Glutamate receptors regulate actin-based plasticity in dendritic spines

nArgBP2 regulates excitatory synapse formation by controlling dendritic spine morphology

Neural Abelson-related gene-binding protein 2 (nArgBP2) was originally identified as a protein that directly interacts with synapse-associated protein 90/postsynaptic density protein 95-associated protein 3 (SAPAP3), a postsynaptic scaffolding protein critical for the assembly of glutamatergic synapses. Although genetic deletion of nArgBP2 in mice leads to manic/bipolar-like behaviors resembling many aspects of symptoms in patients with bipolar disorder, the actual function of nArgBP2 at the synapse is completely unknown. Here, we found that the knockdown (KD) of nArgBP2 by specific small hairpin RNAs (shRNAs) resulted in a dramatic change in dendritic spine morphology. Reintroducing shRNA-resistant nArgBP2 reversed these defects. In particular, nArgBP2 KD impaired spine-synapse formation such that excitatory synapses terminated mostly at dendritic shafts instead of spine heads in spiny neurons, although inhibitory synapse formation was not affected. nArgBP2 KD further caused a marked increase of actin cytoskeleton dynamics in spines, which was associated with increased Wiskott-Aldrich syndrome protein-family verprolin homologous protein 1 (WAVE1)/p21-activated kinase (PAK) phosphorylation and reduced activity of cofilin. These effects of nArgBP2 KD in spines were rescued by inhibiting PAK or activating cofilin combined with sequestration of WAVE. Together, our results suggest that nArgBP2 functions to regulate spine morphogenesis and subsequent spine-synapse formation at glutamatergic synapses. They also raise the possibility that the aberrant regulation of synaptic actin filaments caused by reduced nArgBP2 expression may contribute to the manifestation of the synaptic dysfunction observed in manic/bipolar disorder.

Rewiring neuronal microcircuits of the brain via spine head protrusions--a role for synaptopodin and intracellular calcium stores

Neurological diseases associated with neuronal death are also accompanied by axonal denervation of connected brain regions. In these areas, denervation leads to a decrease in afferent drive, which may in turn trigger active central nervous system (CNS) circuitry rearrangement. This rewiring process is important therapeutically, since it can partially recover functions and can be further enhanced using modern rehabilitation strategies. Nevertheless, the cellular mechanisms of brain rewiring are not fully understood. We recently reported a mechanism by which neurons remodel their local connectivity under conditions of network-perturbance: hippocampal pyramidal cells can extend spine head protrusions (SHPs), which reach out toward neighboring terminals and form new synapses. Since this form of activity-dependent rewiring is observed only on some spines, we investigated the required conditions. We speculated, that the actin-associated protein synaptopodin, which is involved in several synaptic plasticity mechanisms, could play a role in the formation and/or stabilization of SHPs. Using hippocampal slice cultures, we found that ~70 % of spines with protrusions in CA1 pyramidal neurons contained synaptopodin. Analysis of synaptopodin-deficient neurons revealed that synaptopodin is required for the stability but not the formation of SHPs. The effects of synaptopodin could be linked to its role in Ca²⁺ homeostasis, since spines with protrusions often contained ryanodine receptors and synaptopodin. Furthermore, disrupting Ca²⁺ signaling shortened protrusion lifetime. By transgenically reintroducing synaptopodin on a synaptopodin-deficient background, SHP stability could be rescued. Overall, we show that synaptopodin increases the stability of SHPs, and could potentially modulate the rewiring of microcircuitries by making synaptic reorganization more efficient.

The Role of Actin Cytoskeleton in Memory Formation in Amygdala

The central, lateral and basolateral amygdala (BLA) nuclei are essential for the formation of long-term memories including emotional and drug-related memories. Studying cellular and molecular mechanisms of memory in amygdala may lead to better understanding of how memory is formed and of fear and addiction-related disorders. A challenge is to identify molecules activated by learning that subserve cellular changes needed for memory formation and maintenance in amygdala. Recent studies show that activation of synaptic receptors during fear and drug-related learning leads to alteration in actin cytoskeleton dynamics and structure in amygdala. Such changes in actin cytoskeleton in amygdala are essential for fear and drug-related memories formation. Moreover, the actin cytoskeleton subserves, after learning, changes in neuronal morphogenesis and glutamate receptors trafficking in amygdala. These cellular events are involved in fear and drug-related memories formation. Actin polymerization is also needed for the maintenance of drug-associated memories in amygdala. Thus, the actin cytoskeleton is a key mediator between receptor activation during learning and cellular changes subserving long-term memory (LTM) in amygdala. The actin cytoskeleton may serve as a target for pharmacological treatment of fear memory associated with fear and anxiety disorders and drug addiction to prevent the debilitating consequences of these diseases.

Minireview: Food for thought: regulation of synaptic function by metabolic hormones

The peripheral actions of the metabolic hormones, leptin and insulin, are well documented. However, the functions of these hormones are not restricted to the periphery because evidence is growing that both leptin and insulin can readily cross the blood-brain barrier and have widespread central actions. The hippocampus in particular expresses high levels of both insulin and leptin receptors as well as key components of their associated signaling cascades. Moreover, recent studies indicate that both hormones are potential cognitive enhancers. Indeed, it has been demonstrated that both leptin and insulin markedly influence key cellular events that underlie hippocampal learning and memory including activity-dependent synaptic plasticity and the trafficking of glutamate receptors to and away from hippocampal synapses. The hippocampal formation is also a prime site for the neurodegenerative processes that occur during Alzheimer's disease, and impairments in either leptin or insulin function have been linked to central nervous system-driven diseases like Alzheimer's disease. Thus, the capacity of the metabolic hormones, leptin and insulin, to regulate hippocampal synaptic function has significant implications for normal brain function and also central nervous system-driven disease.

Emerging Roles of Filopodia and Dendritic Spines in Motoneuron Plasticity during Development and Disease

Motoneurons develop extensive dendritic trees for receiving excitatory and inhibitory synaptic inputs to perform a variety of complex motor tasks. At birth, the somatodendritic domains of mouse hypoglossal and lumbar motoneurons have dense filopodia and spines. Consistent with Vaughn's synaptotropic hypothesis, we propose a developmental unified-hybrid model implicating filopodia in motoneuron spinogenesis/synaptogenesis and dendritic growth and branching critical for circuit formation and synaptic plasticity at embryonic/prenatal/neonatal period. Filopodia density decreases and spine density initially increases until postnatal day 15 (P15) and then decreases by P30. Spine distribution shifts towards the distal dendrites, and spines become shorter (stubby), coinciding with decreases in frequency and increases in amplitude of excitatory postsynaptic currents with maturation. In transgenic mice, either overexpressing the mutated human Cu/Zn-superoxide dismutase (hSOD1^G93A) gene or deficient in GABAergic/glycinergic synaptic transmission (gephyrin, GAD-67, or VGAT gene knockout), hypoglossal motoneurons develop excitatory glutamatergic synaptic hyperactivity. Functional synaptic hyperactivity is associated with increased dendritic growth, branching, and increased spine and filopodia density, involving actin-based cytoskeletal and structural remodelling. Energy-dependent ionic pumps that maintain intracellular sodium/calcium homeostasis are chronically challenged by activity and selectively overwhelmed by hyperactivity which eventually causes sustained membrane depolarization leading to excitotoxicity, activating microglia to phagocytose degenerating neurons under neuropathological conditions.

