Chanelopathies Flashcards
What plays the biggest role in maintaining ion concentration?
NaK ATPase (2K in 3Na out)
Ionic concentration @ rest
in/out Na high/low K low/high Ca lowlowlow/high Cl low/high
Ion channel families separated based on
cloning of similar genes & selective permeability to ions
Types of ion channel families
1) Resting K+ channel (always open)
2) v-gated channel
3) ligand-gated channel
4) signal-gated channel (open in response to a specific INTRACELLULAR molecule)
@ rest
- 70mV
- K high inside
- K channel leaks K out
@ depol
-45mV
-Na high outside-
-Na enters via v-gated Na channel due to depol
=> Na channel encode fast AP spike
@ hyperpol
-75mV
-Cl high outside
-Cl enter via Cl channel leading to hyperpol
=> reduces excitability
-30mV
- Ca very low inside
- Ca channel open allowing Ca to enter cell when depoled leading to coupling of excitability to other cellular processes e.g. NT release, muscle contraction, gene synthesis, cell death
-60mV
- Non selective cation channels mediate depol that drive cell spiking
- Cardiac pacemaker cells, rod cells and neurons have hyperpol-activated cation channels gated by nucleotides and Vm
- Some NT receptors are non-selective + channels
AP timeline
@rest K open @threshold v-gated Na channel open (Na influx) @depol v-gated Na channel close after hitting equilibrium (AP peak) @repol K channel open (K outflux) @hyperpol K channel close after hitting equilibrium (hyperpol pealk) @afterwards NaK ATPase restores RMP
Ligand gated ion channels convert chemical signal to electrical signal within
ms
Synapses of the brain
60% excitatory, rest inhibitory
30% or more of brain E demand
20% of body E is used by brain
Location of channels
-Na channel Dendrites, axon hillock, node of ranvier, soma -K channel NOR, dendrites, axon terminal, soma -Cl channel Inhibitory dendrites, soma -Ca channel Soma, dendrite, axon terminal -Cation Channels Dendrites, soma, terminal, internal organelles
Receptor trafficking
-Receptor delivery, recycling and diffusion (moving to its location) seems complex and details are not well understood
-Most likely the receptors are all made in the dendrites and delivered to the surface or axons etc
1 channel get made by ribosomes into ER
2 get matured in golgi
3 into secretory vesicles
4 Diffuse to synapse
5 Kick/moved out back inside by clathrin
Squid Giant Axon Size
Human axon diameter 2 um
Squid giant axon almost 1mm
Squid Giant Axon Findings
#1 Na dependence of AP Reducing the Na content of the extracellular medium made AP smaller and rise slower, which was regained when Na was put back #2 Action potential is a wave Thanks to fast Na channel opening followed by slow K channel opening
Na channels structure
1 gene, 4-fold segments
S4 segment have the v-sensitive domain
Eukaryotic NaV channel is structurally more complex than bacterial as for bacterial, 4 segments are the same
K channel structure
Tetramers made of 3 same parts
Ca channel structure
4 fold domains
Without Ca channel, no AP
Recording characteristics
1)Rhythmic firing
Small injection of current => slow rhythmic firing
High injection of current => high rhythmic firing (spike right after absolute refractory period)
2)Spike Trains
-Dynamic stimulus cause more clear on and off of firing times
-CNS can signal by spike or by absence of spike not by flat stimulus
-Even a signal of single neuron is so complicated & subtle yet dynamic
-Precise timing is the key for this dynamic activity
3) Cortical States
When asleep clear on/off phase of firing, when awake constantly firing in the cortex
Summary of receptors
- Selective ionic currents through membranes underlie fast (electric) signalling between excitable cells.
- The action potential is a traveling wave of excitability in all higher organisms
- Spikes encode digital information not the event itself
- Timing and density of channels is critical
What is channelopathy?
