RESUMO
Changes in the balance between glutamate (Glu) release and uptake may stimulate synaptic reorganization and even synapse loss. In the case of neurodegeneration, a mismatch between astroglial Glu uptake and presynaptic Glu release could be detected if both parameters were assessed independently and at a single-synapse level. This has now become possible due to a new imaging assay with the genetically encoded ultrafast Glu sensor iGlu u We report findings from individual corticostriatal synapses in acute slices prepared from mice of either sex that were >1 year of age. Contrasting patterns of short-term plasticity and a size criterion identified two classes of terminals, presumably corresponding to the previously defined IT (intratelencephalic) and PT (pyramidal tract) synapses. The latter exhibited a higher degree of frequency potentiation/residual Glu accumulation and were selected for our first iGlu u single-synapse study in Q175 mice, a model of Huntington's disease (HD). In HD mice, the decay time constant of the perisynaptic Glu concentration (TauD), as an indicator of uptake, and the peak iGlu u amplitude, as an indicator of release, were prolonged and reduced, respectively. Treatment of WT preparations with the astrocytic Glu uptake blocker TFB-TBOA (100 nm) mimicked the TauD changes in homozygotes. Considering the largest TauD values encountered in WT, â¼40% of PT synapses tested in Q175 heterozygotes can be classified as dysfunctional. Moreover, HD but not WT synapses exhibited a positive correlation between TauD and the peak amplitude of iGlu u Finally, EAAT2 (excitatory amino acid transport protein 2) immunoreactivity was reduced next to corticostriatal terminals. Thus, astrocytic Glu transport remains a promising target for therapeutic intervention.SIGNIFICANCE STATEMENT Alterations in astrocytic Glu uptake can play a role in synaptic plasticity and neurodegeneration. Until now, the sensitivity of synaptic responses to pharmacological transport block and the resulting activation of NMDA receptors were regarded as reliable evidence for a mismatch between synaptic uptake and release. But the latter parameters are interdependent. Using a new genetically encoded sensor to monitor extracellular glutamate concentration ([Glu]) at individual corticostriatal synapses, we can now quantify the time constant of perisynaptic [Glu] decay (as an indicator of uptake) and the maximal [Glu] elevation next to the active zone (as an indicator of Glu release). The results provide a positive answer to the hitherto unresolved question of whether neurodegeneration (e.g., Huntington's disease) associates with a glutamate uptake deficit at tripartite excitatory synapses.
Assuntos
Córtex Cerebral/metabolismo , Corpo Estriado/metabolismo , Ácido Glutâmico/metabolismo , Doença de Huntington/metabolismo , Sinapses/metabolismo , Vias Aferentes/metabolismo , Animais , Técnicas Biossensoriais/métodos , Modelos Animais de Doenças , Feminino , Masculino , Potenciais da Membrana , Camundongos Transgênicos , Plasticidade NeuronalRESUMO
UNLABELLED: This study evaluates single-cell indicators of glutamate transport in sulforhodamine 101-positive astrocytes of Q175 mice, a knock-in model of Huntington's disease (HD). Transport-related fluorescent ratio signals obtained with sodium-binding benzofuran isophtalate (SBFI) AM from unperturbed or voltage-clamped astrocytes and respective glutamate transporter currents (GTCs) were induced by photolytic or synaptic glutamate release and isolated pharmacologically. The HD-induced deficit ranged from -27% (GTC maximum at -100 mV in Ba(2+)) to -41% (sodium transients in astrocytes after loading SBFI-AM). Our specific aim was to clarify the mechanism(s) by which Kir4.1 channels can influence glutamate transport, as determined by either Na(+) imaging or transport-associated electrical signals. A decrease of Kir4.1 conductance was mimicked with Ba(2+) (200 µm), and an increase of Kir4.1 expression was obtained by intravenous administration of AAV9-gfaABC1D-Kir4.1-EGFP. The decrease of Kir4.1 conductance reduced the sodium transients but increased the amplitudes of somatic GTCs. Accordingly, after genetic upregulation of Kir4.1, somatic GTCs were found to be decreased. In individual cells, there was a negative correlation between Kir4.1 currents and GTCs. The relative effect of the Kir4.1 conductance was higher in the astrocyte periphery. These and other results suggest that the Kir4.1 conductance affects glutamate transporter activity in a dual manner: (1) by providing the driving force (voltage dependency of the transport itself) and (2) by limiting the lateral charge transfer (thereby reducing the interference with other electrogenic transporter functions). This leads to the testable prediction that restoring the high conductance state of passive astrocytes will not only normalize glutamate uptake but also restore other astrocytic transporter activities afflicted with HD. SIGNIFICANCE STATEMENT: Insufficiency of astrocytic glutamate uptake is a major element in the pathophysiology of neurodegenerative diseases. Considering the heterogeneity of astrocytes and their differential susceptibility to therapeutic interventions, it becomes necessary to evaluate the determinants of transport activity in individual astroglial cells. We have examined intracellular Na(+) transients and glutamate transporter currents as the most telling indicators of glutamate clearance after synaptic or photolytic release of glutamate in striatal slices. The results show that, in Huntington's disease, glutamate uptake activity critically depends on Kir4.1. These channels enable the high conductance state of the astrocytic plasma membrane, which ensures the driving force for glutamate transport and dumps the transport-associated depolarization along the astrocyte processes. This has significant implications for developing therapeutic targets.
