Download Channelopathies

Spatial Distribution of Channels II
The 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 dendrites
Compartmentalization of neurons
and regional distribution of ion channels
Lai H and Jan L,2006
Action Potential Backpropagation
AP 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 Backpropagation
Stuart and Hausser, 1994
Stuart and Sakmann, 1994
Colbert and Johnston, 1996
Failure of action potentials to invade the dendrites of CA1
pyramidal neurons during repetitive action potential firing
Sprustron N et al, 1995
Somatodendritic distribution of Na+ channels
Cell-attached patch recording along the dendritic tree
Single square depolarization:
Na current density uniformly distributed
Rapid activation and inactivation kinetics
Normal voltage-dependent activation and inactivation
Train of depolarization stimulus:
A slow inactivation state that requires seconds for full recovery
composite currents (w/o blockers) from distal dendrite (~170 m from soma)
Colbert C et al., 1997
Somatodendritic distribution of Na+ channels
Distal dendrites
amplitude of
backpropagating
APs during a train
% Na channels
in slow
inactivation state
soma
large hyperpolarization
 remove inactivation
AP amplitude
maintained
Colbert C et al., 1997
Somatodendritic distribution of K+ channels
Cell-attached patch recording of composite currents with TTX
before and after 4-AP
(4-AP sensitive)
Hoffman D et al, 1997
Somatodendritic distribution of K+ channels
5-fold increase of IA current density (KV4.2) from soma to distal dendrites

prevent subthreshold Na+ channel activation

Progressive decrease of backpropagating AP amplitude
Failure of AP backpropagation beyond certain distal branch points
DPP6 establishes the A-type K+ current gradient
Single
transmembrane
K+ channel
auxilliary subunit
Sun et al, 1997
K+ currents in CA1 pyramidal neruons
A-type current, rapid activation/inactivation
Kv4.2
4-AP
Sustained component
delayed-rectifier
TEA
Kv4.2 ko
Chen et al., 2006
A-type K+ current as a key regulator of AP backpropagation
A-type current dendritic bAP amplitude
4-AP
blockade
PKA/PKC
Kv4.2 ko
 activation
 A-current

MAP kinase
inhibition
 activation

Chen et al., 2006
Activity-dependent trafficking of Kv4.2 channels in
dendrites of hippocampal neurons
Basal condition:
Kv4.2 in dendritic spine
100 M AMPA:
Kv4.2 trafficking to dendritic shaft
Ca-dependent
fast onset
plateau in 10 min
Kim et al., 2007
Kv4.2 internalization reduces IA current
Long-term potentiation

PKA/PKC

Kv4.2 internalization
 A-type current
 AP backpropagation
dendritic hyper-excitation

tau

A overexpression
Kim et al., 2007
Summary
The 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.
Channelopathies
Diseases caused by disturbed function of ion channel
subunits or the proteins that regulate them
Congenital: resulting from mutations of ion channels or
regulatory proteins
Acquired: caused by autoantibodies against ion channels
Lambert-Eaton syndrome
Isaacs’ syndrome
How do we study channelopathy?
Genetic studies identify disease associated mutations
Recessive or dominant? Partial or full penetrance?
Variants in normal cohort?
i
Mutations/variations introduced into human cDNA
i
Wild-type and mutant/variant channels expressed in
heterologous systems: Xenopus oocytes, HEK293T cells etc
Loss- or gain-of function mutation?
Genotype-phenotype correlation?
i
expressing mutant channels in neurons
Change of excitability, neurotransmission, morphology, survival etc
i
Characterization of mice carrying the human mutation
human disease-like phenotype?
Sodium channelopathies:
cell-type specific effects of mutant channels
Disease
epilepsy, migraine
epilepsy
TTX-S
TTX-R
periodic paralysis
cardiac arrhythmia
chronic pain
insensitive to pain
Meisler M and Kearney J, 2005
Nav1.1 mutations associated with epilepsy
Nav1.1: TTX-sensitive
< 10% of Na channels in hippocampus
Epilepsy-associated mutations:
autosomal dominant
most are loss-of-function
Nav1.1 mutation severity correlates with disease spectrum
Catterall, 2014
Targeted deletion of Nav1.1 in mice
Heterozygotes: recurring seizure, sporadic death, starting P21-P27
KI mice hz of R1407X mutation exhibit similar phenotype
Ogiwara et al. 2007
Homozygotes: ataxia, seizure, death by P15
Interictal
Myoclonic jerk
hindlimb flexion
Forelimb clonus
Head bobbing
Muscle relax
End of seizure
Yu et al., 2006
Nav1.1 expression in hippocampal neurons
Nav1.1: TTX-sensitive
< 10% of Na channels in hippocampus
~ 75% Na current
in GABAergic interneurons
< 10% Na current
in excitatory pyramidal neurons
Acutely dissociated hippocampal neurons
Interneurons:
fusiform cell body
bipolar processes
Pyramidal neurons: triangular soma
prominent proximal
dendritic extensions
Na currents in dissociated hippocampal neurons
Pyramidal neurons
Bipolar neurons
Nav1.1
Nav1.3
+/+
+/-
-/-
Yu et al., 2006
Loss of Nav1.1 decreases spike frequency and alters
AP shape in bipolar neurons
Yu et al., 2006
Reduction of Nav1.1 expression in inhibitory neurons
recapitulates the seizure phenotype
Floxed Nav1.1 heterozygotes X Dlx1/2-Cre
50% reduction of Nav1.1 expression
in 50% of GABAergic neurons
Christine et al., 2012
Nav1.1, dominant Na channel in inhibitory neurons

