Astrocytes regulate inhibitory synapse formation via Trk-mediated modulation of postsynaptic GABAA receptors

Epigenetic changes in Alzheimer's disease: decrements in DNA methylation

DNA methylation is a vital component of the epigenetic machinery that orchestrates changes in multiple genes and helps regulate gene expression in all known vertebrates. We evaluated immunoreactivity for two markers of DNA methylation and eight methylation maintenance factors in entorhinal cortex layer II, a region exhibiting substantial Alzheimer's disease (AD) pathology in which expression changes have been reported for a wide variety of genes. We show, for the first time, neuronal immunoreactivity for all 10 of the epigenetic markers and factors, with highly significant decrements in AD cases. These decrements were particularly marked in PHF1/PS396 immunoreactive, neurofibrillary tangle-bearing neurons. In addition, two of the DNA methylation maintenance factors, DNMT1 and MBD2, have been reported also to interact with ribosomal RNAs and ribosome synthesis. Consistent with these findings, DNMT1 and MBD2, as well as p66α, exhibited punctate cytoplasmic immunoreactivity that co-localized with the ribosome markers RPL26 and 5.8s rRNA in ND neurons. By contrast, AD neurons generally lacked such staining, and there was a qualitative decrease in RPL26 and 5.8s rRNA immunoreactivity. Collectively, these findings suggest epigenetic dysfunction in AD-vulnerable neurons.

Quantifying synapses: an immunocytochemistry-based assay to quantify synapse number

One of the most important goals in neuroscience is to understand the molecular cues that instruct early stages of synapse formation. As such it has become imperative to develop objective approaches to quantify changes in synaptic connectivity. Starting from sample fixation, this protocol details how to quantify synapse number both in dissociated neuronal culture and in brain sections using immunocytochemistry. Using compartment-specific antibodies, we label presynaptic terminals as well as sites of postsynaptic specialization. We define synapses as points of colocalization between the signals generated by these markers. The number of these colocalizations is quantified using a plug in Puncta Analyzer (written by Bary Wark, available upon request, c.eroglu@cellbio.duke.edu) under the ImageJ analysis software platform. The synapse assay described in this protocol can be applied to any neural tissue or culture preparation for which you have selective pre- and postsynaptic markers. This synapse assay is a valuable tool that can be widely utilized in the study of synaptic development.

Synaptogenesis in adult-generated hippocampal granule cells is affected by behavioral experiences

Adult-generated hippocampal immature neurons play a functional role after integration in functional circuits. Previously, we found that hippocampus-dependent learning in Morris water maze affects survival of immature neurons, even before they are synaptically contacted. Beside learning, this task heavily engages animals in physical activity in form of swimming; physical activity enhances hippocampal neurogenesis. In this article, the effects of training in Morris water maze apparatus on the synapse formation onto new neurons in hippocampus dentate gyrus and on neuronal maturation were investigated in adult rats. Newborn cells were identified using retroviral GFP-expressing virus infusion. In the first week after virus infusion, rats were trained in Morris water maze apparatus in three different conditions (spatial learning, cue test, and swimming). Properties of immature neurons and their synaptic response to perforant pathway stimulation were electrophysiologically investigated early during neuronal maturation. In controls, newborn cells showing GABAergic and glutamatergic responses were found for the first time at 8 and 10 days after mitosis, respectively; no cell with glutamatergic response only was found. Twelve days after virus infusion almost all GFP-positive cells showed both synaptic responses. The main result we found was the anticipated appearance of GABAergic synapses at 6 days in learner, cued and swimmer rats, supported also by immunohistochemical result. Swimmer rats showed the highest percentage of GFP-positive neurons with glutamatergic response at 10 and 12 days postmitosis. Moreover, primary dendrites were more numerous at 7 days in learner, cued and swimmer rats and swimmer rats showed the greatest dendritic tree complexity at 10 days. Finally, voltage-dependent Ca(2+) current was found in a larger number of newborn neurons at 7 days postinfusion in learner, cued and swimmer rats. In conclusion, experiences involving physical activity contextualized in an exploring behavior affect synaptogenesis in adult-generated cells and their early stages of maturation.

In vivo development of outer retinal synapses in the absence of glial contact

Astroglia secrete factors that promote synapse formation and maintenance. In culture, glial contact has also been shown to facilitate synaptogenesis. Here, we examined whether glial contact is important for establishing circuits in vivo by simultaneously monitoring differentiation of glial cells and local synaptogenesis over time. Photoreceptor circuits of the vertebrate retina are particularly suitable for this study because of the relatively simple, laminar organization of their connectivity with their target neurons, horizontal cells and bipolar cells. Also, individual photoreceptor terminals are ensheathed within the outer plexiform layer (OPL) by the processes of one type of glia, Müller glia cells (MGs). We conducted in vivo time-lapse multiphoton imaging of the rapidly developing and relatively transparent zebrafish retina to ascertain the time course of MG development relative to OPL synaptogenesis. The emergence of synaptic triads, indicative of functional photoreceptor circuits, and structural association with glial processes were also examined across ages by electron microscopy. We first show that MG processes form territories that tile within the inner and outer synaptic layers. We then demonstrate that cone photoreceptor synapses are assembled before the elaboration of MG processes in the OPL. Using a targeted cell ablation approach, we also determined whether the maintenance of photoreceptor synapses is perturbed when local MGs are absent. We found that removal of MGs had no appreciable effect on the stability of newly formed cone synapses. Thus, in contrast to other CNS circuits, contact from glia is not necessary for the formation or immediate stabilization of outer retinal synapses.

