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New mechanisms regulating stability and dynamics of AMPA receptors
Abstracts / Neuroscience Research 68S (2010) e4–e52
S1-2-2-1 Unique subcellular distribution patterns of distinct voltage-gated ion channels on the surface of identified nerve cells Zoltan Nusser Institute of Experimetal Medicine In the past decades, anatomical, electrophysiological and molecular approaches revealed morphological and functional heterogeneity of nerve cells. It is still unclear how the morphological diversity of nerve cells is achieved, but it is now a general belief that variation in the intrinsic electrical properties of neurons is the consequence of the expression of distinct sets of ion channels. Understanding diverse electrical properties of nerve cells was one of the incentives driving the tremendous efforts of the past two decades to identify the molecular diversity of ion channels and their expression patterns in the CNS. At the same time, it became also apparent that functional role of an ion channel greatly depends on its subcellular location. Therefore, we investigated the subcellular distribution of hyperpolarizationactivated and cyclic nucleotide gated channels-(HCN), A-type and delayed rectifier potassium channels (Kv) and sodium channels (Nav) in distinct types of nerve cell using immunohistochemical approaches. Our results revealed cell type- and subunit-specific distribution patterns. For example, the density of the HCN1 subunit increases along the proximo-distal axis of hippocampal pyramidal cell dendrites, has a rather uniform density in olfactory bulb external tufted cell dendrites, but is restricted to the axons of cortical basket cells. Considering a single cell type, the distribution patterns of distinct voltage-gated ion channels are very different. Namely, the density of HCN1 increases, whereas that of Nav1.6 decreases in hippocampal pyramidal cell apical dendrites as a function of distance from the soma. The functional consequences of distinct subcellular distribution patterns are also investigated and will be discussed. doi:10.1016/j.neures.2010.07.257
S1-2-2-2 Activity-dependent re-localization of sodium channels at the axon initial segment Hiroshi Kuba Department of Physiology, Kyoto University Graduate School of Medicine The axon initial segment (AIS) is enriched with voltage-gated Na+ (Nav) channels and contributes to initiating spikes in neurons. We have previously shown in avian brainstem auditory neurons that the distribution of Nav channels at the AIS varies depending on the patterns of synaptic inputs and this variation allows each neuron to transform synaptic inputs into spikes efficiently, thereby optimizing its function. This raises a possibility that the distribution of Nav channels at the AIS is effectively coupled with synaptic inputs. In this symposium, I will present our recent data supporting this idea and show how the distribution of Nav channels at the AIS could be modulated by synaptic activity. Chicks were monoaurally deprived at posthatch day 1, and the effects were examined at cochlear nucleus, a second-order nucleus in the auditory pathway. Within 7 days after auditory deprivation, the length of AIS, defined by the distribution of Nav channels and its anchoring proteins, increased by 1.7 times in the deprived side, while its location and density of Nav channels were not altered. Consistently, the auditory deprivation increased the whole-cell Na+ current by 1.5 times without changes in its somatic component, and reduced the spike threshold. Consequently, spontaneous firing appeared in some neurons in slice preparations after auditory deprivation. Thus, the deprivation of synaptic activity rearranged the AIS to increase Nav channels, thereby augmenting the excitability of neurons. This homeostatic regulation of Nav channels may contribute to the maintenance of auditory pathway after hearing loss. Moreover, the plasticity at the spike initiation site may suggest a powerful pathway for refining neuronal computation. doi:10.1016/j.neures.2010.07.258
S1-2-2-3 KCNQ2/3 channels at the axonal initial segment and node of Ranvier Edward C. Cooper Department of Neurology and Neuroscience, Baylor College of Medicine Human mutations in KCNQ2 and KCNQ3 cause epileptic seizures and involuntary muscle twitches (myokymia), indicating that these voltage-gated K+ channels regulate excitability in brain circuits and motoneuron axons. We found that KCNQ2 and KCNQ3 were colocalized with NaV channels at axonal initial segments (AISs) and nodes of Ranvier, axonal subdomains that generate and regenerate action potentials (APs). The axonal colocalization of