Assuntos
Astrócitos/metabolismo , Ácido Glutâmico/metabolismo , Doença de Huntington/metabolismo , Neostriado/metabolismo , Canais de Potássio Corretores do Fluxo de Internalização/metabolismo , Sistema X-AG de Transporte de Aminoácidos/metabolismo , Animais , Astrócitos/efeitos dos fármacos , Benzofuranos/farmacologia , Transportador 2 de Aminoácido Excitatório/genética , Transportador 2 de Aminoácido Excitatório/metabolismo , Potenciais Pós-Sinápticos Excitadores/efeitos dos fármacos , Junções Comunicantes/efeitos dos fármacos , Junções Comunicantes/metabolismo , Técnicas de Introdução de Genes , Proteína Glial Fibrilar Ácida/metabolismo , Camundongos , Neostriado/citologia , Técnicas de Patch-Clamp , Ácidos Ftálicos/farmacologia , Canais de Potássio Corretores do Fluxo de Internalização/antagonistas & inibidoresRESUMO
Huntington's disease (HD) affects both neurons and astrocytes. To target the latter and to ensure brain-wide transgene expression, adeno-associated viral (AAV) vectors can be administered intravenously, as AAV vectors cross the blood-brain barrier (BBB) and enable preferential transduction of astrocytes due to their close association with blood vessels. However, there is a possibility that the subclass of GFAP-expressing astrocytes performs a distinct role in HD and reacts differently to therapeutic measures than the rest of the astrocytes. The gfaABC1D promoter allows specific targeting of the GFAP-expressing astrocytes (~25% of S100ß-expressing astrocytes). We have examined the expression of three different transgenes (GCaMP6f, Kir4.1 and GLT1) and tested the effects of the AAV serotypes 9 and rh8. The AAV vectors were injected into the tail vein of 1-year-old homozygous Z-Q175-KI HD mice and their wild-type (WT) littermates. At this age, HD mice exhibit motor symptoms, including pronounced hypokinesia and circling behaviour. The expression times ranged from 3 to 6weeks. The target cell population was defined as the cells expressing S100ß in addition to GFAP. Viewfields in the dorsal striatum and the overlaying cortex were evaluated and the transduction rate was defined as the percentage of target cells that expressed the reporter transgene (enhanced green fluorescent protein, EGFP, or Tomato). In all cases, the transduction rate was higher in the cortex than in the striatum. AAV9 was more efficient than AAVrh8. One of the injected constructs (AAV9-gfaABC1D-GLT1-Tomato) was tested for the first time. GLT1, the principal astrocytic glutamate transporter, is deficient in HD and therefore considered as a potential target for gene therapy. At a dose of 1.86×1011 vector genome (vg) per animal, the fraction of GLT1-Tomato+ cells in the striatum and the cortex amounted to 30% and 49%, respectively. In individual Tomato+ HD astrocytes, treatment with the GLT1 vector increased the level of GLT1 immunofluorescence by 21% compared to the HD control. The described approach offers new and interesting opportunities to examine the pathophysiological consequences of brain-wide transgene expression in a specific astrocyte subpopulation.
Assuntos
Astrócitos/metabolismo , Dependovirus/genética , Terapia Genética/métodos , Proteína Glial Fibrilar Ácida/metabolismo , Doença de Huntington/terapia , Animais , Corpo Estriado/metabolismo , Transportador 2 de Aminoácido Excitatório/genética , Transportador 2 de Aminoácido Excitatório/metabolismo , Vetores Genéticos/genética , Proteína Glial Fibrilar Ácida/genética , Masculino , Camundongos , Canais de Potássio Corretores do Fluxo de Internalização/genética , Canais de Potássio Corretores do Fluxo de Internalização/metabolismo , Regiões Promotoras Genéticas , Subunidade beta da Proteína Ligante de Cálcio S100/genética , Subunidade beta da Proteína Ligante de Cálcio S100/metabolismo , TransgenesRESUMO
Changes in the activity of striatal output neurons (SONs) have been implicated in the pathogenesis of Huntington's disease (HD). In this inherited polyglutamine disorder, accumulation of intracellular toxins causes a variety of deficits, including synaptic dysfunction, but it is still unclear to what extent striatal GABA release is afflicted as well. Two murine HD models were used, a recently created knock-in mouse (Z_Q175_KI) and an established model of HD (R6/2). In sagittal slices with relatively well-preserved glutamatergic connections throughout the basal ganglia, we have characterized the following: (i) the excitability of SONs; (ii) their spontaneous action potential-dependent GABAergic synaptic activity; (iii) the capacity of exogenous GABA to inhibit spontaneous action potential generation; and (iv) the properties of GABAergic unitary evoked responses (eIPSCs) in response to intrastriatal minimal stimulation at low and high frequency. The HD SONs exhibited enhanced intrisic excitability and higher levels of GABAergic spontaneous activity without presenting evidence for homeostatic upregulation of endogenous or exogenous GABA actions. Unitary eIPSC amplitudes were reduced, with a clear deficit in the probability of release, as indicated by a higher paired-pulse ratio, failure rate and coefficient of variation. In conditions of high-frequency activation, GABAergic connections of HD SONs were prone to asynchronous release and delayed IPSC generation at the expense of synchronized release. Both in wild-type and in HD SONs, GABA was inhibitory. Our results support the conclusion that the enhanced spontaneous synaptic activity in the HD striatum reflects disinhibition. Pharmacological tests identified the HD-related tonic suppression of synaptic inhibition as a glutamate- and endocannabinoid-dependent process.