Quantitative gene-expression analysis of the ligand-receptor system for classical neurotransmitters and neuropeptides in hippocampal CA1, CA3, and dentate gyrus

We have shown quantitative expression levels of genes coding for the "ligand-receptor system" for classical neurotransmitters and neuropeptides in hippocampal subregions CA1, CA3, and dentate gyrus (DG). Using a combination of DNA microarray and quantitative PCR methods, we found that the three subregions have relatively similar expression patterns of ionotropic receptors for classical neurotransmitters. Expression of ionotropic receptors for glutamate and GABA represents more than 90% of all ionotropic receptors for classical neurotransmitters, and the expression ratio between ionotropic receptors for glutamate and GABA is constant (1.2:1-1.6:1) in each subregion. Meanwhile, the three subregions have different expression patterns of neuropeptide receptors. Furthermore, there are asymmetric expression patterns between neuropeptides and their receptors. Expression of Cck, Npy, Sst, and Penk1 represents 90% of neuropeptides derived locally in the hippocampus, whereas expression of these four neuropeptide receptors accounts for 50% of G protein-coupled receptors for neuropeptides. We propose that CA1, CA3, and DG have different modalities based on the ligand-receptor system, particularly the "neuropeptidergic system." Our quantitative gene-expression analysis provides fundamental data to support functional differences between the three hippocampal subregions regarding ligand-receptor interactions.

Astrocytes and synaptic plasticity

Synaptic plasticity, the ability of neurons to change the number and strength of their synapses, has long been considered the sole province of the neuron. Yet neurons do not function in isolation; they are a part of elaborate glial networks where they are intimately associated with astrocytes. Astrocytes make extensive contacts with synaptic sites where they release soluble factors that can increase synapse number, provide synaptic insulation restricting the spread of neurotransmitter to neighboring synapses, and release neuroactive compounds, gliotransmitters, that can directly influence synaptic transmission. During periods of synaptogenesis, astrocyte processes are highly mobile and may contribute to the stabilization of new synapses. As our understanding of the extent of their influence at the synapse unfolds, it is clear that astrocytes are well poised to modulate multiple aspects of synaptic plasticity.

Cellular and synaptic mechanisms of anti-NMDA receptor encephalitis

We recently described a severe, potentially lethal, but treatment-responsive encephalitis that associates with autoantibodies to the NMDA receptor (NMDAR) and results in behavioral symptoms similar to those obtained with models of genetic or pharmacologic attenuation of NMDAR function. Here, we demonstrate that patients' NMDAR antibodies cause a selective and reversible decrease in NMDAR surface density and synaptic localization that correlates with patients' antibody titers. The mechanism of this decrease is selective antibody-mediated capping and internalization of surface NMDARs, as Fab fragments prepared from patients' antibodies did not decrease surface receptor density, but subsequent cross-linking with anti-Fab antibodies recapitulated the decrease caused by intact patient NMDAR antibodies. Moreover, whole-cell patch-clamp recordings of miniature EPSCs in cultured rat hippocampal neurons showed that patients' antibodies specifically decreased synaptic NMDAR-mediated currents, without affecting AMPA receptor-mediated currents. In contrast to these profound effects on NMDARs, patients' antibodies did not alter the localization or expression of other glutamate receptors or synaptic proteins, number of synapses, dendritic spines, dendritic complexity, or cell survival. In addition, NMDAR density was dramatically reduced in the hippocampus of female Lewis rats infused with patients' antibodies, similar to the decrease observed in the hippocampus of autopsied patients. These studies establish the cellular mechanisms through which antibodies of patients with anti-NMDAR encephalitis cause a specific, titer-dependent, and reversible loss of NMDARs. The loss of this subtype of glutamate receptors eliminates NMDAR-mediated synaptic function, resulting in the learning, memory, and other behavioral deficits observed in patients with anti-NMDAR encephalitis.

Brain self-protection: the role of endogenous neural progenitor cells in adult brain after cerebral cortical ischemia

Convincing evidence has shown that brain ischemia causes the proliferation of neural stem cells/neural progenitor cells (NSCs/NPCs) in both the subventricular zone (SVZ) and the subgranular zone (SGZ) of adult brain. The role of brain ischemia-induced NSC/NPC proliferation, however, has remained unclear. Here we have determined whether brain ischemia-induced amplification of the NSCs/NPCs in adult brain is required for brain self-protection. The approach of intracerebroventricular (ICV) infusion of cytosine arabinoside (Ara-C), an inhibitor for cell proliferation, for the first 7days after brain ischemia was used to block ischemia-induced NSC/NPC proliferation. We observed that ICV infusion of Ara-C caused a complete blockade of NSC/NPC proliferation in the SVZ and a dramatic reduction of NSC/NPC proliferation in the SGZ. Additionally, as a result of the inhibition of ischemia-induced NSC/NPC pool amplification, the number of neurons in the hippocampal CA1 and CA3 was significantly reduced, the infarction size was significantly enlarged, and neurological deficits were significantly worsened after focal brain ischemia. We also found that an NSC/NPC-conditioned medium showed neuroprotective effects in vitro and that adult NSC/NPC-released brain-derived neurotrophic factor (BDNF) and vascular endothelial growth factor (VEGF) are required for NSC/NPC-conditioned medium-induced neuroprotection. These data suggest that NSC/NPC-generated trophic factors are neuroprotective and that brain ischemia-triggered NSC/NPC proliferation is crucial for brain protection. This study provides insights into the contribution of endogenous NSCs/NPCs to brain self-protection in adult brain after ischemia injury.

Role of glial cells in the formation and maintenance of synapses

Synaptogenesis is a decisive process for the development of the brain, its plasticity during adulthood and its regeneration after injury and disease. Despite tremendous progress during the last decades, it remains unclear, whether neurons can form synapses autonomously. In this review, I will summarize recent evidence that this is probably not the case and that distinct phases of synapse development depend on help from glial cells. The results supporting this view come from studies on the central and peripheral nervous system and on different experimental models including cultured cells as well as living flies, worms and mice. Our understanding of synapse-glia interactions in the developing, adult and diseased brain is likely to advance more rapidly as new experimental approaches to identify, visualize and manipulate glial cells in vivo become available.

