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Sodium Channel Structure,

Function, Gating and Involvement

in Disease

David R. Marks, M.Sc.

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An Overview

• Sodium Channel Structure

- Current theory and Types of Na+ Channels

• Sodium Channel Function- Current theory of inactivation

- Modulation

- Pharmacology- Activation

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An Overview Cont’d

• Article 2 – Na+ Channel Gating

• Article 1 - Na+ Channels and

Neurodegenerative Disease

• Article 3 – Na+ channel mutation and

physiology

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Sodium Channels - Structure

• Composed of α, β-1 and β-2 subunits, but the large α-subunits carries most of the functional properties

• 4 repeated motifs, each with 6 transmembrane domains

• All linked together

• Contain a voltage “sensor”/ligand binding domain(method of activation)

• The hydrophobic S4 segment (voltage “sensor”) is foundin all voltage gated Na+ channels and is absent in ligand

gated Na+ channels• Selectivity filter (shell of hydration)

• Inactivation gate

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Cartoon representation of the “typical” voltage-activated sodium channel

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Types Of Na+ Channels

• Voltage gated – Changes in membranepolarity open the channel

• Ligand gated (nicotinic acetylcholine

receptor) – Ligand binding alterschannel/receptor conformation and opensthe pore

• Mechanically gated (stretch receptor) – Physical torsion or deformation opens thechannel pore

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Sodium Channels - Function

• Play a central role in the transmission of actionpotentials along a nerve

• Can be in different functional states (3)

-A resting state when it can respond to adepolarizing voltage changes

-Activated, when it allows flow of Na+ ionsthrough the

-Inactivated, when subjected to a“suprathreshold” potential, the channel will notopen

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The theory is

that the inactivation gate

“swings” shut, turning off

the channel

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Please Keep In Mind

• The structure of the Na+ channel is not

100% solved, hence a “working model” is

drawn based on biophysical,

pharmacological, physiological andmolecular assays

• Zhao (2004) writes “The mechanism of

opening and closing is unknown, butstructural studies suggest…”

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Na+ Channel Modulation

• Phosphorylation

• sodium channel function is modulated byserine/threonine and tyrosine kinases as well as tyrosinephosphatases (Yu et al , Science 1997)

• Mutation – altered amino acid sequence/structure canchange the biophysical properties of the Na+ channel

• Pharmacology – block Na+ channel to reduce theconductance

• Proteolysis- (cleavage) Proteases may cleave specificresidues or sequences that inactivate a channel, orsignificantly alter the biophysical properties

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Why Na+ Channels/Modulation Are

Important• Neuronal depolarization, Action Potential

• Neuronal Excitability

• Cardiac Excitability

• Muscle Excitability• The basis of neuronal/cardiac/muscular functionrelies on the propagation of action potentials,down axons, sarcolemma, myocardium, as wellas requiring synaptic transmission.

• Differential excitability alters the electricalconduction/transmission properties of the“circuit”

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Na + Channel

Blockers/Pharmacological Agents

• Tetrodotoxin (TTX)

• Amioderone

• Lidocaine• Procainamide

• Mexilitine

• Ketamine• Many, many others

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Some Na+ Channels Outside The

Nervous System

• Naf – “Funny Current” in pacemaker cells

of the heart (SA node/ectopic

pacemakers)

• Nav in the myocardium, sarcolemma, and

T-tubules and motor endplate

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Na+ Channel Activation

• Change in transmembrane potential results in aconformation change in the Na+ channel

• The four S4 segment alpha helices translocate, thusleading to the opening of the channel pore

• The energy of the conformational change in the channelduring activation is mediated by the reduction in overallentropy of the system.

• The voltage sensor is a highly charged sequence ofamino acids that “aligns” itself according to the electricalfield present

• A change in transmembrane potential createsunfavorable electrodynamic interaction for the voltagesensor, hence a conformational shift lowers the energyof the system and creates more favorable conditions

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Patch Clamping/Transfection

Transfection

1. Kv1.3 cDNA in Plasmid

2. Lipofectamine complexing

3. Add to Dishes

4. Patch 28-48 hrs after

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Transition: A General Overview of

Articles Before Discussion

• From Basic structure/function relationships

to a gating mechanism

• The gating of a bacterial Na+ channel and

application of Na+ channel activation and

biophysical properties

• Article 1 – A gating hinge in Na+ channels:

a molecular switch for electrical signaling

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Proposed conformational shift of A-helix caused by substitution of Proline for G219

Prolines in alpha helices after the first turn (4th residue) cause a kink in the helix.