Vortioxetine promotes maturation of dendritic spines in vitro: A comparative study in hippocampal cultures

Cognitive dysfunction is prevalent in patients with major depressive disorder (MDD), and cognitive impairments can persist after relief of depressive symptoms. The multimodal-acting antidepressant vortioxetine is an antagonist at 5-HT3, 5-HT7, and 5-HT1D receptors, a partial agonist at 5-HT1B receptors, an agonist at 5-HT1A receptors, and an inhibitor of the serotonin (5-HT) transporter (SERT) and has pro-cognitive properties. In preclinical studies, vortioxetine enhances long-term potentiation (LTP), a cellular correlate of neuroplasticity, and enhances memory in various cognitive tasks. However, the molecular mechanisms by which vortioxetine augments LTP and memory remain unknown. Dendritic spines are specialized, actin-rich microdomains on dendritic shafts and are major sites of most excitatory synapses. Since dendritic spine remodeling is implicated in synaptic plasticity and spine size dictates the strength of synaptic transmission, we assessed if vortioxetine, relative to other antidepressants including ketamine, duloxetine, and fluoxetine, plays a role in the maintenance of dendritic spine architecture in vitro. We show that vortioxetine, ketamine, and duloxetine induce spine enlargement. However, only vortioxetine treatment increased the number of spines in contact with presynaptic terminals. In contrast, fluoxetine had no effect on spine remodeling. These findings imply that the various 5-HT receptor mechanisms of vortioxetine may play a role in its effect on spine dynamics and in increasing the proportion of potentially functional synaptic contacts.

Inhibition of actin polymerization in the NAc shell inhibits morphine-induced CPP by disrupting its reconsolidation

Drug-associated contextual cues contribute to drug craving and relapse after abstinence, which is a major challenge to drug addiction treatment. Previous studies showed that disrupting memory reconsolidation impairs drug reward memory. However, the underlying mechanisms remain elusive. Although actin polymerization is involved in memory formation, its role in the reconsolidation of drug reward memory is unknown. In addition, the specific brain areas responsible for drug memory have not been fully identified. In the present study, we found that inhibiting actin polymerization in the nucleus accumbens (NAc) shell, but not the NAc core, abolishes morphine-induced conditioned place preference (CPP) by disrupting its reconsolidation in rats. Moreover, this effect persists for more than 2 weeks by a single injection of the actin polymerization inhibitor, which is not reversed by a morphine-priming injection. Furthermore, the application of actin polymerization inhibitor outside the reconsolidation window has no effect on morphine-associated contextual memory. Taken together, our findings first demonstrate that inhibiting actin polymerization erases morphine-induced CPP by disrupting its reconsolidation. Our study suggests that inhibition of actin polymerization during drug memory reconsolidation may be a potential approach to prevent drug relapse.

Neural plasticity and memory: molecular mechanism

Deciphering the cellular and molecular mechanisms of memory has been an important topic encompassing the learning and memory domain besides the neurodegenerative disorders. Synapses accumulate cognitive information from life-lasting alterations of their molecular and structural composition. Current memory storage models identify posttranslational modification imperative for short-term information storage and mRNA translation for long-term information storage. However, the precise account of these modifications has not been summarized at the individual synapse level. Therefore, herein we describe the spatiotemporal reorganization of synaptic plasticity at the dendritic spine level to elucidate the mechanism through which synaptic substructures are remodeled; though at the molecular level, such mechanisms are still quite unclear. It has thus been concluded that the existing mechanisms do not entirely elaborate memory storage processes. Further efforts are therefore encouraged to delineate the mechanism of neuronal connectivity at the chemical level as well, including inter- or intramolecular bonding patterns at the synaptic level, which may be a permissive and vital step of memory storage.

ROCK1 and ROCK2 inhibition alters dendritic spine morphology in hippocampal neurons

Communication among neurons is mediated through synaptic connections between axons and dendrites, and most excitatory synapses occur on actin-rich protrusions along dendrites called dendritic spines. Dendritic spines are structurally dynamic, and synapse strength is closely correlated with spine morphology. Abnormalities in the size, shape, and number of dendritic spines are prevalent in neurologic diseases, including autism spectrum disorders, schizophrenia, and Alzheimer disease. However, therapeutic targets that influence spine morphology are lacking. Rho-associated coiled-coil containing protein kinases (ROCK) 1 and ROCK2 are potent regulators of the actin cytoskeleton and highly promising drug targets for central nervous system disorders. In this report, we addressed how pharmacologic inhibition of ROCK1 and ROCK2 affects dendritic spine morphology. Hippocampal neurons were transfected with plasmids expressing fluorescently labeled Lifeact, a small actin binding peptide, and then incubated with or without Y-27632, an established pan-ROCK small molecule inhibitor. Using an automated 3D spine morphometry analysis method, we showed that inhibition of ROCK1 and ROCK2 significantly increased the mean protrusion density and significantly reduced the mean protrusion width. A trending increase in mean protrusion length was observed following Y-27632 treatment, and novel effects were observed among spine classes. Exposure to Y-27632 significantly increased the number of filopodia and thin spines, while the numbers of stubby and mushroom spines were similar to mock-treated samples. These findings support the hypothesis that pharmacologic inhibition of ROCK1 and ROCK2 may convey therapeutic benefit for neurologic disorders that feature dendritic spine loss or aberrant structural plasticity.

Expression profiles of inka2 in the murine nervous system

Dynamic rearrangement of the actin cytoskeleton impacts many cellular characteristics in both the developing and adult central nervous systems (CNS), including the migration and adhesion of highly motile neural progenitor cells, axon guidance of immature neurons, and reconstruction of synaptic structures in the adult brain. Inka1, a known regulator of actin cytoskeleton reconstruction, is predominantly expressed by the neural crest cell lineage and regulates the migration and differentiation of these cells. In the present study, we identified a novel gene, designated as inka2, which is related to inka1. Inka2/fam212b is an evolutionarily conserved gene found in different vertebrate species and constitutes a novel gene family together with inka1. Northern blot analysis showed that inka2 mRNA was highly enriched in the nervous system. The spatiotemporal propagation cell profiles of those cells that expressed inka2 transcripts were compatible with those of Olig2-positive oligodendrocyte progenitor cells, which originate in the ventral ventricular zone during embryogenesis. Intense expression of inka2 was also noted in the proliferative neuronal progenitors in the developing cerebellum. On the other hand, immature newborn neurons in the embryonic brain showed no expression of inka2, except for the cells residing in the marginal zone of the embryonic telencephalon, which is known to contain transient cells including the non-subplate pioneer neurons and Cajal-Retzius cells. As brain development proceeds during the postnatal stage, inka2 expression emerged in some populations of immature neurons, including the neocortical pyramidal neurons, hippocampal pyramidal neurons, and granule cells migrating in the cerebellar cortex. In the adult brain, the expression of inka2 was interestingly confined in terminally differentiated neurons in the restricted forebrain regions. Taken together, as a novel regulator of actin cytoskeletons in the CNS, inka2 may be involved in multiple actin-driven processes, including cell migration and establishment of neuronal polarity.