Opposite effects of milnacipran, a serotonin norepinephrine reuptake inhibitor, on the levels of nitric oxide and brain-derived neurotrophic factor in mouse brain cortex

There is a growing body of evidence demonstrating that changes in the brain levels of nitric oxide (NO) and brain-derived neurotrophic factor (BDNF) are implicated in the pathogenesis of major depression. We report here the effects of subchronic treatment of mice with milnacipran, a serotonin norepinephrine reuptake inhibitor, on the levels of NO and BDNF in mice. In vivo administration of milnacipran (10 mg/kg) for 14 days caused a significant decrease in nitrate and nitrite concentrations in the cerebral cortex and hippocampus, but not in the midbrain. Milnacipran (10 mg/kg, 14 days) also decreased the activity of NO synthase in the cerebral cortex. On the other hand, milnacipran (10 mg/kg, 14 days) increased the levels of BDNF protein and mRNA in the cerebral cortex. These findings suggest that milnacipran has opposite effects on the levels of NO and BDNF in the brain cortex, namely, downregulation of NO and upregulation of BDNF.

Look who is weaving the neural web: glial control of synapse formation

Historically, our understanding of synapse formation has been shaped by studies focusing on neurons. However, with advancements in live imaging techniques and molecular and genetic tools we are rapidly uncovering new roles for glia in synapse formation and function. Contact-mediated signals from glia instruct dendrites to become receptive to synaptic partners. Glia-secreted factors coordinate the assembly and apposition of pre-synaptic and post-synaptic specializations. Glial cells also provide cues that are required for synaptic maturation and remodeling of spines both during development and in the adult. As we continue to learn about glial contributions to synapse formation and maintenance, it is likely that glia-derived signals will emerge as potential therapeutic targets for diseases that involve aberrant circuit function such as autism, epilepsy and Alzheimer's Disease.

Astrocyte secreted proteins selectively increase hippocampal GABAergic axon length, branching, and synaptogenesis

Astrocytes modulate the formation and function of glutamatergic synapses in the CNS, but whether astrocytes modulate GABAergic synaptogenesis is unknown. We demonstrate that media conditioned by astrocytes, but not other cells, enhanced GABAergic but not glutamatergic axon length and branching, and increased the number and density of presynaptically active GABAergic synapses in dissociated hippocampal cultures. Candidate mechanisms and factors, such as activity, neurotrophins, and cholesterol were excluded as mediating these effects. While thrombospondins secreted by astrocytes are necessary and sufficient to increase hippocampal glutamatergic synaptogenesis, they do not mediate astrocyte effects on GABAergic synaptogenesis. We show that the factors in astrocyte conditioned media that selectively affect GABAergic neurons are proteins. Taken together, our results show that astrocytes increase glutamatergic and GABAergic synaptogenesis via different mechanisms and release one or more proteins with the novel functions of increasing GABAergic axon length, branching and synaptogenesis.

GABA(B) receptor activation triggers BDNF release and promotes the maturation of GABAergic synapses

GABA, the main inhibitory neurotransmitter in the adult brain, has recently emerged as an important signal in network development. Most of the trophic functions of GABA have been attributed to depolarization of the embryonic and neonatal neurons via the activation of ionotropic GABA(A) receptors. Here we demonstrate a novel mechanism by which endogenous GABA selectively regulates the development of GABAergic synapses in the developing brain. Using whole-cell patch-clamp recordings on newborn mouse hippocampi lacking functional GABA(B) receptors (GABA(B)-Rs) and time-lapse fluorescence imaging on cultured hippocampal neurons expressing GFP-tagged brain-derived neurotrophic factor (BDNF), we found that activation of metabotropic GABA(B) receptors (GABA(B)-Rs) triggers secretion of BDNF and promotes the development of perisomatic GABAergic synapses in the newborn mouse hippocampus. Because activation of GABA(B)-Rs occurs during the characteristic ongoing physiological network-driven synaptic activity present in the developing hippocampus, our results reveal a new mechanism by which synaptic activity can modulate the development of local GABAergic synaptic connections in the developing brain.

BDNF and NT-3 increase excitatory input connectivity in rat hippocampal cultures

The neurotrophic factors brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) have been shown to promote excitatory and inhibitory synapse development. However, a quantitative analysis of their influence on connectivity has proven in general difficult to achieve. In this work we use a novel experimental approach based on percolation concepts that provides a quantification of the average number of connections per neuron. In combination with electrophysiological measurements, we characterize the changes in network connectivity induced by BDNF and NT-3 in rat hippocampal cultures. We show that, on the one hand, BDNF and NT-3 accelerate the maturation of connectivity in the network by about 17 h. On the other hand, BDNF and NT-3 increase the number of excitatory input connections by a factor of about two, but without modifying the number of inhibitory input connections. This scenario of a dominant effect on the excitation is supported by the analysis of spontaneous population bursts in cultures treated with either BDNF or NT-3, which show burst amplitudes that are insensitive to the blockade of inhibition. A leaky integrate-and-fire model reproduces the experimental results well.

Epilepsy, regulation of brain energy metabolism and neurotransmission

Seizures are the result of a sudden and temporary synchronization of neuronal activity, the reason for which is not clearly understood. Astrocytes participate in the control of neurotransmitter storage and neurotransmission efficacy. They provide fuel to neurons, which need a high level of energy to sustain normal and pathological neuronal activities, such as during epilepsy. Various genetic or induced animal models have been developed and used to study epileptogenic mechanisms. Methionine sulfoximine induces both seizures and the accumulation of brain glycogen, which might be considered as a putative energy store to neurons in various animals. Animals subjected to methionine sulfoximine develop seizures similar to the most striking form of human epilepsy, with a long pre-convulsive period of several hours, a long convulsive period during up to 48 hours and a post convulsive period during which they recover normal behavior. The accumulation of brain glycogen has been demonstrated in both the cortex and cerebellum as early as the pre-convulsive period, indicating that this accumulation is not a consequence of seizures. The accumulation results from an activation of gluconeogenesis specifically localized to astrocytes, both in vivo and in vitro. Both seizures and brain glycogen accumulation vary when using different inbred strains of mice. C57BL/6J is the most "resistant" strain to methionine sulfoximine, while CBA/J is the most "sensitive" one. The present review describes the data obtained on methionine sulfoximine dependent seizures and brain glycogen in the light of neurotransmission, highlighting the relevance of brain glycogen content in epilepsies.

