High Ca(2+)-phosphate transfection efficiency enables single neuron gene analysis

MiR-186-5p inhibition restores synaptic transmission and neuronal network activity in a model of chronic stress

Chronic stress exerts profound negative effects on cognitive and emotional behaviours and is a major risk factor for the development of neuropsychiatric disorders. However, the molecular links between chronic stress and its deleterious effects on neuronal and synaptic function remain elusive. Here, using a combination of in vitro and in vivo approaches, we demonstrate that the upregulation of miR-186-5p triggered by chronic stress may be a key mediator of such changes, leading to synaptic dysfunction. Our results show that the expression levels of miR-186-5p are increased both in the prefrontal cortex (PFC) of mice exposed to chronic stress and in cortical neurons chronically exposed to dexamethasone. Additionally, viral overexpression of miR-186-5p in the PFC of naïve mice induces anxiety- and depressive-like behaviours. The upregulation of miR-186-5p through prolonged glucocorticoid receptor activation in vitro, or in a mouse model of chronic stress, differentially affects glutamatergic and GABAergic synaptic transmission, causing an imbalance in excitation/inhibition that leads to altered neuronal network activity. At glutamatergic synapses, we observed both a reduction in synaptic AMPARs and synaptic transmission, whereas GABAergic synaptic transmission was strengthened. These changes could be rescued in vitro by a miR-186-5p inhibitor. Overall, our results establish a novel molecular link between chronic glucocorticoid receptor activation, the upregulation of miR-186-5p and the synaptic changes induced by chronic stress, that may be amenable to therapeutic intervention.

Optimized Calcium-Phosphate-Based Co-transfection of Tau and tdTomato into Human iPSC-Derived Neurons for the Study of Intracellular Distribution of Wild-type and Mutant Human Tau

The study of Tau protein in disease-relevant neuronal cells in culture requires efficient delivery systems for transfection of exogenous Tau and also modulators and interactors of Tau. Transfection of cultivated cells using calcium phosphate precipitation is a simple and cost-effective approach, also for difficult-to-transfect and sensitive cells such as primary neurons. Because of its low cell toxicity and ease of use, the Ca2+-phosphate transfection method is one of the most widely used gene transfer procedures in neuroscience. However, Ca2+-phosphate transfection efficacy in neurons is poor, often in the range of 1-5%, limiting its use in functional investigations. Here, we outline our improved Ca2+-phosphate transfection methodology for human iPSC-derived neurons that yields a reasonable efficiency (20-30% for bright volume markers) without apparent effects on cell health. We have used it to introduce wild-type and mutant human Tau with and without co-transfection of a volume marker (used here: tdTomato). In sum, our procedure can deliver neuronal genes (e.g., MAPT) using typical eukaryotic expression vectors (e.g., using CMV promoter) and is optimized for transfection of human iPSC-derived neurons.

Age-dependent NMDA receptor function is regulated by the amyloid precursor protein

N-methyl-D-aspartate receptors (NMDARs) are critical for the maturation and plasticity of glutamatergic synapses. In the hippocampus, NMDARs mainly contain GluN2A and/or GluN2B regulatory subunits. The amyloid precursor protein (APP) has emerged as a putative regulator of NMDARs, but the impact of this interaction to their function is largely unknown. By combining patch-clamp electrophysiology and molecular approaches, we unravel a dual mechanism by which APP controls GluN2B-NMDARs, depending on the life stage. We show that APP is highly abundant specifically at the postnatal postsynapse. It interacts with GluN2B-NMDARs, controlling its synaptic content and mediated currents, both in infant mice and primary neuronal cultures. Upon aging, the APP amyloidogenic-derived C-terminal fragments, rather than APP full-length, contribute to aberrant GluN2B-NMDAR currents. Accordingly, we found that the APP processing is increased upon aging, both in mice and human brain. Interfering with stability or production of the APP intracellular domain normalized the GluN2B-NMDARs currents. While the first mechanism might be essential for synaptic maturation during development, the latter could contribute to age-related synaptic impairments.

Aberrant hippocampal transmission and behavior in mice with a stargazin mutation linked to intellectual disability

Mutations linked to neurodevelopmental disorders, such as intellectual disability (ID), are frequently found in genes that encode for proteins of the excitatory synapse. Transmembrane AMPA receptor regulatory proteins (TARPs) are AMPA receptor auxiliary proteins that regulate crucial aspects of receptor function. Here, we investigate a mutant form of the TARP family member stargazin, described in an ID patient. Molecular dynamics analyses predicted that the ID-associated stargazin variant, V143L, weakens the overall interface of the AMPAR:stargazin complex and impairs the stability of the complex. Knock-in mice harboring the V143L stargazin mutation manifest cognitive and social deficits and hippocampal synaptic transmission defects, resembling phenotypes displayed by ID patients. In the hippocampus of stargazin V143L mice, CA1 neurons show impaired spine maturation, abnormal synaptic transmission and long-term potentiation specifically in basal dendrites, and synaptic ultrastructural alterations. These data suggest a causal role for mutated stargazin in the pathogenesis of ID and unveil a new role for stargazin in the development and function of hippocampal synapses.

Evolutionary Timeline of Genetic Delivery and Gene Therapy

There are more than 3,500 genes that are being linked to hereditary diseases or correlated with an elevated risk of certain illnesses. As an alternative to conventional treatments with small molecule drugs, gene therapy has arisen as an effective treatment with the potential to not just alleviate disease conditions but also cure them completely. In order for these treatment regimens to work, genes or editing tools intended to correct diseased genetic material must be efficiently delivered to target sites. There have been many techniques developed to achieve such a goal. In this article, we systematically review a variety of gene delivery and therapy methods that include physical methods, chemical and biochemical methods, viral methods, and genome editing. We discuss their historical discovery, mechanisms, advantages, limitations, safety, and perspectives.

Ligand-independent activity of the ghrelin receptor modulates AMPA receptor trafficking and supports memory formation

The biological signals of hunger, satiety, and memory are interconnected. The role of the hormone ghrelin in regulating feeding and memory makes ghrelin receptors attractive targets for associated disorders. We investigated the effects of the high ligand-independent activity of the ghrelin receptor GHS-R1a on the physiology of excitatory synapses in the hippocampus. Blocking this activity produced a decrease in the synaptic content of AMPA receptors in hippocampal neurons and a reduction in GluA1 phosphorylation at Ser⁸⁴⁵ Reducing the ligand-independent activity of GHS-R1a increased the surface diffusion of AMPA receptors and impaired AMPA receptor-dependent synaptic delivery induced by chemical long-term potentiation. Accordingly, we found that blocking this GHS-R1a activity impaired spatial and recognition memory in mice. These observations support a role for the ligand-independent activity of GHS-R1a in regulating AMPA receptor trafficking under basal conditions and in the context of synaptic plasticity that underlies learning.

Regulation of TRPC5 currents by intracellular ATP: Single channel studies

TRPC5 channels are nonselective cation channels activated by G-protein-coupled receptors. It was previously found that recombinant TRPC5 currents are inhibited by intracellular ATP, when studied by whole-cell patch-clamp recording. In the present study, we investigated the mechanism of ATP inhibition at the single-channel level using patches from HEK-293 cells transiently transfected with TRPC5 and the M1 muscarinic receptor. In inside-out patches, application of ATP to the intracellular face of the membrane reduced TRPC5 channel activity at both positive and negative potentials without affecting the unitary current amplitude or open dwell time of the channel. The effect of ATP was rapidly reversible. These results suggest that ATP may bind to the channel protein and affect the ability of the channel to open or to remain in an open, nondesensitized state. The activity of TRPC5 channels may be influenced by cellular metabolism via changes in ATP levels.

BDNF increases synaptic NMDA receptor abundance by enhancing the local translation of Pyk2 in cultured hippocampal neurons

The effects of brain-derived neurotrophic factor (BDNF) in long-term synaptic potentiation (LTP) are thought to underlie learning and memory formation and are partly mediated by local protein synthesis. Here, we investigated the mechanisms that mediate BDNF-induced alterations in the synaptic proteome that are coupled to synaptic strengthening. BDNF induced the synaptic accumulation of GluN2B-containing NMDA receptors (NMDARs) and increased the amplitude of NMDAR-mediated miniature excitatory postsynaptic currents (mEPSCs) in cultured rat hippocampal neurons by a mechanism requiring activation of the protein tyrosine kinase Pyk2 and dependent on cellular protein synthesis. Single-particle tracking using quantum dot imaging revealed that the increase in the abundance of synaptic NMDAR currents correlated with their enhanced stability in the synaptic compartment. Furthermore, BDNF increased the local synthesis of Pyk2 at the synapse, and the observed increase in Pyk2 protein abundance along dendrites of cultured hippocampal neurons was mediated by a mechanism dependent on the ribonucleoprotein hnRNP K, which bound to Pyk2 mRNA and dissociated from it upon BDNF application. Knocking down hnRNP K reduced the BDNF-induced synaptic synthesis of Pyk2 protein, whereas its overexpression enhanced it. Together, these findings indicate that hnRNP K mediates the synaptic distribution of Pyk2 synthesis, and hence the synaptic incorporation of GluN2B-containing NMDARs, induced by BDNF, which may affect LTP and synaptic plasticity.

