NMDA receptors mediate hypoxic spine loss in cultured neurons

Between life and death: the brain twilight zones

Clinically, and legally, death is considered a well-defined state of the organism characterized, at least, by a complete and irreversible cessation of brain activities and functions. According to this pragmatic approach, the moment of death is implicitly represented by a discrete event from which all cerebral processes abruptly cease. However, a growing body of experimental and clinical evidence has demonstrated that cardiorespiratory failure, the leading cause of death, causes complex time-dependent changes in neuronal activity that can lead to death but also be reversed with successful resuscitation. This review synthesizes our current knowledge of the succeeding alterations in brain activities that accompany the dying and resuscitation processes. The anoxia-dependent brain defects that usher in a process of potential death successively include: (1) a set of changes in electroencephalographic (EEG) and neuronal activities, (2) a cessation of brain spontaneous electrical activity (isoelectric state), (3) a loss of consciousness whose timing in relation to EEG changes remains unclear, (4) an increase in brain resistivity, caused by neuronal swelling, concomitant with the occurrence of an EEG deviation reflecting the neuronal anoxic insult (the so-called "wave of death," or "terminal spreading depolarization"), followed by, (5) a terminal isoelectric brain state leading to death. However, a timely restoration of brain oxygen supply-or cerebral blood flow-can initiate a mirrored sequence of events: a repolarization of neurons followed by a re-emergence of neuronal, synaptic, and EEG activities from the electrocerebral silence. Accordingly, a recent study has revealed a new death-related brain wave: the "wave of resuscitation," which is a marker of the collective recovery of electrical properties of neurons at the beginning of the brain's reoxygenation phase. The slow process of dying still represents a terra incognita, during which neurons and neural networks evolve in uncertain states that remain to be fully understood. As current event-based models of death have become neurophysiologically inadequate, I propose a new mixed (event-process) model of death and resuscitation. It is based on a detailed description of the different phases that succeed each other in a dying brain, which are generally described separately and without mechanistic linkage, in order to integrate them into a continuum of declining brain activity. The model incorporates cerebral twilight zones (with still unknown neuronal and synaptic processes) punctuated by two characteristic cortical waves providing real-time biomarkers of death- and resuscitation.

Dendritic spines: Revisiting the physiological role

Dendritic spines are small, thin, specialized protrusions from neuronal dendrites, primarily localized in the excitatory synapses. Sophisticated imaging techniques revealed that dendritic spines are complex structures consisting of a dense network of cytoskeletal, transmembrane and scaffolding molecules, and numerous surface receptors. Molecular signaling pathways, mainly Rho and Ras family small GTPases pathways that converge on actin cytoskeleton, regulate the spine morphology and dynamics bi-directionally during synaptic activity. During synaptic plasticity the number and shapes of dendritic spines undergo radical reorganizations. Long-term potentiation (LTP) induction promote spine head enlargement and the formation and stabilization of new spines. Long-term depression (LTD) results in their shrinkage and retraction. Reports indicate increased spine density in the pyramidal neurons of autism and Fragile X syndrome patients and reduced density in the temporal gyrus loci of schizophrenic patients. Post-mortem reports of Alzheimer's brains showed reduced spine number in the hippocampus and cortex. This review highlights the spine morphogenesis process, the activity-dependent structural plasticity and mechanisms by which synaptic activity sculpts the dendritic spines, the structural and functional changes in spines during learning and memory using LTP and LTD processes. It also discusses on spine status in neurodegenerative diseases and the impact of nootropics and neuroprotective agents on the functional restoration of dendritic spines.

Chronic asthma-induced behavioral and hippocampal neuronal morphological changes are concurrent with BDNF, cofilin1 and Cdc42/RhoA alterations in immature mice

INTRODUCTION

Recent studies have found that persistent hypoxia caused by chronic asthma, especially during childhood, affects the development and function of the brain, but the mechanism is unclear. In the present study, BDNF and its signal pathway was investigated in mediating chronic asthma induced-neuronal changes that lead to behavior alterations.