Roles of glial cells in synapse development

Brain function relies on communication among neurons via highly specialized contacts, the synapses, and synaptic dysfunction lies at the heart of age-, disease-, and injury-induced defects of the nervous system. For these reasons, the formation-and repair-of synaptic connections is a major focus of neuroscience research. In this review, I summarize recent evidence that synapse development is not a cell-autonomous process and that its distinct phases depend on assistance from the so-called glial cells. The results supporting this view concern synapses in the central nervous system as well as neuromuscular junctions and originate from experimental models ranging from cell cultures to living flies, worms, and mice. Peeking at the future, I will highlight recent technical advances that are likely to revolutionize our views on synapse-glia interactions in the developing, adult and diseased brain.

AMPA receptor antibodies in limbic encephalitis alter synaptic receptor location

OBJECTIVE

To report the clinical and immunological features of a novel autoantigen related to limbic encephalitis (LE) and the effect of patients' antibodies on neuronal cultures.

METHODS

We conducted clinical analyses of 10 patients with LE. Immunoprecipitation and mass spectrometry were used to identify the antigens. Human embryonic kidney 293 cells expressing the antigens were used in immunocytochemistry and enzyme-linked immunoabsorption assay. The effect of patients' antibodies on cultures of live rat hippocampal neurons was determined with confocal microscopy.

RESULTS

Median age was 60 (38-87) years; 9 were women. Seven had tumors of the lung, breast, or thymus. Nine patients responded to immunotherapy or oncological therapy, but neurological relapses, without tumor recurrence, were frequent and influenced the long-term outcome. One untreated patient died of LE. All patients had antibodies against neuronal cell surface antigens that by immunoprecipitation were found to be the glutamate receptor 1 (GluR1) and GluR2 subunits of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR). Human embryonic kidney 293 cells expressing GluR1/2 reacted with all patients' sera or cerebrospinal fluid, providing a diagnostic test for the disorder. Application of antibodies to cultures of neurons significantly decreased the number of GluR2-containing AMPAR clusters at synapses with a smaller decrease in overall AMPAR cluster density; these effects were reversed after antibody removal.

INTERPRETATION

Antibodies to GluR1/2 associate with LE that is often paraneoplastic, treatment responsive, and has a tendency to relapse. Our findings support an antibody-mediated pathogenesis in which patients' antibodies alter the synaptic localization and number of AMPARs.

Sex differences and laterality in astrocyte number and complexity in the adult rat medial amygdala

The posterodorsal portion of the medial amygdala (MePD) is sexually dimorphic in several rodent species. In several other brain nuclei, astrocytes change morphology in response to steroid hormones. We visualized MePD astrocytes using glial-fibrillary acidic protein (GFAP) immunocytochemistry. We compared the number and process complexity of MePD astrocytes in adult wildtype male and female rats and testicular feminized mutant (TFM) male rats that lack functional androgen receptors (ARs) to determine whether MePD astrocytes are sexually differentiated and whether ARs have a role. Unbiased stereological methods revealed laterality and sex differences in MePD astrocyte number and complexity. The right MePD contained more astrocytes than the left in all three genotypes, and the number of astrocytes was also sexually differentiated in the right MePD, with males having more astrocytes than females. In contrast, the left MePD contained more complex astrocytes than did the right MePD in all three genotypes, and males had more complex astrocytes than females in this hemisphere. TFM males were comparable to wildtype females, having fewer astrocytes on the right and simpler astrocytes on the left than do wildtype males. Taken together, these results demonstrate that astrocytes are sexually dimorphic in the adult MePD and that the nature of the sex difference is hemisphere-dependent: a sex difference in astrocyte number in the right MePD and a sex difference in astrocyte complexity in the left MePD. Moreover, functional ARs appear to be critical in establishing these sex differences in MePD astrocyte morphology.

Anti-NMDA-receptor encephalitis: case series and analysis of the effects of antibodies

BACKGROUND

A severe form of encephalitis associated with antibodies against NR1-NR2 heteromers of the NMDA receptor was recently identified. We aimed to analyse the clinical and immunological features of patients with the disorder and examine the effects of antibodies against NMDA receptors in neuronal cultures.

METHODS

We describe the clinical characteristics of 100 patients with encephalitis and NR1-NR2 antibodies. HEK293 cells ectopically expressing single or assembled NR1-NR2 subunits were used to determine the epitope targeted by the antibodies. Antibody titres were measured with ELISA. The effect of antibodies on neuronal cultures was determined by quantitative analysis of NMDA-receptor clusters.

FINDINGS

Median age of patients was 23 years (range 5-76 years); 91 were women. All patients presented with psychiatric symptoms or memory problems; 76 had seizures, 88 unresponsiveness (decreased consciousness), 86 dyskinesias, 69 autonomic instability, and 66 hypoventilation. 58 (59%) of 98 patients for whom results of oncological assessments were available had tumours, most commonly ovarian teratoma. Patients who received early tumour treatment (usually with immunotherapy) had better outcome (p=0.004) and fewer neurological relapses (p=0.009) than the rest of the patients. 75 patients recovered or had mild deficits and 25 had severe deficits or died. Improvement was associated with a decrease of serum antibody titres. The main epitope targeted by the antibodies is in the extracellular N-terminal domain of the NR1 subunit. Patients' antibodies decreased the numbers of cell-surface NMDA receptors and NMDA-receptor clusters in postsynaptic dendrites, an effect that could be reversed by antibody removal.

INTERPRETATION

A well-defined set of clinical characteristics are associated with anti-NMDA-receptor encephalitis. The pathogenesis of the disorder seems to be mediated by antibodies.

