. Intense f-actin staining was consistently found in synaptic complexes of the mushroom body calyces (calycal microglomeruli [MG]). These MG contain a central core of presynaptic boutons, predominantly belonging to deutocerebral cholinergic excitatory projection neurons, which are surrounded by a shell of numerous Kenyon cell (KC) dendritic tips. In the cricket Gryllus bimaculatus, high-resolution confocal laser scanning imaging revealed colocalization of f-actin with KC dendritic spine parts within MG. Although presynaptic boutons appear to be mainly devoid of f-actin-phalloidin fluorescence, there appears to be an accumulation of f-actin in KC dendritic spines synaptically contacting the boutons. Electron microscopy of boutons and dextran-stained KC dendrites revealed their pre- and postsynaptic sites, with KCs being strictly postsynaptic elements. Their subsynaptic membrane appositions are considered to be associated with f-actin. Focal accumulation of f-actin in the dendritic tips of KCs was found to be a general feature of MG, with either spheroidal or indented boutons of different sizes, as encountered in the mushroom bodies of the cricket, honey bee, ant, and fruit fly. The structural similarities of calycal MG and f-actin accumulation in KC dendrites with cerebellar microglomeruli are considered comparatively. The accumulation of f-actin in KC dendrites is discussed in view of mushroom body plasticity and its potential role in learning and memory formation.

Structural basis of long-term potentiation in single dendritic spines

Dendritic spines of pyramidal neurons in the cerebral cortex undergo activity-dependent structural remodelling that has been proposed to be a cellular basis of learning and memory. How structural remodelling supports synaptic plasticity, such as long-term potentiation, and whether such plasticity is input-specific at the level of the individual spine has remained unknown. We investigated the structural basis of long-term potentiation using two-photon photolysis of caged glutamate at single spines of hippocampal CA1 pyramidal neurons. Here we show that repetitive quantum-like photorelease (uncaging) of glutamate induces a rapid and selective enlargement of stimulated spines that is transient in large mushroom spines but persistent in small spines. Spine enlargement is associated with an increase in AMPA-receptor-mediated currents at the stimulated synapse and is dependent on NMDA receptors, calmodulin and actin polymerization. Long-lasting spine enlargement also requires Ca2+/calmodulin-dependent protein kinase II. Our results thus indicate that spines individually follow Hebb's postulate for learning. They further suggest that small spines are preferential sites for long-term potentiation induction, whereas large spines might represent physical traces of long-term memory.

Developmental regulation of spine and filopodial motility in primary visual cortex: reduced effects of activity and sensory deprivation

Dendritic protrusions are highly motile during postnatal development. Although spine morphological plasticity could be associated with synaptic plasticity, the function of rapid spine/filopodial motility is still unknown. To investigate the role of spine motility in the development of the visual cortex and its relation with critical periods, we used two-photon imaging of neurons from layers receiving visual input in developing mouse primary visual cortex and compared motility between control and visually deprived animals. Spine and filopodia motility was prominent during early synaptogenesis (P11-P13) but greatly decreased after P15. This "switch" was coincident with a 2.5-fold increase in protrusion density and spine formation. Spine motility was not regulated during the critical period for monocular deprivation (P19-P34). Moreover, delaying the critical period by dark rearing did not delay the normal developmental decrease of spine motility, but caused a modest further reduction in motility at P28-P35. Dark rearing and enucleation also mildly reduced spine motility before eye opening and dark rearing reduced the proportion of filopodia. We conclude that (1) rapid spine motility is not related to critical period plasticity, but is likely to play a role in early synaptogenesis, and (2) neuronal activity stimulates spine motility during synaptogenesis and promotes the appearance of dendritic filopodia.

Behavioral training interferes with the ability of gonadal hormones to increase CA1 spine synapse density in ovariectomized female rats

Estradiol benzoate (EB) has repeatedly been shown to increase hippocampal CA1 spine synapse density in ovariectomized female rats. Although this increase has been assumed to enhance memory, a direct link between increased spine synapse density and memory has not been demonstrated. Furthermore, while androgens, such as testosterone propionate (TP) and dihydrotestosterone (DHT), also increase spine synapse density in females, their effects on memory have yet to be investigated. In the present study, ovariectomized female rats were given two injections, 24 h apart, of sesame oil (control), 10 microg EB, 500 microg TP or 500 microg DHT. Forty-eight hours after the second injection, rats were tested in a 1-day spatial Morris water maze task and then immediately perfused for analysis of CA1 spine synapse density (using electron microscopy and unbiased stereology). In the spatial acquisition phase of testing, EB, but not TP or DHT, significantly impaired memory relative to controls. Hormone treatment did not affect spatial retention or performance in the non-spatial phase of testing. In contrast to previous work, spine synapse density was not increased by EB, TP or DHT. We therefore examined a new set of EB-treated females, only half of which were water maze tested. Consistent with previous work, EB significantly increased spine synapse density among behaviorally naïve females. In contrast, spine synapse densities did not differ among behaviorally tested control and EB females, although they were higher than behaviorally naïve controls. These data indicate that 1-day water maze testing can eliminate the hormone-induced increases in CA1 spine synapse density typically observed in behaviorally naïve females.

AMPA-receptor activation regulates the diffusion of a membrane marker in parallel with dendritic spine motility in the mouse hippocampus

Dendritic spines are the site of most excitatory connections in the hippocampus. We have investigated the diffusibility of a membrane-bound green fluorescent protein (mGFP) within the inner leaflet of the plasma membrane using Fluorescence Recovery After Photobleaching. In dendritic spines the diffusion of mGFP was significantly retarded relative to the dendritic shaft. In parallel, we have assessed the motility of dendritic spines, and found an inverse correlation between spine motility and the rate of diffusion of mGFP. We then tested the influence of glutamate receptor activation or blockade, and the involvement of the actin cytoskeleton (using latrunculin A) on spine motility and mGFP diffusion. These results show that glutamate receptors regulate the mobility of molecules in the inner leaflet of the plasma membrane through an action upon the actin cytoskeleton, suggesting a novel mechanism for the regulation of postsynaptic receptor density and composition.