Abnormal synaptic Ca(2+) homeostasis and morphology in cortical neurons of familial hemiplegic migraine type 1 mutant mice

OBJECTIVE

Migraine is among the most common and debilitating neurological conditions. Familial hemiplegic migraine type 1 (FHM1), a monogenic migraine subtype, is caused by gain-of-function of voltage-gated CaV 2.1 calcium channels. FHM1 mice carry human pathogenic mutations in the α1A subunit of CaV 2.1 channels and are highly susceptible to cortical spreading depression (CSD), the electrophysiologic event underlying migraine aura. To date, however, the mechanism underlying increased CSD/migraine susceptibility remains unclear.

METHODS

We employed in vivo multiphoton microscopy of the genetically encoded Ca(2+)-indicator yellow cameleon to investigate synaptic morphology and [Ca(2+)]i in FHM1 mice. To study CSD-induced cerebral oligemia, we used in vivo laser speckle flowmetry and multimodal imaging. With electrophysiologic recordings, we investigated the effect of the CaV 2.1 gating modifier tert-butyl dihydroquinone on CSD in vivo.

RESULTS

FHM1 mutations elevate neuronal [Ca(2+)]i and alter synaptic morphology as a mechanism for enhanced CSD susceptibility that we were able to normalize with a CaV 2.1 gating modifier in hyperexcitable FHM1 mice. At the synaptic level, axonal boutons were larger, and dendritic spines were predominantly of the mushroom type, which both provide a structural correlate for enhanced neuronal excitability. Resting neuronal [Ca(2+)]i was elevated in FHM1, with loss of compartmentalization between synapses and neuronal shafts. The percentage of calcium-overloaded neurons was increased. Neuronal [Ca(2+)]i surge during CSD was faster and larger, and post-CSD oligemia and hemoglobin desaturation were more severe in FHM1 brains.

INTERPRETATION

Our findings provide a mechanism for enhanced CSD susceptibility in hemiplegic migraine. Abnormal synaptic Ca(2+) homeostasis and morphology may contribute to chronic neurodegenerative changes as well as enhanced vulnerability to ischemia in migraineurs.

ICAM-5 affects spine maturation by regulation of NMDA receptor binding to α-actinin

ICAM-5 is a negative regulator of dendritic spine maturation and facilitates the formation of filopodia. Its absence results in improved memory functions, but the mechanisms have remained poorly understood. Activation of NMDA receptors induces ICAM-5 ectodomain cleavage through a matrix metalloproteinase (MMP)-dependent pathway, which promotes spine maturation and synapse formation. Here, we report a novel, ICAM-5-dependent mechanism underlying spine maturation by regulating the dynamics and synaptic distribution of α-actinin. We found that GluN1 and ICAM-5 partially compete for the binding to α-actinin; deletion of the cytoplasmic tail of ICAM-5 or ablation of the gene resulted in increased association of GluN1 with α-actinin, whereas internalization of ICAM-5 peptide perturbed the GluN1/α-actinin interaction. NMDA treatment decreased α-actinin binding to ICAM-5, and increased the binding to GluN1. Proper synaptic distribution of α-actinin requires the ICAM-5 cytoplasmic domain, without which α-actinin tended to accumulate in filopodia, leading to F-actin reorganization. The results indicate that ICAM-5 retards spine maturation by preventing reorganization of the actin cytoskeleton, but NMDA receptor activation is sufficient to relieve the brake and promote the maturation of spines.

Sex Hormones Regulate Cytoskeletal Proteins Involved in Brain Plasticity

In the brain of female mammals, including humans, a number of physiological and behavioral changes occur as a result of sex hormone exposure. Estradiol and progesterone regulate several brain functions, including learning and memory. Sex hormones contribute to shape the central nervous system by modulating the formation and turnover of the interconnections between neurons as well as controlling the function of glial cells. The dynamics of neuron and glial cells morphology depends on the cytoskeleton and its associated proteins. Cytoskeletal proteins are necessary to form neuronal dendrites and dendritic spines, as well as to regulate the diverse functions in astrocytes. The expression pattern of proteins, such as actin, microtubule-associated protein 2, Tau, and glial fibrillary acidic protein, changes in a tissue-specific manner in the brain, particularly when variations in sex hormone levels occur during the estrous or menstrual cycles or pregnancy. Here, we review the changes in structure and organization of neurons and glial cells that require the participation of cytoskeletal proteins whose expression and activity are regulated by estradiol and progesterone.

PICK1 links AMPA receptor stimulation to Cdc42

Rho-family GTPases control numerous cell biological processes via effects on actin dynamics, such as cell migration, cell adhesion, morphogenesis and vesicle traffic. In neurons, they are involved in dendritic spine morphogenesis and other aspects of neuronal morphology via regulation of the actin cytoskeleton. The Rho-family member Cdc42 regulates dendritic spine morphology via its effector N-WASP, which activates the actin-nucleating Arp2/3 complex. Excitatory synaptic transmission is known to regulate actin dynamics in dendritic spines to bring about changes in spine morphology or motility, however, the details of the signalling pathways that transduce glutamate receptor activation to Rho GTPase function are unclear. PICK1 is a PDZ and BAR domain protein that interacts with the Arp2/3 complex and the GTPase Arf1 to regulate actin polymerisation in dendritic spines. PICK1 also binds AMPA receptor subunits GluA2/3 and is involved in GluA2-dependent AMPAR trafficking. Here, we show that PICK1 binds Rac1 and Cdc42, via distinct but overlapping binding sites. Furthermore, AMPAR stimulation deactivates Cdc42 and alters its detergent solubility in neurons via a PICK1-dependent process. This work suggests a novel role for PICK1 in transducing AMPAR stimulation to Cdc42 function in neurons.

Investigating dynamic structural and mechanical changes of neuroblastoma cells associated with glutamate-mediated neurodegeneration

Glutamate-mediated neurodegeneration resulting from excessive activation of glutamate receptors is recognized as one of the major causes of various neurological disorders such as Alzheimer's and Huntington's diseases. However, the underlying mechanisms in the neurodegenerative process remain unidentified. Here, we investigate the real-time dynamic structural and mechanical changes associated with the neurodegeneration induced by the activation of N-methyl-D-aspartate (NMDA) receptors (a subtype of glutamate receptors) at the nanoscale. Atomic force microscopy (AFM) is employed to measure the three-dimensional (3-D) topography and mechanical properties of live SH-SY5Y cells under stimulus of NMDA receptors. A significant increase in surface roughness and stiffness of the cell is observed after NMDA treatment, which indicates the time-dependent neuronal cell behavior under NMDA-mediated neurodegeneration. The present AFM based study further advance our understanding of the neurodegenerative process to elucidate the pathways and mechanisms that govern NMDA induced neurodegeneration, so as to facilitate the development of novel therapeutic strategies for neurodegenerative diseases.

