Sodium Channels Flashcards
Name the six main types of ion channels.
Voltage-gated.
Ligand-gated.
Mechanically-gated.
Temperature-gated.
G-protein-gated.
Phosphorylation-gated.
Define gating.
The process of opening and closing the gates of an ion channel in response to external signals.
What is an active channel?
A channel that has gates that can open or close the channel.
What is a passive channel?
A channel that is always open, so ions can pass through continuously.
Name a specific passive channel that contributes to neuron’s RMP.
KCNK 2-pore potassium channel.
A change in voltage across the membrane activates voltage-gated sodium channels, and sodium rushes into the neuron. Why is this inward current only transient?
Voltage-gated sodium channels inactivate despite continued depolarisation of the membrane potential.
Which voltage-gated sodium channels contribute to the rising phase of an AP?
Nav1.6.
Nav1.7.
Nav1.8.
What voltage-gated sodium channels contribute to the amplification of subthreshold stimuli of an AP?
Nav1.3.
Nav1.7.
Nav1.9.
Describe the basic structure of the voltage-gated sodium channel.
Large multimeric complex.
Alpha subunit and one or more smaller auxiliary beta subunits.
Where is the ion-conducting aqueous pore found in a voltage-gated sodium channel?
Within the alpha subunit.
What is the role of the auxiliary beta subunits in a voltage-gated sodium channel?
Modify the kinetics and voltage-dependence of the gating.
How is an Nav a tetramer-mimicking structure?
It looks like it’s composed of multiple subunits but it’s actually a single polypeptide chain folded into a 3D structure (monomer of around 2000 amino acids).
Describe the domains of an Nav.
DI-DIV.
Each domain consists of 6 transmembrane spanning alpha helices (S1-S6).
Which helices make up the voltage-sensing domain (VSD) of an Nav?
S1-S4.
Which helices make up part of the pore domain (PD) of an Nav?
S5 and S6.
A sequence of amino acids that connect domains III and IV of an Nav make up what?
The portion of the channel responsible for inactivation.
Why is it important to identify regions of a gene or protein that have been conserved over time?
It is an important clue regarding the parts of a gene or protein that are most important.
What is the function of the arginine repeats on S4 of an Nav?
It is a key evolutionary feature that ensures that the arginine residues align at the same point on each bend in the coil of S4, forming a positively-charged band along the length of the alpha helix.
What is the characteristic and conserved pattern in the location of arginine residues on S4 of an Nav?
Arginine repeats every 3 amino acids.
Describe the sliding helix model of channel activation.
S4 is pulled inwards when the neuron is at rest.
It is repelled outwards when the membrane is depolarised.
How are S4 and S5 connected to each other?
Via a sequence known as a linker.
How does the physical movement of S4 when it is repelled due to depolarisation lead to a change in conformation of the Nav?
The physical movement of S4 pulls on the S4-S5 linker and shifts the position of S5 and S6.
Why is the S4-S5 linker essential for electromechanical coupling between the voltage sensor and the pore domain?
It physically connects the voltage sensor to the pore domain.
How is the selectivity filter of an Nav formed?
Four key residues form the selectivity filter.
They are an aspartate-glutamate-lysine-alanine (DEKA) motif.
Each domain contributes an amino acid to this motif.
Is the selectivity filter positively- or negatively-charged?
Negatively-charged.
Why is the fast inactivation (~1msec) of Nav channels important?
It prevent hyperexcitability.
What is the role of the hydrophobic IFM motif?
It acts intracellularly as the gating particle that binds and occludes the pore during fast inactivation.
Describe the hinged-lid mechanism for inactivation.
The IFM-motif acts as a hydrophobic latch that binds to sites on the S4-S5 linkers of DIII and DIV, as well as the cytoplasmic end of the DIV-S6 helix to close the activation gate from the inside.
Why does deletion or mutation of the IFM-motif impair Nav channels?
It is required for Nav channel inactivation.
How do B1 and B2 subunits modify the kinetics and voltage-dependence of alpha subunit gating?
They increase the peak sodium current carried by Nav1.2.
They accelerate inactivation.
They shift the voltage-dependence of activation and inactivation to more negative potentials.
How do B1-B4 subunits affect the activation and inactivation of Nav channels?
They accelerate it.
How does PKA phosphorylation of Nav1.1 and Nav1.2 affect sodium currents?
Reduces peak sodium currents.
How does PKA phosphorylation of Nav1.8 affect sodium currents?
Increases sodium currents.
How does PKC phosphorylation of Nav1.1 and Nav1.2 affect channel inactivation and sodium currents, and why?
Slows channel inactivation due to phosphorylation of a site in the inactivation gate.
Reduces peak sodium current due to phosphorylation of sites in the intracellular loop between domains I and II.
How does PKC phosphorylation of Nav1.7 and Nav1.8 affect sodium currents?
Increases sodium currents.
What is the mechanism of action of local anaesthetics?
Block Nav channels in sensory nerves (pain).
What is the mechanism of action of class I cardiac antiarrhythmics?
Block Nav channels in the heart (arrhythmia).
What is the mechanism of action for tetrodotoxin (TTX)?
Inhibits Nav by binding to multiple sites in the pore forming domain and block deep within the pore.
Can lead to heart failure and death.
Name the two distinct mechanisms that toxins that influence Nav channel function use.
Pore-blocking.
Gating-modifying.
Describe the mechanism of action of pore-blocking toxins.
Inhibit the flow of sodium ions by binding to the outer vestibule or inside the ion conduction pore.
Describe the mechanism of action of gating-modifier toxins.
Interact with a region of the channel that changes conformation during channel opening to alter the gating mechanism.
Which three Nav channels are involved in the nociceptive pathway (pain)?
Nav1.7.
Nav1.8.
Nav1.9.
What does deletion of Nav1.7 or Nav1.8 or Nav1.9 lead to?
Reduces inflammatory pain.
In which neurons are Nav1.7, Nav1.8 and Nav1.9 expressed?
Peripheral DRG neurons.
In which neurons are Nav1.1 and Nav1.2 expressed?
GABAergic interneurons.
What is the mechanism of action for anticonvulsants?
Block Nav channels or enhance inactivation.
What does loss-of-function mutations of Nav1.1 and Nav1.2 lead to?
Impairs the excitability of GABAergic inhibitory neurons, leading to neural hyperexcitability and seizures.
How do mutations of Nav1.5 lead to cardiovascular diseases?
Gain-of-function mutations prevent channel inactivation, generating persistent sodium currents which underlies Long QT syndrome.
Loss-of-function mutations can cause arrhythmia.
How does an epilepsy-causing channelopathy point mutation affect Nav channels?
Impairs inactivation of the Nav channel, leading to an increased persistent sodium current (INaP) and excessive sodium influx at steady state leading to hyperexcitability.
