Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Slide 39Slide 40Slide 41Slide 42Slide 43Spatial Distribution of Channels IIThe spatial distribution of ion channels play important role in controlling neuronal excitability and their computational power.Action potential (AP) generation at axon initial segment (AIS) AP backpropagation from soma to distal dendritesLai H and Jan L,2006Compartmentalization of neuronsand regional distribution of ion channelsAP backpropagation occurs in most, but not all, neurons.Backpropagating APs provide a retrograde signal to the dendritic tree indicating the level of neuronal output. This might serve as an associative link between presynaptic excitation and the postsynaptic response of a neuron necessary for some forms of synaptic plasticity.The back-propagating AP in the dendrites becomes progressively smaller in amplitude the farther it travels from the soma, and the AP may actually fail to propagate beyond certain distal branch points.Action Potential BackpropagationAction Potential BackpropagationStuart and Hausser, 1994Stuart and Sakmann, 1994Colbert and Johnston, 1996Sprustron N et al, 1995Failure of action potentials to invade the dendrites of CA1 pyramidal neurons during repetitive action potential firingCell-attached patch recording along the dendritic treeSingle square depolarization:Na current density uniformly distributed Rapid activation and inactivation kineticsNormal voltage-dependent activation and inactivationTrain of depolarization stimulus:A slow inactivation state that requires seconds for full recoverySomatodendritic distribution of Na+ channelsColbert C et al., 1997composite currents (w/o blockers) from distal dendrite (~170 m from soma)Somatodendritic distribution of Na+ channelsColbert C et al., 1997somaDistal dendrites% Na channels in slow inactivation statelarge hyperpolarization remove inactivationamplitude of backpropagating APs during a trainAP amplitudemaintainedCell-attached patch recording of composite currents with TTXbefore and after 4-AP Somatodendritic distribution of K+ channelsHoffman D et al, 1997(4-AP sensitive)Somatodendritic distribution of K+ channels5-fold increase of IA current density (KV4.2) from soma to distal dendritesprevent subthreshold Na+ channel activationProgressive decrease of backpropagating AP amplitudeFailure of AP backpropagation beyond certain distal branch pointsSun et al, 1997DPP6 establishes the A-type K+ current gradientSingle transmembraneK+ channel auxilliary subunitKv4.2 koK+ currents in CA1 pyramidal neruonsA-type current, rapid activation/inactivation Kv4.2 4-APSustained component delayed-rectifier TEA Chen et al., 2006Chen et al., 2006A-type K+ current as a key regulator of AP backpropagation4-AP blockadePKA/PKC  activation Kv4.2 ko  A-current MAP kinase  activation inhibitionA-type current dendritic bAP amplitudeActivity-dependent trafficking of Kv4.2 channels in dendrites of hippocampal neuronsKim et al., 2007Basal condition:Kv4.2 in dendritic spine100 mM AMPA:Kv4.2 trafficking to dendritic shaftCa-dependentfast onsetplateau in 10 minKv4.2 internalization reduces IA currentKim et al., 2007Long-term potentiationPKA/PKCtauA overexpressionKv4.2 internalizationA-type current AP backpropagationdendritic hyper-excitationSummaryThe spatial distribution of ion channels play important role in controlling neuronal excitability.The high density of Na channels at axon initial segment is responsible for action potential generation. The increase of A-type K channel (KV4.2) from soma to distal dendrites regulates AP backpropagation.ChannelopathiesDiseases caused by disturbed function of ion channel subunits or the proteins that regulate themCongenital: resulting from mutations of ion channels or regulatory proteinsAcquired: caused by autoantibodies against ion channelsLambert-Eaton syndromeIsaacs’ syndromeHow do we study channelopathy?Genetic studies identify disease associated mutationsRecessive or dominant? Partial or full penetrance?Variants in normal cohort?iMutations/variations introduced into human cDNAiWild-type and mutant/variant channels expressed in heterologous systems: Xenopus oocytes, HEK293T cells etcLoss- or gain-of function mutation?Genotype-phenotype correlation?iexpressing mutant channels in neuronsChange of excitability, neurotransmission, morphology, survival etciCharacterization of mice carrying the human mutationhuman disease-like phenotype?Meisler M and Kearney J, 2005TTX-STTX-RSodium channelopathies: cell-type specific effects of mutant channelsDiseaseepilepsy, migraineepilepsyperiodic paralysiscardiac arrhythmiachronic paininsensitive to painNav1.1 mutations associated with epilepsyNav1.1: TTX-sensitive< 10% of Na channels in hippocampusEpilepsy-associated mutations: autosomal dominantmost are loss-of-functionNav1.1 mutation severity correlates with disease spectrumCatterall, 2014Targeted deletion of Nav1.1 in miceHeterozygotes: recurring seizure, sporadic death, starting P21-P27Homozygotes: ataxia, seizure, death by P15Myoclonic jerkhindlimb flexionForelimb clonusHead bobbingMuscle relaxEnd of seizureInterictalYu et al., 2006KI mice hz of R1407X mutation exhibit similar phenotype Ogiwara et al. 2007Nav1.1: TTX-sensitive< 10% of Na channels in hippocampus~ 75% Na current in GABAergic interneurons< 10% Na current in excitatory pyramidal neuronsAcutely dissociated hippocampal neuronsInterneurons: fusiform cell bodybipolar processesPyramidal neurons: triangular somaprominent proximal dendritic extensionsNav1.1 expression in hippocampal neuronsYu et al., 2006Na currents in dissociated hippocampal neuronsPyramidal neurons Bipolar neurons+/+ +/- -/-Nav1.1Nav1.3Loss of Nav1.1 decreases spike frequency and altersAP shape in bipolar neuronsYu et al., 2006Reduction of Nav1.1 expression in inhibitory neuronsrecapitulates the seizure phenotypeFloxed Nav1.1 heterozygotes X Dlx1/2-Cre50% reduction of Nav1.1 expression in 50% of GABAergic neuronsChristine et al., 2012Nav1.1, dominant Na channel in inhibitory neuronsLoss-of-function Nav1.1 mutationspreferentially affect GABAergic neuronsreduction of total Na currentsdecrease in spike frequencyEpilepsyL858HErythermalgia: an