Stargazin Dephosphorylation Mediates Homeostatic Synaptic Downscaling of Excitatory Synapses

Synaptic scaling is a form of homeostatic plasticity that is critical for maintaining neuronal activity within a dynamic range, and which alters synaptic strength through changes in postsynaptic AMPA-type glutamate receptors. Homeostatic scaling down of excitatory synapses has been shown to occur during sleep, and to contribute to synapse remodeling and memory consolidation, but the underlying mechanisms are only partially known. Here, we report that synaptic downscaling in cortical neurons is accompanied by dephosphorylation of the transmembrane AMPA receptor regulatory protein stargazin, and by an increase in its cell surface mobility. The changes in stargazin surface diffusion were paralleled by an increase in the mobility of GluA1-containing AMPA receptors at synaptic sites. In addition, stargazin dephosphorylation was required for the downregulation of surface levels of GluA1-containing AMPA receptors promoted by chronic elevation of neuronal activity, specifically by mediating the interaction with the adaptor proteins AP-2 and AP-3A. Disruption of the stargazin-AP-3A interaction was sufficient to prevent the decrease in cell surface GluA1-AMPA receptor levels associated with chronically enhanced synaptic activity, suggesting that scaling down is accomplished through decreased AMPA receptor recycling and enhanced lysosomal degradation. Thus, synaptic downscaling is associated with both increased stargazin and AMPA receptor cell surface diffusion, as well as with stargazin-mediated AMPA receptor endocytosis and lysosomal degradation.

G protein signaling-biased agonism at the κ-opioid receptor is maintained in striatal neurons

Biased agonists of G protein-coupled receptors may present a means to refine receptor signaling in a way that separates side effects from therapeutic properties. Several studies have shown that agonists that activate the κ-opioid receptor (KOR) in a manner that favors G protein coupling over β-arrestin2 recruitment in cell culture may represent a means to treat pain and itch while avoiding sedation and dysphoria. Although it is attractive to speculate that the bias between G protein signaling and β-arrestin2 recruitment is the reason for these divergent behaviors, little evidence has emerged to show that these signaling pathways diverge in the neuronal environment. We further explored the influence of cellular context on biased agonism at KOR ligand-directed signaling toward G protein pathways over β-arrestin-dependent pathways and found that this bias persists in striatal neurons. These findings advance our understanding of how a G protein-biased agonist signal differs between cell lines and primary neurons, demonstrate that measuring [³⁵S]GTPγS binding and the regulation of adenylyl cyclase activity are not necessarily orthogonal assays in cell lines, and emphasize the contributions of the environment to assessing biased agonism.

Discovery and characterization of two novel CB1 receptor splice variants with modified N-termini in mouse

Numerous studies have been carried out in the mouse model, investigating the role of the cannabinoid receptor type 1 (CB1). However, mouse CB1 (mCB1) receptor differs from human CB1 (hCB1) receptor in 13 amino acid residues. Two splice variants, hCB1a and hCB1b, diverging in their amino-termini, have been reported to be unique for hCB1 and, via different signaling properties, contribute to CB1 receptor physiology and pathophysiology. We hypothesized that splice variants also exist for the mCB1 receptor and have different signaling properties. On murine hippocampal cDNA, we identified two novel mCB1 receptor splice variants generated by splicing of introns with 117 bp and 186 bp in the N-terminal domain, corresponding to deletions of 39 or 62 amino acids, respectively. The mRNAs for the splice variants mCB1a and mCB1b are expressed at low levels in different brain regions. Western blot analysis of protein extracts from stably transfected HEK293 cells indicates a strongly reduced glycosylation because of the absence of two glycosylation sites in mCB1b. On-cell western analysis in these stable lines revealed increased internalization of mCB1a and mCB1b upon stimulation with the agonist WIN55,212-2 as compared to mCB1. Results also point toward an increased affinity to SR141716 for mCB1a, as well as slightly enhanced inhibition of neurotransmission compared to mCB1. In mCB1b, agonist-induced MAPK phosphorylation was decreased compared to mCB1 and mCB1a. Identification of mouse CB1 receptor splice variants may help to explain differences found between human and mouse endocannabinoid systems and improve the understanding of CB1 receptor signaling and trafficking in different species.

Ataxin-3 phosphorylation decreases neuronal defects in spinocerebellar ataxia type 3 models

Ataxin-3, the protein involved in spinocerebellar ataxia type 3 or Machado-Joseph disease, causes dendritic and synapse loss in cultured neurons when expanded, and mutation of phosphorylation site S12 reduces aggregation, neuronal loss, and synapse loss.

Different neurodegenerative diseases are caused by aberrant elongation of repeated glutamine sequences normally found in particular human proteins. Although the proteins involved are ubiquitously distributed in human tissues, toxicity targets only defined neuronal populations. Changes caused by an expanded polyglutamine protein are possibly influenced by endogenous cellular mechanisms, which may be harnessed to produce neuroprotection. Here, we show that ataxin-3, the protein involved in spinocerebellar ataxia type 3, also known as Machado-Joseph disease, causes dendritic and synapse loss in cultured neurons when expanded. We report that S12 of ataxin-3 is phosphorylated in neurons and that mutating this residue so as to mimic a constitutive phosphorylated state counters the neuromorphologic defects observed. In rats stereotaxically injected with expanded ataxin-3–encoding lentiviral vectors, mutation of serine 12 reduces aggregation, neuronal loss, and synapse loss. Our results suggest that S12 plays a role in the pathogenic pathways mediated by polyglutamine-expanded ataxin-3 and that phosphorylation of this residue protects against toxicity.

Interleukin-1β has trophic effects in microglia and its release is mediated by P2X7R pore

Background

Enhanced expression of the purinergic P2X7 receptor (P2X7R) occurs in several neuroinflammatory conditions where increased microglial activation is a co-existing feature. P2X7 receptors can function either as a cation channel or, upon continued stimulation, a large pore. P2X7R-over-expression alone is sufficient to drive microglial activation and proliferation in a process that is P2X7R pore dependent, although the biological signaling pathway through which this occurs remains unclear. Once activated, microglia are known to release a number of bioactive substances that include the proinflammatory cytokine interleukin-1β (IL-1β). Previous studies have linked P2X7R stimulation to the processing and release of IL-1β, but whether the channel or pore state of P2X7R is predominant in driving IL-1β release is unknown and is a major aim of this study. In addition, we will determine whether IL-1β has trophic effects on surrounding microglia.

Methods

Electron microscopy and immunohistochemistry were used to delineate the sub-cellular localization of P2X7R and IL-1β in primary hippocampal rat cultures. FM1-43 fluorescent dye and confocal microscopy were used to quantify vesicular exocytosis from microglia expressing the pore-forming P2X7R versus a non-pore-forming point mutant, P2X7RG345Y. IL-1β in culture was quantified with an enzyme-linked immunosorbent assay (ELISA). IL-1β intracellular processing was blocked with inhibition of caspase 1 (with a synthetic peptide antagonist), and its extracellular form was neutralized with an IL-1β neutralizing antibody. Microglial activation and proliferation was quantified immunohistochemically with confocal microscopy.

Results

P2X7R and IL-1β were co-localized in lysosomes. Vesicular exocytosis was higher in microglia expressing the pore-forming P2X7R compared to those expressing the non-pore-forming mutant. There was increased IL-1β in cultures expressing the pore-forming P2X7R, and this proinflammatory cytokine was found to mediate the trophic effects of P2X7R pore in microglia. Inhibition of IL-1β production and function resulted in a significant decrease in P2X7R-mediated microglial activation and proliferation.

Conclusions

IL-1β is a mediator of microglial activation and proliferation, and its release/production is P2X7R pore dependent. Blockade of P2X7R pore could serve as a therapeutic target in alleviating the degree of inflammation seen in neurodegenerative and neoplastic conditions.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-016-0621-8) contains supplementary material, which is available to authorized users.