METHODS

The chronic asthma model was induced by sensitization with ovalbumin for more than 9 weeks in immature mice. Morris water maze test (MWMT), open field test (OFT) and elevated plus maze test (EPMT) were used to conduct behavioral evaluation. Neuronal morphology in hippocampal CA1, CA3 and DG was assessed using ImageJ's Sholl plugin and RESCONSTRUCT software. BDNF signaling pathway related molecules was determined by Western blotting.

RESULTS

Chronic asthma does affect the behavioral performances of immature mice evaluated in MWMT, OFT, and EPMT. The analysis by three-dimensional reconstruction software found that following the behavioral alteration of asthmatic mice, dendritic changes also occurred in hippocampal neurons, including shortened dendrite length, significantly reduced number of dendritic branches, decreased density of dendritic spines, and reduced percentage of functional dendritic spine types. At the same time, by immunofluorescence and western blotting, we also found that alterations in dendritic morphology were consistent with activation of cofilin1 and changes in BDNF-Cdc42/RhoA levels. Some of the changes mentioned above can be alleviated by intranasal administration of budesonide.

CONCLUSION

Our data suggest that response similar to nicotine withdrawal or/and hypoxia induced by childhood chronic asthma enhances the BDNF-Cdc42/RhoA signaling pathway and activates cofilin1, leading to the remodeling of actin, causing the loss of dendritic spines and atrophy of dendrites, eventually resulting in behavioral alterations.

Role of Akt-independent mTORC1 and GSK3β signaling in sublethal NMDA-induced injury and the recovery of neuronal electrophysiology and survival

Glutamate-induced excitotoxicity, mediated by overstimulation of N-methyl-D-aspartate (NMDA) receptors, is a mechanism that causes secondary damage to neurons. The early phase of injury causes loss of dendritic spines and changes to synaptic activity. The phosphatidylinositol-4,5-bisphosphate 3-kinase/Akt/ mammalian target of rapamycin (PI3K/Akt/mTOR) pathway has been implicated in the modulation and regulation of synaptic strength, activity, maturation, and axonal regeneration. The present study focuses on the physiology and survival of neurons following manipulation of Akt and several downstream targets, such as GSK3β, FOXO1, and mTORC1, prior to NMDA-induced injury. Our analysis reveals that exposure to sublethal levels of NMDA does not alter phosphorylation of Akt, S6, and GSK3β at two and twenty four hours following injury. Electrophysiological recordings show that NMDA-induced injury causes a significant decrease in spontaneous excitatory postsynaptic currents at both two and twenty four hours, and this phenotype can be prevented by inhibiting mTORC1 or GSK3β, but not Akt. Additionally, inhibition of mTORC1 or GSK3β promotes neuronal survival following NMDA-induced injury. Thus, NMDA-induced excitotoxicity involves a mechanism that requires the permissive activity of mTORC1 and GSK3β, demonstrating the importance of these kinases in the neuronal response to injury.

Molecular regulation of dendritic spine dynamics and their potential impact on synaptic plasticity and neurological diseases

The structure and dynamics of dendritic spines reflect the strength of synapses, which are severely affected in different brain diseases. Therefore, understanding the ultra-structure, molecular signaling mechanism(s) regulating dendritic spine dynamics is crucial. Although, since last century, dynamics of spine have been explored by several investigators in different neurological diseases, but despite countless efforts, a comprehensive understanding of the fundamental etiology and molecular signaling pathways involved in spine pathology is lacking. The purpose of this review is to provide a contextual framework of our current understanding of the molecular mechanisms of dendritic spine signaling, as well as their potential impact on different neurodegenerative and psychiatric diseases, as a format for highlighting some commonalities in function, as well as providing a format for new insights and perspectives into this critical area of research. Additionally, the potential strategies to restore spine structure-function in different diseases are also pointed out. Overall, these informations should help researchers to design new drugs to restore the structure-function of dendritic spine, a "hot site" of synaptic plasticity.