Assuntos
Corpo Estriado/fisiologia , Doença de Huntington/fisiopatologia , Proteínas do Tecido Nervoso/fisiologia , Proteínas Nucleares/fisiologia , Receptor CB1 de Canabinoide/fisiologia , Receptores de Glutamato Metabotrópico/fisiologia , Animais , Proteína Huntingtina , Técnicas In Vitro , Potenciais da Membrana , Camundongos , Camundongos Transgênicos , Mutação , Neurônios/fisiologia , Receptor de Glutamato Metabotrópico 5 , Ácido gama-Aminobutírico/fisiologiaRESUMO
Rapid removal of glutamate from the sites of glutamate release is an essential step in excitatory synaptic transmission. However, despite many years of research, the molecular mechanisms underlying the intracellular regulation of glutamate transport at tripartite synapses have not been fully uncovered. This limits the options for pharmacological treatment of glutamate-related motor disorders, including Huntington's disease (HD). We therefore investigated the possible binding partners of transgenic EAAT2 and their alterations under the influence of mutant huntingtin (mHTT). Mass spectrometry analysis after pull-down of striatal YFP-EAAT2 from wild-type (WT) mice and heterozygote (HET) Q175 mHTT-knock-in mice identified a total of 148 significant (FDR < 0.05) binders to full-length EAAT2. Of them 58 proteins exhibited mHTT-related differences. Most important, in 26 of the 58 mHTT-sensitive cases, protein abundance changed back toward WT levels when the mice expressed a C-terminal-truncated instead of full-length variant of EAAT2. These findings motivated new attempts to clarify the role of astrocytic EAAT2 regulation in cortico-basal movement control. Striatal astrocytes of Q175 HET mice were targeted by a PHP.B vector encoding EAAT2 with different degree of C-terminal modification, i.e., EAAT2-S506X (truncation at S506), EAAT2-4KR (4 lysine to arginine substitutions) or EAAT2 (full-length). The results were compared to HET and WT injected with a tag-only vector (CTRL). It was found that the presence of a C-terminal-modified EAAT2 transgene (i) increased the level of native EAAT2 protein in striatal lysates and perisynaptic astrocyte processes, (ii) enhanced the glutamate uptake of transduced astrocytes, (iii) stimulated glutamate clearance at individual corticostriatal synapses, (iv) increased the glutamate uptake of striatal astrocytes and (iv) alleviated the mHTT-related hypokinesia (open field indicators of movement initiation). In contrast, over-expression of full-length EAAT2 neither facilitated glutamate uptake nor locomotion. Together, our results support the new hypothesis that preventing abnormal protein-protein interactions at the C-terminal of EAAT2 could eliminate the mHTT-related deficits in corticostriatal synaptic glutamate clearance and movement initiation.
RESUMO
Small GTPases of the Rho family play versatile roles in the formation and development of axons and dendrites, effects often studied by the Rho-inactivating C3 transferase (C3bot) from Clostridium botulinum. Recently, we reported that transferase-deficient C3bot also exerted axonotrophic activity. Using overlapping peptides from the C3bot sequence, we identified a small peptide of 29 amino acids (covering residues 154-182) from the C-terminal region of C3bot that promotes both axonal and dendritic growth, as well as branching of hippocampal neurons, at submicromolar concentrations. Several C3bot constructs, including the short peptide, enhanced the number of axonal segments from mid- to higher-order segments. C3bot(154-182) also increased the number of synaptophysin-expressing terminals, up-regulated various synaptic proteins, and functionally increased the glutamate uptake. Staining against the vesicular glutamate and GABA transporters further revealed that the effect was attributable to a higher number of glutamatergic and GABAergic inputs on proximal dendrites of enhanced green fluorescent protein (EGFP)-transfected neurons. Using organotypical slice cultures, we also detected trophic effects of C3bot(154-182) on length and density of outgrowing fibers from the entorhinal cortex that were comparable to the effects elicited by full-length C3bot. In addition, an enhanced reinnervation was observed in a hippocampal-entorhinal lesion model. In summary, the neurotrophic effect of C3bot is executed by a C-terminal peptide fragment covering aa 154-182 of C3; it triggers dendritic and axonal growth and branching as well as increased synaptic connectivity. In contrast to full-length C3, this C3 peptide selectively acts on neurons but not on glial cells.
Assuntos
ADP Ribose Transferases/metabolismo , Aminoácidos/farmacologia , Axônios/fisiologia , Toxinas Botulínicas/metabolismo , Dendritos/fisiologia , Neurônios/fisiologia , ADP Ribose Transferases/química , ADP Ribose Transferases/genética , Aminoácidos/química , Animais , Axônios/efeitos dos fármacos , Axônios/metabolismo , Biomarcadores/metabolismo , Toxinas Botulínicas/química , Toxinas Botulínicas/genética , Células Cultivadas , Clostridium botulinum/genética , Clostridium botulinum/metabolismo , Técnicas de Cocultura , Dendritos/efeitos dos fármacos , Dendritos/genética , Dendritos/metabolismo , Embrião de Mamíferos , Glutationa Transferase/metabolismo , Proteínas de Fluorescência Verde/metabolismo , Hipocampo/citologia , Hipocampo/embriologia , Imuno-Histoquímica , Camundongos , Camundongos Endogâmicos , Proteínas Associadas aos Microtúbulos/metabolismo , Peso Molecular , Proteínas de Neurofilamentos/química , Proteínas de Neurofilamentos/metabolismo , Neurônios/efeitos dos fármacos , Neurônios/metabolismo , Proteínas Recombinantes/química , Proteínas Recombinantes/isolamento & purificação , Proteínas Recombinantes/metabolismo , Fatores de TempoRESUMO
Synapses are highly compartmentalized functional units that operate independently on each other. In Huntington's disease (HD) and other neurodegenerative disorders, this independence might be compromised due to insufficient glutamate clearance and the resulting spill-in and spill-out effects. Altered astrocytic coverage of the presynaptic terminals and/or dendritic spines as well as a reduced size of glutamate transporter clusters at glutamate release sites have been implicated in the pathogenesis of diseases resulting in symptoms of dys-/hyperkinesia. However, the mechanisms leading to the dysfunction of glutamatergic synapses in HD are not well understood. Improving and applying synapse imaging we have obtained data shedding new light on the mechanisms impeding the initiation of movements. Here, we describe the principle elements of a relatively inexpensive approach to achieve single synapse resolution by using the new genetically encoded ultrafast glutamate sensor iGluu, wide-field optics, a scientific CMOS (sCMOS) camera, a 473 nm laser and a laser positioning system to evaluate the state of corticostriatal synapses in acute slices from age appropriate healthy or diseased mice. Glutamate transients were constructed from single or multiple pixels to obtain estimates of i) glutamate release based on the maximal elevation of the glutamate concentration [Glu] next to the active zone and ii) glutamate uptake as reflected in the time constant of decay (TauD) of the perisynaptic [Glu]. Differences in the resting bouton size and contrasting patterns of short-term plasticity served as criteria for the identification of corticostriatal terminals as belonging to the intratelencephalic (IT) or the pyramidal tract (PT) pathway. Using these methods, we discovered that in symptomatic HD mice ~40% of PT-type corticostriatal synapses exhibited insufficient glutamate clearance, suggesting that these synapses might be at risk to excitotoxic damage. The results underline the usefulness of TauD as a biomarker of dysfunctional synapses in Huntington mice with a hypokinetic phenotype.