Astrocytes from acyclic female rats exhibit lowered capacity for neuronal differentiation

Astrocytes comprise a large proportion of the central nervous system support cells and play a critical role in neural injury and repair. The present study examined the impact of ovarian aging using an ex vivo model system, where astrocytes were derived from the olfactory bulb of young, reproductively competent females and reproductive senescent females. Cellular morphology and the spatial pattern of laminin deposition was altered in astrocyte cultures derived from reproductive senescent females. Young adult astrocytes had a flattened polygonal shape with actin bundles at the cell edges, while reproductive senescent astrocytes had a contractile appearance with thick stress fibers visible throughout the cell. Moreover, in reproductive senescent astrocytes, BDNF was elevated with a concomitant reduction in expression of the BDNF receptor, TrkB. To examine the ability of astrocytes derived from young adult and reproductive senescent females to promote neuronal differentiation, neural progenitor cells (NPCs) were co-cultured with astrocytes derived from these groups. At day 4 in vitro, MAP-2(+) NPCs were located in smaller clusters when co-cultured with young adult astrocytes and in large clusters when co-cultured with older astrocytes. At days 6 and 10, neuronal differentiation was significantly reduced in reproductive senescent astrocyte-NPC co-cultures, as determined by NeuN(+) cell numbers and MAP-2(+) process lengths. Furthermore, estrogen only enhanced neuronal differentiation in young adult-NPC co-cultures. The ovarian age-related astrocyte phenotype thus limits the ability of this cell to promote neuronal differentiation in NPC populations and suggests that the astrocyte-mediated microenvironment in older acyclic females is less conducive to repair following neurovascular injury.

SDF1alpha/CXCR4 signaling, via ERKs and the transcription factor Egr1, induces expression of a 67-kDa form of glutamic acid decarboxylase in embryonic hippocampal neurons

Stromal cell-derived factor alpha (SDF1alpha) and its cognate receptor CXCR4 play an important role in neuronal development in the hippocampus, but the genes directly regulated by SDF1alpha/CXCR4 signaling are unknown. To study the role of CXCR4 targeted genes in neuronal development, we used neuronal cultures established from embryonic day 18 rats. Hippocampal neurons express CXCR4 receptor proteins and are stimulated by SDF1alpha resulting in activation of extracellular signal-regulated kinase (ERK)1/2 and the transcription factor cAMP-response element-binding protein. SDF1alpha rapidly induces the expression of the early growth response gene Egr1, a transcription factor involved in activity-dependent neuronal responses, in a concentration-dependent manner. Gel-shift analysis showed that SDF1alpha enhances DNA binding activity to the Egr1-containing promoter for GAD67. Chromatin immunoprecipitation analysis using an Egr1 antibody indicated that SDF1alpha stimulation increases binding of Egr1 to a GAD67 promoter DNA sequence. SDF1alpha stimulation increases the expression of GAD67 at both the mRNA and protein levels, and increases the amount and neurite localization of gamma-aminobutyric acid (GABA) in neurons already expressing GABA. SDF1alpha-induced Egr1/GAD67 expression is mediated by the G protein-coupled CXCR4 receptor and activation of the ERK pathway. Reduction of Egr1 gene expression using small interfering RNA technology lowers the level of GAD67 transcripts and inhibits SDF1alpha-induced GABA production. Inhibition of CXCR4 activation in the developing mouse brain in utero greatly reduced Egr1 and GAD67 mRNA levels and GAD67 protein levels, suggesting a pivotal role for CXCR4 signaling in the development of GABAergic neurons in vivo. Our data suggest that SDF1alpha/CXCR4/G protein/ERK signaling induces the expression of the GAD67 system via Egr1 activation, a mechanism that may promote the maturation of GABAergic neurons during development.

Distribution of glial-associated proteins in the developing chick auditory brainstem

In the avian brainstem, nucleus magnocellularis (NM) projects bilaterally to nucleus laminaris (NL) in a pathway that facilitates sound localization. The distribution of glia during the development of this pathway has not previously been characterized. Radial glia, astrocytes, and oligodendrocytes facilitate many processes including axon pathfinding, synaptic development, and maturation. Here we determined the spatiotemporal expression patterns of glial cell types in embryonic development of the chick auditory brainstem using glial-specific antibodies and histological markers. We found that vimentin-positive processes are intercalated throughout the NL cell layer. Astrocytes are found in two domains: one in the ventral neuropil region and the other dorsolateral to NM. GFAP-positive processes are primarily distributed along the ventral margin of NL. Astrocytic processes penetrate the NL cell layer following the onset of synaptogenesis, but before pruning and maturation. The dynamic, nonoverlapping expression patterns of GFAP and vimentin suggest that distinct glial populations are found in dorsal versus ventral regions of NL. Myelination occurs after axons have reached their targets. FluoroMyelin and myelin basic protein (MBP) gradually increase along the mediolateral axis of NL starting at E10. Multiple GFAP-positive processes are directly apposed to NM-NL axons and MBP, which suggests a role in early myelinogenesis. Our results show considerable changes in glial development after initial NM-NL connections are made, suggesting that glia may facilitate maturation of the auditory circuit.

Phosphatidylinositide 3-kinase and protein kinase C zeta mediate retinoic acid induction of DARPP-32 in medium size spiny neurons in vitro

Mature striatal medium size spiny neurons express the dopamine and cAMP-regulated phosphoprotein, 32 kDa (DARPP-32), but little is known about the mechanisms regulating its levels, or the specification of fully differentiated neuronal subtypes. Cell extrinsic molecules that increase DARPP-32 mRNA and/or protein levels include retinoic acid (RA), brain-derived neurotrophic factor, and estrogen (E(2)). We now demonstrate that RA regulates DARPP-32 mRNA and protein in primary striatal neuronal cultures. Furthermore, DARPP-32 induction by RA in vitro requires phosphatidylinositide 3-kinase, but is independent of tropomyosin-related kinase B, cyclin-dependent kinase 5, and protein kinase B. Using pharmacologic inhibitors of various isoforms of protein kinase C (PKC), we also demonstrate that DARPP-32 induction by RA in vitro is dependent on PKC zeta (PKCzeta). Thus, the signal transduction pathways mediated by RA are very different than those mediating DARPP-32 induction by brain-derived neurotrophic factor. These data support the presence of multiple signal transduction pathways mediating expression of DARPP-32 in vitro, including a novel, important pathway via which phosphatidylinositide 3-kinase regulates the contribution of PKCzeta.