Chronic stress alters the dendritic morphology of callosal neurons and the acute glutamate stress response in the rat medial prefrontal cortex

We have previously reported that interhemispheric regulation of medial prefrontal cortex (PFC)-mediated stress responses is subserved by glutamate (GLU)- containing callosal neurons. Evidence of chronic stress-induced dendritic and spine atrophy among PFC pyramidal neurons led us to examine how chronic restraint stress (CRS) might alter the apical dendritic morphology of callosal neurons and the acute GLU stress responses in the left versus right PFC. Morphometric analyses of retrogradely labeled, dye-filled PFC callosal neurons revealed hemisphere-specific CRS-induced dendritic retraction; whereas significant dendritic atrophy occurred primarily within the distal arbor of left PFC neurons, it was observed within both the proximal and distal arbor of right PFC neurons. Overall, CRS also significantly reduced spine densities in both hemispheres with the greatest loss occurring among left PFC neurons, mostly at the distal extent of the arbor. While much of the overall decrease in dendritic spine density was accounted by the loss of thin spines, the density of mushroom-shaped spines, despite being fewer in number, was halved. Using microdialysis we found that, compared to controls, basal PFC GLU levels were significantly reduced in both hemispheres of CRS animals and that their GLU response to 30 min of tail-pinch stress was significantly prolonged in the left, but not the right PFC. Together, these findings show that a history of chronic stress alters the dendritic morphology and spine density of PFC callosal neurons and suggest a mechanism by which this might disrupt the interhemispheric regulation of PFC-mediated responses to subsequent stressors.

The nuclear calcium signaling target, activating transcription factor 3 (ATF3), protects against dendrotoxicity and facilitates the recovery of synaptic transmission after an excitotoxic insult

The focal swellings of dendrites ("dendritic beading") are an early morphological hallmark of neuronal injury and dendrotoxicity. They are associated with a variety of pathological conditions, including brain ischemia, and cause an acute disruption of synaptic transmission and neuronal network function, which contribute to subsequent neuronal death. Here, we show that increased synaptic activity prior to excitotoxic injury protects, in a transcription-dependent manner, against dendritic beading. Expression of activating transcription factor 3 (ATF3), a nuclear calcium-regulated gene and member of the core gene program for acquired neuroprotection, can protect against dendritic beading. Conversely, knockdown of ATF3 exacerbates dendritic beading. Assessment of neuronal network functions using microelectrode array recordings revealed that hippocampal neurons expressing ATF3 were able to regain their ability for functional synaptic transmission and to participate in coherent neuronal network activity within 48 h after exposure to toxic concentrations of NMDA. Thus, in addition to attenuating cell death, synaptic activity and expression of ATF3 render hippocampal neurons more resistant to acute dendrotoxicity and loss of synapses. Dendroprotection can enhance recovery of neuronal network functions after excitotoxic insults.

Isoflurane post-conditioning protects primary cultures of cortical neurons against oxygen and glucose deprivation injury via upregulation of Slit2/Robo1

Different mechanisms have been suggested to contribute to isoflurane-mediated neuroprotection. Previous studies have suggested that the protein Slit can abrogate neuronal death in mixed neuronal-glial cultures exposed to oxygen-glucose deprivation (OGD) and reperfusion (OGD/R). We hypothesized that isoflurane increases the expression of Slit and its receptor Robo when cortical neurons are exposed to OGD/R. To test this hypothesis, we exposed primary cortical neurons to OGD for 90 min and reperfusion for 24h and investigated how isoflurane post-conditioning affected cell survival and expression of Slit2 and receptors Robo1 and Robo4. Cell survival increased after administration of isoflurane, as assessed by the lactate dehydrogenase assay, trypan blue analysis, and propidium iodide staining. Western blot analysis showed that cleaved caspase-3 was increased after OGD/R(P<0.01) but reduced by isoflurane post-conditioning. Real-time PCR and Western blot analysis showed that the expression levels of Slit2 and Robo1, but not Robo4, were increased after OGD/R (P<0.5) and increased even further by isoflurane post-conditioning (P<0.01). Our results suggest that isoflurane post-conditioning markedly attenuates apoptosis and necrosis of cortical neurons exposed to OGD/R possibly in part via elevation of Slit2 and Robo1 expression. These findings provide a novel explanation for the pleiotropic effects of isoflurane that could benefit the central nervous system.