- A defect in an ion channel that leads to a diseaseCommonly due to impeded, damaged or less selected ion permeation
- > 300 human genes encode channels
- Provides insight to basic disease mechanisms
- Wide genome coverage => huge penetration (defect is big)
- Best case scenario, by understanding the mechanism, there could be a finding where already approved drugs can be applied to other diseases
example of channelopathies
Wide study in peripheral tissues
1) Long QT syndromeArrhythmic heartbeat which have a potential to cause syncope (fainting), seizures or sudden death. Only find out after working out or change in heartbeat, as otherwise it is symptom less.Very well studied and understood because heart is easy
2) Diabetes
3) Congenital MyastheniaDue to defect in various channelsVery well studied and understood because muscle is easy
Complexity of channelopathy in the CNS/brain
- Brain is like an non-conducted orchestra; play together without a lead
- It affects so many disease: epileptology,movement disorders(epilepsy: mutation in SCN1a),headache/migraine,peripheral nerves, pain,myology
- Combinations of defect can be non linear (e.g. severe*mild = healthy)
- Sanger sequencing (DNA sequencing) does not help as healthy and disease patients may have equal amounts of mutations
Mechanisms of channel dysfunction
For a neuron, they care only about the inhibited function by inhibited current I = N.P.i = number of channels * open probability of “a” channel * unitary conductance of a channel Also the issue could be caused due to the right change in current amount but by wrong ion
Types of channel dysfunction
alteration is selectivity or sensitivity is extremely bad
overexpression of channels e.g. autism
Example of channel dysfunction
1) omega pore mutations (omega current): increase/decrease the voltage sensitivity leading to dysfunctional opening of the pore
slowed inactivation of Na C => mytonia
incomplete inactivation of Na C => periodic paralysis
- HypoPP is characterised by episodes of muscle weakness or paralysis associated with reduced serum K levels
- Cause: The resting membrane potential of muscle fibers from patients with HypoPP is excessively depolarized, leading to inactivation of voltage-gated sodium channels, inexcitability, and paralysis of the muscle
spider toxin can inhibit its omega current caused by excessive depol
2) resurgent sodium current
One channel dysfunction may cause
multiple unwanted outcome (multitasking)
Potential effect of channel mutation
Proliferation -> cell cycle defects, cell fate
Migration -> heterotopias, dyslamination
Differentiation -> excitability, arborization
Synaptogenesis -> aberrant density, positioning
Stabilisation -> molecular remodelling, plasticity
Survival -> selective vulnerability
Channelopathy in diabetes
Player: KATP channel (ATP activated K channel)
- KATP channels are K+ channels that are controlled by ATP.
- plays a role in insulin secretion
- mutation in the KATP channel can lead to type 2 diabetes
- KATP also plays a role in the CNS –> result: disrupted KATP in muscle - channel was not sensitive to ATP –> motor deficits
- Connecting metabolism to secretion
- Motor coordination only affected by CNS and not muscle expression (nerve is affected but not the muscle)
- Channel properties (of turning off) in purkinje neurons are affected (reversible by drug: tolbutamide)
- Depending on the type of mutation, different outcome on insulin secretion via B cell occurs
- Polygenic state of KATP-C makes is normal, monogenic causes increase/decrease in insulin secretion
Non genetic vs genetic forms
- Non-genetic epilepsy, migraine and chronic pain are amongst the most common disabling neurological complaints
- usually acquired
- Genetic forms are very rare, but give insights into mechanism because they are, to some extent, stereotyped/have some commonalities meaning the findings can be applied for non-genetic forms
Channelopathies in epilepsy
Generalised epilepsy can result from
1) Excessive neuronal excitation
- Increased activity of principal neurons or excitatory neurotransmission
2) Insufficient synaptic inhibition
- Reduced activity of interneurons of inhibitory neurotransmission