Efficient Transfection by Using PDMAEMA-Modified SiNWAs as a Platform for Ca(2+)-Dependent Gene Delivery

The major bottleneck for gene delivery lies in the lack of safe and efficient gene vectors and delivery systems. In order to develop a much safer and efficient transfection system, a novel strategy of combining traditional Ca(2+)-dependent transfection with cationic polymer poly(N,N-dimethylamino)ethyl methacrylate (PDMAEMA) modified silicon nanowire arrays (SiNWAs) was proposed in this work. Detailed studies were carried out on the effects of the PDMAEMA polymerization time, the Ca(2+) concentration, and the incubation time of Ca(2+)@DNA complex with PDMAEMA-modified SiNWAs (SN-PDM) on the gene transfection in the cells. The results demonstrated that the transfection efficiency of SN-PDM assisted traditional Ca(2+)-dependent transfection was significantly enhanced compared to those without any surface assistance, and SN-PDM with polymerization time 24 h exhibited the highest efficiency. Moreover, the optimal transfection efficiency was found at the system of a complex containing Ca(2+) (100 mM) and plasmid DNA (pDNA) incubated on SN-PDM for 20 min. Compared with unmodified SiNWAs, SN-PDM has little cytotoxicity and can improve cell attachment. All of these results demonstrated that SN-PDM could significantly enhance Ca(2+)-dependent transfection; this process depends on the amino groups' density of PDMAEMA on the surface, the Ca(2+) concentration, and the available Ca(2+)@DNA complex. Our study provides a potential novel and excellent means of gene delivery for therapeutic applications.

Could tuning of the inhibitory tone involve graded changes in neuronal chloride transport?

Hyperpolarizing synaptic inhibition through GABAA and glycine receptors depends on the presence of the neuronal cation-chloride-cotransporter protein, KCC2. Several transcriptional and post-transcriptional mechanisms have been shown to regulate KCC2 and thereby influence the polarity and efficacy of inhibitory synaptic transmission. It is unclear however whether regulation of KCC2 enables the transporter to attain different levels of activity thus allowing a neuron to modulate the strength of inhibitory synaptic transmission to its changing requirements. We therefore investigated whether phosphorylation can allow KCC2 to achieve distinct levels of [Cl(-)]i in neurons. We generated a variety of KCC2 alanine dephosphorylation mimics and used NH4(+)-induced pHi shifts in cultured hippocampal neurons to quantify the rate of KCC2 transport activity exhibited by these mutants. To explore the relationship between KCC2 transport and GABAA receptor-mediated current amplitudes we performed gramicidine perforated-patch recordings. The correlation between EGABA and NH4(+)-induced pHi shifts enabled an estimate of the range of chloride extrusion possible by kinase/phosphatase regulation of KCC2. Our results demonstrate that KCC2 transport can vary considerably in magnitude depending on the combination of alanine mutations present on the protein. Transport can be enhanced to sufficiently high levels that hyperpolarizing GABAA responses may be obtained even in neurons with an extremely negative resting membrane potential and at high extracellular K(+) concentrations. Our findings highlight the significant potential for regulating the inhibitory tone by KCC2-mediated chloride extrusion and suggest that cellular signaling pathways may act combinatorially to alter KCC2 phosphorylation/dephosphorylation and thereby tune the strength of synaptic inhibition.

SUMOylation of the brain-predominant Ataxin-3 isoform modulates its interaction with p97

BACKGROUND

Machado-Joseph Disease (MJD), a form of dominantly inherited ataxia belonging to the group of polyQ expansion neurodegenerative disorders, occurs when a threshold value for the number of glutamines in Ataxin-3 (Atx3) polyglutamine region is exceeded. As a result of its modular multidomain architecture, Atx3 is known to engage in multiple macromolecular interactions, which might be unbalanced when the polyQ tract is expanded, culminating in the aggregation and formation of intracellular inclusions, a unifying fingerprint of this group of neurodegenerative disorders. Since aggregation is specific to certain brain regions, localization-dependent posttranslational modifications that differentially affect Atx3 might also contribute for MJD.

METHODS

We combined in vitro and cellular approaches to address SUMOylation in the brain-predominant Atx3 isoform and assessed the impact of this posttranslational modification on Atx3 self-assembly and interaction with its native partner, p97.

RESULTS

We demonstrate that Atx3 is SUMOylated at K356 both in vitro and in cells, which contributes for decreased formation of amyloid fibrils and for increased affinity towards p97.

CONCLUSIONS AND GENERAL SIGNIFICANCE

These findings highlight the role of SUMOylation as a regulator of Atx3 function, with implications on Atx3 protein interaction network and self-assembly, with potential impact for further understanding the molecular mechanisms underlying MJD pathogenesis.

Cannabinoid receptor-interacting protein 1a modulates CB1 receptor signaling and regulation

Cannabinoid CB1 receptors (CB1Rs) mediate the presynaptic effects of endocannabinoids in the central nervous system (CNS) and most behavioral effects of exogenous cannabinoids. Cannabinoid receptor-interacting protein 1a (CRIP1a) binds to the CB1R C-terminus and can attenuate constitutive CB1R-mediated inhibition of Ca(2+) channel activity. We now demonstrate cellular colocalization of CRIP1a at neuronal elements in the CNS and show that CRIP1a inhibits both constitutive and agonist-stimulated CB1R-mediated guanine nucleotide-binding regulatory protein (G-protein) activity. Stable overexpression of CRIP1a in human embryonic kidney (HEK)-293 cells stably expressing CB1Rs (CB1-HEK), or in N18TG2 cells endogenously expressing CB1Rs, decreased CB1R-mediated G-protein activation (measured by agonist-stimulated [(35)S]GTPγS (guanylyl-5'-[O-thio]-triphosphate) binding) in both cell lines and attenuated inverse agonism by rimonabant in CB1-HEK cells. Conversely, small-interfering RNA-mediated knockdown of CRIP1a in N18TG2 cells enhanced CB1R-mediated G-protein activation. These effects were not attributable to differences in CB1R expression or endocannabinoid tone because CB1R levels did not differ between cell lines varying in CRIP1a expression, and endocannabinoid levels were undetectable (CB1-HEK) or unchanged (N18TG2) by CRIP1a overexpression. In CB1-HEK cells, 4-hour pretreatment with cannabinoid agonists downregulated CB1Rs and desensitized agonist-stimulated [(35)S]GTPγS binding. CRIP1a overexpression attenuated CB1R downregulation without altering CB1R desensitization. Finally, in cultured autaptic hippocampal neurons, CRIP1a overexpression attenuated both depolarization-induced suppression of excitation and inhibition of excitatory synaptic activity induced by exogenous application of cannabinoid but not by adenosine A1 agonists. These results confirm that CRIP1a inhibits constitutive CB1R activity and demonstrate that CRIP1a can also inhibit agonist-stimulated CB1R signaling and downregulation of CB1Rs. Thus, CRIP1a appears to act as a broad negative regulator of CB1R function.

Rapid genotyping of animals followed by establishing primary cultures of brain neurons

High-resolution analysis of the morphology and function of mammalian neurons often requires the genotyping of individual animals followed by the analysis of primary cultures of neurons. We describe a set of procedures for: labeling newborn mice to be genotyped, rapid genotyping, and establishing low-density cultures of brain neurons from these mice. Individual mice are labeled by tattooing, which allows for long-term identification lasting into adulthood. Genotyping by the described protocol is fast and efficient, and allows for automated extraction of nucleic acid with good reliability. This is useful under circumstances where sufficient time for conventional genotyping is not available, e.g., in mice that suffer from neonatal lethality. Primary neuronal cultures are generated at low density, which enables imaging experiments at high spatial resolution. This culture method requires the preparation of glial feeder layers prior to neuronal plating. The protocol is applied in its entirety to a mouse model of the movement disorder DYT1 dystonia (ΔE-torsinA knock-in mice), and neuronal cultures are prepared from the hippocampus, cerebral cortex and striatum of these mice. This protocol can be applied to mice with other genetic mutations, as well as to animals of other species. Furthermore, individual components of the protocol can be used for isolated sub-projects. Thus this protocol will have wide applications, not only in neuroscience but also in other fields of biological and medical sciences.