Assuntos
Encéfalo/metabolismo , Ácido Glutâmico/metabolismo , Doença de Huntington/metabolismo , Sinapses/metabolismo , Animais , Artefatos , Astrócitos/metabolismo , Axônios/metabolismo , Encéfalo/patologia , Camundongos , Fenótipo , Terminações Pré-Sinápticas/metabolismoRESUMO
In an attempt to characterize the molecular components by which electric activity influences the development of synapses, we searched for cell surface proteins modulated by calcium influx and glutamate receptor activity. Here, we report that neuronal depolarization facilitates the conversion of CALEB, which results in a truncated transmembrane form with an exposed EGF domain. To characterize the role of CALEB in synapse development, synaptic features were investigated in slices of the colliculus superior from CALEB-deficient mice. In the absence of CALEB, the number of synapses and their morphological characteristics remained unchanged. However, in CALEB-deficient mice, synapses displayed higher paired-pulse ratios, less depression during prolonged repetitive activation, a lower rate of spontaneous postsynaptic currents, and a lower release probability at early but not mature postnatal stages. Our findings indicate that CALEB provides a molecular basis for maintaining normal release probability at early developmental stages.
Assuntos
Fator de Crescimento Epidérmico/deficiência , Glicoproteínas de Membrana/deficiência , Proteínas do Tecido Nervoso/deficiência , Sinapses/fisiologia , Sequência de Aminoácidos , Animais , Western Blotting , Embrião de Galinha , Potenciais Pós-Sinápticos Excitadores , Imuno-Histoquímica , Camundongos , Camundongos Knockout , Microscopia Eletrônica de Transmissão , Dados de Sequência Molecular , Técnicas de Cultura de Órgãos , Colículos Superiores/crescimento & desenvolvimento , Sinapses/ultraestruturaRESUMO
Striatal GABAergic signaling has been shown to be essential for basal ganglia output and proper motor performance. In the mouse neostriatum GABA transporter 1 (GAT-1) was previously found to assist in the clearance of GABA from the extracellular space and influence both phasic and tonic GABAergic inhibition of medium-sized striatal output neurons (SONs). It currently remains unknown whether GAT subtypes other than GAT-1 participate in the modulation of GABAergic transmission in this brain structure. In this study, we aimed at assessing the role of GAT-3 in the mouse neostriatum. To this end, we recorded GABAergic inhibitory postsynaptic currents (IPSCs) from SONs in brain slices at different developmental stages (postnatal days (P) 7-9, 12-14, and 28-34) using the whole-cell patch-clamp technique. When applied under control conditions, SNAP-5114 (40 microM), a specific GAT-3 blocker, did not affect miniature or evoked IPSCs (m/eIPSCs) and produced no significant effect on tonic GABAA receptor-mediated conductances in SONs. However, in the presence of NO-711 (10 microM), a specific GAT-1 blocker, SNAP-5114 reduced mIPSC frequencies without affecting mIPSC amplitudes or kinetics. In addition, SNAP-5114 reduced the mean amplitude of eIPSCs and increased the paired-pulse ratio. These effects were entirely abolished by CGP55845 (1 microM), a specific GABAB receptor blocker, indicating that they were mediated by presynaptic GABAB receptors. Similar results were obtained from all age groups. We conclude that GAT-3 is functionally expressed in the mouse neostriatum. Whereas an acute block of GAT-3 under resting conditions is fully compensated for by GAT-1, GAT-3 might provide an additional uptake capacity when neuronal activity and GABA release are increased.
Assuntos
Corpo Estriado/citologia , Proteínas da Membrana Plasmática de Transporte de GABA/metabolismo , Neurônios/fisiologia , Ácido gama-Aminobutírico/metabolismo , Fatores Etários , Animais , Animais Recém-Nascidos , Anisóis/farmacologia , Biofísica , Corpo Estriado/crescimento & desenvolvimento , Relação Dose-Resposta a Droga , Interações Medicamentosas , Estimulação Elétrica/métodos , Antagonistas GABAérgicos/farmacologia , Inibidores da Captação de GABA , Técnicas In Vitro , Potenciais Pós-Sinápticos Inibidores/efeitos dos fármacos , Potenciais Pós-Sinápticos Inibidores/fisiologia , Camundongos , Camundongos Endogâmicos C57BL , Neurônios/classificação , Neurônios/efeitos dos fármacos , Ácidos Nipecóticos/farmacologia , Técnicas de Patch-ClampRESUMO
Layer I of the developing cortex contains a dense GABAergic fiber plexus. These fibers provide excitatory inputs to Cajal-Retzius (CR) cells, the early born neurons in layer I. CR cells possess an extensive axonal projection and form synaptic contacts with excitatory, presumably pyramidal, neurons before birth. Interestingly, activity of CR cells declines during the first postnatal week, but mechanism(s) underlying this phenomenon is not yet known. Here we recorded inhibitory postsynaptic currents (IPSCs) in CR cells at postnatal day (P) 1-2 and P5-7. Blockade of adenosine A(1) receptors (A(1)Rs) increased the amplitude of evoked IPSCs (eIPSCs) and decreased paired-pulse ratio at P5-7 but not at P1-2. A(1)R activation decreased the mean eIPSC amplitude at P5-7, but failed to affect eIPSCs at P1-2. Ecto-adenosine triphosphatase (ATPase) inhibition completely abolished the A(1)R-mediated effects suggesting that extracellular ATP is the main source of adenosine. Because A(1)R blockade did not affect the median miniature IPSC amplitude, our results demonstrate that adenosine reduces gamma-aminiobutyric acid (GABA) release probability via presynaptic A(1)Rs at P5-7. As neuronal activity in layer I can depolarize pyramidal neurons influencing thereby glutamatergic synaptogenesis in the lower cortical layers, postnatal weakening of GABAergic transmission by adenosinergic system might reflect a developmental downregulation of this excitatory drive when glutamatergic synapses are formed.