Kinin-B2 receptor expression and activity during differentiation of embryonic rat neurospheres

Neural progenitor cells were isolated from rat fetal telencephalon and proliferate as neurospheres in the presence of EGF, FGF-2, and heparin. In the absence of these growth factors, neurospheres differentiate into neurons, astrocytes, and oligodendrocytes. Using an embryonal carcinoma cell line as in vitro differentiation model, we have already demonstrated the presence of an autocrine loop system between kinin-B2 receptor activity and secretion of its ligand bradykinin (BK) as prerequisites for final neuronal differentiation (Martins et al., J Biol Chem 2005; 280: 19576-19586). The aim of this study was to verify the activity of the kallikrein-kinin system (KKS) during neural progenitor cell differentiation. Immunofluorescence studies and flow cytometry analysis revealed increases in glial fibrillary acidic protein and beta-3 tubulin expression and decrease in the number of nestin-positive cells along neurospheres differentiation, indicating the transition of neural progenitor cells to astrocytes and neurons. Kinin-B2 receptor expression and activity, secretion of BK into the medium, and presence of high-molecular weight kininogen suggest the participation of the KKS in neurosphere differentiation. Functional kinin-B2 receptors and BK secretion indicate an autocrine loop during neurosphere differentiation to neurons, astrocytes, and oligodendrocytes, reflecting events occurring during early brain development.

Developmental expression of FMRP in the astrocyte lineage: implications for fragile X syndrome

One of the most common causes of mental retardation in humans, Fragile X syndrome, results from the absence of FMRP, the protein product of the FMR1 gene. In the nervous system, expression of FMRP has been thought to be confined mainly to neurons as little research has examined FMRP expression in non-neuronal lineages. We present evidence that, in addition to neuronal expression, FMRP is expressed in developing CNS glial cells in vitro and in vivo. The neurosphere assay was used to establish cultures of stem and progenitor cells from the brains of wildtype and FMRP knockout (B6.129.FMR1/FvBn) mouse pups. When the neurospheres were differentiated in vitro, approximately 50% of the FMRP positive cells also expressed GFAP. Immunocytochemical studies of the embryonic and postnatal mouse brain revealed coexpression of FMRP and GFAP in the developing hippocampus. Prominent coexpression was also observed in ependymal cells surrounding the third ventricle and astrocytes of the glia limitans. No double-labeled cells were evident in the brains of young adult mice. Cells coexpressing FMRP and the oligodendrocyte precursor marker NG2 were also identified in the hippocampus and corpus callosum of the early postnatal brain. Our results suggest that FMRP is expressed in cells of non-neuronal lineage(s) during development. This represents potential involvement of glial cells in the neural development of fragile X syndrome.

Conditional knock-out of Kir4.1 leads to glial membrane depolarization, inhibition of potassium and glutamate uptake, and enhanced short-term synaptic potentiation

During neuronal activity, extracellular potassium concentration ([K+]out) becomes elevated and, if uncorrected, causes neuronal depolarization, hyperexcitability, and seizures. Clearance of K+ from the extracellular space, termed K+ spatial buffering, is considered to be an important function of astrocytes. Results from a number of studies suggest that maintenance of [K+]out by astrocytes is mediated by K+ uptake through the inward-rectifying Kir4.1 channels. To study the role of this channel in astrocyte physiology and neuronal excitability, we generated a conditional knock-out (cKO) of Kir4.1 directed to astrocytes via the human glial fibrillary acidic protein promoter gfa2. Kir4.1 cKO mice die prematurely and display severe ataxia and stress-induced seizures. Electrophysiological recordings revealed severe depolarization of both passive astrocytes and complex glia in Kir4.1 cKO hippocampal slices. Complex cell depolarization appears to be a direct consequence of Kir4.1 removal, whereas passive astrocyte depolarization seems to arise from an indirect developmental process. Furthermore, we observed a significant loss of complex glia, suggestive of a role for Kir4.1 in astrocyte development. Kir4.1 cKO passive astrocytes displayed a marked impairment of both K+ and glutamate uptake. Surprisingly, membrane and action potential properties of CA1 pyramidal neurons, as well as basal synaptic transmission in the CA1 stratum radiatum appeared unaffected, whereas spontaneous neuronal activity was reduced in the Kir4.1 cKO. However, high-frequency stimulation revealed greatly elevated posttetanic potentiation and short-term potentiation in Kir4.1 cKO hippocampus. Our findings implicate a role for glial Kir4.1 channel subunit in the modulation of synaptic strength.

Glia-induced neuronal differentiation by transcriptional regulation

There is increasing evidence that different phases of brain development depend on neuron-glia interactions including postnatal key events like synaptogenesis. To address how glial cells influence synapse development, we analyzed whether and how glia-derived factors affect gene expression in primary cultures of immunoisolated rat retinal ganglion cells (RGCs) by oligonucleotide microarrays. Our results show that the transcript pattern matched the developmental stage and characteristic properties of RGCs in vitro. Glia-conditioned medium (GCM) and cholesterol up- and downregulated a limited number of genes that influence the development of dendrites and synapses and regulate cholesterol and fatty acid metabolism. The oligonucleotide microarrays detected the transcriptional regulation of neuronal cholesterol homeostasis in response to GCM and cholesterol treatment. Surprisingly, our study revealed neuronal expression and glial regulation of matrix gla protein (Mgp). Together, our results suggest that glial cells promote different aspects of neuronal differentiation by regulating transcription of distinct classes of genes.

In vitro methods to prepare astrocyte and motoneuron cultures for the investigation of potential in vivo interactions

This protocol details methods to isolate and purify astrocytes and motoneurons (MNs) from the chick lumbar spinal cord. In addition, an approach to study the influences of astrocyte secreted factors on MNs is provided. Astrocytes are isolated between embryonic days 10 and 12 (E10-12), propagated in serum (2-3 h) and differentiated in chemically defined medium (3-4 h). When prepared according to this protocol, astrocyte cultures are more than 98% pure when assessed using the astrocyte-specific markers glial fibrillary acidic protein (GFAP) and S100beta. MNs are isolated between E5.5 and 6.0 (3-4 h) using a procedure that takes selective advantage of the large size of these cells. These cultures can be maintained using individual trophic factors, target-derived factors or astrocyte-derived factors, the preparation of which is also described (5-6 h). All or part of these techniques can be used to investigate a variety of processes that occur during nervous system development and disease or after injury.