Transthyretin Induces Insulin-like Growth Factor I Nuclear Translocation Regulating Its Levels in the Hippocampus

Transthyretin (TTR) is the carrier protein of thyroxine (T₄) and binds to retinol-binding protein (RBP)-retinol complex. It is mainly synthesized by both liver and choroid plexuses of the brain. Besides these properties, it has a neuroprotective role in several contexts such as Alzheimer’s disease (AD) and cerebral ischemia. Activation of insulin-like growth factor receptor I (IGF-IR) pathways and increased levels of TTR are associated with absence of neurodegeneration in an AD mouse model. In the present study, we verified that young/adult TTR null mice had decreased levels of IGF-IR in the hippocampus, but not in choroid plexus when compared with wild-type age-matched controls. Moreover, we could also demonstrate that conditional silencing of peripheral TTR did not have any influence in hippocampal IGF-IR levels, indicating that TTR effect on IGF-IR levels is due to TTR mainly synthesized in the choroid plexus. In vitro cellular studies, using NIH3T3 cell line and primary cultured hippocampal neurons, we showed that TTR upregulates IGF-IR at the transcription and translation levels and that is dependent on receptor internalization. Using a GFP-IGF-IR fusion protein, we also found that TTR triggers IGF-IR nuclear translocation in cultured neurons. We could also see an enrichment of IGF-IR in the nuclear fraction, after TTR stimulation in NIH3T3 cells, indicating that IGF-IR regulation, triggered by TTR is induced by nuclear translocation. In summary, the results provide evidence of a new role of TTR as a transcription inducer of IGF-IR in central nervous system (CNS), unveiling a new role in neuroprotection.

Upregulation of dendritic arborization by N-acetyl-D-glucosamine kinase is not dependent on its kinase activity

N-acetylglucosamine kinase (GlcNAc kinase or NAGK; EC 2.7.1.59) is highly expressed and plays a critical role in the development of dendrites in brain neurons. In this study, the authors conducted structure-function analysis to verify the previously proposed 3D model structure of GlcNAc/ ATP-bound NAGK. Three point NAGK mutants with different substrate binding capacities and reaction velocities were produced. Wild-type (WT) NAGK showed strong substrate preference for GlcNAc. Conversion of Cys143, which does not make direct hydrogen bonds with GlcNAc, to Ser (i.e., C143S) had the least affect on the enzymatic activity of NAGK. Conversion of Asn36, which plays a role in domain closure by making a hydrogen bond with GlcNAc, to Ala (i.e., N36A) mildly reduced NAGK enzyme activity. Conversion of Asp107, which makes hydrogen bonds with GlcNAc and would act as a proton acceptor during nucleophilic attack on the γ-phosphate of ATP, to Ala (i.e., D107A), caused a total loss in enzyme activity. The overexpression of EGFP-tagged WT or any of the mutant NAGKs in rat hippocampal neurons (DIV 5-9) increased dendritic architectural complexity. Finally, the overexpression of the small, but not of the large, domain of NAGK resulted in dendrite degeneration. Our data show the effect of structure on the functional aspects of NAGK, and in particular, that the small domain of NAGK, and not its NAGK kinase activity, plays a critical role in the upregulation of dendritogenesis.

Phosphorylation of Ser1166 on GluN2B by PKA is critical to synaptic NMDA receptor function and Ca2+ signaling in spines

The NMDA-type glutamate receptor (NMDAR) is essential for synaptogenesis, synaptic plasticity, and higher cognitive function. Emerging evidence indicates that NMDAR Ca(2+) permeability is under the control of cAMP/protein kinase A (PKA) signaling. Whereas the functional impact of PKA on NMDAR-dependent Ca(2+) signaling is well established, the molecular target remains unknown. Here we identify serine residue 1166 (Ser1166) in the carboxy-terminal tail of the NMDAR subunit GluN2B to be a direct molecular and functional target of PKA phosphorylation critical to NMDAR-dependent Ca(2+) permeation and Ca(2+) signaling in spines. Activation of β-adrenergic and D1/D5-dopamine receptors induces Ser1166 phosphorylation. Loss of this single phosphorylation site abolishes PKA-dependent potentiation of NMDAR Ca(2+) permeation, synaptic currents, and Ca(2+) rises in dendritic spines. We further show that adverse experience in the form of forced swim, but not exposure to fox urine, elicits striking phosphorylation of Ser1166 in vivo, indicating differential impact of different forms of stress. Our data identify a novel molecular and functional target of PKA essential to NMDAR-mediated Ca(2+) signaling at synapses and regulated by the emotional response to stress.

The non-canonical effect of N-acetyl-D-glucosamine kinase on the formation of neuronal dendrites

N-acetylglucosamine kinase (GlcNAc kinase or NAGK; EC 2.7.1.59) is a N-acetylhexosamine kinase that belong to the sugar kinase/heat shock protein 70/actin superfamily. In this study, we investigated both the expression and function of NAGK in neurons. Immunohistochemistry of rat brain sections showed that NAGK was expressed at high levels in neurons but at low levels in astrocytes. Immunocytochemistry of rat hippocampal dissociate cultures confirmed these findings and showed that NAGK was also expressed at low levels in oligodendrocytes. Furthermore, several NAGK clusters were observed in the nucleoplasm of both neuron and glia. The overexpression of EGFP- or RFP (DsRed2)-tagged NAGK in rat hippocampal neurons (DIV 5-9) increased the complexity of dendritic architecture by increasing the numbers of primary dendrites and dendritic branches. In contrast, knockdown of NAGK by shRNA resulted in dendrite degeneration, and this was prevented by the co-expression of RFP-tagged NAGK. These results suggest that the upregulation of dendritic complexity is a non-canonical function of NAGK.

Diacylglycerol lipaseα (DAGLα) and DAGLβ cooperatively regulate the production of 2-arachidonoyl glycerol in autaptic hippocampal neurons

Cannabinoids are part of an endogenous signaling system consisting of cannabinoid receptors and endogenous cannabinoids as well as the enzymatic machinery for their synthesis and degradation. Depolarization-induced suppression of excitation (DSE) is a form of cannabinoid CB(1) receptor-mediated inhibition of synaptic transmission that involves the production of the endogenous cannabinoid 2-arachidonoyl glycerol (2-AG). Both diacylglycerol lipase α (DAGLα) and DAGLβ can produce 2-AG in vitro, but evidence from knockout animals argues strongly for a predominant, even exclusive, role for DAGLα in regulation of 2-AG-mediated synaptic plasticity. What role, if any, might be played by DAGLβ remains largely unknown. Cultured autaptic hippocampal neurons exhibit robust DSE. With the ability to rapidly modulate expression of DAGLα and DAGLβ in these neurons with short hairpin RNA, they are well suited for a comparative study of the roles of each isoform in mediating DSE. We find that RNA interference knockdown of DAGLα substantially reduces autaptic DSE, shifting the "depolarization-response curve" from an ED(50) value of 1.7 seconds to 3.0 seconds. Surprisingly, DAGLβ knockdown diminishes DSE as much or more (ED(50) 6.4 seconds), suggesting that DAGLβ is also responsible for a portion of 2-AG production in autaptic neurons. Similarly, the two DAGLs both contribute to the production of 2-AG via group I metabotropic glutamate receptors. Our results provide the first explicit evidence for a role of DAGLβ in modulating neurotransmission.

Dance of the SNAREs: assembly and rearrangements detected with FRET at neuronal synapses

Soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptors (SNAREs) mediate vesicle fusion with the plasma membrane on activation by calcium binding to synaptotagmin. In the present study, we used fluorescence resonance energy transfer (FRET) and fluorescence lifetime imaging microscopy between fluorescently labeled SNARE proteins expressed in cultured rat hippocampal neurons to detect resting SNARE complexes, their conformational rearrangement on exocytosis, their disassembly before endocytosis of vesicular proteins, and SNARE assembly at newly docked vesicles. Assembled SNAREs are not only present in docked vesicles; unexpected residual "orphan SNARE complexes" also reside in para-active zone regions. Real-time changes in FRET between N-terminally labeled SNAP-25 and VAMP reported a reorientation of the SNARE motif upon exocytosis, SNARE disassembly in the active zone periphery, and SNARE reassembly in newly docked vesicles. With VAMP labeled C-terminally, decreased fluorescence in C-terminally labeled syntaxin (extracellular) reported trans-cis-conformational changes in SNAREs on vesicle fusion. After fusion SNAP-25 and syntaxin disperse along with VAMP, as well as the FRET signal itself, indicating diffusion of intact SNAREs after vesicle fusion but before their peripheral disassembly. Our measurements of spatiotemporal dynamics of SNARE conformational changes and movements refine models of SNARE function. Technical advances required to detect tiny changes in fluorescence in small fractions of labeled proteins in presynaptic boutons on a time scale of seconds permit the detection of rapid intermolecular interactions between small proportions of protein partners in cellular subcompartments.

Compartmentalizing genetically encoded calcium sensors

Within single cells there is a complex myriad of signaling which controls physiological process many of which are modulated, or signaled directly, by intracellular calcium ions. Understanding the exquisitely sensitive, and spatially restricted, changes in calcium has been of interest to the researcher for a number of years. Recent advances in this field have been driven by the development of genetically encoded calcium probes for detecting calcium changes within the cells specifically targeting organelles such as mitochondria, endoplasmic reticulum, and the nucleus. In this chapter the authors outline some of the available fluorescent probes, with particular emphasis on an endoplasmic reticulum targeted calcium biosensor in cell signaling studies with astrocytes, detailing experimental protocols and the interpretation of data from such probes.