Orchestration of synaptic plasticity through AKAP signaling complexes

Significant progress has been made toward understanding the mechanisms by which organisms learn from experiences and how those experiences are translated into memories. Advances in molecular, electrophysiological and genetic technologies have permitted great strides in identifying biochemical and structural changes that occur at synapses during processes that are thought to underlie learning and memory. Cellular events that generate the second messenger cyclic AMP (cAMP) and activate protein kinase A (PKA) have been linked to synaptic plasticity and long-term memory. In this review we will focus on the role of PKA in synaptic plasticity and discuss how the compartmentalization of PKA through its association with A-Kinase Anchoring Proteins (AKAPs) affect PKA function in this process.

Differential targeting of the L-type Ca2+ channel alpha1C (CaV1.2) to synaptic and extrasynaptic compartments in hippocampal neurons

In central nervous system neurons L-type Ca2+ channels are involved in developmental processes, the integration and conduction of postsynaptic electric activity and synaptic plasticity. However, little is known about the channel isoforms underlying each of these functions or about the exact localization and targeting properties of the major L-type channel isoform alpha1C (CaV1.2) in neurons. We addressed these questions using high-resolution immunofluorescence analysis of the endogenous alpha1C and epitope-tagged recombinant channel isoforms expressed in mouse hippocampal neurons. Endogenous alpha1C and surface-expressed hemagglutinin (HA)-tagged alpha1C-HA were localized in small clusters distributed between the axon initial segment and the apical branches of the dendritic tree. The average cluster size was estimated to be eight channels per alpha1C-HA cluster. Analysis of the subcellular localization of alpha1C-HA clusters relative to known synaptic markers suggested the existence of two distinct populations of alpha1C clusters, extrasynaptic and synaptic, the latter associated with glutamatergic synapses in dendritic spines. Both glutamatergic and GABAergic neurons expressed alpha1C in the soma and dendrites. In contrast to the N-type channel GFP-alpha1B, GFP-alpha1C was excluded from distal axons and nerve terminals of mature neurons. In developing neurons, however, alpha1C and alpha1C-HA were robustly expressed in the growth cone, indicating that specific targeting properties of neuronal compartments change during differentiation. Synaptic and extrasynaptic localizations of alpha1C correspond to putative roles of L-type Ca2+ currents in synaptic modulation and in the propagation of dendritic Ca2+ spikes, respectively. The transiently expressed alpha1C in the growth cone may be involved in neurite extension and axonal pathfinding.

Temporal dynamics of NMDA receptor-induced changes in spine morphology and AMPA receptor recruitment to spines

Recent studies have shown that the activation of NMDA receptors can induce rapid changes in dendritic morphology and synaptic recruitment of AMPA receptors in dendritic spines. Here, we analyze the time course of NMDA receptor-induced changes in dendrite morphology and recruitment of AMPA receptors to synapses in cultured neurons. Activation of NMDA receptors causes a rapid transient increase in the size of preexisting spines and then the gradual formation of new dendritic protrusions and spines. NMDA receptor activation also induced GFP-tagged AMPA receptors to cluster in dendrites and to be inserted into the surface of dendritic spines. These results indicate that NMDA receptor activation induces several phases of dendritic plasticity, initial expansion of dendritic spines, followed by the de novo formation of spines and AMPA receptor dendritic clustering and surface expression on spines. Each of these forms of plasticity may have significant effects on the efficacy of synaptic transmission.

Stability of dendritic spines and synaptic contacts is controlled by alpha N-catenin

Morphological plasticity of dendritic spines and synapses is thought to be crucial for their physiological functions. Here we show that alpha N-catenin, a linker between cadherin adhesion receptors and the actin cytoskeleton, is essential for stabilizing dendritic spines in rodent hippocampal neurons in culture. In the absence of alpha N-catenin, spine heads were abnormally motile, actively protruding filopodia from their synaptic contact sites. Conversely, alpha N-catenin overexpression in dendrites reduced spine turnover, causing an increase in spine and synapse density. Tetrodotoxin (TTX), a neural activity blocker, suppressed the synaptic accumulation of alpha N-catenin, whereas bicuculline, a GABA antagonist, promoted it. Furthermore, excess alpha N-catenin rendered spines resistant to the TTX treatment. These results suggest that alpha N-catenin is a key regulator for the stability of synaptic contacts.

The culture of chick forebrain neurons

Dissociation of the forebrain of a single 8-day chick embryo produces > 10(7) neurons in nearly pure culture. Our methods allow 50-70% of these neurons to develop an axon and typical pyrimidal shape after 3-4 days in culture at low density (10(4) cells/cm2) by a stereotyped developmental sequence similar to that of rat hippocampal neurons. The culture method for chick forebrain neurons is unusually rapid, inexpensive, simple, and could be used in undergraduate laboratory exercises. The dissection and dissociation of the tissue are easy and rapid, requiring less than 30 min from cracking open the chicken egg to plating the cells. Axonal development by these neurons and growth for about a week do not require glial support. The neurons are grown on polylysine-treated culture surfaces in either CO2-dependent (Medium 199) or -independent (Liebovitz L15) media with 10% fetal bovine serum and a supplement based on the classic N2 supplement for neuronal culture.