Cell-autonomous TrkB signaling in presynaptic retinal ganglion cells mediates axon arbor growth and synapse maturation during the establishment of retinotectal synaptic connectivity

BDNF contributes to the activity-dependent establishment and refinement of visual connectivity. In Xenopus, BDNF applications in the optic tectum influence retinal ganglion cell (RGC) axon branching and promote synapse formation and stabilization. The expression patterns of BDNF and TrkB suggest that BDNF specifically regulates the maturation of RGC axons at the target. It is possible, however, that BDNF modulates retinotectal synaptic connectivity by differentially influencing presynaptic RGC axons and postsynaptic tectal cells. Here, we combined single-cell expression of a dominant-negative TrkB-enhanced green fluorescent protein (GFP) fusion protein with confocal microscopy imaging in live Xenopus tadpoles to differentiate between presynaptic and postsynaptic actions of BDNF. Disruption of TrkB signaling in individual RGCs influenced the branching and synaptic maturation of presynaptic axon arbors. Specifically, GFP-TrkB.T1 overexpression increased the proportion of axons with immature, growth cone-like morphology, decreased axon branch stability, and increased axon arbor degeneration. In addition, GFP-TrkB.T1 overexpression reduced the number of red fluorescent protein-synaptobrevin-labeled presynaptic specializations per axon terminal. In contrast, overexpression of GFP-TrkB.T1 in tectal neurons did not alter synaptic number or the morphology or dynamic behavior of their dendritic arbors. Electron microscopy analysis revealed a significant decrease in the number of mature synaptic profiles and in the number of docked synaptic vesicles at retinotectal synapses made by RGC axons expressing GFP-TrkB.T1. Together, our results demonstrate that presynaptic TrkB signaling in RGCs is a key determinant in the establishment of visual connectivity and indicate that changes in tectal neuron synaptic connectivity are secondary to the BDNF-elicited enhanced stability and growth of presynaptic RGCs.

Regional variations in the glial influence on synapse development in the mouse CNS

There is increasing evidence that synapse function depends on interactions with glial cells, namely astrocytes. Studies on specific neurons of the central nervous system (CNS) indicated that glial signals also control synapse development, but it remained unclear whether this is a general principle that applies to other neuronal cell types. To address this question, we developed new methods to immunoisolate neurons from different brain regions of postnatal mice and to culture them in a chemically defined medium. Electrophysiological recordings and immunocytochemical staining revealed vigorous synaptogenesis in hippocampal and cerebellar neurons, but not in retinal ganglion cells (RGCs) in the absence of glial cells. Co-culture with glia promoted synapse formation in RGCs as indicated by a strong increase in the incidence and frequency of action potential-independent miniature synaptic currents, but showed no such effects in hippocampal or cerebellar neurons. On the other hand, glial signals promoted the efficacy of excitatory synapses in all regions as indicated by an increase in the size of spontaneous synaptic events in cerebellar cultures and of miniature synaptic currents in hippocampal neurons and RGCs. Inhibitory synaptic currents remained largely unaffected by glia. Our results indicate that in the mammalian CNS, the way that glial signals promote the development of excitatory synapses depends on the type of neuron.

Dissociated cortical networks show spontaneously correlated activity patterns during in vitro development

In vitro cultured neuronal networks coupled to microelectrode arrays (MEAs) constitute a valuable experimental model for studying changes in the neuronal dynamics at different stages of development. After a few days in culture, neurons start to connect each other with functionally active synapses, forming a random network and displaying spontaneous electrophysiological activity. The patterns of collective rhythmic activity change in time spontaneously during in vitro development. Such activity-dependent modifications play a key role in the maturation of the network and reflect changes in the synaptic efficacy, fact widely recognized as a cellular basis of learning, memory and developmental plasticity. Getting advantage from the possibilities offered by the MEAs, the aim of our study is to analyze and characterize the natural changes in dynamics of the electrophysiological activity at different ages of the culture, identifying peculiar steps of the spontaneous evolution of the network. The main finding is that between the second and the third week of culture, the network completely changes its electrophysiological patterns, both in terms of spiking and bursting activity and in terms of cross-correlation between pairs of active channels. Then the maturation process can be characterized by two main phases: modulation and shaping in the synaptic functional connectivity of the network (within the first and second week) and general moderate correlated activity, spread over the entire network, with connections properly formed and stabilized (within the fourth and fifth week).

Development of gamma-aminobutyric acidergic synapses in cultured hippocampal neurons

The formation and maturation of gamma-aminobutyric acid (GABA)-ergic synapses was studied in cultured hippocampal pyramidal neurons by both performing immunocytochemistry for GABAergic markers and recording miniature inhibitory postsynaptic currents (mIPSCs). Nascent GABAergic synapses appeared between 3 and 8 days in vitro (DIV), with GABAA receptor subunit clusters appearing first, followed by GAD-65 puncta, then functional synapses. The number of GABAergic synapses increased from 7 to 14 DIV, with a corresponding increase in frequency of mIPSCs. Moreover, these new GABAergic synapses formed on neuronal processes farther from the soma, contributing to decreased mIPSC amplitude and slowed mIPSC 19-90% rise time. The mIPSC decay quickened from 7 to 14 DIV, with a parallel change in the distribution of the alpha5 subunit from diffuse expression at 7 DIV to clustered expression at 14 DIV. These alpha5 clusters were mostly extrasynaptic. The alpha1 subunit was expressed as clusters in none of the neurons at 7 DIV, in 20% at 14 DIV, and in 80% at 21 DIV. Most of these alpha1 clusters were expressed at GABAergic synapses. In addition, puncta of GABA transporter 1 (GAT-1) were localized to GABAergic synapses at 14 DIV but were not expressed at 7 DIV. These studies demonstrate that mIPSCs appear after pre- and postsynaptic elements are in place. Furthermore, the process of maturation of GABAergic synapses involves increased synapse formation at distal processes, expression of new GABAA receptor subunits, and GAT-1 expression at synapses; these changes are reflected in altered frequency, kinetics, and drug sensitivity of mIPSCs.