Endogenous nitric oxide induces activation of apoptosis signal-regulating kinase 1 via S-nitrosylation in rat hippocampus during cerebral ischemia-reperfusion

Apoptosis signal-regulating kinase 1 (ASK1) is a general mediator of cell death in response to a variety of stimuli, including reactive oxygen species, tumor necrosis factor α, lipopolysaccharide, endoplasmic reticulum stress, calcium influx and ischemia. Here we reported ASK1 was activated by nitric oxide (NO) through S-nitrosylation during cerebral ischemia-reperfusion. The reagents that abrogate neuronal nitric oxide synthase (nNOS) activity such as nNOS inhibitor 7NI and N-methyl-D-aspartate receptor antagonist MK801 prevented ASK1 activation via decreasing ASK1 S-nitrosylation. In HEK293 cells, over-expressed ASK1 could be S-nitrosylated by both exogenous and endogenous NO and Cys869 was identified as the site of ASK1 S-nitrosylation. S-nitrosylation increased the level of ASK1 phosphorylation at Thr845, which represents ASK1 activation. Our results further confirmed that S-nitrosylation led to the increment of ASK1 dimerization. S-nitrosylation of ASK1 also activated the downstream JNK signaling and JNK-mediated nucleic pathway. The exogenous NO (SNP and GSNO) reversed the effect of endogenous NO by suppressing S-nitrosylation of ASK1 and exerted neuroprotection during ischemia-reperfusion. These results suggest that inhibiting ASK1 S-nitrosylation may be a novel approach for stroke therapy.

Genetically controlled upregulation of adenosine A(1) receptor expression enhances the survival of primary cortical neurons

Adenosine has a key endogenous neuroprotective role in the brain, predominantly mediated by the adenosine A(1) receptor (A(1)R). This has been mainly explored using pharmacological tools and/or receptor knockout mice strains. It has long been suggested that the neuroprotective effects of A(1)R are increased following receptor upregulation, thus attenuating neuronal damage in pathological conditions. We have previously shown that the neuroprotective and neuromodulatory actions of the cytokines IL-6 and oncostatin M are mediated by induction of neuronal A(1)R expression. In order to investigate the direct effects of A(1)R upregulation in neurons, we have generated a tetracycline-regulated expression system with a bidirectional promoter, directing the simultaneous expression of the mouse A(1)R and GFP/mCherry reporter genes. In a first step, we tested the efficacy of the system in transiently transfected human embryonic kidney 293 cells. In addition, we confirmed the functional integrity of the expressed A(1)R by whole-cell patch clamp recordings. We demonstrated that A(1)R-transfected primary neurons show enhanced survival against N-methyl-D-aspartate-induced excitotoxicity. Pretreatment with an A(1)R-selective agonist additionally strongly decreased neuronal cell death, while an A(1)R antagonist completely abolished the neuroprotective effects of A(1)R upregulation. The presented data provide for the first time direct evidence that the upregulation of A(1)R enhances neuronal survival.

Differential signalling in human cannabinoid CB1 receptors and their splice variants in autaptic hippocampal neurones

BACKGROUND AND PURPOSE

Cannabinoids such as Δ(9) - tetrahydrocannabinol, the major psychoactive component of marijuana and hashish, primarily act via cannabinoid CB(1) and CB(2) receptors to produce characteristic behavioural effects in humans. Due to the tractability of rodent models for electrophysiological and behavioural studies, most of the studies of cannabinoid receptor action have used rodent cannabinoid receptors. While CB(1) receptors are relatively well-conserved among mammals, human CB(1) (hCB(1) ) differs from rCB(1) and mCB(1) receptors at 13 residues, which may result in differential signalling. In addition, two hCB(1) splice variants (hCB(1a) and hCB(1b) ) have been reported, diverging in their amino-termini relative to hCB(1) receptors. In this study, we have examined hCB(1) signalling in neurones.

EXPERIMENTAL APPROACH

hCB(1) , hCB(1a) hCB(1b) or rCB(1) receptors were expressed in autaptic cultured hippocampal neurones from CB(1) (-/-) mice. Such cells express a complete endogenous cannabinoid signalling system. Electrophysiological techniques were used to assess CB(1) receptor-mediated signalling. KEY RESULTS Expressed in autaptic hippocampal neurones cultured from CB(1) (-/-) mice, hCB(1) , hCB(1a) and hCB(1b) signal differentially from one another and from rodent CB(1) receptors. Specifically, hCB(1) receptors inhibit synaptic transmission less effectively than rCB(1) receptors.

CONCLUSIONS AND IMPLICATIONS

Our results suggest that cannabinoid receptor signalling in humans is quantitatively very different from that in rodents. As the problems of marijuana and hashish abuse occur in humans, our results highlight the importance of studying hCB(1) receptors. They also suggest further study of the distribution and function of hCB(1) receptor splice variants, given their differential signalling and potential impact on human health.

LINKED ARTICLES

This article is part of a themed section on Cannabinoids in Biology and Medicine. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.165.issue-8. To view Part I of Cannabinoids in Biology and Medicine visit http://dx.doi.org/10.1111/bph.2011.163.issue-7.

The CB1 cannabinoid receptor C-terminus regulates receptor desensitization in autaptic hippocampal neurones

BACKGROUND AND PURPOSE

The cannabinoid CB(1) receptor is the chief mediator of the CNS effects of cannabinoids. In cell culture model systems, CB(1) receptors both desensitize and internalize on activation. Previous work suggests that the extreme carboxy-terminus of this receptor regulates internalization via phosphorylation of residues clustered within this region. Mutational analysis of the carboxy-terminus of CB(1) receptors has demonstrated that the last six serine/threonine residues are necessary for agonist-induced internalization. However, the structural determinants of CB(1) receptor internalization are also dependent on the local cellular environment. The importance of cell context on CB(1) receptor function calls for an investigation of the functional roles of these residues in neurones.

EXPERIMENTAL APPROACH

To determine the structural requirements of CB(1) internalization in neurones, we evaluated the signalling properties of carboxy-terminal mutated CB(1) receptors expressed in cultured autaptic hippocampal neurones, using electrophysiological methods.

KEY RESULTS

CB(1) receptors transfected into CB(1) knockout neurones signalled and desensitized as did wild-type neurones, allowing us to test specific CB(1) receptor mutations. Deletion of the last 13 residues yielded a CB(1) receptor that inhibited excitatory postsynaptic currents but did not desensitize. Furthermore, mutation of the final six serine and threonine residues to alanines resulted in a non-desensitizing receptor. In contrast, CB(1) receptors lacking residues 419-460, leaving the last 14 residues intact, did desensitize.

CONCLUSIONS AND IMPLICATIONS

The distal thirteen residues of CB(1) receptors are crucial for their desensitization in cultured neurones. Furthermore, this desensitization is likely to follow phosphorylation of serines and threonines within this region.

LINKED ARTICLES

This article is part of a themed section on Cannabinoids in Biology and Medicine. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.165.issue-8. To view Part I of Cannabinoids in Biology and Medicine visit http://dx.doi.org/10.1111/bph.2011.163.issue-7.

Sustained translational repression by eIF2α-P mediates prion neurodegeneration

The mechanisms leading to neuronal death in neurodegenerative disease are poorly understood. Many of these disorders, including Alzheimer's, Parkinson's and prion diseases, are associated with the accumulation of misfolded disease-specific proteins. The unfolded protein response is a protective cellular mechanism triggered by rising levels of misfolded proteins. One arm of this pathway results in the transient shutdown of protein translation, through phosphorylation of the α-subunit of eukaryotic translation initiation factor, eIF2. Activation of the unfolded protein response and/or increased eIF2α-P levels are seen in patients with Alzheimer's, Parkinson's and prion diseases, but how this links to neurodegeneration is unknown. Here we show that accumulation of prion protein during prion replication causes persistent translational repression of global protein synthesis by eIF2α-P, associated with synaptic failure and neuronal loss in prion-diseased mice. Further, we show that promoting translational recovery in hippocampi of prion-infected mice is neuroprotective. Overexpression of GADD34, a specific eIF2α-P phosphatase, as well as reduction of levels of prion protein by lentivirally mediated RNA interference, reduced eIF2α-P levels. As a result, both approaches restored vital translation rates during prion disease, rescuing synaptic deficits and neuronal loss, thereby significantly increasing survival. In contrast, salubrinal, an inhibitor of eIF2α-P dephosphorylation, increased eIF2α-P levels, exacerbating neurotoxicity and significantly reducing survival in prion-diseased mice. Given the prevalence of protein misfolding and activation of the unfolded protein response in several neurodegenerative diseases, our results suggest that manipulation of common pathways such as translational control, rather than disease-specific approaches, may lead to new therapies preventing synaptic failure and neuronal loss across the spectrum of these disorders.