SHN-1, a Shank homologue inC. elegans, affects defecation rhythm via the inositol-1,4,5-trisphosphate receptor

Protein localization in the postsynaptic density (PSD) of neurons is mediated by scaffolding proteins such as PSD-95 and Shank, which ensure proper function of receptors at the membrane. The Shank family of scaffolding proteins contain PDZ (PSD-95, Dlg, and ZO-1) domains and have been implicated in the localizations of many receptor proteins including glutamate receptors in mammals. We have identified and characterized shn-1, the only homologue of Shank in Caenorhabditis elegans. The shn-1 gene shows approximately 40% identity over 1000 amino acids to rat Shanks. SHN-1 protein is localized in various tissues including neurons, pharynx and intestine. RNAi suppression of SHN-1 did not cause lethality or developmental abnormality. However, suppression of SHN-1 in the itr-1 (sa73) mutant, which has a defective inositol-1,4,5-trisphosphate (IP(3)) receptor, resulted in animals with altered defecation rhythm. Our data suggest a possible role of SHN-1 in affecting function of IP(3) receptors in C. elegans.

Estrogen alters hippocampal dendritic spine shape and enhances synaptic protein immunoreactivity and spatial memory in female mice

Estrogen (E) treatment induces axospinous synapses in rat hippocampus in vivo and in cultured hippocampal neurons in vitro. To better explore the molecular mechanisms underlying this phenomenon, we have established a mouse model for E action in the hippocampus by using Golgi impregnation to examine hippocampal dendritic spine morphology, radioimmunocytochemistry (RICC) and silver-enhanced immunocytochemistry to examine expression levels of synaptic protein markers, and hippocampal-dependent object-placement memory as a behavioral readout for the actions of E. In ovariectomized mice of several strains and F(1) hybrids, the total dendritic spine density on neurons in the CA1 region was not enhanced by E treatment, a finding that differs from that in the female rat. E treatment of ovariectomized C57BL/6J mice, however, caused an increase in the number of spines with mushroom shapes. By RICC and silver-enhanced immunocytochemistry, we found that the immunoreactivity of postsynaptic markers (PSD95 and spinophilin) and a presynaptic marker (syntaxin) were enhanced by E treatment throughout all fields of the dorsal hippocampus. In the object-placement tests, E treatment enhanced performance of object placement, a spatial episodic memory task. Taken together, the morphology and RICC results suggest a previously uncharacterized role of E in synaptic structural plasticity that may be interpreted as a facilitation of the spine-maturation process and may be associated with enhancement of hippocampal-dependent memory.

Adenovirus-mediated gene transfer into the brain stem to examine cardiovascular function: role of nitric oxide and Rho-kinase

The central nervous system plays an important role in the regulation of blood pressure via the sympathetic nervous system. Abnormal regulation of the sympathetic nerve activity is involved in the pathophysiology of hypertension. In particular, the brain stem, including the nucleus tractus solitarii (NTS) and the rostral ventrolateral medulla (RVLM), is a key site that controls and maintains blood pressure via the sympathetic nervous system. Nitric oxide (NO) is a unique molecule that influences sympathetic nerve activity. Rho-kinase is a downstream effector of the small GTPase, Rho, and is implicated in various cellular functions. We developed a technique to transfer adenovirus vectors encoding endothelial nitric oxide synthase and dominant-negative Rho-kinase into the NTS or the RVLM of rats in vivo. We applied this technique to hypertensive rats to explore the physiological significance of NO and Rho-kinase.

Ca 2+ -independent spine dynamics in cultured hippocampal neurons

Developing spines are highly dynamic processes that undergo rapid morphologic changes. We have investigated whether basal developmentally regulated spine motility is triggered by spontaneous changes in intracellular Ca(2+) ([Ca(2+)](i)). To address the link between Ca(2+) transients and motility, we have used the cell-permeable chelator BAPTA-AM. Consistent with others, we found that young cultured hippocampal neurons [7-13 days in vitro (DIV)] display significantly more spine motility than older neurons (14-21 days in vitro). Control experiments indicate that BAPTA-AM can lower basal [Ca(2+)](i) as well as block synaptically mediated Ca(2+) transients. However, globally buffering [Ca(2+)](i) by BAPTA-AM treatment failed to significantly alter the basal motility of developing spines measured at either relatively slow (5 min) or faster sampling times (every 20 s). We conclude that the basal spine motility of cultured hippocampal neurons may represent an intrinsic feature of developing neurons and is not necessarily choreographed by ongoing changes in [Ca(2+)](i) levels.