This kink is caused by proline being unable to complete the

H-bonding chain of the helix and steric or rotamer effects that keep proline fromadapting the prefered helical geometry

Conserved glycine

In the S6 domain

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Na+ Channel Gating

• Current theory holds that a change intransmembrane potential “flips” the conformationof the voltage sensor, thereby opening thechannel pore

• A mutation, G219P, glycine 219 changed toproline alters the conformation of the S6 domain

• The mutant channel now favors a state much

like the “open” state of a wild-type channel• NOTE: these bacterial Na+ channels are

homotetramers of identical subunits

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Mutation alters the biophysical properties of the channel

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The G219P mutant activates significantly earlier (activates at much more

negative voltages) than the wild-type

V ½ : Voltage at which ½ of channels present are in the open state

Comparable to Km in that it is a measure of the ability of a channel to activate

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Other mutations to the Na+ channel

Do not exert as significant effects in the

activation (V ½)

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Influence of hybrid Na+ channel subunits on gating and biophysical properties

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Article 2 - Na+ Channels And

Neurodegenerative Disease

• Overview – Multiple Sclerosis (MS)

displays a remission-relapse course.

Some axons are able to maintain minimal

conduction velocity, while othersdegenerate completely.

• Definition: Experimental autoimmune

encephalomytis (EAE) – animal model ofMS

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MS can display remission-relapsing

course. This is believed to be the

result of the expression of two

distinct isoformsof voltage-gated Na+ channels

NaV 1.2/1.6 are expressed over

long distances (> 10μm)

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B-amyloid are pepties associated with neurodegenerative diseases, and can accumulate

in fibrillar aggregates

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What is Important About This

Article

• Nav 1.6 is colocalized with a Na/Ca

exchanger

• Nav

1.2 is NOT colocalized with B-amyloid

proteins

• Nav 1.2 help restore conduction in

demyelinated axons

• Nav 1.6 is seen in degenerating axons

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An increase in

NaV1.6 yields an

Increase in Na/Ca

exchangers, elevating

intracellular Ca2+

to harmful levels

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Article 3 - Na+ Channels and the

Conduction System of the Heart

• Long QT syndrome – disease where the entire

cycle of excitation-contraction coupling of the

myocardium is prolonged

• Patient had G-A substitution at codon 1763 ofthe Nav 1.5 channel gene, which changed a

valine (GTG) to a methionine (ATG)

• This mutation produced a persistently active and

fast recovering Na+ channel

• Mutant was INSENSITIVE to lidocaine

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Article 3

• Authors generated a similar mutant by site-directed mutagenesis

• Examined the mutant in a heterologous

expression system to obtain biophysical and

other properties

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The Nav 1.5 V1763M mutant is

Sensitive to TTX, but resistant to

lidocaine

TTX eliminates lidocaine-insensitive current

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Why this is important:

Other than traumatic cardiac arrest,

arrhythmias degenerate into ventricular

fibrillation or ventricular tachycardias.

“circus movement” whereby tissuebecomes “hyper -excitable”

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Extension and Application of Na+

Channel Properties and FunctionRelating to Article 3

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Advanced Cardiac Life Support

(ACLS) Targets Na+ Channels Extensively

• “Please Shock Shock Shock, Everybody

Shock, And Lets Make Patients Better”

The purpose of defibrillation of ventricular arrhythmias is to apply a controlled electricalshock to the heart, which leads to depolarization of the entire electrical conduction

system of the heart. When the heart repolarizes, the normal electrical conduction

may restore itself

Depolarization theoretically inactivates all voltage-gated Na+ channels, and allows

Voltage-gated potassium channels to activate, and help hyperpolarize the membrane

+40 mv

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+40 mv

-70 mv

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After Administration

Of Procainamide

V FIB/V TACH

After phosphorylation/

phosphate cleavage

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• Use-dependent block of sodium channels.

• Blocks potassium channels.

• Blocks alpha-adrenergic receptors.

• Blocks muscarinic receptors.

• Used to attempt to terminate persistent reentrantarrhythmias

• Reduces automaticity of ALL pacemakers (boththe SA node and ANY tissue capable ofgenerating a pacemaker potential)

• Slows Down Conduction of depolarization in ALL

tissues of the heart and decreases cardiacexcitability

• This is your last resort. Giving this drug maystop the arrhythmia, but make it almost

impossible for the heart to spread impulses after

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Summary For the Lecture

• Na+ channels are comprised subunits, the Alpha of 4repeating motifs, each motif with 6 transmembranedomains

• There are voltage, ligand, and mechanically-gated Na +

channels• Na+ channels are involved in the depolarization of

excitable membranes

• Na+ channels have multiple modalities of modulation,which can alter neuronal/membrane excitability

• Na+ channels are the target of a multitude ofpharmacological agents

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Summary

• Na+ channels Are involved in the

remission-relapse of MS

• Na+ channel gating can be significantly

affected by modulation (phosphorylation,

mutation, proteolytic cleavage)

• Mutation in Nav 1.5 is implicated in Long

QT syndrome, generating persistent andslow inactivating sodium current

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