Assuntos
Potenciais Pós-Sinápticos Excitadores/fisiologia , Regulação da Expressão Gênica no Desenvolvimento/fisiologia , Neocórtex/fisiologia , Terminações Pré-Sinápticas/metabolismo , Receptor A1 de Adenosina/metabolismo , Transmissão Sináptica/fisiologia , Ácido gama-Aminobutírico/metabolismo , Animais , Animais Recém-Nascidos , Regulação para Baixo , Camundongos , Camundongos Endogâmicos C57BL , Rede Nervosa/fisiologiaRESUMO
The function of supramedullary glycine receptors (GlyRs) is still unclear. Using Wistar rat collicular slices, we demonstrate GlyR-mediated inhibition of spike discharge elicited by low glycine (10 microM). Searching for the molecular basis of this phenomenon, we identified a new GlyR isoform. GlyR alpha3(P185L), a result of cytidine 554 deamination, confers high glycine sensitivity (EC50 approximately 5 microM) to neurons and thereby promotes the generation of sustained chloride conductances associated with tonic inhibition. The level of GlyR alpha3-C554U RNA editing is sensitive to experimentally induced brain lesion, inhibition of cytidine deamination by zebularine and inhibition of mRNA transcription by actinomycin D, but not to blockade of protein synthesis by cycloheximide. Conditional regulation of GlyR alpha3(P185L) is thus likely to be part of a post-transcriptional adaptive mechanism in neurons with enhanced excitability.
Assuntos
Potenciais de Ação/genética , Inibição Neural/genética , Neurônios/metabolismo , Edição de RNA/genética , Receptores de Glicina/genética , Colículos Superiores/metabolismo , Potenciais de Ação/efeitos dos fármacos , Sequência de Aminoácidos , Animais , Animais Recém-Nascidos , Agonistas dos Canais de Cloreto , Canais de Cloreto/efeitos dos fármacos , Canais de Cloreto/genética , Citidina/metabolismo , Desaminação , Inibidores Enzimáticos/farmacologia , Regulação da Expressão Gênica/efeitos dos fármacos , Regulação da Expressão Gênica/genética , Glicina/metabolismo , Glicina/farmacologia , Dados de Sequência Molecular , Inibição Neural/efeitos dos fármacos , Neurônios/efeitos dos fármacos , Técnicas de Cultura de Órgãos , Técnicas de Patch-Clamp , Isoformas de Proteínas/agonistas , Isoformas de Proteínas/genética , Isoformas de Proteínas/isolamento & purificação , Processamento de Proteína Pós-Traducional/efeitos dos fármacos , Processamento de Proteína Pós-Traducional/genética , Inibidores da Síntese de Proteínas/farmacologia , RNA Mensageiro/antagonistas & inibidores , RNA Mensageiro/metabolismo , Ratos , Ratos Wistar , Receptores de Glicina/agonistas , Receptores de Glicina/isolamento & purificação , Colículos Superiores/anatomia & histologia , Colículos Superiores/efeitos dos fármacosRESUMO
GABAergic medium-sized striatal output neurons (SONs) provide the principal output for the neostriatum. In vitro and in vivo data indicate that spike discharge of SONs is tightly controlled by effective synaptic inhibition. Although phasic GABAergic transmission critically depends on ambient GABA levels, the role of GABA transporters (GATs) in neostriatal GABAergic synaptic transmission is largely unknown. In the present study we aimed at elucidating the role of GAT-1 in the developing mouse neostriatum (postnatal day (P) 7-34). We recorded GABAergic postsynaptic currents (PSCs) using the whole-cell patch-clamp technique. Based on the effects of NO-711, a specific GAT-1 blocker, we demonstrate that GAT-1 is operative at this age and influences GABAergic synaptic transmission by presynaptic and postsynaptic mechanisms. Presynaptic GABA(B)R-mediated suppression of GABA release was found to be functional at all ages tested; however, there was no evidence for persistent GABA(B)R activity under control conditions, unless GAT-1 was blocked (P12-34). In addition, whereas no tonic GABA(A)R-mediated conductances were detected in SONs until P14, application of a specific GABA(A)R antagonist caused distinct tonic outward currents later in development (P19-34). In the presence of NO-711, tonic GABA(A)R-mediated currents were also observed at P7-14 and were dramatically increased at more mature stages. Furthermore, GAT-1 block reduced the median amplitude of GABAergic miniature PSCs indicating a decrease in quantal size. We conclude that in the murine neostriatum GAT-1 operates in a net uptake mode. It prevents the persistent activation of presynaptic GABA(B)Rs (P12-34) and prevents (P7-14) or reduces (P19-34) tonic postsynaptic GABA(A)R activity.