Synaptic plasticity, astrocytes and morphological homeostasis

Recent discoveries suggest that astrocytes are an integral part of synaptic connections, as they sense and modulate synaptic activity. Moreover, there is evidence that astrocytes change the number of synaptic connections directly via synaptogenic signals or indirectly, by modifying the morphology of axons and dendrites. Here, we formulate the hypothesis that astrocytes mediate the morphological homeostasis of nerve cells, which is any adaptation of the morphology of a neuron to preserve its ability to respond to and generate synaptic activity during learning and memory-induced changes. We argue that astrocytes control neuronal morphology locally and across long-ranging assemblies of neurons and that on the other hand, astrocytes are part of the engram with plasticity-related changes affecting their morphology.

Signaling between glia and neurons: focus on synaptic plasticity

Glial cells are now emerging from the shadows cast by their more excitable CNS counterparts. Within the developing nervous system, astrocytes and Schwann cells actively help to promote synapse formation and function, and have even been implicated in synapse elimination. In the adult brain, astrocytes respond to synaptic activity by releasing transmitters that modulate synaptic activity. Thus, glia are active participants in brain function. Many questions remain about the identity of glial-neuronal signals and their significance.

Brain-derived neurotrophic factor modulates GABAergic synaptic transmission by enhancing presynaptic glutamic acid decarboxylase 65 levels, promoting asynchronous release and reducing the number of activated postsynaptic receptors

Brain-derived neurotrophic factor is known to modulate the function of GABAergic synapses, but the site of brain-derived neurotrophic factor action is still a matter of controversy. This study was aimed at further dissecting the functional alterations produced by brain-derived neurotrophic factor treatment of GABAergic synaptic connections in cultures of the murine superior colliculus. The functional consequences of long-term brain-derived neurotrophic factor treatment were assessed by analysis of unitary evoked and delayed inhibitory postsynaptic currents in response to high frequency stimulation of single axons. It was found that brain-derived neurotrophic factor facilitated the asynchronous release, but had no effect on the probability of evoked release, the size of the readily releasable pool, and the paired-pulse behavior of evoked inhibitory postsynaptic currents. However, the amplitudes of evoked inhibitory postsynaptic currents, delayed inhibitory postsynaptic currents and miniature inhibitory postsynaptic currents were significantly reduced. Non-stationary fluctuation analysis revealed a decrease in the open channel number at the miniature/evoked inhibitory postsynaptic current peak, but no effect on the mean GABA(A) receptor single channel conductance. Quantitative immunocytochemistry uncovered a significant elevation of presynaptic levels of glutamic acid decarboxylase 65. Together, these findings indicate that brain-derived neurotrophic factor treatment induces pre- as well as postsynaptic changes. What effect predominates will depend on the presynaptic activity pattern: at low activation rates brain-derived neurotrophic factor-treated synapses display a pronounced postsynaptic depression, but at high frequencies this depression is fully compensated by an enhancement of asynchronous release.

In vitro formation and activity-dependent plasticity of synapses between Helix neurons involved in the neural control of feeding and withdrawal behaviors

Short-term activity-dependent synaptic plasticity has a fundamental role in short-term memory and information processing in the nervous system. Although the neuronal circuitry controlling different behaviors of land snails of the genus Helix has been characterized in some detail, little is known about the activity-dependent plasticity of synapses between identified neurons regulating specific behavioral acts. In order to study homosynaptic activity-dependent plasticity of behaviorally relevant Helix synapses independently of heterosynaptic influences, we sought to reconstruct them in cell culture. To this aim, we first investigated in culture the factors regulating synapse formation between Helix neurons, and then we studied the short-term plasticity of in vitro-reconstructed monosynaptic connections involved in the neural control of salivary secretion and whole-body withdrawal. We found that independently of extrinsic factors, cell-cell interactions are seemingly sufficient to trigger the formation of electrical and chemical synapses, although mostly inappropriate--in their type or association--with respect to the in vivo synaptic connectivity. The presence of ganglia-derived factors in the culture medium was required for the in vitro reestablishment of the appropriate in vivo-like connectivity, by reducing the occurrence of electrical connections and promoting the formation of chemical excitatory synapses, while apparently not influencing the formation of inhibitory connections. These heat-labile factors modulated electrical and chemical synaptogenesis through distinct protein tyrosine kinase signal transduction pathways. Taking advantage of in vitro-reconstructed synapses, we have found that feeding interneuron-efferent neuron synapses and mechanosensory neuron-withdrawal interneuron synapses display multiple forms of short-term enhancement-like facilitation, augmentation and posttetanic potentiation as well as homosynaptic depression. These forms of plasticity are thought to be relevant in the regulation of Helix feeding and withdrawal behaviors by inducing dramatic activity-dependent changes in the strength of input and output synapses of high-order interneurons with a crucial role in the control of Helix behavioral hierarchy.

Neurotrophin signaling among neurons and glia during formation of tripartite synapses

Synapse formation in the CNS is a complex process that involves the dynamic interplay of numerous signals exchanged between pre- and postsynaptic neurons as well as perisynaptic glia. Members of the neurotrophin family, which are widely expressed in the developing and mature CNS and are well-known for their roles in promoting neuronal survival and differentiation, have emerged as key synaptic modulators. However, the mechanisms by which neurotrophins modulate synapse formation and function are poorly understood. Here, we summarize our work on the role of neurotrophins in synaptogenesis in the CNS, in particular the role of these signaling molecules and their receptors, the Trks, in the development of excitatory and inhibitory hippocampal synapses. We discuss our results that demonstrate that postsynaptic TrkB signaling plays an important role in modulating the formation and maintenance of NMDA and GABAA receptor clusters at central synapses, and suggest that neurotrophin signaling coordinately modulates these receptors as part of mechanism that promotes the balance between excitation and inhibition in developing circuits. We also discuss our results that demonstrate that astrocytes promote the formation of GABAergic synapses in vitro by differentially regulating the development of inhibitory presynaptic terminals and postsynaptic GABAA receptor clusters, and suggest that glial modulation of inhibitory synaptogenesis is mediated by neurotrophin-dependent and -independent signaling. Together, these findings extend our understanding of how neuron-glia communication modulates synapse formation, maintenance and function, and set the stage for defining the cellular and molecular mechanisms by which neurotrophins and other cell-cell signals direct synaptogenesis in the developing brain.