Skeletal muscle channelopathies L1

Question	Answer
What do ion channels determine?	Tissue excitability
Which four groups of tissue can ion channel dysfunction affect?	1) Neurological 2) Neuromuscular 3) Cardiac 4) Epithelial and mucosal tissues
What is a channelopathy?	Inherited diseases caused by mutations in ion channels.
Which ion channel is responsible for the positive upstroke in skeletal muscle AP?	VG Na v1.4
Which channel is responsible for membrane repolarisation in skeletal muscle?	VG K+
Which channel is responsible for buffering of after potentials/maintaining repolarisation?	Voltage gated Cl-
Which channels are responsible for setting the membrane potential in skeletal muscle?	Inward rectifier K+ channels - 'leak' channels
Which channel plays a role in channelopathies but isn't mutated?	K+ leak channel
What two structures are required for the release of Ca2+ from the SR?	1) Sarcolemmal voltage gated Ca2+ channels/dihydropyridine receptor (DHPR) 2) SR ryanodine receptor RyR
Why is RyR called RyR?	Can be opened using ryanodine (plant based substance)
Describe the relationship between the end plate and sarcolemma. What are the key differences?	Physically continuous but different profile of protein expression. 1) nAChR expressed on endplate not sarcolemma 2) VG Na+ channels expressed on sarcolemma not endplate.
Describe the conduction of an AP at the neuromuscular junction.	1) Ach released by a motoneuron 2) Ach binds to nAChR 3) Triggers endplate potential 4) Action potential if enough VG Na+ channels are activated. 5) AP travels through T-tubule and depolarises DHPR
Where is Ca2+ released from in skeletal muscle? How?	1) Sarcoplasmic reticulum 2) AP depolarises DHPR causing conformational change. DHPR is physically coupled to RyR. Conformational change causes RyR to open releasing Ca2+.
What are the different channels expressed on the surface membrane of skeletal muscle and what are they responsible for? (3)	1) K+ leak channel - sets MP 2) Na/K-ATPase, maintains ion homeostasis. Accounts for 40% ATP consumption 3) Cl- channel, maintains negative potential following repolarisation.
Give four examples of channelopathies.	1) Myotonia (muscle stiffness) 2) Paramyotonia 3) Muscle weakness (periodic paralysis) 4) Malignant hyperthermia
What happens in myotonia? Why? What can help to alleviate the symptoms?	1) Uncontrolled repetitive APs resulting in involuntary contractions 2) Something is keeping the membrane depolarised 3) Warm-up phenomenon, gentle & repetitive exercise helps relax muscle
What is paramyotonia? How does it differ to myotonia?	1) Paradoxical myotonia 2) Worsens with exercise
Describe periodic paralysis/what causes it.	Lack of APs and excitability
What causes malignant hyperthermia?	Abnormal Ca2+ release by mutated RyR. Increased metabolism of Ca2+ (as increased Ca2+) results in hyperthermia.
What are the three steps in determining the cause of a disease/channelopathy?	1) Genetic linkage studies 2) Animal Models of disease 3) Structure-function studies
What are the two parts of genetic linkage studies?	1) Family pedigrees 2) Chromosomal sequence associated with disease ie identify individual with conditions. Compare genetic make up of affected individual with non-affected and affected family members to determine chromosomal linkage.
How can animal models of disease be used?	To determine/build up ideas about electrophysiology - ie record from skeletal muscle of affected animal such as fainting goat.
Once the gene causing the condition has been identified what can be done? What can this be used to study?	1) Transfect gene into cell line to perform studies of mutation 2) Channel kinetics - ie slow deactivation or faster recovery from inactivation.
What causes myotonia in fainting goats? What is observed? (3)	Cl- channels. 1) No loss of consciousness 2) No change in BP 3) Stiffening/contraction of back and leg muscles
What are the two types of myotonia which are due to mutations in Cl- channels? What is the difference between the two?	1) Thomsen and Becker 2) Thomsen is an autosomally dominant condition whereas Becker is a recessive disease.
What phenomena do both types of Cl- myotonia show? What mutations are seen and where?	1) Warm-up phenomena 2) Mis-sense and non-sense mutations in CIC-1.