Ionising radiation immediately impairs synaptic plasticity-associated cytoskeletal signalling pathways in HT22 cells and in mouse brain: an in vitro/in vivo comparison study

Patients suffering from brain malignancies are treated with high-dose ionising radiation. However, this may lead to severe learning and memory impairment. Preventive treatments to minimise these side effects have not been possible due to the lack of knowledge of the involved signalling pathways and molecular targets. Mouse hippocampal neuronal HT22 cells were irradiated with acute gamma doses of 0.5 Gy, 1.0 Gy and 4.0 Gy. Changes in the cellular proteome were investigated by isotope-coded protein label technology and tandem mass spectrometry after 4 and 24 hours. To compare the findings with the in vivo response, male NMRI mice were irradiated on postnatal day 10 with a gamma dose of 1.0 Gy, followed by evaluation of the cellular proteome of hippocampus and cortex 24 hours post-irradiation. Analysis of the in vitro proteome showed that signalling pathways related to synaptic actin-remodelling were significantly affected at 1.0 Gy and 4.0 Gy but not at 0.5 Gy after 4 and 24 hours. We observed radiation-induced reduction of the miR-132 and Rac1 levels; miR-132 is known to regulate Rac1 activity by blocking the GTPase-activating protein p250GAP. In the irradiated hippocampus and cortex we observed alterations in the signalling pathways similar to those in vitro. The decreased expression of miR-132 and Rac1 was associated with an increase in hippocampal cofilin and phospho-cofilin. The Rac1-Cofilin pathway is involved in the modulation of synaptic actin filament formation that is necessary for correct spine and synapse morphology to enable processes of learning and memory. We suggest that acute radiation exposure leads to rapid dendritic spine and synapse morphology alterations via aberrant cytoskeletal signalling and processing and that this is associated with the immediate neurocognitive side effects observed in patients treated with ionising radiation.

Fine structure of synapses on dendritic spines

Camillo Golgi's "Reazione Nera" led to the discovery of dendritic spines, small appendages originating from dendritic shafts. With the advent of electron microscopy (EM) they were identified as sites of synaptic contact. Later it was found that changes in synaptic strength were associated with changes in the shape of dendritic spines. While live-cell imaging was advantageous in monitoring the time course of such changes in spine structure, EM is still the best method for the simultaneous visualization of all cellular components, including actual synaptic contacts, at high resolution. Immunogold labeling for EM reveals the precise localization of molecules in relation to synaptic structures. Previous EM studies of spines and synapses were performed in tissue subjected to aldehyde fixation and dehydration in ethanol, which is associated with protein denaturation and tissue shrinkage. It has remained an issue to what extent fine structural details are preserved when subjecting the tissue to these procedures. In the present review, we report recent studies on the fine structure of spines and synapses using high-pressure freezing (HPF), which avoids protein denaturation by aldehydes and results in an excellent preservation of ultrastructural detail. In these studies, HPF was used to monitor subtle fine-structural changes in spine shape associated with chemically induced long-term potentiation (cLTP) at identified hippocampal mossy fiber synapses. Changes in spine shape result from reorganization of the actin cytoskeleton. We report that cLTP was associated with decreased immunogold labeling for phosphorylated cofilin (p-cofilin), an actin-depolymerizing protein. Phosphorylation of cofilin renders it unable to depolymerize F-actin, which stabilizes the actin cytoskeleton. Decreased levels of p-cofilin, in turn, suggest increased actin turnover, possibly underlying the changes in spine shape associated with cLTP. The findings reviewed here establish HPF as an appropriate method for studying the fine structure and molecular composition of synapses on dendritic spines.

Experience-dependent plasticity of visual cortical microcircuits

The recent decade testified a tremendous increase in our knowledge on how cell-type-specific microcircuits process sensory information in the neocortex and on how such circuitry reacts to manipulations of the sensory environment. Experience-dependent plasticity has now been investigated with techniques endowed with cell resolution during both postnatal development and in adult animals. This review recapitulates the main recent findings in the field using mainly the primary visual cortex as a model system to highlight the more important questions and physiological principles (such as the role of non-competitive mechanisms, the role of inhibition in excitatory cell plasticity, the functional importance of spine and axonal plasticity on a microscale level). I will also discuss on which scientific problems the debate and controversies are more pronounced. New technologies that allow to perturbate cell-type-specific subcircuits will certainly shine new light in the years to come at least on some of the still open questions.

Rapid regulation of endoplasmic reticulum dynamics in dendritic spines by NMDA receptor activation

Endoplasmic reticulum (ER) is motile within dendritic spines, but the mechanisms underlying its regulation are poorly understood. To address this issue, we have simultaneously imaged morphology and ER content of dendritic spines in cultured dissociated mouse hippocampal neurons. Over a 10 min period, spines were highly dynamic, with spines both increasing and decreasing in volume. ER was present in approximately 50% of spines and was also highly dynamic, with a net increase over this period of time. Inhibition of the endogenous activation of NMDA receptors resulted in a reduction in ER growth. Conversely, augmentation of the synaptic activation of NMDA receptors, by elimination of striatal-enriched protein tyrosine phosphatase (STEP), resulted in enhanced ER growth. Therefore, NMDA receptors rapidly regulate spine ER dynamics.

Quantitative image analysis tool to study the plasma membrane localization of proteins and cortical actin in neuroendocrine cells

BACKGROUND

Adrenal chromaffin cells are a widely used model system to study regulated exocytosis and other membrane-associated processes. Alterations in the amount and localization of the proteins involved in these processes can be visualized with fluorescent probes that report the effect of different stimuli or genetic modifications. However, the quantitative analysis of such images remains difficult, especially when focused on specific locations, such as the plasma membrane.

NEW METHOD

We developed an image analysis algorithm, named plasma membrane analysis in chromaffin cells (PlasMACC). PlasMACC enables automatic detection of the plasma membrane region and quantitative analysis of multi-fluorescent signals from spherical cells. PlasMACC runs in the image analysis software ImageJ environment, it is user-friendly and freely available.

RESULTS

PlasMACC delivers detailed information about intensity, thickness and density of fluorescent signals at the plasma membrane of both living and fixed cells. Individual signals can be compared between cells and different signals within one cell can be correlated. PlasMACC can process conventional laser-scanning confocal images as well as data obtained by super-resolution methods such as structured illumination microscopy.

COMPARISON WITH EXISTING METHOD(S)

By comparing PlasMACC to methods currently used in the field, we show more consistent quantitative data due to the fully automated algorithm. PlasMACC also provides an expanded set of novel analysis parameters.

CONCLUSION

PlasMACC enables a detailed quantification of fluorescent signals at the plasma membrane of spherical cells in an unbiased and reliable fashion.

Isoflurane reversibly destabilizes hippocampal dendritic spines by an actin-dependent mechanism

General anesthetics produce a reversible coma-like state through modulation of excitatory and inhibitory synaptic transmission. Recent evidence suggests that anesthetic exposure can also lead to sustained cognitive dysfunction. However, the subcellular effects of anesthetics on the structure of established synapses are not known. We investigated effects of the widely used volatile anesthetic isoflurane on the structural stability of hippocampal dendritic spines, a postsynaptic structure critical to excitatory synaptic transmission in learning and memory. Exposure to clinical concentrations of isoflurane induced rapid and non-uniform shrinkage and loss of dendritic spines in mature cultured rat hippocampal neurons. Spine shrinkage was associated with a reduction in spine F-actin concentration. Spine loss was prevented by either jasplakinolide or cytochalasin D, drugs that prevent F-actin disassembly. Isoflurane-induced spine shrinkage and loss were reversible upon isoflurane elimination. Thus, isoflurane destabilizes spine F-actin, resulting in changes to dendritic spine morphology and number. These findings support an actin-based mechanism for isoflurane-induced alterations of synaptic structure in the hippocampus. These reversible alterations in dendritic spine structure have important implications for acute anesthetic effects on excitatory synaptic transmission and synaptic stability in the hippocampus, a locus for anesthetic-induced amnesia, and have important implications for anesthetic effects on synaptic plasticity.