Vezatin is essential for dendritic spine morphogenesis and functional synaptic maturation

Vezatin is an integral membrane protein associated with cell-cell adhesion complex and actin cytoskeleton. It is expressed in the developing and mature mammalian brain, but its neuronal function is unknown. Here, we show that Vezatin localizes in spines in mature mouse hippocampal neurons and codistributes with PSD95, a major scaffolding protein of the excitatory postsynaptic density. Forebrain-specific conditional ablation of Vezatin induced anxiety-like behavior and impaired cued fear-conditioning memory response. Vezatin knock-down in cultured hippocampal neurons and Vezatin conditional knock-out in mice led to a significantly increased proportion of stubby spines and a reduced proportion of mature dendritic spines. PSD95 remained tethered to presynaptic terminals in Vezatin-deficient hippocampal neurons, suggesting that the reduced expression of Vezatin does not compromise the maintenance of synaptic connections. Accordingly, neither the amplitude nor the frequency of miniature EPSCs was affected in Vezatin-deficient hippocampal neurons. However, the AMPA/NMDA ratio of evoked EPSCs was reduced, suggesting impaired functional maturation of excitatory synapses. These results suggest a role of Vezatin in dendritic spine morphogenesis and functional synaptic maturation.

Excessive Na+/H+ exchange in disruption of dendritic Na+ and Ca2+ homeostasis and mitochondrial dysfunction following in vitro ischemia

Neuronal dendrites are vulnerable to injury under diverse pathological conditions. However, the underlying mechanisms for dendritic Na(+) overload and the selective dendritic injury remain poorly understood. Our current study demonstrates that activation of NHE-1 (Na(+)/H(+) exchanger isoform 1) in dendrites presents a major pathway for Na(+) overload. Neuronal dendrites exhibited higher pH(i) regulation rates than soma as a result of a larger surface area/volume ratio. Following a 2-h oxygen glucose deprivation and a 1-h reoxygenation, NHE-1 activity was increased by ∼70-200% in dendrites. This elevation depended on activation of p90 ribosomal S6 kinase. Moreover, stimulation of NHE-1 caused dendritic Na(+)(i) accumulation, swelling, and a concurrent loss of Ca(2+)(i) homeostasis. The Ca(2+)(i) overload in dendrites preceded the changes in soma. Inhibition of NHE-1 or the reverse mode of Na(+)/Ca(2+) exchange prevented these changes. Mitochondrial membrane potential in dendrites depolarized 40 min earlier than soma following oxygen glucose deprivation/reoxygenation. Blocking NHE-1 activity not only attenuated loss of dendritic mitochondrial membrane potential and mitochondrial Ca(2+) homeostasis but also preserved dendritic membrane integrity. Taken together, our study demonstrates that NHE-1-mediated Na(+) entry and subsequent Na(+)/Ca(2+) exchange activation contribute to the selective dendritic vulnerability to in vitro ischemia.

The role of the ubiquitin proteasome system in ischemia and ischemic tolerance

Ubiquitin modification targets a protein for rapid degradation by the proteasome. However, polyubiquitination of proteins can result in multiple functions depending on the topology of the ubiquitin chain. Therefore, ubiquitin signaling offers a more complex and versatile biology compared with many other posttranslational modifications. One area of potential for the application of this knowledge is the field of ischemia-induced brain damage, as occurs following a stroke. The ubiquitin proteasome system may exert a dual role on neuronal outcome following ischemia. Harmful ischemia results in an overload of the ubiquitin proteasome system, and blocking the proteasome reduces brain infarction following ischemia. However, the rapid and selective degradation of proteins following brief ischemia results in endogenous protection against ischemia. Therefore, further understanding of the molecular signaling mechanisms that regulate the ubiquitin proteasome system may reveal novel therapeutic targets to reduce brain damage when ischemia is predicted or reduce the activation of the cell death mechanisms and the inflammatory response following stroke. The aim of this review is to discuss some of the recent advances in the understanding of protein ubiquitination and its implications for novel stroke therapies.