Integrated technology for evaluation of brain function and neural plasticity

The study of neural plasticity has expanded rapidly in the past decades and has shown the remarkable ability of the developing, adult, and aging brain to be shaped by environmental inputs in health and after a lesion. Robust experimental evidence supports the hypothesis that neuronal aggregates adjacent to a lesion in the sensorimotor brain areas can take over progressively the function previously played by the damaged neurons. It definitely is accepted that such a reorganization modifies sensibly the interhemispheric differences in somatotopic organization of the sensorimotor cortices. This reorganization largely subtends clinical recovery of motor performances and sensorimotor integration after a stroke. Brain functional imaging studies show that recovery from hemiplegic strokes is associated with a marked reorganization of the activation patterns of specific brain structures. To regain hand motor control, the recovery process tends over time to bring the bilateral motor network activation toward a more normal intensity/extent, while overrecruiting simultaneously new areas, perhaps to sustain this process. Considerable intersubject variability exists in activation/hyperactivation pattern changes over time. Some patients display late-appearing dorsolateral prefrontal cortex activation, suggesting the development of "executive" strategies to compensate for the lost function. The AH in stroke often undergoes a significant "remodeling" of sensory and motor hand somatotopy outside the "normal" areas, or enlargement of the hand representation. The UH also undergoes reorganization, although to a lesser degree. Although absolute values of the investigated parameters fluctuate across subjects, secondary to individual anatomic variability, variation is minimal with regards to interhemispheric differences, due to the fact that individual morphometric characters are mirrored in the two hemispheres. Excessive interhemispheric asymmetry of the sensorimotor hand areas seems to be the parameter with highest sensitivity in describing brain reorganization after a monohemispheric lesion, and mapping motor and somatosensory cortical areas through focal TMS, fMRI, PET, EEG, and MEG is useful in studying hand representation and interhemispheric asymmetries in normal and pathologic conditions. TMS and MEG allow the detection of sensorimotor areas reshaping, as a result of either neuronal reorganization or recovery of the previously damaged neural network. These techniques have the advantage of high temporal resolution but also have limitations. TMS provides only bidimensional scalp maps, whereas MEG, even if giving three-dimensional mapping of generator sources, does so by means of inverse procedures that rely on the choice of a mathematical model of the head and the sources. These techniques do not test movement execution and sensorimotor integration as used in everyday life. fMRI and PET may provide the ideal means to integrate the findings obtained with the other two techniques. This multitechnology combined approach is at present the best way to test the presence and amount of plasticity phenomena underlying partial or total recovery of several functions, sensorimotor above all. Dynamic patterns of recovery are emerging progressively from the relevant literature. Enhanced recruitment of the affected cortex, be it spared perilesional tissue, as in the case of cortical stroke, or intact but deafferented cortex, as in subcortical strokes, seems to be the rule, a mechanism especially important in early postinsult stages. The transfer over time of preferential activation toward contralesional cortices, as observed in some cases, seems, however, to reflect a less efficient type of plastic reorganization, with some aspects of maladaptive plasticity. Reinforcing the use of the affected side can cause activation to increase again in the affected side with a corresponding enhancement of clinical function. Activation of the UH MI may represent recruitment of direct (uncrossed) corticospinal tracts and relate more to mirror movements, but it more likely reflects activity redistribution within preexisting bilateral, large-scale motor networks. Finally, activation of areas not normally engaged in the dysfunctional tasks, such as the dorsolateral prefrontal cortex or the superior parietal cortex in motor paralysis, might reflect the implication of compensatory cognitive strategies. An integrated approach with technologies able to investigate functional brain imaging is of considerable value in providing information on the excitability, extension, localization, and functional hierarchy of cortical brain areas. Deepening knowledge of the mechanisms regulating the long-term recovery (even if partial), observed for most neurologic sequelae after neural damage, might prompt newer and more efficacious therapeutic and rehabilitative strategies for neurologic diseases.

Nervous Wreck, an SH3 Adaptor Protein that Interacts with Wsp, Regulates Synaptic Growth in Drosophila

We describe the isolation and characterization of nwk (nervous wreck), a temperature-sensitive paralytic mutant that causes excessive growth of larval neuromuscular junctions (NMJs), resulting in increased synaptic bouton number and branch formation. Ultrastructurally, mutant boutons have reduced size and fewer active zones, associated with a reduction in synaptic transmission. nwk encodes an FCH and SH3 domain-containing adaptor protein that localizes to the periactive zone of presynaptic terminals and binds to the Drosophila ortholog of Wasp (Wsp), a key regulator of actin polymerization. wsp null mutants display synaptic overgrowth similar to nwk and enhance the nwk morphological phenotype in a dose-dependent manner. Evolutionarily, Nwk belongs to a previously undescribed family of adaptor proteins that includes the human srGAPs, which regulate Rho activity downstream of Robo receptors. We propose that Nwk controls synapse morphology by regulating actin dynamics downstream of growth signals in presynaptic terminals.

Rho/Rho-kinase pathway in the brainstem contributes to hypertension caused by chronic nitric oxide synthase inhibition

Central nervous system mechanisms are involved in hypertension caused by chronic inhibition of nitric oxide (NO) synthesis. Chronic inhibition of NO synthesis might also activate the Rho/Rho-kinase pathway in the vasculature. We recently demonstrated that activation of the Rho/Rho-kinase pathway in the nucleus tractus solitarii (NTS) contributes to hypertensive mechanisms in spontaneously hypertensive rats. The aim of the present study was to determine whether activation of this pathway also contributes to neurogenic hypertensive mechanisms caused by chronic NO synthesis inhibition. The NO synthase inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME) was administered to Wistar-Kyoto rats in their drinking water (1 mg/mL) for 2 weeks. Bilateral microinjection of Y-27632, a specific Rho-kinase inhibitor, into the NTS elicited decreases in arterial pressure, heart rate, and renal sympathetic nerve activity in control rats and L-NAME-treated rats. The magnitude of the decrease, however, was significantly greater in L-NAME-treated than in control rats. In another group of rats, the specific Rho-kinase inhibitor, Y-27632, was administered intracisternally for 2 weeks with a mini-osmotic pump from the beginning of treatment with L-NAME. Y-27632 co-treatment significantly attenuated the increase in arterial pressure. Furthermore, the expression level of membranous RhoA and phosphorylation of the target proteins of Rho-kinase, the ERM (ezrin, radixin, moesin) family members, was significantly greater in L-NAME-treated rats than in control rats. These results indicate that activation of the Rho/Rho-kinase pathway in the NTS contributes to neurogenic hypertension caused by chronic NO synthase inhibition.