Assuntos
Neostriado/crescimento & desenvolvimento , Neurônios/fisiologia , Receptores de GABA/fisiologia , Transmissão Sináptica/fisiologia , Potenciais de Ação/efeitos dos fármacos , Potenciais de Ação/fisiologia , Animais , Animais Recém-Nascidos , Baclofeno/farmacologia , Bicuculina/análogos & derivados , Bicuculina/farmacologia , Impedância Elétrica , Estimulação Elétrica , Eletrofisiologia , Antagonistas GABAérgicos/farmacologia , Antagonistas de Receptores de GABA-A , Antagonistas de Receptores de GABA-B , Técnicas In Vitro , Potenciais da Membrana/efeitos dos fármacos , Potenciais da Membrana/fisiologia , Camundongos , Camundongos Endogâmicos C57BL , Neostriado/citologia , Neostriado/fisiologia , Neurônios/citologia , Neurônios/efeitos dos fármacos , Ácidos Nipecóticos/farmacologia , Oximas/farmacologia , Técnicas de Patch-Clamp , Ácidos Fosfínicos/farmacologia , Terminações Pré-Sinápticas/fisiologia , Propanolaminas/farmacologia , Piridazinas/farmacologia , Receptores de GABA-A/fisiologia , Receptores de GABA-B/fisiologia , Transmissão Sináptica/efeitos dos fármacos , Tetrodotoxina/farmacologiaRESUMO
Notch and neurotrophins control neuronal shape, but it is not known whether their signaling pathways intersect. Here we report results from hippocampal neuronal cultures that are in support of this possibility. We found that low cell density or blockade of Notch signaling by a soluble Delta-Fc ligand decreased the mRNA levels of the nuclear targets of Notch, the homologues of enhancer-of-split 1 and 5 (Hes1/5). This effect was associated with enhanced sprouting of new dendrites or dendrite branches. In contrast, high cell density or exposure of low-density cultures to NGF increased the Hes1/5 mRNA, reduced the number of primary dendrites and promoted dendrite elongation. The NGF effects on both Hes1/5 expression and dendrite morphology were prevented by p75-antibody (a p75NTR-blocking antibody) or transfection with enhancer-of-split 6 (Hes6), a condition known to suppress Hes activity. Nuclear translocation of NF-kappaB was identified as a link between p75NTR and Hes1/5 because it was required for the up-regulation of these two genes. The convergence of the Notch and p75NTR signaling pathways at the level of Hes1/5 illuminates an unexpected mechanism through which a diffusible factor (NGF) could regulate dendrite growth when cell-cell interaction via Notch is not in action.
Assuntos
Proteínas de Ligação a DNA/metabolismo , Dendritos/metabolismo , Hipocampo/citologia , Proteínas de Homeodomínio/metabolismo , Proteínas de Membrana/metabolismo , Fator de Crescimento Neural/metabolismo , Receptores de Fator de Crescimento Neural/metabolismo , Proteínas Repressoras/metabolismo , Transporte Ativo do Núcleo Celular , Animais , Axônios/metabolismo , Fatores de Transcrição Hélice-Alça-Hélice Básicos , Western Blotting , Comunicação Celular , Núcleo Celular/metabolismo , Células Cultivadas , Hipocampo/embriologia , Processamento de Imagem Assistida por Computador , Ligantes , Camundongos , Modelos Biológicos , NF-kappa B/metabolismo , Fatores de Crescimento Neural/metabolismo , Ligação Proteica , RNA Mensageiro/metabolismo , Receptor de Fator de Crescimento Neural , Receptores Notch , Reação em Cadeia da Polimerase Via Transcriptase Reversa , Transdução de Sinais , Fatores de Transcrição HES-1RESUMO
One of the main functions of astrocytes is to ensure glutamate homeostasis by glutamate uptake and glutamine synthesis. However, during the past ten years it has become clear that astrocytes may also induce changes in synaptic glutamate release when respective pathways must cope with the consequences of brain damage or other alterations in their functional requirements. The loss of glutamatergic synapses in Parkinson's and Huntington's disease is likely to associate with a continuous redistribution of presynaptic activity within the pool of surviving synapses, and astrocytes may have a role in the maintenance of independent control at individual glutamate release sites. The rodent striatum should be a good model structure to analyse astrocyte-synapse interactions underlying disease-related plasticity, because it does not itself contain any glutamatergic neurons. Here we examine recent results that may shed light on the mechanisms underlying pathway-specific alterations in the corticostriatal or thalamostriatal synaptic transmission with a possible involvement of astrocytic release or uptake of glutamate. The conclusions emphasize the need of exploring the highly compartmentalised and presumably heterogeneous synapse astrocyte-interactions at a single synapse level.
Assuntos
Astrócitos/fisiologia , Corpo Estriado/fisiologia , Plasticidade Neuronal/fisiologia , Terminações Pré-Sinápticas/fisiologia , Animais , Corpo Estriado/fisiopatologia , HumanosRESUMO
Astrocytes are abundant within mature neural circuits and are involved in brain disorders. Here, we summarize our current understanding of astrocytes and Huntington's disease (HD), with a focus on correlative and causative dysfunctions of ion homeostasis, calcium signaling, and neurotransmitter clearance, as well as on the use of transplanted astrocytes to produce therapeutic benefit in mouse models of HD. Overall, the data suggest that astrocyte dysfunction is an important contributor to the onset and progression of some HD symptoms in mice. Additional exploration of astrocytes in HD mouse models and humans is needed and may provide new therapeutic opportunities to explore in conjunction with neuronal rescue and repair strategies.