Contactin-associated protein 1 (Caspr1) regulates the traffic and synaptic content of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors

Glutamate receptors of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) type mediate fast excitatory synaptic transmission in the CNS. Synaptic strength is modulated by AMPA receptor binding partners, which regulate receptor synaptic targeting and functional properties. We identify Contactin-associated protein 1 (Caspr1) as an AMPA receptor interactor. Caspr1 is present in synapses and interacts with AMPA receptors in brain synaptic fractions. Coexpression of Caspr1 with GluA1 increases the amplitude of glutamate-evoked currents. Caspr1 overexpression in hippocampal neurons increases the number and size of synaptic GluA1 clusters, whereas knockdown of Caspr1 decreases the intensity of synaptic GluA1 clusters. Hence, Caspr1 is a regulator of the trafficking of AMPA receptors to synapses.

COX-2 and fatty acid amide hydrolase can regulate the time course of depolarization-induced suppression of excitation

BACKGROUND AND PURPOSE

Depolarization-induced suppression of inhibition (DSI) and excitation (DSE) are two forms of cannabinoid CB(1) receptor-mediated inhibition of synaptic transmission, whose durations are regulated by endocannabinoid (eCB) degradation. We have recently shown that in cultured hippocampal neurons monoacylglycerol lipase (MGL) controls the duration of DSE, while DSI duration is determined by both MGL and COX-2. This latter result suggests that DSE might be attenuated, and excitatory transmission enhanced, during inflammation and in other settings where COX-2 expression is up-regulated.

EXPERIMENTAL APPROACH

To investigate whether it is possible to control the duration of eCB-mediated synaptic plasticity by varied expression of eCB-degrading enzymes, we transfected excitatory autaptic hippocampal neurons with putative 2-AG metabolizing enzymes: COX-2, fatty acid amide hydrolase (FAAH), α/β hydrolase domain 6 (ABHD6), α/β hydrolase domain 12 (ABHD12) or MGL.

KEY RESULTS

We found that overexpression of either COX-2 or FAAH shortens the duration of DSE while ABHD6 or ABHD12 do not. In contrast, genetic deletion (MGL(-/-)) and overexpression of MGL both radically altered eCB-mediated synaptic plasticity.

CONCLUSIONS AND IMPLICATIONS

We conclude that both FAAH and COX-2 can be trafficked to neuronal sites where they are able to degrade eCBs to modulate DSE duration and, by extension, net endocannabinoid signalling at a given synapse. The results for COX-2, which is often up-regulated under pathological conditions, are of particular note in that they offer a mechanism by which up-regulated COX-2 may promote neuronal excitation by suppressing DSE while enhancing conversion of 2-AG to PGE(2) -glycerol ester under pathological conditions.

Cleavage of the vesicular GABA transporter under excitotoxic conditions is followed by accumulation of the truncated transporter in nonsynaptic sites

GABA is the major inhibitory neurotransmitter in the CNS and changes in GABAergic neurotransmission affect the overall activity of neuronal networks. The uptake of GABA into synaptic vesicles is mediated by the vesicular GABA transporter (VGAT), and changes in the expression of the transporter directly regulate neurotransmitter release. In this work we investigated the changes in VGAT protein levels during ischemia and in excitotoxic conditions, which may affect the demise process. We found that VGAT is cleaved by calpains following excitotoxic stimulation of hippocampal neurons with glutamate, giving rise to a stable truncated cleavage product (tVGAT). VGAT cleavage was also observed after transient middle cerebral artery occlusion in mice, a cerebral ischemia model, and following intrahippocampal injection of kainate, but no effect was observed in transgenic mice overexpressing calpastatin, a calpain inhibitor. Incubation of isolated cerebrocortical synaptic vesicles with recombinant calpain also induced the cleavage of VGAT and formation of stable tVGAT. Immunoblot experiments using antibodies targeting different regions of VGAT and N-terminal sequencing analysis showed that calpain cleaves the transporter in the N-terminal region, at amino acids 52 and 60. Immunocytochemistry of GABAergic striatal neurons expressing GFP fusion proteins with the full-length VGAT or tVGAT showed that cleavage of the transporter induces a loss of synaptic delivery, leading to a homogeneous distribution of the protein along neurites. Our results show that excitotoxicity downregulates full-length VGAT, with a concomitant generation of tVGAT, which is likely to affect GABAergic neurotransmission and may influence cell death during ischemia.

Protocol for recombinant RBD-based SARS vaccines: protein preparation, animal vaccination and neutralization detection

Based on their safety profile and ability to induce potent immune responses against infections, subunit vaccines have been used as candidates for a wide variety of pathogens. Since the mammalian cell system is capable of post-translational modification, thus forming properly folded and glycosylated proteins, recombinant proteins expressed in mammalian cells have shown the greatest potential to maintain high antigenicity and immunogenicity. Although no new cases of SARS have been reported since 2004, future outbreaks are a constant threat; therefore, the development of vaccines against SARS-CoV is a prudent preventive step and should be carried out. The RBD of SARS-CoV S protein plays important roles in receptor binding and induction of specific neutralizing antibodies against virus infection. Therefore, in this protocol, we describe novel methods for developing a RBD-based subunit vaccine against SARS. Briefly, the recombinant RBD protein (rRBD) was expressed in culture supernatant of mammalian 293T cells to obtain a correctly folded protein with proper conformation and high immunogenicity. The transfection of the recombinant plasmid encoding RBD to the cells was then performed using a calcium phosphate transfection method with some modifications. Compared with the lipid transfection method, this modified calcium phosphate transfection method is cheaper, easier to handle, and has the potential to reach high efficacy once a transfection complex with suitable size and shape is formed. Finally, a SARS pseudovirus neutralization assay was introduced in the protocol and used to detect the neutralizing activity of sera of mice vaccinated with rRBD protein. This assay is relatively safe, does not involve an infectious SARS-CoV, and can be performed without the requirement of a biosafety-3 laboratory. The protocol described here can also be used to design and study recombinant subunit vaccines against other viruses with class I fusion proteins, for example, HIV, respiratory syncytial virus (RSV), Ebola virus, influenza virus, as well as Nipah and Handra viruses. In addition, the methods for generating a pseudovirus and subsequently establishing a pseudovirus neutralization assay can be applied to all these viruses.

Processing-dependent trafficking of Sonic hedgehog to the regulated secretory pathway in neurons

Neurons are an important source of the secreted morphogen Sonic hedgehog (Shh), however, little is known about neuron-specific regulation of Shh transport and secretion. To study this process, we investigated the subcellular distribution of Shh in primary neurons and differentiated cells of a neuroendocrine cell line by fluorescence microscopy and biochemical fractionation. In retinal ganglion cells, endogenous Shh was distributed as intra- and extracellular puncta at the soma, dendrites, axons and neurite terminals. Shh(+) puncta move bidirectionally and colocalize with markers of synaptic vesicles (SVs) and dense core granules. Lipid modification and proteolysis were required for Shh sorting to SVs and cell surface association. Finally, consistent with its association with regulated secretory vesicles, Shh secretion could be induced under depolarizing conditions. Taken together, these observations suggest that long-range Shh transport and signalling in neurons involves trafficking to the regulated secretory pathway and cell surface accumulation of Shh on axons and suggests a link between neuronal activity and Shh release.

Serotonin, but not N-methyltryptamines, activates the serotonin 2A receptor via a ß-arrestin2/Src/Akt signaling complex in vivo

Hallucinogens mediate many of their psychoactive effects by activating serotonin 2A receptors (5-HT(2A)R). Although serotonin is the cognate endogenous neurotransmitter and is not considered hallucinogenic, metabolites of serotonin also have high affinity at 5-HT(2A)R and can induce hallucinations in humans. Here we report that serotonin differs from the psychoactive N-methyltryptamines by its ability to engage a β-arrestin2-mediated signaling cascade in the frontal cortex. Serotonin and 5-hydroxy-L-tryptophan (5-HTP) induce a head-twitch response in wild-type (WT) mice that is a behavioral proxy for 5-HT(2A)R activation. The response in β-arrestin2 knock-out (βarr2-KO) mice is greatly attenuated until the doses are elevated, at which point, βarr2-KO mice display a head-twitch response that can exceed that of WT mice. Direct administration of N-methyltryptamines also produces a greater response in βarr2-KO mice. Moreover, the inhibition of N-methyltransferase blocks 5-HTP-induced head twitches in βarr2-KO mice, indicating that N-methyltryptamines, rather than serotonin, primarily mediate this response. Biochemical studies demonstrate that serotonin stimulates Akt phosphorylation in the frontal cortex and in primary cortical neurons through the activation of a β-arrestin2/phosphoinositide 3-kinase/Src/Akt cascade, whereas N-methyltryptamines do not. Furthermore, disruption of any of the components of this cascade prevents 5-HTP-induced, but not N-methyltryptamine-induced, head twitches. We propose that there is a bifurcation of 5-HT(2A)R signaling that is neurotransmitter and β-arrestin2 dependent. This demonstration of agonist-directed 5-HT(2A)R signaling in vivo may significantly impact drug discovery efforts for the treatment of disorders wherein hallucinations are part of the etiology, such as schizophrenia, or manifest as side effects of treatment, such as depression.