The physiological effect of mutation depends on the biophysics and the selective expression pattern creating the Dual Polarity of excitatory and inhibitory neurons
Excitatory neurons:
Na C gain of function
K C loss of fx
Inhibitory neurons:
Na C loss of fx
K C gain of fx
IG: Channelopathy in Timothy Syndrome
- Ca channel disease
- Neuropsychiatric disease including lower score on Global Assessment of Functioning (functioning of daily lives) and autism
Channelopathy in Migraine
- Ca Channel
- Common, disabling brain disorder of episodic headache pain of unknown etiology that affects >10% of the population
- Unknown mechanism
- Autonomic attacks and headaches are the most common symptoms
- Types:
1) MO - migraine without aura e.g. Lashley’s visual aura associated with cortical spreading depression, meaning the vision loss caused by decreased blood flow in the responding cortical area
2) MA - migraine with aura
3) FHM - very rare autosomal disorder that seems to relate to calcium channels and/or ATPases
More about FHM
Familial Hemiplegic Migraine
FHM1
Ca C (CaV2.1)
-Increased activity in the cerebellum and cortex “supertransmission”
-CSD is facilitated and correlates with severity of symptoms
FHM2
a2-isoform of the NA-K ATPase, autosomal dominant
-alpha-2 isoform expressed mainly in glia
-inhibitory mutations affect catalysis (rate) not affinity for transported ions
-Loss of a single allele can be enough to induce the symptoms
FHM3 Na C
Channelopathy in Charcot Marie Tooth
- Charcot-Marie-Tooth disease (CMT) is one of the most common inherited neurological disorders, affecting approximately 1 in 2,500 people in the United States.
- disorder of the peripheral nerves; muscle weakness
- Connexins
- TRPV4 seemed to become cytotoxic
- Mechanism wise nothing much is known
Channelopathy in Hyperekplexia
- aka Stiff baby syndrome, startle disease
- point mutations in the glycine receptor and transporter (GLRA, GLRB, GLYT1)
- extremely rare, Autosomal dominant
- acoustic startle response - brainstem.
- mutations reveal functions in pseudogenes -> are they pseudo then?
CNS channelopathies conclusion
- almost all channels can give rise to channelopathies
- type of problem depends on selective expression
- severity of symptoms has little relation to biophysical properties
- Channels should be studied in the context of the developing cortex
- you cure very few… (may not be suited for CNS)
CNS channelopathy examples
diabetes, epilepsy, timothy syndrome, migraine, Charcot Marie tooth, Hyperekplexia
Type of seizures in epilepsy
-Simple partial seizure Small seizure in one side of brain Conciousness not impaired -Complex partial seizure Larger seizure in one side of brain -Partial seizures with secondary generalisation Seizure in both sides -Generalised seizures Larger seizure in both sides
Generalised seizures
Generalized epilepsy with type of seizures listed below usually resolve after 6 years of age: febrile seizures plus (GEFS+) Tonic-clonic myoclonic (jerks) absence atonic seizures
Brain regions involved in epilepsy
It’s complex!!!
Simple & complex partial seizures include:
cortex, temporal lobe (hippocampus, dentate gyrus, entorhinal cortex, amygdala, piriform cortex, perirhinal cortex), nucleus accumbens, striatum, globus pallidus, subthalamic nucleus
Secondary generalised partial seizures involve:
substantia nigra pars reticulata, thalamus, superior colliculus, pedunculopontine nucleus
EEG
Polarity of signal depends on depth and sign
=> cannot say whether signal is excitatory or inhibitory
How to read EEG
- Slow wave activity in the awake: Epileptic focus F7
- The majority (85%) of patients with complex partial seizures have temporal lobe onset, whereas a small proportion (15%) have frontal lobe onset partial epilepsy.
- Temporal lobe epilepsy is the most common type of focal epilepsy in adults
- The peak of the potential field involves inferior frontal (F7 or F8), midtemporal (T3 or T4), and the ear lobe electrodes (A1 or A2).
Berger effect
Phenomenon of seeing various oscillations when eyes are closed. Those underlying rhythms are blocked when eyes are open due to synchrony (orchestra)
theta and gamma cycle
Active memories (sensory memory) are repeated in each theta (4-10 Hz) cycle within one gamma (20-80 Hz)
What does HCN do?