Brain plasticity in health and disease

Research during the last decades has greatly increased our understanding of brain plasticity, i.e. how neuronal circuits can be modified by experience, learning and in response to brain lesions. Currently available neuroimaging techniques that make it possible to study the function of the human brain in vivo have had an important impact. Cross-modal plasticity during development is demonstrated by cortical reorganization in blind or deaf children. Early musical training has lasting effects in shaping the brain. Albeit the plasticity is largest during childhood, the adult brain retains a capacity for functional and structural reorganization that earlier has been underestimated. Recent research on Huntington's disease has revealed the possibility of environmental interaction even with dominant genes. Scientifically based training methods are now being applied in rehabilitation of patients after stroke and trauma, and in the sensory retraining techniques currently applied in the treatment of focal hand dystonia as well as in sensory re-education after nerve repair in hand surgery. There is evidence that frequent participation in challenging and stimulating activities is associated with reduced cognitive decline during aging. The current concept of brain plasticity has wide implication for areas outside neuroscience and for all human life.

Protein synthesis is necessary for dendritic spine proliferation in adult brain slices

Dendritic spines, small protrusions from dendritic shafts, receive most of the excitatory synapses in cortical regions. Spines are highly plastic structures that can be rapidly produced or lost in response to a wide array of internal and external stimuli, and they proliferate in acute slice preparations [J. Neurosci. 19 (1999) 2876]. The goal of the present study was to determine if protein synthesis is necessary for this spine proliferation. We found that the addition of protein synthesis inhibitors to acute slices (in which spines otherwise proliferate) blocked new spine growth. Furthermore, a population of longer spines was observed after 2 h but these did not develop during protein synthesis blockade. These data suggest that protein synthesis is necessary for new spine growth in acute brain slice preparations and support literature suggesting that newly produced spines develop from filopodia-like protrusions.

Dynamic presynaptic varicosities: a role in activity-dependent synaptogenesis

Recent developments in confocal imaging techniques have opened new avenues for investigating how synaptic networks evolve over time. These studies have revealed not only that synaptic structures are motile but also, most importantly, that a fraction of synapses undergo a continuous elimination and formation process, and that these mechanisms are markedly enhanced by activity. Turnover of dendritic spines was recently demonstrated in the somatosensory cortex upon sensory stimulation. De Paola et al. have now provided evidence for correlated remodelling of presynaptic structures. These results, together with others, indicate that activation of neuronal networks enhances dynamic mechanisms at both the presynaptic and the postsynaptic level, resulting in an increased turnover of synapses and, accordingly, a reorganization of synaptic circuitry.

Motility of dendritic spines in visual cortex in vivo: changes during the critical period and effects of visual deprivation

Cortical dendritic spines are highly motile postsynaptic structures onto which most excitatory synapses are formed. It has been postulated that spine dynamics might reflect synaptic plasticity of cortical neurons. To test this hypothesis, we have investigated spine dynamics during the critical period in mouse visual cortex in vivo with and without sensory deprivation. The motility of spines on apical dendrites of layer 5 neurons was assayed by time-lapse two-photon microscopy. Spines were motile at the ages examined, postnatal days (P)21-P42, although motility decreased between P21 and P28 and then remained stable through P42. Binocular deprivation from before the time of eye-opening up-regulated spine motility during the peak of the critical period (P28), without affecting average spine length, class distribution, or density. Deprivation at the start of the critical period had no effect on spine motility, whereas continued deprivation through the end of the critical period appeared to reduce spine motility slightly. We conclude that spine motility might be involved in critical-period plasticity and that reduction of activity during the critical period enhances spine dynamics.

Kalirin, a multifunctional Rho guanine nucleotide exchange factor, is necessary for maintenance of hippocampal pyramidal neuron dendrites and dendritic spines

The structures of dendritic spines and the dendritic tree, key determinants of neuronal function, are regulated by diverse inputs that affect many scaffolding and signaling molecules. Nevertheless, here we show that reduced expression of a single gene results in loss of dendritic spines and a decrease in dendritic complexity. Kalirin, a dual Rho GDP-GTP exchange factor, causes spine formation when overexpressed. Reduced expression of Kalirin in CA1 hippocampal neurons resulted in a reduction in linear spine density, with dispersion of postsynaptic density markers and elimination of presynaptic endings. Simplification of the apical dendritic tree preceded simplification of basal dendrites. Pyramidal cell axons were not dramatically altered. Although many factors determine dendrite shape and spine formation, expression of Kalirin is necessary for the normal function of these many regulatory elements.

Localization of a beta-actin messenger ribonucleoprotein complex with zipcode-binding protein modulates the density of dendritic filopodia and filopodial synapses

The dendritic transport and local translation of mRNA may be an essential mechanism to regulate synaptic growth and plasticity. We investigated the molecular mechanism and function of beta-actin mRNA localization in dendrites of cultured hippocampal neurons. Previous studies have shown that beta-actin mRNA localization to the leading edge of fibroblasts or the growth cones of developing neurites involved a specific interaction between a zipcode sequence in the 3' untranslated region and the mRNA-binding protein zipcode-binding protein-1 (ZBP1). Here, we show that ZBP1 is required for the localization of beta-actin mRNA to dendrites. Knock-down of ZBP1 using morpholino antisense oligonucleotides reduced dendritic levels of ZBP1 and beta-actin mRNA and impaired growth of dendritic filopodia in response to BDNF treatment. Transfection of an enhanced green fluorescent protein (EGFP)-beta-actin construct, which contained the zipcode, increased the density of dendritic filopodia and filopodial synapses. Transfection of an EGFP construct, also with the zipcode, resulted in recruitment of endogenous ZBP1 and beta-actin mRNA into dendrites and similarly increased the density of dendritic filopodia. However, the beta-actin zipcode did not affect filopodial length or the density of mature spines. These results reveal a novel function for an mRNA localization element and its binding protein in the regulation of dendritic morphology and synaptic growth via dendritic filopodia.