e7
NaV and KCNQ2/3 channels depends on analogous anchor motifs that bind the channels to the cytoskeleton. Voltage-clamp studies indicate that nodal KCNQ channels regulate excitability of peripheral myelinated fibers, mediating a period of reduced excitability following single spikes and spike trains. We used computational modeling and electrophysiology to analyze how AIS localization contributes to the ability of KCNQ2 and KCNQ3 to regulate neuronal responsiveness. In the model, a relatively small amplitude KCNQ conductance placed in the AIS near the site of AP initiation exerts considerable influence over firing. The same amplitude of KCNQ conductance placed at the soma and/or dendrites exerts less influence. We tested this model using acutely dissociated Purkinje cells (adPCs), which fire spontaneously (50–100 Hz) in vitro. When prepared with procedures that preserve their AISs, adPC firing is strongly sensitive to KCNQ channel modulators. Cells lacking AISs show markedly diminished influence of KCNQ modulators on firing. doi:10.1016/j.neures.2010.07.259
S1-2-2-4 New mechanisms regulating stability and dynamics of AMPA receptors Michisuke Yuzaki , Shinji Matsuda Department of Physiol, Sch of Med, Keio University, Tokyo The AMPA-type ionotropic glutamate receptor mainly mediates fast excitatory neurotransmission in the vertebrate CNS. Increased neuronal activities cause long-term potentiation and long-term depression (LTD) of neurotransmission, which serve as the basis for learning and memory. Increasing lines of evidence have indicated that clathrin-mediated endocytosis of the AMPA receptor at postsynaptic membranes is the key molecular event leading to the induction of LTD. Although the current model explains how AMPA receptors are released from its postsynaptic anchoring protein GRIP during the LTD-induction stimulus, it remains unclear how the endocytosis step itself is coordinated. At presynaptic sites, Ca2+ influx through the voltage-gated Ca2+ channels is shown to activate calcineurin, which dephosphorylates several components of the endocytic machinery, such as amphiphysin, epsin, and AP-180. In addition, calcineurin is reported to activate phosphatidylinositol 4-phosphate 5-kinase (PIP5K), which produces phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2], a key membrane phospholipid that recruits several components of the endocytic machinery to endocytic hot spots at presynaptic terminals. In contrast, although the GluA2 subunit of the AMPA receptor was reported to bind to AP-2 at postsynaptic membranes, whether and how such binding was regulated by neuronal activities is unclear. In this talk, we would like to introduce and discuss some of our recent findings on several key steps that regulate clathrin-mediated endocytosis of AMPA receptors at postsynaptic membranes during the LTD-inducing stimulus. We found several mechanisms, distinct from those at presynaptic sites, were activated by Ca2+ influx through the NMDA receptor, not through the voltagegated Ca2+ channel, leading to the recruitment of AP-2 specifically near AMPA receptors at postsynaptic sites. doi:10.1016/j.neures.2010.07.260
S1-2-2-5 Dynamic localization of synaptic and extrasynaptic AMPA-type glutamate receptors in physiological learning process Ryuichi Shigemoto NIPS Glutamate receptors serve as key molecules for synaptic plasticity such as long-term potentiation and depression, which are believed to underlie various types of physiological learning processes. Although many lines of evidence obtained with in vitro studies and knock out mice support essential roles of NMDA- and AMPA-type glutamate receptor subunits in synaptic plasticity and learning, we have rather sparse evidence demonstrating what kind of alteration occurs in vivo in localization and function of glutamate receptors and synapses during physiological learning. I will introduce a newly developed quantitative freeze-fracture replica immunolabeling and dynamic changes of synapses and synaptic receptors detected with this and conventional electron microscopic methods after physiological spatial and motor learning, and discuss about roles of glutamate receptors in physiological learning processes. doi:10.1016/j.neures.2010.07.261