SNAP-25 is a target of protein kinase C phosphorylation critical to NMDA receptor trafficking

Protein kinase C (PKC) enhances NMDA receptor (NMDAR)-mediated currents and promotes NMDAR delivery to the cell surface via SNARE-dependent exocytosis. Although the mechanisms of PKC potentiation are established, the molecular target of PKC is unclear. Here we show that synaptosomal-associated protein of 25 kDa (SNAP-25), a SNARE protein, is functionally relevant to PKC-dependent NMDAR insertion, and identify serine residue-187 as the molecular target of PKC phosphorylation. Constitutively active PKC delivered via the patch pipette potentiated NMDA (but not AMPA) whole-cell currents in hippocampal neurons. Expression of RNAi targeting SNAP-25 or mutant SNAP-25(S187A) and/or acute disruption of the SNARE complex by treatment with BoNT A, BoNT B or SNAP-25 C-terminal blocking peptide abolished NMDAR potentiation. A SNAP-25 peptide and function-blocking antibody suppressed PKC potentiation of NMDA EPSCs at mossy fiber-CA3 synapses. These findings identify SNAP-25 as the target of PKC phosphorylation critical to PKC-dependent incorporation of synaptic NMDARs and document a postsynaptic action of this major SNARE protein relevant to synaptic plasticity.

Postsynaptic scaffolding molecules modulate the localization of neuroligins

Previous work has shown an important role for neuroligins in promoting the formation of synaptic connections in cultured cells. Although neuroligins enhance both excitatory and inhibitory synapse formation, individual neuroligin isoforms have been shown to preferentially localize to either glutamatergic or GABAergic synapses. Current evidence points to an important role for both the extracellular and intracellular domains of neuroligins in their synaptic localization. Although postsynaptic density protein 95 (PSD-95) has been shown to be involved in the recruitment of neuroligin 1 to excitatory synapses, the localization of neuroligin 2 (NL2) and neuroligin 3 (NL3) to excitatory and inhibitory synapses is less well defined. We assessed the roles of gephyrin and PSD-95, postsynaptic scaffolding molecules exclusively localized to inhibitory and excitatory synapses, respectively, in localizing NL2 and NL3 in primary neuronal cultures. We demonstrate that knockdown of gephyrin results in a significant shift of NL2 from inhibitory to excitatory synaptic contacts, while knockdown of PSD-95 leads to a partial shift of NL2 and NL3 from excitatory to inhibitory contacts. Furthermore, analysis of specific domain deletions within the C-terminal, intracellular domain of NL2 reveals that the region between amino acids 716 and 782 is required for the normal synaptic clustering of this protein. Together, these data suggest that intracellular mechanisms are involved in the targeting of different neuroligin family members to synapses (216).

The P2X7 receptor drives microglial activation and proliferation: a trophic role for P2X7R pore

Microglial activation is an integral part of neuroinflammation associated with many neurodegenerative conditions. Interestingly, a number of neurodegenerative conditions exhibit enhanced P2X(7) receptor (P2X(7)R) expression in the neuroinflammatory foci where activated microglia are a coexisting feature. Whether P2X(7)R overexpression is driving microglial activation or, conversely, P2X(7)R overexpression is a consequence of microglial activation is not known. We report that overexpression alone of a purinergic P2X(7)R, in the absence of pathological insults, is sufficient to drive the activation and proliferation of microglia in rat primary hippocampal cultures. The trophic responses observed in microglia were found to be P2X(7)R specific as the P2X(7)R antagonist, oxidized ATP (oxATP), was effective in markedly attenuating microgliosis. oxATP treatment of primary hippocampal cultures expressing exogenous P2X(7)Rs resulted in a significant decrease in the number of activated microglia. P2X(7)R is unusual in exhibiting two conductance states, a cation channel and a plasma membrane pore, and there are no pharmacological agents capable of cleanly discriminating between these two states. We used a point mutant of P2X(7)R (P2X7RG345Y) with intact channel function but ablated pore-forming capacity to establish that the trophic effects of increased P2X(7)R expression are exclusively mediated by the pore conductance. Collectively, and contrary to previous reports describing P2X(7)R as a "death receptor," we provide evidence for a novel trophic role for P2X(7)R pore in microglia.

Trafficking and fusion of neuropeptide Y-containing dense-core granules in astrocytes

It is becoming clear that astrocytes are active participants in synaptic functioning and exhibit properties, such as the secretion of classical transmitters, previously thought to be exclusively neuronal. Whether these similarities extend to the release of neuropeptides, the other major class of transmitters, is less clear. Here we show that cortical astrocytes can synthesize both native and foreign neuropeptides and can secrete them in a stimulation-dependent manner. Reverse transcription-PCR and mass spectrometry indicate that cortical astrocytes contain neuropeptide Y (NPY), a widespread neuronal transmitter. Immunocytochemical studies reveal NPY-immunoreactive (IR) puncta that colocalize with markers of the regulated secretory pathway. These NPY-IR puncta are distinct from the synaptic-like vesicles that contain classical transmitters, and the two types of organelles are differentially distributed. After activation of metabotropic glutamate receptors and the release of calcium from intracellular stores, the NPY-IR puncta fuse with the cell membrane, and the peptide-containing dense cores are displayed. To determine whether peptide secretion subsequently occurred, exocytosis was monitored from astrocytes expressing NPY-red fluorescent protein (RFP). In live cells, after activation of glutamate receptors, the intensity of the NPY-RFP-labeled puncta declined in a step-like manner indicating a regulated release of the granular contents. Because NPY is a widespread and potent regulator of synaptic transmission, these results suggest that astrocytes could play a role in the peptidergic modulation of synaptic signaling in the CNS.

ErbB4-neuregulin signaling modulates synapse development and dendritic arborization through distinct mechanisms

Perturbations in neuregulin-1 (NRG1)/ErbB4 function have been associated with schizophrenia. Affected patients exhibit altered levels of these proteins and display hypofunction of glutamatergic synapses as well as altered neuronal circuitry. However, the role of NRG1/ErbB4 in regulating synapse maturation and neuronal process formation has not been extensively examined. Here we demonstrate that ErbB4 is expressed in inhibitory interneurons at both excitatory and inhibitory postsynaptic sites. Overexpression of ErbB4 postsynaptically enhances size but not number of presynaptic inputs. Conversely, knockdown of ErbB4 using shRNA decreases the size of presynaptic inputs, demonstrating a specific role for endogenous ErbB4 in synapse maturation. Using ErbB4 mutant constructs, we demonstrate that ErbB4-mediated synapse maturation requires its extracellular domain, whereas its tyrosine kinase activity is dispensable for this process. We also demonstrate that depletion of ErbB4 decreases the number of primary neurites and that stimulation of ErbB4 using a soluble form of NRG1 results in exuberant dendritic arborization through activation of the tyrosine kinase domain of ErbB4 and the phosphoinositide 3-kinase pathway. These findings demonstrate that NRG1/ErbB4 signaling differentially regulates synapse maturation and dendritic morphology via two distinct mechanisms involving trans-synaptic signaling and tyrosine kinase activity, respectively.

Characterization of alternatively spliced isoforms of AMPA receptor subunits encoding truncated receptors

Glutamate receptors of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type play an important role in synaptic plasticity and contribute to cell death under excitotoxic conditions. AMPA receptors form heterotetramers of four homologous subunits (GluR1-4), which exist in two functionally different isoforms, flip and flop, generated by alternative splicing. We identified transcripts for alternatively spliced isoforms of AMPA receptor subunits which lack both the flip and the flop exons, in hippocampal and retinal cultures. These transcripts originate AMPA receptor subunits lacking the flip/flop cassette, the fourth transmembrane domain and the intracellular C-terminus. Truncated GluR1 associates with full-length GluR1 and exerts a dominant negative effect, giving rise to non-functional receptors. Moreover, truncated GluR1 reaches the cell surface, but is not efficiently targeted to the synapse. Hippocampal neuronal transfection with truncated GluR1 resulted in a significant reduction in apoptotic neuronal death triggered by toxic concentrations of glutamate. Furthermore, mRNA coding for the truncated subunits is consistently detected in some regions of the brain in epileptic rats and in hippocampal neurons submitted to toxic concentrations of glutamate. The existence of truncated AMPA receptor subunits may constitute an intrinsic neuroprotective mechanism.