Assuntos
Astrócitos/fisiologia , Astrócitos/transplante , Doença de Huntington/fisiopatologia , Doença de Huntington/terapia , Animais , Astrócitos/patologia , Humanos , Doença de Huntington/patologiaRESUMO
Spinal cord neurons release glycine and GABA and accumulate glycine receptors (GlyRs) and GABA(A) receptors in the same postsynaptic densities. In contrast, supramedullar neurons prefer GABA as a neurotransmitter and exclude GlyRs from postsynaptic anchoring. The general aim of the present study was to elucidate the mechanisms underlying transmitter-appropriate receptor accumulation at inhibitory synapses. Specifically, we intended to clarify the molecular basis for the prohibition of GlyR accumulation in the postsynaptic densities of GABAergic synapses. A green fluorescent protein (GFP)-tagged gephyrin-binding loop of the GlyR beta subunit (GFP::betaL) was used as a surrogate for full-length receptors to characterize the GlyR binding capacity of postsynaptic gephyrins in transfected neurons. Both in spinal cord neurons (SCNs) and hippocampal neurons (HNs) GFP::betaL distribution displayed transmitter specificity; i.e., postsynaptic accumulation of GFP::betaL was high opposite terminals able to release glycine and low opposite purely GABAergic terminals. When comparing SCN and HN cultures we found that the level of mRNA coding for gephyrin splice variants containing the cassette C5 (C5-gephyrins) was significantly higher in HNs. In HNs depleted of C5-gephyrins, both GFP::betaL and endogenous GlyRs accumulated at postsynaptic GABAergic sites. Accordingly in SCNs, GFP-tagged C5-gephyrin displayed a preferential postsynaptic accumulation opposite GABAergic synapses. Comparison of glycinergic, mixed, and GABAergic synapses in SCNs showed that the degree of GlyR accumulation was inversely related to the amount of postsynaptic C5-gephyrin. These results identify the C5 splice variant of gephyrin as a factor regulating the transmitter-appropriate degree of GlyR accumulation at inhibitory synapses.
Assuntos
Proteínas de Transporte/metabolismo , Proteínas de Membrana/metabolismo , Neurônios/metabolismo , Receptores de Glicina/metabolismo , Sinapses/metabolismo , Ácido gama-Aminobutírico/metabolismo , Processamento Alternativo , Animais , Células Cultivadas , Proteínas de Fluorescência Verde , Hipocampo/citologia , Proteínas Luminescentes/genética , Substâncias Macromoleculares , Inibição Neural/fisiologia , Neurônios/citologia , Fragmentos de Peptídeos/genética , Fragmentos de Peptídeos/metabolismo , Subunidades Proteicas/genética , Subunidades Proteicas/metabolismo , Ratos , Ratos Wistar , Receptores de Glicina/genética , Proteínas Recombinantes de Fusão/genética , Proteínas Recombinantes de Fusão/metabolismo , Reação em Cadeia da Polimerase Via Transcriptase Reversa , Medula Espinal/citologia , TransfecçãoRESUMO
Patients with akinesia benefit from chronic high frequency stimulation (HFS) of the subthalamic nucleus (STN). Among the mechanisms contributing to the therapeutic success of HFS-STN might be a suppression of activity in the output region of the basal ganglia. Indeed, recordings in the substantia nigra pars reticulata (SNr) of fully adult mice revealed that HFS-STN consistently produced a reduction of compound glutamatergic excitatory postsynaptic currents at a time when the tetrodotoxin-sensitive components of the local field potentials had already recovered after the high frequency activation. These observations suggest that HFS-STN not only alters action potential conduction on the way towards the SNr but also modifies synaptic transmission within the SNr. A classical conditioning-test paradigm was then designed to better separate the causes from the indicators of synaptic depression. A bipolar platinum-iridium macroelectrode delivered conditioning HFS trains to a larger group of fibers in the STN, while a separate high-ohmic glass micropipette in the rostral SNr provided test stimuli at minimal intensity to single fibers. The conditioning-test interval was set to 100 ms, i.e. the time required to recover the excitability of subthalamo-nigral axons after HFS-STN. The continuity of STN axons passing from the conditioning to the test sites was examined by an action potential occlusion test. About two thirds of the subthalamo-nigral afferents were occlusion-negative, i.e. they were not among the fibers directly activated by the conditioning STN stimulation. Nonetheless, occlusion-negative afferents exhibited signs of presynaptic depression that could be eliminated by blocking GABA(B) receptors with CGP55845 (1 µM). Further analysis of single fiber-activated responses supported the proposal that the heterosynaptic depression of synaptic glutamate release during and after HFS-STN is mainly caused by the tonic release of GABA from co-activated striato-nigral afferents to the SNr. This mechanism would be consistent with a gain-of-function hypothesis of DBS.
Assuntos
Vias Aferentes/fisiologia , Estimulação Elétrica , Plasticidade Neuronal/efeitos dos fármacos , Terminações Pré-Sinápticas/fisiologia , Substância Negra/fisiologia , Núcleo Subtalâmico/fisiologia , Ácido gama-Aminobutírico/farmacologia , Potenciais de Ação/fisiologia , Vias Aferentes/efeitos dos fármacos , Animais , Condicionamento Psicológico , Potenciais Pós-Sinápticos Excitadores/fisiologia , Ácido Glutâmico/farmacologia , Potenciais Pós-Sinápticos Inibidores/efeitos dos fármacos , Ativação do Canal Iônico/efeitos dos fármacos , Camundongos Endogâmicos C57BL , Modelos Neurológicos , Terminações Pré-Sinápticas/efeitos dos fármacos , Receptores de GABA/metabolismo , Substância Negra/efeitos dos fármacos , Núcleo Subtalâmico/efeitos dos fármacosRESUMO
The extracellular concentration of the two main neurotransmitters glutamate and GABA is low but not negligible which enables a number of tonic actions. The effects of ambient GABA vary in a region-, cell-type, and age-dependent manner and can serve as indicators of disease-related alterations. Here we explored the tonic inhibitory actions of GABA in Huntington's disease (HD). HD is a devastating neurodegenerative disorder caused by a mutation in the huntingtin gene. Whole cell patch clamp recordings from striatal output neurons (SONs) in slices from adult wild type mice and two mouse models of HD (Z_Q175_KI homozygotes or R6/2 heterozygotes) revealed an HD-related reduction of the GABA(A) receptor-mediated tonic chloride current (I(Tonic(GABA))) along with signs of reduced GABA(B) receptor-mediated presynaptic depression of synaptic GABA release. About half of I(Tonic(GABA)) depended on tetrodotoxin-sensitive synaptic GABA release, but the remaining current was still lower in HD. Both in WT and HD, I(Tonic(GABA)) was more prominent during the first 4 h after preparing the slices, when astrocytes but not neurons exhibited a transient depolarization. All further tests were performed within 1-4 h in vitro. Experiments with SNAP5114, a blocker of the astrocytic GABA transporter GAT-3, suggest that in WT but not HD GAT-3 operated in the releasing mode. Application of a transportable substrate for glutamate transporters (D-aspartate 0.1-1 mM) restored the non-synaptic GABA release in slices from HD mice. I(Tonic(GABA)) was also rescued by applying the hyperagonist gaboxadol (0.33 µM). The results lead to the hypothesis that lesion-induced astrocyte depolarization facilitates non-synaptic release of GABA through GAT-3. However, the capacity of depolarized astrocytes to provide GABA for tonic inhibition is strongly reduced in HD.