Hypobaric hypoxia induces dendritic plasticity in cortical and hippocampal pyramidal neurons in rat brain

Hypobaric hypoxia (HH), a predisposing environmental condition at high altitude (HA) encountered by many mountaineers jeopardizes their normal physiology like motor coordination and cognitive functions. Our previous studies revealed that the HH induces oxidative stress and neurodegeneration, which is associated with spatial memory impairment in rats. However, the dendritic changes after exposure to different duration of HH remain largely unknown. The aim of the present study was to investigate the duration-dependent dendritic changes in CA1, CA3 and entorhinal cortex (EC) of hippocampus and layer II of prefrontal cortex (PFC) with spatial memory functions in rats on exposure to different duration of HH. The rats were exposed to simulated HA of 6100 m for 3, 7, 14 and 21 days and the spatial reference memory was investigated using Morris water maze (MWM) and the morphological alteration of CA1, CA3, EC and layer II of PFC were investigated. There was a significant decrease in dendritic arborization and spine number along with increased number of damaged neurons, after 3, 7 and 14 days of HH but after 21 days of HH exposure the structural recovery was noted in all the regions. There was impairment of spatial memory after 3 and 7 days of exposure, but slight improvement of spatial memory was noted after 14 and 21 days of exposure. Our studies suggested that HH induces dendritic plasticity of PFC and hippocampal pyramidal neurons of rat brain, which might be associated with improvement of spatial memory function after 21 days of HH exposure.

Important role of matrix metalloproteinase 9 in epileptogenesis

Temporal lobe epilepsy (TLE) is a devastating disease in which aberrant synaptic plasticity plays a major role. We identify matrix metalloproteinase (MMP) 9 as a novel synaptic enzyme and a key pathogenic factor in two animal models of TLE: kainate-evoked epilepsy and pentylenetetrazole (PTZ) kindling–induced epilepsy. Notably, we show that the sensitivity to PTZ epileptogenesis is decreased in MMP-9 knockout mice but is increased in a novel line of transgenic rats overexpressing MMP-9. Immunoelectron microscopy reveals that MMP-9 associates with hippocampal dendritic spines bearing asymmetrical (excitatory) synapses, where both the MMP-9 protein levels and enzymatic activity become strongly increased upon seizures. Further, we find that MMP-9 deficiency diminishes seizure-evoked pruning of dendritic spines and decreases aberrant synaptogenesis after mossy fiber sprouting. The latter observation provides a possible mechanistic basis for the effect of MMP-9 on epileptogenesis. Our work suggests that a synaptic pool of MMP-9 is critical for the sequence of events that underlie the development of seizures in animal models of TLE.

Ubiquitin proteasome-mediated synaptic reorganization: a novel mechanism underlying rapid ischemic tolerance

Ischemic tolerance is an endogenous neuroprotective mechanism in brain and other organs, whereby prior exposure to brief ischemia produces resilience to subsequent normally injurious ischemia. Although many molecular mechanisms mediate delayed (gene-mediated) ischemic tolerance, the mechanisms underlying rapid (protein synthesis-independent) ischemic tolerance are relatively unknown. Here we describe a novel mechanism for the induction of rapid ischemic tolerance mediated by the ubiquitin-proteasome system. Rapid ischemic tolerance is blocked by multiple proteasome inhibitors [carbobenzoxy-L-leucyl-L-leucyl-L-leucinal (MG132), MG115 (carbobenzoxy-L-leucyl-L-leucyl-L-norvalinal), and clasto-lactacystin-beta-lactone]. A proteomics strategy was used to identify ubiquitinated proteins after preconditioning ischemia. We focused our studies on two actin-binding proteins of the postsynaptic density that were ubiquitinated after rapid preconditioning: myristoylated, alanine-rich C-kinase substrate (MARCKS) and fascin. Immunoblots confirm the degradation of MARCKS and fascin after preconditioning ischemia. The loss of actin-binding proteins promoted actin reorganization in the postsynaptic density and transient retraction of dendritic spines. This rapid and reversible synaptic remodeling reduced NMDA-mediated electrophysiological responses and renders the cells refractory to NMDA receptor-mediated toxicity. The dendritic spine retraction and NMDA neuroprotection after preconditioning ischemia are blocked by actin stabilization with jasplakinolide, as well as proteasome inhibition with MG132. Together these data suggest that rapid tolerance results from changes to the postsynaptic density mediated by the ubiquitin-proteasome system, rendering neurons resistant to excitotoxicity.