High Ca2+-phosphate transfection efficiency in low-density neuronal cultures

This protocol describes a high-efficiency Ca2+-phosphate transfection method with low cell toxicity. The Ca2+-phosphate transfection method is widely used in transfecting neurons because of its low cell toxicity and simplicity in use, but the efficiency is typically low (approximately 1-5%). To solve this problem we have developed a new Ca2+-phosphate transfection protocol that increases the efficiency by 10-fold (< or = 60%), while maintaining low cell toxicity. First, it is critical to have gentle mixing of the DNA-Ca2+ solution with phosphate buffer to form a homogeneous snowlike precipitate (particle size 1-3 microm). Second, the precipitate should be dissolved using a slightly acidic culture medium to reduce cell toxicity. The high efficiency of this new protocol makes it possible to transfect single autaptic neurons as well as mature neurons (15-82 days in vitro) for gene functional analysis. The total time required for the protocol is 2-4 h (including 45 min-3 h incubation time).

Mitochondrial dysfunction and dendritic beading during neuronal toxicity

Mitochondrial dysfunction (depolarization and structural collapse), cytosolic ATP depletion, and neuritic beading are early hallmarks of neuronal toxicity induced in a variety of pathological conditions. We show that, following global exposure to glutamate, mitochondrial changes are spatially and temporally coincident with dendritic bead formation. During oxygen-glucose deprivation, mitochondrial depolarization precedes mitochondrial collapse, which in turn is followed by dendritic beading. These events travel as a wave of activity from distal dendrites toward the neuronal cell body. Despite the spatiotemporal relationship between dysfunctional mitochondria and dendritic beads, mitochondrial depolarization and cytoplasmic ATP depletion do not trigger these events. However, mitochondrial dysfunction increases neuronal vulnerability to these morphological changes during normal physiological activity. Our findings support a mechanism whereby, during glutamate excitotoxicity, Ca(2+) influx leads to mitochondrial depolarization, whereas Na(+) influx leads to an unsustainable increase in ATP demand (Na(+),K(+)-ATPase activity). This leads to a drop in ATP levels, an accumulation of intracellular Na(+) ions, and the subsequent influx of water, leading to microtubule depolymerization, mitochondrial collapse, and dendritic beading. Following the removal of a glutamate challenge, dendritic recovery is dependent upon the integrity of the mitochondrial membrane potential, but not on a resumption of ATP synthesis or Na(+),K(+)-ATPase activity. Thus, dendritic recovery is not a passive reversal of the events that induce dendritic beading. These findings suggest that the degree of calcium influx and mitochondrial depolarization inflicted by a neurotoxic challenge, determines the ability of the neuron to recover its normal morphology.

Naturally occurring variations in the human 5-HT3A gene profoundly impact 5-HT3 receptor function and expression

BACKGROUND

The serotonin [5-hydroxytryptamine (5-HT)]-gated ion channel 5-HT3 is involved in the mediation of postoperative and radiotherapy/chemotherapy-induced nausea/emesis and in irritable bowel syndrome. It has also been suggested to play a role in various psychiatric diseases. Five naturally occurring single nucleotide polymorphisms leading to amino acid changes have been identified in the human 5-HT3A gene.

METHODS AND RESULTS

We investigated the functional effects of these polymorphisms on the 5-HT3A receptor using fluorescence-based cellular assays. Notably, variants A33T, S253N, and M257I displayed 5-HT-induced maximal responses of 3-64% of the wild-type response, whereas R344H and P391R exhibited wild-type-like function. All variants displayed wild-type-like potencies of 5-HT and three 5-HT3 antagonists. Furthermore, all variants displayed Kd values similar to that of the wild-type receptor in a [H]GR65630-binding assay. The surface expression of A33T, M257I, and R344H was reduced 2-4-fold compared with the wild-type, despite similar total expression levels. Finally, coexpression of wild-type 5-HT3A or 5-HT3B subunits with 5-HT3A variants A33T, S253N, or M257I resulted in mixed or heteromeric receptors, characterized by significantly reduced maximal responses to 5-HT compared with the wild-type receptors.

CONCLUSIONS

Three polymorphisms of the 5-HT3A gene gave rise to functionally impaired receptors whose function could not be rescued by either wild-type 5-HT3A or 5-HT3B. Three of the variant receptors were surface-expressed at reduced levels in spite of total expression levels similar to wild-type, indicating that these variants affect receptor biogenesis and/or trafficking. These severe single nucleotide polymorphism effects hold promise for identification of new 5-HT3A gene-disease causalities.

Generation and functional characterization of fluorescent, N-terminally tagged CB1 receptor chimeras for live-cell imaging

N-terminally tagged CB1 receptor fusion proteins, incorporating enhanced green fluorescent protein (GFP) or super-ecliptic pHluorin (SEP), were generated to study CB1 receptor trafficking and cell surface receptor expression in live COS7 and HEK293 cells and hippocampal neurons. An artificial signal sequence (SS) was required for efficient surface expression of CB1 receptor chimeras, which behaved like wild-type CB1 receptors in functional assays. Treatment with cannabinoid ligands led to a rapid down-regulation of SS-GFP-CB1 from the plasma membrane in COS7 and HEK293 cells, associated with trafficking into cytosolic vesicles. Activation of CB1 receptors was also linked with a time-dependent reduction in cell surface SEP-CB1 fluorescence and incorporation of the construct into acidic endosomes, revealed following exposure to NH4Cl. In live hippocampal neurons, SEP-CB1 fluorescence was largely restricted to the axon, consistent with its polarised surface expression. Thus, these new molecular tools are well suited for studying CB1 receptor trafficking and a new generation of GPCR chimeras incorporating SEP at the N-terminus will be especially useful for monitoring dynamic changes in cell surface receptor expression in living cells.

Calpain-Mediated mGluR1α Truncation: A Key Step in Excitotoxicity

Excitotoxicity mediated by glutamate receptors plays crucial roles in ischemia and other neurodegenerative diseases. Whereas overactivation of ionotropic glutamate receptors is neurotoxic, the role of metabotropic glutamate receptors (mGluRs), and especially mGluR1, remains equivocal. Here we report that activation of NMDA receptors results in calpain-mediated truncation of the C-terminal domain of mGluR1alpha at Ser(936). The truncated mGluR1alpha maintains its ability to increase cytosolic calcium while it no longer activates the neuroprotective PI(3)K-Akt signaling pathways. Full-length and truncated forms of mGluR1alpha play distinct roles in excitotoxic neuronal degeneration in cultured neurons. A fusion peptide derived from the calpain cleavage site of mGluR1alpha efficiently blocks NMDA-induced truncation of mGluR1alpha in primary neuronal cultures and exhibits neuroprotection against excitotoxicity both in vitro and in vivo. These findings shed light on the relationship between NMDA and mGluR1alpha and indicate the existence of a positive feedback regulation in excitotoxicity involving calpain and mGluR1alpha.

An essential role for constitutive endocytosis, but not activity, in the axonal targeting of the CB1 cannabinoid receptor

In central neurons, the cell-surface distribution of cannabinoid receptor subtype-1 (CB(1)) is highly polarized toward axons and is associated with synaptic terminals, in which it is well-positioned to modulate neurotransmitter release. It has been suggested that high levels of constitutive activity mediate CB(1) receptor axonal targeting, leading to domain-specific endocytosis. We have investigated further the mechanisms that underlie CB(1) receptor axonal polarization in hippocampal neurons and found that constitutive activity is not an essential requirement for this process. We demonstrate that the cell-surface distribution of an N-terminally tagged, fluorescent CB(1) receptor fusion-protein is almost exclusively localized to the axon when expressed in cultured hippocampal neurons. Inhibition of endocytosis by cotransfection with a dominant-negative dynamin-1 (K44A) mutant traps both recombinant and endogenous CB(1) receptors at the somatodendritic cell surface. However, this effect could not be mimicked by inhibiting constitutive activity or receptor activation, either by expressing mutant receptors that lack these properties or by treatment with CB(1) receptor antagonists possessing inverse agonist activity. These data are consistent with a revised model in which domain-specific endocytosis regulates the functional polarization of CB(1) receptors, but this process is distinct from constitutive activity.