The role of nicotinic receptors in shaping and functioning of the glutamatergic system: a window into cognitive pathology

The involvement of the cholinergic system in learning, memory and attention has long been recognized, although its neurobiological mechanisms are not fully understood. Recent evidence identifies the endogenous cholinergic signaling via nicotinic acetylcholine receptors (nAChRs) as key players in determining the morphological and functional maturation of the glutamatergic system. Here, we review the available experimental and clinical evidence of nAChRs contribution to the establishment of the glutamatergic system, and therefore to cognitive function. We provide some clues of the putative underlying molecular mechanisms and discuss recent human studies that associate genetic variability of the genes encoding nAChR subunits with cognitive disorders. Finally, we discuss the new avenues to therapeutically targeting nAChRs in persons with cognitive dysfunction for which the α7-nAChR subunit is an important etiological mechanism.

Responses to cell loss become restricted as the supporting cells in mammalian vestibular organs grow thick junctional actin bands that develop high stability

Sensory hair cell (HC) loss is a major cause of permanent hearing and balance impairments for humans and other mammals. Yet, fish, amphibians, reptiles, and birds readily replace HCs and recover from such sensory deficits. It is unknown what prevents replacement in mammals, but cell replacement capacity declines contemporaneously with massive postnatal thickening of F-actin bands at the junctions between vestibular supporting cells (SCs). In non-mammals, SCs can give rise to regenerated HCs, and the bands remain thin even in adults. Here we investigated the stability of the F-actin bands between SCs in ears from chickens and mice and Madin-Darby canine kidney cells. Pharmacological experiments and fluorescence recovery after photobleaching (FRAP) of SC junctions in utricles from mice that express a γ-actin-GFP fusion protein showed that the thickening F-actin bands develop increased resistance to depolymerization and exceptional stability that parallels a sharp decline in the cell replacement capacity of the maturing mammalian ear. The FRAP recovery rate and the mobile fraction of γ-actin-GFP both decreased as the bands thickened with age and became highly stabilized. In utricles from neonatal mice, time-lapse recordings in the vicinity of dying HCs showed that numerous SCs change shape and organize multicellular actin purse strings that reseal the epithelium. In contrast, adult SCs appeared resistant to deformation, with resealing responses limited to just a few neighboring SCs that did not form purse strings. The exceptional stability of the uniquely thick F-actin bands at the junctions of mature SCs may play an important role in restricting dynamic repair responses in mammalian vestibular epithelia.

Rapid, transient synaptic plasticity in addiction

Chronic use of addictive drugs produces enduring neuroadaptations in the corticostriatal glutamatergic brain circuitry. The nucleus accumbens (NAc), which integrates cortical information and regulates goal-directed behavior, undergoes long-term morphological and electrophysiological changes that may underlie the increased susceptibility for relapse in drug-experienced individuals even after long periods of withdrawal. Additionally, it has recently been shown that exposure to cues associated with drug use elicits rapid and transient morphological and electrophysiological changes in glutamatergic synapses in the NAc. This review highlights these dynamic drug-induced changes in this pathway that are specific to a drug seeking neuropathology, as well as how these changes impair normal information processing and thereby contribute to the uncontrollable motivation to relapse. Future directions for relapse prevention and pharmacotherapeutic targeting of the rapid, transient synaptic plasticity in relapse are discussed. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Improved method for the quantification of motility in glia and other morphologically complex cells

Cells such as astrocytes and radial glia with many densely ramified, fine processes pose particular challenges for the quantification of structural motility. Here we report the development of a method to calculate a motility index for individual cells with complex, dynamic morphologies. This motility index relies on boxcar averaging of the difference images generated by subtraction of images collected at consecutive time points. An image preprocessing step involving 2D projection, edge detection, and dilation of the raw images is first applied in order to binarize the images. The boxcar averaging of difference images diminishes the impact of artifactual pixel fluctuations while accentuating the group-wise changes in pixel values which are more likely to represent real biological movement. Importantly, this provides a value that correlates with mean process elongation and retraction rates without requiring detailed reconstructions of very complex cells. We also demonstrate that additional increases in the sensitivity of the method can be obtained by denoising images using the temporal frequency power spectra, based on the fact that rapid intensity fluctuations over time are mainly due to imaging artifact. The MATLAB programs implementing these motility analysis methods, complete with user-friendly graphical interfaces, have been made publicly available for download.

Measurement of saturation processes in glutamatergic and GABAergic synapse densities during long-term development of cultured rat cortical networks

The aim of this study was to clarify the saturation processes of excitatory and inhibitory synapse densities during the long-term development of cultured neuronal networks. For this purpose, we performed a long-term culture of rat cortical cells for 35 days in vitro (DIV). During this culture period, we labeled glutamatergic and GABAergic synapses separately using antibodies against vesicular glutamate transporter 1 (VGluT1) and vesicular transporter of γ-aminobutyric acid (VGAT). The densities and distributions of both types of synaptic terminals were measured simultaneously. Observations and subsequent measurements of immunofluorescence demonstrated that the densities of both types of antibody-labeled terminals increased gradually from 7 to 21-28 DIV. The densities did not show a further increase at 35 DIV and tended to become saturated. Triple staining with VGluT1, VGAT, and microtubule-associated protein 2 (MAP2) enabled analysis of the distribution of both types of synapses, and revealed that the densities of the two types of synaptic terminals on somata were not significantly different, but that glutamatergic synapses predominated on the dendrites during long-term culture. However, some neurons did not fall within this distribution, suggesting differences in synapse distribution on target neurons. The electrical activity also showed an initial increase and subsequent saturation of the firing rate and synchronized burst rate during long-term culture, and the number of days of culture to saturation from the initial increase followed the same pattern under this culture condition.

Leptin regulation of neuronal morphology and hippocampal synaptic function

The central actions of the hormone leptin in regulating energy homeostasis via the hypothalamus are well documented. However, evidence is growing that this hormone can also modify the structure and function of synapses throughout the CNS. The hippocampus is a region of the forebrain that plays a crucial role in associative learning and memory and is an area also highly vulnerable to neurodegenerative processes. Recent studies indicate that leptin is a potential cognitive enhancer as it modulates the cellular processes underlying hippocampal-dependent learning and memory including dendritic morphology, glutamate receptor trafficking and activity-dependent synaptic plasticity. Here, we review the recent evidence implicating the hormone leptin as a key regulator of hippocampal synaptic function and discuss the role of leptin receptor-driven lipid signaling pathways involved in this process.

Calcium release-dependent actin flow in the leading process mediates axophilic migration

Proper assembly of neural circuits requires newly born neurons to migrate from their place of origin to their final location. Little is known about the mechanisms of axophilic neuronal migration, whereby neurons travel along axon pathways to navigate to their destinations. Gonadotropin-releasing hormone (GnRH)-expressing neurons migrate along olfactory axons from the nose into the forebrain during development, and were used as a model of axophilic migration. After migrating, GnRH neurons are located in the hypothalamus and are essential for puberty and maintenance of reproductive function. To gain a better understanding of the mechanisms underlying axophilic migration, we investigated in mice the regulation of movement from calcium signals to cytoskeletal dynamics. Live imaging revealed robust calcium activity during axophilic migration, and calcium release through IP3 receptors was found to stimulate migration. This occurred through a signaling pathway involving the calcium sensor calcium/calmodulin protein kinase kinase, AMP-activated kinase, and RhoA/ROCK. By imaging GnRH neurons expressing actin-GFP or Lifeact-RFP, calcium release was found to stimulate leading process actin flow away from the cell body. In contrast, actin contractions at the cell rear were unaffected by this calcium signaling pathway. These findings are the first to test the regulation of cytoskeletal dynamics in axophilic migration, and reveal mechanisms of movement that have broad implications for the migration of other CNS populations.