Permissive and repulsive cues and signalling pathways of axonal outgrowth and regeneration

Successful axonal outgrowth in the adult central nervous system (CNS) is central to the process of nerve regeneration and brain repair. To date, much of the knowledge on axonal guidance and outgrowth comes from studies on neuritogenesis and patterning during development where distal growth cones constantly sample the local environment and respond to specific physical and trophic influences. Opposing permissive (e.g., growth factors) and hostile signals (e.g., repulsive cues) are processed, leading to growth cone remodelling, and a concomitant restructuring of the cytoskeleton, thereby permitting pioneering extension and a potential for establishing synaptic connections. Repulsive cues, such as semaphorins, ephrins and myelin-secreted inhibitory glycoproteins, act through their respective receptors to affect the collapsing or turning of growth cones via several pathways, such as the Rho GTPases signalling which precipitates the cytoskeletal changes. One of the direct modulators of microtubules is the family of brain-specific proteins, collapsin response mediator protein (CRMP). Exciting evidence emerged recently that cleavage of CRMPs in response to injury-activated proteases, such as calpain, signals axonal retraction and neuronal death in adult post-mitotic neurons, while blocking this signal transduction prevents axonal retraction and death following excitotoxic insult and cerebral ischemia. Regeneration is minimal in injured postnatal CNS, albeit the occurrence of some limited remodelling in areas where synaptic plasticity is prevalent. Frequently in the absence of axonal regeneration, there is not only an inevitable loss of functional connections, but also a loss of neurons, such as through the actions of dependence receptors. Deciphering the cues and signalling pathways of axonal guidance and outgrowth may hold the key to fully understanding nerve regeneration and brain repair, thereby opening the way for developing potential therapeutics.

Slit modulates cerebrovascular inflammation and mediates neuroprotection against global cerebral ischemia

Cerebrovascular inflammation contributes to secondary brain injury following ischemia. Recent in vitro studies of cell migration and molecular guidance mechanisms have indicated that the Slit family of secreted proteins can exert repellant effects on leukocyte recruitment in response to chemoattractants. Utilizing intravital microscopy, we addressed the role of Slit in modulating leukocyte dynamics in the mouse cortical venular microcirculation in vivo following TNFalpha application or global cerebral ischemia. We also studied whether Slit affected neuronal survival in the mouse global ischemia model as well as in mixed neuronal-glial cultures subjected to oxygen-glucose deprivation. We found that systemically administered Slit significantly attenuated cerebral microvessel leukocyte-endothelial adherence occurring 4 h after TNFalpha and 24 h after global cerebral ischemia. Administration of RoboN, the soluble receptor for Slit, exacerbated the acute chemotactic response to TNFalpha. These findings are indicative of a tonic repellant effect of endogenous Slit in brain under acute proinflammatory conditions. Three days of continuous systemic administration of Slit following global ischemia significantly attenuated the delayed neuronal death of hippocampal CA1 pyramidal cells. Moreover, Slit abrogated neuronal death in mixed neuronal-glial cultures exposed to oxygen-glucose deprivation. The ability of Slit to reduce the recruitment of immune cells to ischemic brain and to provide cytoprotective effects suggests that this protein may serve as a novel anti-inflammatory and neuroprotective target for stroke therapy.