Loss-of-function Nav1.1 mutations
preferentially affect GABAergic neurons

reduction of total Na currents
decrease in spike frequency

Epilepsy
Erythermalgia: an autosomal dominant painful neuropathy
associated with gain-of-function Nav1.7 mutations
Intermittent burning sensation of extremities
hyper-sensitivity of dorsal root ganglion (DRG) neurons
Redness of the skin
vasodilation, inhibition of sympathetic tone
hypo-sensitivity of sympathetic neurons in
superior cervical ganglion (SCG)?
L858H
Nav1.7 L858H and I848H mutations:
f voltage-dependent activation
h ramp current (-100 mV ~ 0 mV, 500 ms)
h persistent Na current
Cummins et al, 2004
500 ms ramp
-100 mV ~ 0 mV
Hyper-excitability of DRG neurons expressing L858H mutant
Rush et al, 2006
Hypo-excitability of SCG neurons expressing L858H mutant
Rush et al, 2006
Q1. Is L858H gain-of-function in both DRG and SCG?
Likely, similar depolarization shift of Vrest in DRG and SCG
Q2. How can depolarization shift of Vrest in SCG
lead to SCG hypo-excitability?
More Na channel undergo steady-state inactivation
i
AP threshold increase, amplitude decrease
Hyper-polarization shift of Vrest
restores SCG excitability
Rush et al, 2006
Q1. Is L858H gain-of-function in both DRG and SCG?
Likely, similar depolarization shift of Vrest in DRG and SCG
Q2. How can depolarization shift of Vrest in SCG
lead to SCG hypo-excitability?
More Na channel undergo steady-state inactivation
i
AP threshold increase, amplitude decrease
Q3. Is Na channel density higher in DRG neurons?
Maybe not, AP current threshold DRG > SCG
Q4. Do DRG and SCG neurons express different types
of Na channels?
DRG: Nav 1.1, 1.3, 1.6, 1.7, 1.8, 1.9
SCG: Nav 1.1, 1.3, 1.6, 1.7
Rush et al, 2006
SCG excitability can be restored by
co-expression of Nav1.8 and L858H
Rush et al, 2006
Nav1.7 L858H mutation:
gain-of-function for channel property
cell-type-specific effect on excitability
Hyper-excitation of DRG neurons
Intermittent burning sensation of extremities
Hypo-excitability of SCG neurons
Inhibition of sympathetic tone
Vasodilation, redness of the skin
i
Erythermalgia
Summary
Cell type-specific effect of ion channel mutations
can result from
1. the cell-type specific expression of the
affected channel per se
2. the cell-type specific expression of other ion
channels.
Dysregulation of ion channel activities occur beyond
conventional channelopathies
Fragile X syndrome (FXS):
most common inherited form of human intellectual
disability and autism
learning disabilities
language deficits
disrupted sleep
aggression
hypersensitivity to sensory stimuli
hyperarousal to seizures
CNS hyper-excitability
Abnormal channel activity?
FXS: expansion of CGG repeat in Fmr1 gene
i
Loss of expression of FMRP
Polyribosome-associated RNA binding protein
i
Abnormal RNA translation
Contractor et al., 2015
FMRP R138Q mutation
Intellectural disability, seizure
RNA-binding and protein translation unaffected
FMRP directly regulates ion channel activities
N-terminus: h BKCa channels
h Slack KNa channels
C-terminus: i N-type Ca channels
BKCa: big conductance, Ca2+-activated K+ channel
action potential (AP)
FMRP
+
BKCa channels
AP repolarization
Fast afterhyperpolarization (AHP)
Regulate AP shape and firing pattern
Vrest
Vrest
FMRP regulates AP width through activating BK channels
in hippocampal CA3 pyramidal neurons
AP width
EPSC
Loss of FMRP: excessive h AP width, h EPSC
Sensitive to BK blocker, independent of protein translation
Deng et al., 2013
FMRP R138Q mutation
Intellectural disability, seizure
RNA-binding and protein translation unaffected
Binding to BK 4 subunit disrupted
No effect on AP width increase
Myrick et al., 2015
FMRP regulates AP width through activation of BK channels
in hippocampal CA3 pyramidal neurons
Contractor et al., 2013
Summary
Cell type-specific effect of channel mutations may result
from the differential expression pattern of the affected
channel per se and/or other unaffected channels.
Dysregulation of ion channel activities occur beyond
conventional channelopathies.