Continued expression of GATA3 is necessary for cochlear neurosensory development

Hair cells of the developing mammalian inner ear are progressively defined through cell fate restriction. This process culminates in the expression of the bHLH transcription factor Atoh1, which is necessary for differentiation of hair cells, but not for their specification. Loss of several genes will disrupt ear morphogenesis or arrest of neurosensory epithelia development. We previously showed in null mutants that the loss of the transcription factor, Gata3, results specifically in the loss of all cochlear neurosensory development. Temporal expression of Gata3 is broad from the otic placode stage through the postnatal ear. It therefore remains unclear at which stage in development Gata3 exerts its effect. To better understand the stage specific effects of Gata3, we investigated the role of Gata3 in cochlear neurosensory specification and differentiation utilizing a LoxP targeted Gata3 line and two Cre lines. Foxg1^Cre∶Gata3^f/f mice show recombination of Gata3 around E8.5 but continue to develop a cochlear duct without differentiated hair cells and spiral ganglion neurons. qRT-PCR data show that Atoh1 was down-regulated but not absent in the duct whereas other hair cell specific genes such as Pou4f3 were completely absent. In addition, while Sox2 levels were lower in the Foxg1^Cre:Gata3^f/f cochlea, Eya1 levels remained normal. We conclude that Eya1 is unable to fully upregulate Atoh1 or Pou4f3, and drive differentiation of hair cells without Gata3. Pax2-Cre∶Gata3^f/f mice show a delayed recombination of Gata3 in the ear relative to Foxg1^Cre:Gata3^f/f. These mice exhibited a cochlear duct containing patches of partially differentiated hair cells and developed only few and incorrectly projecting spiral ganglion neurons. Our conditional deletion studies reveal a major role of Gata3 in the signaling of prosensory genes and in the differentiation of cochlear neurosenory cells. We suggest that Gata3 may act in combination with Eya1, Six1, and Sox2 in cochlear prosensory gene signaling.

The effect of kainic acid on hippocampal dendritic spine motility at the early and late stages of brain development

Dendrites and spines undergo dynamic changes in physiological conditions, such as learning and memory, and in pathological conditions, such as epilepsy. Abnormalities in dendritic spines have commonly been observed in brain specimens from epilepsy patients and animal models of epilepsy. However, the functional implications and clinical consequences of this dendritic pathology for epilepsy are uncertain. Motility of dendritic spines and axonal filopodia has been recently discovered by the advanced imaging techniques, and remains to a large degree an exciting phenomenology in search of function. Here we demonstrate the effect of kainic acid (KA), which is a structural analog of glutamate, on dendritic spine motility in hippocampal CA1 area at the different stages of brain development. In order to reveal the changes that take place in spine and filopodial motility in the epileptic model of brain, time-lapse imaging of acute hippocampal slices treated with various concentrations of KA after different incubation time points was performed. The effects of KA exposure were tested on the slices from young (postnatal day (P)7-P10) and adolescent (P28-P30) Thy1-YFPH transgenic mice. Slices were treated with either 50 μM or 100 μM of KA, for either 30 or 100 min. The results obtained in our experiments show diverse effects of KA in 2 different age groups. According to our results, 100 μM/100 min KA treatment increases spine motility at early stage of brain development (P10) by 41.5%, while in P30 mice spine motility is increased only by 3%. Our findings also indicate that effect of KA on hippocampal dendritic spine motility is predominantly time- rather than concentration-dependent.

Abl2/Arg controls dendritic spine and dendrite arbor stability via distinct cytoskeletal control pathways

Rho family GTPases coordinate cytoskeletal rearrangements in neurons, and mutations in their regulators are associated with mental retardation and other neurodevelopmental disorders (Billuart et al., 1998; Kutsche et al., 2000; Newey et al., 2005; Benarroch, 2007). Chromosomal microdeletions encompassing p190RhoGAP or its upstream regulator, the Abl2/Arg tyrosine kinase, have been observed in cases of mental retardation associated with developmental defects (Scarbrough et al., 1988; James et al., 1996; Takano et al., 1997; Chaabouni et al., 2006; Leal et al., 2009). Genetic knock-out of Arg in mice leads to synapse, dendritic spine, and dendrite arbor loss accompanied by behavioral deficits (Moresco et al., 2005; Sfakianos et al., 2007). To elucidate the cell-autonomous mechanisms by which Arg regulates neuronal stability, we knocked down Arg in mouse hippocampal neuronal cultures. We find that Arg knockdown significantly destabilizes dendrite arbors and reduces dendritic spine density by compromising dendritic spine stability. Inhibiting RhoA prevents dendrite arbor loss following Arg knockdown in neurons, but does not block spine loss. Interestingly, Arg-deficient neurons exhibit increased miniature EPSC amplitudes, and their remaining spines exhibit larger heads deficient in the actin stabilizing protein cortactin. Spine destabilization in Arg knockdown neurons is prevented by blocking NMDA receptor-dependent relocalization of cortactin from spines, or by forcing cortactin into spines via fusion to an actin-binding region of Arg. Thus, Arg employs distinct mechanisms to selectively regulate spine and dendrite stability: Arg dampens activity-dependent disruption of cortactin localization to stabilize spines and attenuates Rho activity to stabilize dendrite arbors.

Effects of progesterone and medroxyprogesterone on actin remodeling and neuronal spine formation

Sex steroids are important regulators of neuronal cell morphology, and this is critical for gender differences in brain function and dysfunction. Neuronal morphology is controlled by multiprotein complexes including moesin (a member of the ezrin/radixin/moesin family), focal adhesion kinase (FAK), or the Wiskott-Aldrich syndrome protein-family verprolin homologous (WAVE1) protein, controlling dynamic remodeling of the cytoskeleton and cell membrane. We investigated the actions of natural progesterone (P) and of the synthetic progestin medroxyprogesterone acetate (MPA) on actin remodeling, focal adhesion complex formation, and actin branching in rat cortical neurons. Treatment with P and, to a lesser extent, MPA, increases the number and density of dendritic spines. P increases the phosphorylation of moesin, FAK, and WAVE1, and their redistribution toward cell membrane sites where spines are formed. Signaling to moesin is achieved by PR via a Gα/Gβ-dependent signaling to the small GTPase Ras homolog gene family, member A and its related kinase, Rho-associated kinase-2. In parallel, WAVE1 recruitment is triggered by a Gαi/Gβ-dependent signaling of PR to c-Src, FAK, and Rac1 GTPase. Rac1 recruits cyclin-dependent kinase-5, which phosphorylates WAVE1. Silencing of moesin, FAK, or WAVE1 abrogates the increase in dendritic spines induced by progesterone. In all applications, MPA is found to act similar to P, albeit with a lower efficacy. In conclusion, our findings indicate that the control of actin polymerization and branching and focal adhesion complex formation via moesin, FAK, and WAVE1 is a key function of progesterone receptor in neurons, which may be relevant for the regulation of dendritic spine turnover and neuronal plasticity.