Focal motility determines the geometry of dendritic spines

The geometry of dendritic spines has a major impact on signal transmission at excitatory synapses. To study it in detail we raised transgenic mice expressing an intrinsic green fluorescent protein-based plasma membrane marker that directly visualizes the cell surface of living neurons throughout the brain. Confocal imaging of developing hippocampal slices showed that as dendrites mature they switch from producing labile filopodia and polymorphic spine precursors to dendritic spines with morphologies similar to those reported from studies of adult brain. In images of live dendrites these mature spines are fundamentally stable structures, but retain morphological plasticity in the form of actin-rich lamellipodia at the tips of spine heads. In live mature dendrites up to 50% of spines had cup-shaped heads with prominent terminal lamellipodia whose motility produced constant alterations in the detailed geometry of the synaptic contact zone. The partial enveloping of presynaptic terminals by these cup-shaped spines coupled with rapid actin-driven changes in their shape may operate to fine-tune receptor distribution and neurotransmitter cross-talk at excitatory synapses.

Local calcium signaling in neurons

Transient rises in the cytoplasmic concentration of calcium ions serve as second messenger signals that control many neuronal functions. Selective triggering of these functions is achieved through spatial localization of calcium signals. Several qualitatively different forms of local calcium signaling can be distinguished by the location of open calcium channels as well as by the distance between these channels and the calcium binding proteins that serve as the molecular targets of calcium action. Local calcium signaling is especially prominent at presynaptic active zones and postsynaptic densities, structures that are distinguished by highly organized macromolecular arrays that yield precise spatial arrangements of calcium signaling proteins. Similar forms of local calcium signaling may be employed throughout the nervous system, though much remains to be learned about the molecular underpinnings of these events.

Presynaptic remodeling contributes to activity-dependent synaptogenesis

Induction of long-term potentiation and application of short periods of anoxia/hypoglycemia result in the growth of dendritic filopodia and formation of new spines. Here we investigated whether these conditions also affected the morphology of presynaptic structures. Using confocal imaging of DiI-labeled axons, electron microscopy, and stereological analyses, we show that short anoxia/hypoglycemia and theta burst stimulation induced rapid, calcium-dependent growth of presynaptic filopodia-like protrusions and remodeling of presynaptic varicosities. Three-dimensional reconstruction of axonal outgrowths revealed that, within 30 min, they made contacts and triggered the formation of a postsynaptic density on the target cell. Interestingly, these axonal filopodia first established synapses with the dendritic shaft and later mostly with spines. They also contributed to the formation of multi-innervated spines. Because these presynaptic growth mechanisms depended on NMDA receptor activation, we investigated whether a diffusing messenger could be involved. We found that blockade of nitric oxide synthase prevented these changes, and conversely, a nitric oxide donor could reproduce them. A model is presented that proposes that activation of NMDA receptors and subsequent release of nitric oxide could trigger the growth of presynaptic filopodia, which, in turn, play an active role in synaptogenesis and spine formation.

Activity-induced targeting of profilin and stabilization of dendritic spine morphology

Morphological changes in dendritic spines have been implicated in connective plasticity in brain circuitry, but the underlying pathway leading from synaptic transmission to structural change is unknown. Using primary neurons expressing GFP-tagged proteins, we found that profilin, a regulator of actin polymerization, is targeted to spine heads when postsynaptic NMDA receptors are activated and that actin-based changes in spine shape are concomitantly blocked. Profilin targeting was triggered by electrical stimulation patterns known to induce the long-term changes in synaptic responsiveness associated with memory formation. These results suggest that, in addition to electrophysiological changes, NMDA receptor activation initiates changes in the actin cytoskeleton of dendritic spines that stabilize synaptic structure.

Development of the outer retina in the mouse

Mice represent a valuable species for studies of development and disease. With the availability of transgenic models for retinal degeneration in this species, information regarding development and structure of mouse retina has become increasingly important. Of special interest is the differentiation and synaptogenesis of photoreceptors since these cells are predominantly involved in hereditary retinal degenerations. Thus, some of the keys to future clinical management of these retinal diseases may lie in understanding the molecular mechanisms of outer retinal development. In this study, we describe the expression of markers for photoreceptors (recoverin), horizontal cells (calbindin), bipolar cells (protein kinase C; PKC) and cytoskeletal elements pivotal to axonogenesis (beta-tubulin and actin) during perinatal development of mouse retina. Immunocytochemical localization of recoverin, calbindin, PKC and beta-tubulin was monitored in developing mouse retina (embryonic day (E) 18.5 to postnatal day (PN) 14), whereas f-actin was localized by Phalloidin binding. Recoverin immunoreactive cells, presumably the photoreceptors, were observed embryonically (E 18.5) and their number increased until PN 14. Neurite projections from the immunoreactive cells towards the outer plexiform layer (OPL) were noted at PN 0 and these processes reached the OPL at PN 7 coincident with histological evidence for the differentiation of the OPL. Outer segments, all the cell bodies in the ONL, as well as the OPL were immunoreactive to recoverin at PN 14. Calbindin immunoreactive horizontal cells were also present in E 18.5 retinas. These cells became progressively displaced proximally as the ONL developed. A calbindin immunoreactive plexus was seen in the OPL at PN 7. PKC immunoreactive bipolar cells developed postnatally, becoming distinguished at PN 7. Both beta-tubulin and actin immunoreactive cells were present in the IPL as early as E 18.5; however, appearance of processes labeled with these markers in the OPL was delayed until PN 7, concurrent with the first appearance of photoreceptor neurites, development of the horizontal cell plexus, and development of synaptophysin immunoreactivity at this location. These results provide a developmental timeframe for the expression of recoverin, calbindin, synaptophysin, beta-tubulin and actin. Our findings suggest that the time between PN 3 and PN 7 represents a critical period during which elements of the OPL are assembled.