S1-2-2-1 Unique subcellular distribution patterns of distinct voltage-gated ion channels on the surface of identified nerve cells Zoltan Nusser Institute of Experimetal Medicine In the past decades, anatomical, electrophysiological and molecular approaches revealed morphological and functional heterogeneity of nerve cells. It is still unclear how the morphological diversity of nerve cells is achieved, but it is now a general belief that variation in the intrinsic electrical properties of neurons is the consequence of the expression of distinct sets of ion channels. Understanding diverse electrical properties of nerve cells was one of the incentives driving the tremendous efforts of the past two decades to identify the molecular diversity of ion channels and their expression patterns in the CNS. At the same time, it became also apparent that functional role of an ion channel greatly depends on its subcellular location. Therefore, we investigated the subcellular distribution of hyperpolarizationactivated and cyclic nucleotide gated channels-(HCN), A-type and delayed rectifier potassium channels (Kv) and sodium channels (Nav) in distinct types of nerve cell using immunohistochemical approaches. Our results revealed cell type- and subunit-specific distribution patterns. For example, the density of the HCN1 subunit increases along the proximo-distal axis of hippocampal pyramidal cell dendrites, has a rather uniform density in olfactory bulb external tufted cell dendrites, but is restricted to the axons of cortical basket cells. Considering a single cell type, the distribution patterns of distinct voltage-gated ion channels are very different. Namely, the density of HCN1 increases, whereas that of Nav1.6 decreases in hippocampal pyramidal cell apical dendrites as a function of distance from the soma. The functional consequences of distinct subcellular distribution patterns are also investigated and will be discussed. doi:10.1016/j.neures.2010.07.257
S1-2-2-2 Activity-dependent re-localization of sodium channels at the axon initial segment Hiroshi Kuba Department of Physiology, Kyoto University Graduate School of Medicine The axon initial segment (AIS) is enriched with voltage-gated Na+ (Nav) channels and contributes to initiating spikes in neurons. We have previously shown in avian brainstem auditory neurons that the distribution of Nav channels at the AIS varies depending on the patterns of synaptic inputs and this variation allows each neuron to transform synaptic inputs into spikes efficiently, thereby optimizing its function. This raises a possibility that the distribution of Nav channels at the AIS is effectively coupled with synaptic inputs. In this symposium, I will present our recent data supporting this idea and show how the distribution of Nav channels at the AIS could be modulated by synaptic activity. Chicks were monoaurally deprived at posthatch day 1, and the effects were examined at cochlear nucleus, a second-order nucleus in the auditory pathway. Within 7 days after auditory deprivation, the length of AIS, defined by the distribution of Nav channels and its anchoring proteins, increased by 1.7 times in the deprived side, while its location and density of Nav channels were not altered. Consistently, the auditory deprivation increased the whole-cell Na+ current by 1.5 times without changes in its somatic component, and reduced the spike threshold. Consequently, spontaneous firing appeared in some neurons in slice preparations after auditory deprivation. Thus, the deprivation of synaptic activity rearranged the AIS to increase Nav channels, thereby augmenting the excitability of neurons. This homeostatic regulation of Nav channels may contribute to the maintenance of auditory pathway after hearing loss. Moreover, the plasticity at the spike initiation site may suggest a powerful pathway for refining neuronal computation. doi:10.1016/j.neures.2010.07.258
S1-2-2-3 KCNQ2/3 channels at the axonal initial segment and node of Ranvier Edward C. Cooper Department of Neurology and Neuroscience, Baylor College of Medicine Human mutations in KCNQ2 and KCNQ3 cause epileptic seizures and involuntary muscle twitches (myokymia), indicating that these voltage-gated K+ channels regulate excitability in brain circuits and motoneuron axons. We found that KCNQ2 and KCNQ3 were colocalized with NaV channels at axonal initial segments (AISs) and nodes of Ranvier, axonal subdomains that generate and regenerate action potentials (APs). The axonal colocalization of