Microdomains in forebrain spines: an ultrastructural perspective

Glutamatergic axons in the mammalian forebrain terminate predominantly onto dendritic spines. Long-term changes in the efficacy of these excitatory synapses are tightly coupled to changes in spine morphology. The reorganization of the actin cytoskeleton underlying this spine "morphing" involves numerous proteins that provide the machinery needed for adaptive cytoskeletal remodeling. Here, we review recent literature addressing the chemical architecture of the spine, focusing mainly on actin-binding proteins (ABPs). Accumulating evidence suggests that ABPs are organized into functionally distinct microdomains within the spine cytoplasm. This functional compartmentalization provides a structural basis for regulation of the spinoskeleton, offering a novel window into mechanisms underlying synaptic plasticity.

Glucocorticoid suppresses dendritic spine development mediated by down-regulation of caldesmon expression

Glucocorticoids (GCs) mediate the effects of stress to cause structural plasticity in brain regions such as the hippocampus, including simplification of dendrites and shrinkage of dendritic spines. However, the molecular mechanics linking stress and GCs to these effects remain largely unclear. Here, we demonstrated that corticosterone (CORT) reduces the expression levels of caldesmon (CaD), causing dendritic spines to become vulnerable. CaD regulates cell motility by modulating the actin-myosin system and actin filament stability. In cultured rat hippocampal neurons, CaD localized to dendritic spines by binding to filamentous actin (F-actin), and CaD expression levels increased during spine development. CaD stabilized the F-actin dynamics in spines, thereby enlarging the spine heads, whereas CaD knockdown decreased the spine-head size via destabilization of the F-actin dynamics. CaD was also required for chemical LTP-induced actin stabilization. The CaD expression levels were markedly decreased by exposure to CORT mediated by suppression of serum response factor-dependent transcription. High CORT levels reduced both the spine-head size and F-actin stability similarly to CaD knockdown, and overexpressing CaD abolished the detrimental effect of CORT on dendritic spine development. These results indicate that CaD enlarges the spine-head size by stabilizing F-actin dynamics, and that CaD is a critical target in the GC-induced detrimental effects on dendritic spine development.

Structural plasticity of spines at giant mossy fiber synapses

The granule cells of the dentate gyrus give rise to thin unmyelinated axons, the mossy fibers. They form giant presynaptic boutons impinging on large complex spines on the proximal dendritic portions of hilar mossy cells and CA3 pyramidal neurons. While these anatomical characteristics have been known for some time, it remained unclear whether functional changes at mossy fiber synapses such as long-term potentiation (LTP) are associated with structural changes. Since subtle structural changes may escape a fine-structural analysis when the tissue is fixed by using aldehydes and is dehydrated in ethanol, rapid high-pressure freezing (HPF) of the tissue was applied. Slice cultures of hippocampus were prepared and incubated in vitro for 2 weeks. Then, chemical LTP (cLTP) was induced by the application of 25 mM tetraethylammonium (TEA) for 10 min. Whole-cell patch-clamp recordings from CA3 pyramidal neurons revealed a highly significant potentiation of mossy fiber synapses when compared to control conditions before the application of TEA. Next, the slice cultures were subjected to HPF, cryosubstitution, and embedding in Epon for a fine-structural analysis. When compared to control tissue, we noticed a significant decrease of synaptic vesicles in mossy fiber boutons and a concomitant increase in the length of the presynaptic membrane. On the postsynaptic side, we observed the formation of small, finger-like protrusions, emanating from the large complex spines. These short protrusions gave rise to active zones that were shorter than those normally found on the thorny excrescences. However, the total number of active zones was significantly increased. Of note, none of these cLTP-induced structural changes was observed in slice cultures from Munc13-1 deficient mouse mutants showing severely impaired vesicle priming and docking. In conclusion, application of HPF allowed us to monitor cLTP-induced structural reorganization of mossy fiber synapses.

Presynaptic GABA(B) Receptor Regulates Activity-Dependent Maturation and Patterning of Inhibitory Synapses through Dynamic Allocation of Synaptic Vesicles

Accumulating evidence indicate that GABA regulates activity-dependent development of inhibitory synapses in the vertebrate brain, but the underlying mechanisms remain unclear. Here we combined live imaging of cortical GABAergic axons with single cell genetic manipulation to dissect the role of presynaptic GABA_B receptors (GABA_BRs) in inhibitory synapse formation in mouse. Developing GABAergic axons form a significant number of transient boutons but only a subset was stabilized. Synaptic vesicles in these nascent boutons are often highly mobile in the course of tens of minutes. Activation of presynaptic GABA_BRs stabilized mobile vesicles in nascent boutons through the local enhancement of actin polymerization. Inactivation of GABA_BRs in developing basket interneurons resulted in aberrant pattern of bouton size distribution, reduced bouton density and reduced axon branching, as well as reduced frequency of miniature inhibitory currents in postsynaptic pyramidal neurons. These results suggest that GABA_BRs along developing inhibitory axons act as a local sensor of GABA release and promote presynaptic maturation through increased recruitment of mobile vesicle pools. Such release-dependent validation and maturation of nascent terminals is well suited to sculpt the pattern of synapse formation and distribution along axon branches.

Street rabies virus causes dendritic injury and F-actin depolymerization in the hippocampus

Rabies is an acute viral infection of the central nervous system and is typically fatal in humans and animals; however, its pathogenesis remains poorly understood. In this study, the morphological changes of dendrites and dendritic spines in the CA1 region of the hippocampus were investigated in mice that were infected intracerebrally with an MRV strain of the street rabies virus. Haematoxylin and eosin and fluorescence staining analysis of brain sections from the infected mice showed very few morphological changes in the neuronal bodies and neuronal processes. However, we found a significant decrease in the number of dendritic spines. Primary neuronal cultures derived from the hippocampus of mice (embryonic day 16.5) that were infected with the virus also showed an obvious decrease in the number of dendritic spines. Furthermore, the decrease in the number of dendritic spines was related to the depolymerization of actin filaments (F-actin). We propose that the observed structural changes can partially explain the severe clinical disease that was found in experimental models of street rabies virus infections.

Behavioral effects of Rho GTPase modulation in a model of Alzheimer's disease

Small GTPases of the Rho family, including Rho, Rac and CDC42 subfamilies, play key role in neural connectivity and cognition. The pharmacological modulation of these regulatory proteins is associated with enhancement of learning and memory. We sought to determine whether the modulation of cerebral Rho GTPases may correct behavioral disturbances in a mouse model of Alzheimer's disease (AD). TgCRND8 mice show early-onset Abeta amyloid deposits associated with deficits in several cognitive tasks. We report that four-month old TgCRND8 mice display (a) increased locomotor activity in an open field, (b) mild deficits in the learning of a fixed platform position in a water maze task. More markedly, after displacement of the escape platform, TgCRND8 mice exhibit impairment in the learning of the novel position (reversal learning), as they perseverate searching in the familiar position. The administration of the Rho GTPase activator Cytotoxic Necrotizing Factor 1 (CNF1, 1.0 fmol kg(-1) intracerebroventricularly) reduces locomotor hyperactivity and corrects the deficits in reversal learning, thus re-establishing normal behavioral plasticity. We conclude that the pharmacological modulation of Rho GTPase signaling might be beneficial for the treatment of AD. Reversal learning in TgCRND8 mice may represent a convenient pre-clinical assay for the efficacy of therapeutic interventions in AD.