Ganglioside/calmodulin kinase II signal inducing cdc42-mediated neuronal actin reorganization

Cell surface glycoconjugates are thought to mediate cell-cell recognition and play roles in neuronal development and functions. We demonstrated here that exposure of neuronal cells to nanomolar levels of gangliosides Neu5Acalpha 8Neu5Acalpha 3Galbeta 4GlcCer, Galbeta 3GalNAcbeta 4(Neu5Acalpha 8Neu5Acalpha 3)Galbeta 4GlcCer (GD1b), Neu5Acalpha 3Galbeta 3GalNAcbeta 4(Neu5Acalpha 8Neu5Acalpha 3)Galbeta 4GlcCer (GT1b) or its oligosaccharide portion induced a rapid and transient activation of Ca2+/calmodulin-dependent protein kinase II (CaM-KII) in the subplasmalemma. Galbeta 3GalNAcbeta 4(Neu5Acalpha 3)Galbeta 4GlcCer (GM1), GalNAcbeta 4(Neu5Acalpha 3)Galbeta 4GlcCer, Neu5Acalpha 3Galbeta 4GlcCer, Neu5Acalpha 3Galbeta 3GalNAcbeta 4(Neu5Acalpha 3)Galbeta 4GlcCer (GD1a), and Neu5Acalpha 8Neu5Acalpha 3Galbeta 3GalNAcbeta 4(Neu5Acalpha 8Neu5Acalpha 3)-Galbeta 4GlcCer were ineffective. GT1b and GD1b stimulated transient elevation of bulk cytosolic Ca2+ levels while GM1 slightly elevated the levels and GD1a did not. Thus, the cytosolic Ca2+ elevation by the gangliosides may trigger the CaM-KII activation. The treatment was accompanied by peripheral actin polymerization and filopodia formation in NG108-15 cells and primary hippocampal neurons, but not in glial cells. CaM-KII inhibitors blocked both CaM-KII activation and the subsequent filopodia formation. A small G-protein cdc42 was a potential downstream target of CaM-KII activated by the gangliosides. These results suggest that oligosaccharides of the gangliosides serve as potential regulators of the filopodia formation in neuronal cells by triggering the activation of CaM-KII followed by cdc42 up-regulation via a cell surface receptor-like component. The filopodia formation induced by the gangliosides may have a physiological relevance because long-term exposure of hippocampal neurons to GT1b oligosaccharide induced advanced dendritogenesis. Furthermore, exposure of cerebellar neurons to GT1b oligosaccharide facilitated CaM-KII-dependent dendritic outgrowth and branch formation of cerebellar Purkinje neurons, in which actin isoforms were localized to motile structures in dendrites. Thus, the ganglioside/CaM-KII signal plays a role in modulating dendritic morphogenesis by inducing cdc42-mediated actin reorganization.

Activity-regulated dynamic behavior of early dendritic protrusions: evidence for different types of dendritic filopodia

Dendritic filopodia are long and thin protrusions that occur predominantly during early development of the mammalian CNS. The function of dendritic filopodia is unknown, but they could serve to form early synapses, to generate spines, or to regulate dendritic branching and growth. We used two-photon imaging to characterize the motile behavior of dendritic protrusions during early postnatal development (P2-P12) in pyramidal neurons from acute slices of mouse neocortex. Dendritic protrusions in immature neurons are highly dynamic, and this motility is actin based. Motility and turnover of these early protrusions decreases throughout development, mirroring an increase in their average lifetime and density. Interestingly, density, motility, and length of filopodia are greater in dendritic growth cones than in dendritic shafts. These growth cones disappear after P5. Blocking synaptic transmission globally using TTX or calcium-free solutions led to a 40-120% increase in the density and length of dendritic filopodia in shafts but not in growth cones. Moreover, blocking ionotropic glutamate receptors resulted in an approximately 35% decrease in the density and turnover of shaft filopodia, whereas focal glutamate application led to a 75% increase in the length of shaft filopodia, but neither manipulation affected growth cone filopodia. Our results support the existence of two populations of filopodia, in growth cones and shafts, which are differentially regulated by neuronal activity. We propose that filopodia in dendritic growth cones are involved in dendritic growth and branching in an activity-independent manner, whereas shaft filopodia are responsible for activity-dependent synaptogenesis and, in some cases, may become dendritic spines.

Drebrin-dependent actin clustering in dendritic filopodia governs synaptic targeting of postsynaptic density-95 and dendritic spine morphogenesis

Dendritic spines have two major structural elements: postsynaptic densities (PSDs) and actin cytoskeletons. PSD proteins are proposed to regulate spine morphogenesis. However, other molecular mechanisms should govern spine morphogenesis, because the initiation of spine morphogenesis precedes the synaptic clustering of these proteins. Here, we show that synaptic clustering of drebrin, an actin-binding protein highly enriched in dendritic spines, governs spine morphogenesis. We immunocytochemically analyzed developing hippocampal neurons of low-density cultures. Filopodia-like dendritic protrusions were classified into two types: diffuse-type filopodia, which have diffuse distribution of drebrin, and cluster-type filopodia, which have drebrin clusters with filamentous actin (F-actin). Most cluster-type filopodia were synaptic filopodia. Postsynaptic drebrin clusters were found in both most synaptic filopodia and spines. Postsynaptic PSD-95 clusters, however, were found in only one-half of synaptic filopodia but in most spines. These data indicate that cluster-type filopodia are not mature spines but their precursors. Suppression of the upregulation of drebrin adult isoform (drebrin A) by antisense oligonucleotides against it attenuated synaptic clustering of PSD-95, as well as clustering of drebrin and F-actin. Furthermore, the restoration of drebrin A expression by injection of the expression vectors of drebrin A tagged with green fluorescent protein into the neurons treated with the antisense oligonucleotides induced synaptic reclustering of PSD-95 on clusters of the labeled drebrin A. These data indicated that the synaptic clustering of drebrin is necessary for that of PSD-95 in developing neurons. Together, these data suggest that synaptic clustering of drebrin is an essential step for spine morphogenesis.