Dendritic pathology in prion disease starts at the synaptic spine

Spine loss represents a common hallmark of neurodegenerative diseases. However, little is known about the underlying mechanisms, especially the relationship between spine elimination and neuritic destruction. We imaged cortical dendrites throughout a neurodegenerative disease using scrapie in mice as a model. Two-photon in vivo imaging over 2 months revealed a linear decrease of spine density. Interestingly, only persistent spines (lifetime > or = 8 d) disappeared, whereas the density of transient spines (lifetime < or = 4 d) was unaffected. Before spine loss, dendritic varicosities emerged preferentially at sites where spines protrude from the dendrite. These results implicate that the location where the spine protrudes from the dendrite may be particularly vulnerable and that dendritic varicosities may actually cause spine loss.

Hypoxic injury of isolated axons is independent of ionotropic glutamate receptors

Axonal injury in white matter is an important consequence of many acute neurological diseases including ischemia. A role for glutamate-mediated excitotoxicity is suggested by observations from in vitro and in situ models that AMPA/kainate blockers can reduce axonal injury. We assessed axonal vulnerability in primary murine neuronal cultures, with axons isolated from their cell bodies using a compartmented chamber design. Transient removal of oxygen and glucose in the axon compartment resulted in irreversible loss of axon length and neurofilament labeling. This injury was not prevented by addition of ionotropic glutamate receptor blockers and could not be reproduced by glutamate receptor agonists. However, hypoxic injury was prevented by blockade of voltage-gated sodium channels, inhibition of calpain and removal of extracellular calcium. These results suggest that isolated, unmyelinated axons are vulnerable to hypoxic injury which is mediated by influx of sodium and calcium but is independent of glutamate receptor activation.

NMDA receptor mediated dendritic plasticity in cortical cultures after oxygen-glucose deprivation

Dendrites and spines undergo dynamic changes in physiological and pathological conditions. Dendritic outgrowth has been observed in surviving neurons months after ischemia, which is associated with the functional compensation. It remains unclear how dendrites in surviving neurons are altered shortly after ischemia, which might reveal the mechanisms underlying neuronal survival. Using primary cortical cultures, we monitored the dendritic changes in individual neurons after oxygen-glucose deprivation (OGD). Two to four hours of OGD induced approximately 30-50% cell death in 24 h. However, the total dendritic length in surviving neurons was significantly increased after OGD with a peak at 6 h after re-oxygenation. The increase of dendritic length after OGD was mainly due to the sprouting rather than the extension of the dendrites. The dendritic outgrowth after 2 h of OGD was greater than that after 4 h of OGD. Application of NMDA receptor blocker MK-801 abolished OGD-induced dendritic outgrowth, whereas application of AMPA receptor antagonist CNQX had no significant effects. These results demonstrate a NMDA receptor-dependent dendritic plasticity shortly after OGD, which provides insights into the early response of surviving neurons after ischemia.

Actin redistribution underlies the sparing effect of mild hypothermia on dendritic spine morphology after in vitro ischemia

Brain hypothermia is at present the most effective neuroprotective treatment against brain ischemia in man. Ischemia induces a redistribution of proteins involved in synaptic functions, which is markedly diminished by therapeutic hypothermia (33 degrees C). Dendritic spines at excitatory synapses are motile and show both shape changes and rearrangement of synaptic proteins as a consequence of neuronal activity. We investigated the effect of reduced temperature (33 degrees C and 27 degrees C compared with 37 degrees C), on spine motility, length and morphology by studying the distribution of GFP-actin before, during and after induction of in vitro ischemia. Because high-concentration actin filaments are located inside spines, dissociated hippocampal neurons (7-11 DIV) from transgenic mice expressing GFP-actin were used in this study. The movement of the spines and the distribution of GFP-actin were recorded using time-lapse fluorescence microscopy. Under normal conditions rapid rearrangement of GFP-actin was seen in dendritic spines, indicating highly motile spines at 37 degrees C. Decreasing the incubation temperature to 33 degrees C or 27 degrees C, dramatically reduces actin dynamics (spine motility) by approximately 50% and 70%, respectively. In addition, the length of the spine shaft was reduced by 20%. We propose that decreasing the temperature from 37 degrees C to 33 degrees C during ischemia decreases the neuronal actin polymerization rate, which reduces spine calcium kinetics, disrupts detrimental cell signaling and protects neurons against damage.