What does CIC-1 exist as? How does this affect mutation patterns?	1) Dimer 2) Dominant - only one allele is affected but that mutation cause dysfunction. Recessive - both alleles have to be effected for mutation to cause dysfunction.
What is CIC-1?	Skeletal muscle Cl- channel.
Where would you expect to the mutation in the goat CIC-1 channel?	Myotonic goat mutation (Ala-Pro) is in C-terminus
Where are the a helices of channels? What characteristic do they have?	1) Transmembrane spanning 2) Stable
What is the overall type of mutation in CIC-1? What does this result in/how does it cause myotonia? (3)	1) Loss-of-function 2) Reduced Cl- conductance 3) Lack of buffering of after potentials 4) New premature APs can be triggered causing myotonia.
What does the CIC-1 channel consist of? What does this mean for ion flow?	1) Two monomers - ie is a dimer. 2) Two possible pathways
What is cryo-electron microscopy?	Very high resolution electron microscopy.
How do Thomsen and Becker Myotonias affect ion flow?	1) Thomsen disrupts ion flow through either/or pathway 2) Becker disrupts flow through both pathways. Either way non-functional and no stabilisation of repolarisation.
Unlike most channels what do we not know about Cl- channels?	1) How the gating works 2) How ion selectivity arises
In terms of inheritance what are all Na+ channel myotonias? Why is this?	Autosomal dominant - Na+ channel exists as a monomer therefore only need one allele to be affected to impact physiology.
What are the five different types of Na+ channel myotonia?	1) Potassium-aggravated myotonia (PAM) 2) Paramyotonia congenita (PMC) 3) Hyperkalaemic periodic paralysis (HyperPP) 4) Hypokalaemic periodic paralysis (HypoPP) 5) Congenital myasthenic syndrome (CMS)
What helps PAM? What isn't it associated with?	1) Exercise 2) Weakness/paralysis
How does PMC differ to PAM? Why is this?	PMC is associated with short periods of weakness and is exacerbated by exercise. Not known.
What is hyperPP similar too? What triggers it? How does this differ to hypoPP? What are the associated recommendations?	1) PMC 2) Possibly high K+ rich food, as high K+ seen during periods of weakness. 3) K+ deficiency seen during periods of weakness in hypoPP. 4) Eat K+ rich foods for hypoPP and evade them for hyperPP
What is the problem in CMS? What can also cause this?	Defective neuromuscular transmission - abnormalities in nAChR
Describe the structure of an VG Na+ channel (2).	Consists of four subunits each made up of 6 subunits. 2) Intracellular N and C terminus
What forms the ion selective pore?	P loop between S5 and S6 of each domain.
Where is the voltage sensor? What role does the voltage sensor play?	1) TM4 of each subunit. 2) Moves/conformational change with depolarisation which allows the pore to open. Reverse is true with repolarisation
What is responsible for inactivating the VG Na+ channel? Where is it located? How does it do this?	1) Inactivation particle 2) Between subunits III and IV 3) Moves into pore
What are the key mutations which cause sodium channel myotonias and where are they located? What type of myotonia do these each cause?	1) Glycine at 1306 for Ala/val/glu on the P loop of the III domain (ie inactivating linker) - Potassium-aggravated myotonia 2) Arginine at 669 for His on TM4 of domain II - HypoPP
How was potassium-aggravated myotonia studied? What were the results? PART 1	1) Whole cell patch clamp, produced family of currents to various different depolarising voltages 2) In PAM gain of function mutation causes slower inactivation - 'gain of function' mutation
How was potassium-aggravated myotonia studied? What were the results? PART 2 What is the cause of this?	1) Single channel patch clamp recordings 2) wt channel opened once and was then inactivated. Mutated PAM channel showed multiple openings during depolarisation indicating that channels weren't inactivating allowing for multiple reopenings. 3) Mutations in 3-4 linker (inactivator) cause slow inactivation allowing channel to remain activated.

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Skeletal muscle channelopathies L1

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	Created by Anna Hogarth about 8 years ago