Synaptopodin-deficient mice lack a spine apparatus and show deficits in synaptic plasticity

The spine apparatus is a cellular organelle that is present in many dendritic spines of excitatory neurons in the mammalian forebrain. Despite its discovery >40 years ago, the function of the spine apparatus is still unknown although calcium buffering functions as well as roles in synaptic plasticity have been proposed. We have recently shown that the 100-kDa protein synaptopodin is associated with the spine apparatus. Here, we now report that mice homozygous for a targeted deletion of the synaptopodin gene completely lack spine apparatuses. Interestingly, this absence of the spine apparatus is accompanied by a reduction in hippocampal long-term potentiation (LTP) in the CA1 region of the hippocampus and by an impairment of spatial learning in the radial arm maze test. This genetic analysis points to a role of the spine apparatus in synaptic plasticity.

Ethanol and brain plasticity: receptors and molecular networks of the postsynaptic density as targets of ethanol

Brain plasticity refers to the ability of the brain to undergo structural and functional changes. It is a necessary process that allows us to adapt and learn from our environment and is fundamental to our survival. However, under certain conditions, these neuroadaptive responses can have adverse consequences. In particular, increasing evidence indicates that plastic processes are coopted by drugs of abuse, leading to addiction and associated drug-seeking behaviors. An extensive and diverse group of studies ranging from the molecular to the behavioral level has also strongly implicated glutamatergic neurotransmission as a critical mediator of experience-dependent synaptic plasticity. Thus, it is vital to understand how drugs of abuse interact and potentially alter glutamatergic neurotransmission and associated signal transduction processes. This review will focus on the cellular and molecular neuroscience of alcoholism, with emphasis on events at the glutamatergic postsynaptic density (PSD) and dendritic spine dynamics that appear to underlie much of the structural and functional plasticity of the CNS.

Brain-derived neurotrophic factor mediates activity-dependent dendritic growth in nonpyramidal neocortical interneurons in developing organotypic cultures

Brain-derived neurotrophic factor (BDNF) promotes postnatal maturation of GABAergic inhibition in the cerebral and cerebellar cortices, and its expression and release are enhanced by neuronal activity, suggesting that it acts in a feedback manner to maintain a balance between excitation and inhibition during development. BDNF promotes differentiation of cerebellar, hippocampal, and neostriatal inhibitory neurons, but its effects on the dendritic development of neocortical inhibitory interneurons remain unknown. Here, we show that BDNF mediates depolarization-induced dendritic growth and branching in neocortical interneurons. To visualize inhibitory interneurons, we biolistically transfected organotypic cortical slice cultures from neonatal mice with green fluorescent protein (GFP) driven by the glutamic acid decarboxylase (GAD)67 promoter. Nearly all GAD67-GFP-expressing neurons were nonpyramidal, many contained GABA, and some expressed markers of neurochemically defined GABAergic subtypes, indicating that GAD67-GFP-expressing neurons were GABAergic. We traced dendritic trees from confocal images of the same GAD67-GFP-expressing neurons before and after a 5 d growth period, and quantified the change in total dendritic length (TDL) and total dendritic branch points (TDBPs) for each neuron. GAD67-GFP-expressing neurons growing in control medium exhibited a 20% increase in TDL, but in 200 ng/ml BDNF or 10 mm KCl, this increase nearly doubled and was accompanied by a significant increase in TDBPs. Blocking action potentials with TTX did not prevent the BDNF-induced growth, but antibodies against BDNF blocked the growth-promoting effect of KCl. We conclude that BDNF, released by neocortical pyramidal neurons in response to depolarization, enhances dendritic growth and branching in nearby inhibitory interneurons.

Overview of image analysis, image importing, and image processing using freeware

Quantitative image analysis turns microscopic data that might otherwise be merely descriptive into reliable mechanistic information. This unit provides an overview of the types of analysis that have been useful in the past, as well as descriptions of emerging applications. In addition, two protocols are included for importing data and for the first steps of data manipulation in the more common analytical applications of the freeware, NIH Image and ImageJ, as well as commercially available imaging software, Adobe Photoshop.

Systematic regulation of spine sizes and densities in pyramidal neurons

Dendritic spines receive most excitatory inputs in the CNS. Recent evidence has demonstrated that the spine head volume is linearly correlated with the readily releasable pool of neurotransmitter and the PSD size. These correlations can be used to functionally interpret spine morphology. Using Golgi impregnations and light microscopy, we reconstructed 23000 spines from pyramidal neurons in layers 2/3, 4, 5 and 6 of mouse primary visual cortex and CA1 hippocampal region and measured their spine head diameters and densities. Spine head diameters and densities are variable within and across cells, although they are similar between apical and basal dendrites. When compared to other regions, layer 5 neurons have larger spine heads and CA1 neurons higher spine densities. Interestingly, we detect a correlation between spine head diameter and interspine distance within and across cells, whereby larger spines are spaced further away from each other than smaller spines. Finally, in CA1 neurons, spine head diameters are larger, and spine density lower, in distal apical dendrites (>200 microm from soma) compared to proximal regions. These results reveal that spine morphologies and densities, and therefore synaptic properties, are jointly modulated with respect to cortical region, laminar position, and, in some cases, even the position of the spine along the dendritic tree. Individual neurons also appear to regulate their apical and basal spine densities and morphologies in concert. Our data provide evidence for a homeostatic control of excitatory synaptic strength.