Effect of excess extracellular glutamate on dendrite growth from cerebral cortical neurons at 3 days in vitro: Involvement of NMDA receptors

Glutamate is an important regulator of dendrite development; however, during cerebral ischemia, massive glutamate release can lead to neurodegeneration and death. An early consequence of glutamate excitotoxicity is dendrite injury, which often precedes cell death. We examined the effect of glutamate on dendrite growth from embryonic day 18 (E18) mouse cortical neurons grown for 3 days in vitro (DIV) and immunolabeled with anti-microtubule-associated protein (MAP)2 and anti-neurofilament (NF)-H, to identify dendrites and axons, respectively. Cortical neurons exposed to excess extracellular glutamate (100 microM) displayed reduced dendrite growth, which occurred in the absence of cell death. This effect was mimicked by the ionotropic glutamate receptor agonist N-methyl-D-aspartate (NMDA) and blocked by the ionotropic glutamate receptor antagonist kynurenic acid and the NMDA receptor-specific antagonist MK-801. The non-NMDA receptor agonist AMPA, however, did not affect process growth. Neither NMDA nor AMPA influenced neuron survival. Immunolabeling and Western blot analysis of NMDA receptors using antibodies against the NR1 subunit, demonstrated that immature cortical neurons used in this study, express NMDA receptors. These results suggest that excess glutamate decreases dendrite growth through a mechanism resulting from NMDA receptor subclass activation. Furthermore, these data support the possibility that excess glutamate activation of NMDA receptors mediate both cell death in mature neurons and the inhibitory effect of excess glutamate on dendrite growth in immature neurons or in the absence of cell death.

Effect of Acute Calcium Influx after Mechanical Stretch InjuryIn Vitroon the Viability of Hippocampal Neurons

We use a new in vitro model to examine the effect of mechanical deformation on neurons. We examined acute changes in cytosolic calcium concentrations ([Ca(2+)](i)) caused by a rapid stretch of cultured hippocampal neurons, using mechanical loading conditions that mimic brain deformations during trauma. We found that stretch-injury of neurons induces a strain-dependent increase in [Ca(2+)](i). Remarkably, the extent of this calcium response exceeded the levels initiated by chemical toxicity with NMDA (100 microM) or glutamate (5 mM) exposure. Propidium iodide labeling at 24 h following stretch showed neuronal death occurred only at the most severe level of mechanical injury. Although NMDA-induced toxicity could be inhibited in calcium free media or by treatment with MK-801, stretch-induced neuronal death was not similarly reduced with either treatment. Unexpectedly, reduction of the acute stretch-induced calcium transient with calcium-free media or MK-801 resulted in an increase in neuronal death at lower stretch levels. These data suggest that mechanical stretch can initiate calcium influx in hippocampal neurons, but substantially modulating the early calcium flux from the extracellular space or through the NMDA channel does not provide an effective means for improving neuronal survival.

A vinca alkaloid enhances morphological dynamics of dendritic spines of neocortical layer 2/3 pyramidal cells

We imaged neocortical layer 2/3 pyramidal cells in rat brain slices with two-photon laser scanning microscopy to investigate that spine motility can be influenced by the voltage-dependent Na(+) and Ca(2+) channel inhibitor, vinpocetine, which exhibited positive cognitive effects in human studies. Veratridine, which enhances sodium influx, was also tested on dendritic spine motility. Perfusion with vinpocetine, a derivative of vinca alkaloids, caused a substantial increase in the structural dynamics of dendritic spines measured by the changes in length or the number of new/retracted spines. In contrast, enhancement of sodium influx with veratridine failed to change spine motility. Our results indicate that the rapid changes in spine shape and size could occur, when calcium and sodium influx has been decreased by this vinca alkaloid. Spine motility induced by vinpocetine may be associated to microtubule alterations, an effect that was described for other vinca alkaloids. On the other hand, the potential of vinpocetine to enhance cognition in clinical studies suggests that the increased spine motility may be related to cognitive functions.