Structural plasticity of hippocampal mossy fiber synapses as revealed by high-pressure freezing

Despite recent progress in fluorescence microscopy techniques, electron microscopy (EM) is still superior in the simultaneous analysis of all tissue components at high resolution. However, it is unclear to what extent conventional fixation for EM using aldehydes results in tissue alteration. Here we made an attempt to minimize tissue alteration by using rapid high-pressure freezing (HPF) of hippocampal slice cultures. We used this approach to monitor fine-structural changes at hippocampal mossy fiber synapses associated with chemically induced long-term potentiation (LTP). Synaptic plasticity in LTP has been known to involve structural changes at synapses including reorganization of the actin cytoskeleton and de novo formation of spines. While LTP-induced formation and growth of postsynaptic spines have been reported, little is known about associated structural changes in presynaptic boutons. Mossy fiber synapses are assumed to exhibit presynaptic LTP expression and are easily identified by EM. In slice cultures from wildtype mice, we found that chemical LTP increased the length of the presynaptic membrane of mossy fiber boutons, associated with a de novo formation of small spines and an increase in the number of active zones. Of note, these changes were not observed in slice cultures from Munc13-1 knockout mutants exhibiting defective vesicle priming. These findings show that activation of hippocampal mossy fibers induces pre- and postsynaptic structural changes at mossy fiber synapses that can be monitored by EM.

A role for hippocampal actin rearrangement in object placement memory in female rats

Actin rearrangement, the polymerization of globular actin (G-actin) to filamentous actin, causes morphological changes in dendritic spines and is hypothesized to be a substrate of learning and memory. The ovarian hormone estradiol promotes hippocampal actin rearrangement and enhances performance on hippocampus-dependent tasks, including object placement memory. The goals of the current study were to determine a role for actin rearrangement and its regulatory pathway in object placement memory in female rats and to determine if estradiol impacts actin rearrangement in ovariectomized rats during the performance of the task. In an initial experiment, young adult Long-Evans rats were ovariectomized and implanted with capsules containing either cholesterol vehicle or estradiol. Bilateral intrahippocampal infusions of aCSF vehicle or the actin rearrangement inhibitor, latrunculin A, were administered 15 min prior to initiation of the object placement task. Latrunculin A dose-dependently impaired object placement memory. Estradiol had no impact on the ability of latrunculin A to affect performance. In a second experiment, rats were ovariectomized and received implants containing cholesterol or estradiol. Half of each hormone treatment group was exposed to the object placement memory task and half underwent control procedures. Immediately following completion of behavior, rats were euthanized and hippocampi removed. Western blotting was used to measure hippocampal levels of phosphorylated and total levels of a regulator of actin polymerization, the actin depolymerization factor cofilin. Exposure to the object placement memory task resulted in significant increases in phosphorylated levels of cofilin. Estradiol treatment had no impact on protein levels. These data support a role for hippocampal actin rearrangement and its regulatory proteins in object placement memory in female rats and suggest that chronic estradiol treatment does not impact hippocampal actin arrangement.

Neurochemical and neurostructural plasticity in alcoholism

The behavioral manifestations of alcoholism are primarily attributable to the numerous and lasting adaptations that occur in the brain as a result of chronic heavy alcohol consumption. As will be reviewed here, these adaptations include alcohol-induced plasticity in chemical neurotransmission, density and morphology of dendritic spines, as well as neurodegeneration and cerebral atrophy. Within the context of these neuroadaptations that have been observed in both human and animal studies, we will discuss how these changes potentially contribute to the cognitive and behavioral dysfunctions that are hallmark features of alcoholism, as well as how they reveal novel potential pharmacological targets for the treatment of this disorder.

Cranial irradiation alters dendritic spine density and morphology in the hippocampus

Therapeutic irradiation of the brain is a common treatment modality for brain tumors, but can lead to impairment of cognitive function. Dendritic spines are sites of excitatory synaptic transmission and changes in spine structure and number are thought to represent a morphological correlate of altered brain functions associated with hippocampal dependent learning and memory. To gain some insight into the temporal and sub region specific cellular changes in the hippocampus following brain irradiation, we investigated the effects of 10 Gy cranial irradiation on dendritic spines in young adult mice. One week or 1 month post irradiation, changes in spine density and morphology in dentate gyrus (DG) granule and CA1 pyramidal neurons were quantified using Golgi staining. Our results showed that in the DG, there were significant reductions in spine density at both 1 week (11.9%) and 1 month (26.9%) after irradiation. In contrast, in the basal dendrites of CA1 pyramidal neurons, irradiation resulted in a significant reduction (18.7%) in spine density only at 1 week post irradiation. Analysis of spine morphology showed that irradiation led to significant decreases in the proportion of mushroom spines at both time points in the DG as well as CA1 basal dendrites. The proportions of stubby spines were significantly increased in both the areas at 1 month post irradiation. Irradiation did not alter spine density in the CA1 apical dendrites, but there were significant changes in the proportion of thin and mushroom spines at both time points post irradiation. Although the mechanisms involved are not clear, these findings are the first to show that brain irradiation of young adult animals leads to alterations in dendritic spine density and morphology in the hippocampus in a time dependent and region specific manner.

Linkage of N-cadherin to multiple cytoskeletal elements revealed by a proteomic approach in hippocampal neurons

The CNS synapse is an adhesive junction differentiated for chemical neurotransmission and is equipped with presynaptic vesicles and postsynaptic neurotransmitter receptors. Cell adhesion molecule cadherins not only maintain connections between pre- and postsynaptic membranes but also modulate the efficacy of synaptic transmission. Although the components of the cadherin-mediated adhesive apparatus have been studied extensively in various cell systems, the complete picture of these components, particularly at the synaptic junction, remains elusive. Here, we describe the proteomic assortment of the N-cadherin-mediated synaptic adhesion apparatus in cultured hippocampal neurons. N-cadherin immunoprecipitated from Triton X-100-solubilized neuronal extract contained equal amounts of β- and α-catenins, as well as F-actin-related membrane anchor proteins such as integrins bridged with α-actinin-4, and Na(+)/K(+)-ATPase bridged with spectrins. A close relative of β-catenin, plakoglobin, and its binding partner, desmoplakin, were also found, suggesting that a subset of the N-cadherin-mediated adhesive apparatus also anchors intermediate filaments. Moreover, dynein heavy chain and LEK1/CENPF/mitosin were found. This suggests that internalized pools of N-cadherin in trafficking vesicles are conveyed by dynein motors on microtubules. In addition, ARVCF and NPRAP/neurojungin/δ2-catenin, but not p120ctn/δ1-catenin or plakophilins-1, -2, -3, -4 (p0071), were found, suggesting other possible bridges to microtubules. Finally, synaptic stimulation by membrane depolarization resulted in an increased 93-kDa band, which corresponded to proteolytically truncated β-catenin. The integration of three different classes of cytoskeletal systems found in the synaptic N-cadherin complex may imply a dynamic switching of adhesive scaffolds in response to synaptic activity.