Assuntos
Astrócitos/metabolismo , Corpo Estriado/metabolismo , Proteínas da Membrana Plasmática de Transporte de GABA/metabolismo , Doença de Huntington/metabolismo , Inibição Neural/fisiologia , Neurônios/metabolismo , Ácido gama-Aminobutírico/metabolismo , Animais , Anisóis/farmacologia , Astrócitos/efeitos dos fármacos , Corpo Estriado/efeitos dos fármacos , Corpo Estriado/fisiopatologia , Agonistas GABAérgicos/farmacologia , Inibidores da Captação de GABA/farmacologia , Doença de Huntington/genética , Doença de Huntington/fisiopatologia , Potenciais Pós-Sinápticos Inibidores/efeitos dos fármacos , Potenciais Pós-Sinápticos Inibidores/fisiologia , Isoxazóis/farmacologia , Masculino , Camundongos , Camundongos Transgênicos , Inibição Neural/efeitos dos fármacos , Neurônios/efeitos dos fármacos , Ácidos Nipecóticos/farmacologia , Técnicas de Patch-Clamp , Transmissão Sináptica/efeitos dos fármacos , Transmissão Sináptica/fisiologiaRESUMO
Functional impairment of the adult brain can result from deficits in the ontogeny of GABAergic synaptic transmission. Gene defects underlying autism spectrum disorders, Rett's syndrome or some forms of epilepsy, but also a diverse set of syndromes accompanying perinatal trauma, hormonal imbalances, intake of sleep-inducing or mood-improving drugs or, quite common, alcohol intake during pregnancy can alter GABA signaling early in life. The search for therapeutically relevant endogenous molecules or exogenous compounds able to alleviate the consequences of dysfunction of GABAergic transmission in the embryonic or postnatal brain requires a clear understanding of its site- and state-dependent development. At the level of single synapses, it is necessary to discriminate between presynaptic and postsynaptic alterations, and to define parameters that can be regarded as both suitable and accessible for the quantification of developmental changes. Here we focus on the performance of GABAergic synapses in two brain structures, the hippocampus and the superior colliculus, describe some novel aspects of neurotrophin effects during the development of GABAergic synaptic transmission and examine the applicability of the following rules: (1) synaptic transmission starts with GABA, (2) nascent/immature GABAergic synapses operate in a ballistic mode (multivesicular release), (3) immature synaptic terminals release vesicles with higher probability than mature synapses, (4) immature GABAergic synapses are prone to paired pulse and tetanic depression, (5) synapse maturation is characterized by an increasing dominance of synchronous over asynchronous release, (6) in immature neurons GABA acts as a depolarizing transmitter, (7) synapse maturation implies inhibitory postsynaptic current shortening due to an increase in alpha1 subunit expression, (8) extrasynaptic (tonic) conductances can inhibit the development of synaptic (phasic) GABA actions.
RESUMO
An increasing number of epilepsy patients are afflicted with drug-resistant temporal lobe epilepsy (TLE) and require alternative therapeutic approaches. High-affinity glycine receptors (haGlyRs) are functionally adapted to tonic inhibition due to their response to hippocampal ambient glycine, and their synthesis is activity-dependent. Therefore, in our study, we scanned TLE hippocampectomies for expression of haGlyRs and characterized the effects mediated by these receptors using primary hippocampal neurons. Increased haGlyR expression occurred in TLE hippocampi obtained from patients with a severe course of disease. Furthermore, in TLE patients, haGlyR and potassium chloride cotransporter 2 (KCC2) expressions were inversely regulated. To examine this potential causal relationship with respect to TLE histopathology, we established a hippocampal cell culture system utilising tonic inhibition mediated by haGlyRs in response to hippocam-pal ambient glycine and in the context of a high Cl equilibrium potential, as is the case in TLE hippocampal neurons. We showed that hypoactive neurons increase their ratio between glutamatergic and GABAergic synapses, reduce their dendrite length and finally undergo excitotoxicity. Pharmacological dissection of the underlying processes revealed ionotropic glutamate and TrkB receptors as critical mediators between neuronal hypoactivity and the emergence of these TLE-characteristic histopathological signs. Moreover, our results indicate a beneficial role for KCC2, because decreasing the Cl- equilibrium potential by KCC2 expression also rescued hypoactive hippocampal neurons. Thus, our data support a causal relationship between increased haGlyR expression and the emergence of histopathological TLE-characteristic signs, and they establish a pathophysiological role for neuronal hypoactivity in the context of a high Cl- equilibrium potential.