e7
NaV and KCNQ2/3 channels depends on analogous anchor motifs that bind the channels to the cytoskeleton. Voltage-clamp studies indicate that nodal KCNQ channels regulate excitability of peripheral myelinated fibers, mediating a period of reduced excitability following single spikes and spike trains. We used computational modeling and electrophysiology to analyze how AIS localization contributes to the ability of KCNQ2 and KCNQ3 to regulate neuronal responsiveness. In the model, a relatively small amplitude KCNQ conductance placed in the AIS near the site of AP initiation exerts considerable influence over firing. The same amplitude of KCNQ conductance placed at the soma and/or dendrites exerts less influence. We tested this model using acutely dissociated Purkinje cells (adPCs), which fire spontaneously (50–100 Hz) in vitro. When prepared with procedures that preserve their AISs, adPC firing is strongly sensitive to KCNQ channel modulators. Cells lacking AISs show markedly diminished influence of KCNQ modulators on firing. doi:10.1016/j.neures.2010.07.259
S1-2-2-4 New mechanisms regulating stability and dynamics of AMPA receptors Michisuke Yuzaki , Shinji Matsuda Department of Physiol, Sch of Med, Keio University, Tokyo The AMPA-type ionotropic glutamate receptor mainly mediates fast excitatory neurotransmission in the vertebrate CNS. Increased neuronal activities cause long-term potentiation and long-term depression (LTD) of neurotransmission, which serve as the basis for learning and memory. Increasing lines of evidence have indicated that clathrin-mediated endocytosis of the AMPA receptor at postsynaptic membranes is the key molecular event leading to the induction of LTD. Although the current model explains how AMPA receptors are released from its postsynaptic anchoring protein GRIP during the LTD-induction stimulus, it remains unclear how the endocytosis step itself is coordinated. At presynaptic sites, Ca2+ influx through the voltage-gated Ca2+ channels is shown to activate calcineurin, which dephosphorylates several components of the endocytic machinery, such as amphiphysin, epsin, and AP-180. In addition, calcineurin is reported to activate phosphatidylinositol 4-phosphate 5-kinase (PIP5K), which produces phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2], a key membrane phospholipid that recruits several components of the endocytic machinery to endocytic hot spots at presynaptic terminals. In contrast, although the GluA2 subunit of the AMPA receptor was reported to bind to AP-2 at postsynaptic membranes, whether and how such binding was regulated by neuronal activities is unclear. In this talk, we would like to introduce and discuss some of our recent findings on several key steps that regulate clathrin-mediated endocytosis of AMPA receptors at postsynaptic membranes during the LTD-inducing stimulus. We found several mechanisms, distinct from those at presynaptic sites, were activated by Ca2+ influx through the NMDA receptor, not through the voltagegated Ca2+ channel, leading to the recruitment of AP-2 specifically near AMPA receptors at postsynaptic sites. doi:10.1016/j.neures.2010.07.260
S1-2-2-5 Dynamic localization of synaptic and extrasynaptic AMPA-type glutamate receptors in physiological learning process Ryuichi Shigemoto NIPS Glutamate receptors serve as key molecules for synaptic plasticity such as long-term potentiation and depression, which are believed to underlie various types of physiological learning processes. Although many lines of evidence obtained with in vitro studies and knock out mice support essential roles of NMDA- and AMPA-type glutamate receptor subunits in synaptic plasticity and learning, we have rather sparse evidence demonstrating what kind of alteration occurs in vivo in localization and function of glutamate receptors and synapses during physiological learning. I will introduce a newly developed quantitative freeze-fracture replica immunolabeling and dynamic changes of synapses and synaptic receptors detected with this and conventional electron microscopic methods after physiological spatial and motor learning, and discuss about roles of glutamate receptors in physiological learning processes. doi:10.1016/j.neures.2010.07.261