Hyperpolarisation-activated channel (HCN) sustain the depol state allowing the neuron to fire again faster in the refractory period (Ih)
Spike timing within the cycle
- Spike is most likely to occur when depolarising (highest probability at depol peak)
- Least likely when hyperpolarised, due to IPSP of interneurons and shunting (loss of input resistance)
- The stronger the excitatory input, the earlier the spike. ⇒ intensity of excitatory drive is converted into a time-code
Communication of connected brain areas
❖ Oscillatory neuronal activity adjusts spike timing and sets the time window for integrating distributed activity
❖ Abnormalities in the ability of neuronal networks to synchronize and to engage in high frequency oscillations associate with: Schizophrenia, Alzheimer Disease, Autism, Parkinson and, obviously, Epilepsy
Long range synchronisation
- The slow sleep (1Hz) oscillations are scene in sleep or epilepsy
- ThalamoCortical spikes orchestrate the cortical neurons and NucleusReticularThalamic interneurons
- NRT interneurons terminate the TC volley
- Bistable membrane potential (in both TC and NRT) comes from Ih, low threshold calcium currents freed by hyperpolarization, and a Ca-activated non-selective cation current
Key player of oscillations
Interneurons!!!
- Physiological theta-gamma network activity allows the RECURRENT INHIBITION to occur
- If recurrent inhibition of the interneurons is lost, it causes hypersynchronous gamma oscillation. No on/off but firing constantly
- Epilepsy could be explained by hypersynchronous neural activity not just simply an overexcitation of neurons
- Hypersynchronous γ-Oscillation Precedes Seizures
Type of microcircuits
1) feed forward inhibition: like turning on followed by immediate off
2) feedback inhibition
3) counter inhibition: two neurons inhibiting each other
4) recurrent inhibition: two neurons exciting each other; major mode of connectivity in cortical network
Where does seizure occur
@ Cortex, hippocampus & dentate gyrus, thalamus
Seizure not only needs to occur but also propagate
to other neurons which can be assessed by
Ca2+ imaging
Epileptic seizure and microcircuits
epileptic seizures emerge from dysfunction of specific microcircuits, which then progressively engage other microcircuits to activate the full seizure network—an overall process known as ictogenesis
Epilepsy and thalamus
Thalamic dysrhythmia: Uncontrolled burst firing in the ‘awake‘ thalamus may trigger some types of seizure activity and spread of epileptic attacks
Epileptic activity and sleep
Most of interictal epileptiform activity occurs during sleep, especially stage II
Epilepsy and genetics: K channel
1) KCNQ2 (voltage-gated potassium channel)
Phenotype (seizures):
Benign familial neonatal convulsions (BFNC)
Generalized seizures, 1st week of life, usually resolving by 6 weeks, attacks are frequent and include tonic posturing, staring, blinking, automatisms or chronic seizures Function: Muscarinic current – Reduction in K current amplitude cause epilepsy
2) NKCC
Phenotype: seizure
Function: regulate intracellular Cl- concentration
GABA depolarizes neurons early in development (and in epilepsy)
@immature decreases intracellular Cl- content reciprocal regulation of NKCC1 and KCC2
@matured Direction of Cl- flow through channels switches during development (depol)
Epilepsy and genetics: Na Channel
SCN1A gain of function by impairing channel inactivation; prolonged Na+ influx no clustering (defects everywhere) and no mechanistic link, just problems SCN1B gain of function
Epilepsy and Genetics: GABA-R
GABRG2 Phenotype: seizure Function: loss of function mutation Characteristics: Timing-sensitive Temperature-sensitive
Homeostatic Regulation
Homeostatic Regulation of Excitation - Inhibition Balance
Ethanol-Withdrawal (EW) SeizuresEthanol potentiates GABAergic currents
Epilepsy and channelopathy summary
❖ Because epilepsy is a malady of excitability, we must consider all elements that contribute.
❖ There are many cellular and anatomical mechanisms of generation.
❖ Numerous genetic defects and behaviours can wreck excitation-inhibition balance
