Potassium channels

Question 1

What is the basic structure of a potassium channel?

Answer 1

• Four similar protein subunits come together to form a single por for potassium permeation

Question 2

Describe the nature, structure and role of “A-type” K+ channels

Answer 2

Nature of current:

• activated when membrane of the cell starts to depolarise
• after activation the current rapidly declines because channel inactivates quicky due to ball and chain mechnaism
• Some experiments now suggest that although the ball region interacts with the channel pore, it may induce a channel conformational change, rather than having a plugging mechanism
• In either case, residues forming part of the channel pore interact with the protein’s N-terminal region
• Peak of current gets bigger as potential inside cell increases as more channels open and there is also a greater force driving potassium out of the cell

Structure:

• each subunt contains six helical membrane-spanning regions (S1-6)
• Fourth transmembrane element contains positive charges –> voltage sensor
• Pore formed by S5 and S6 (P-loop connects them)
• P-loop contains signature sequence (TXGYGD) –> glycine residues always conserved but there are a number of variations in this sequence

Function

• Translates intensity to frequency
• At relatively negative potentials type A become activated (after repolarisation and closure of delayed rectifiers)
• i.e. they open in response to the stimulus before the cell reaches its action potential threshold :.exerts hyperpolarising effect
• effect transient due to rapid inactivation
• the bigger the stimulus the faster it overcomes the ‘A’ current and hence more frequent action potential
• ‘A’ type channels recover from their inactivation when the cell membrane potential becomes strongly hyperpolarised again (by delayed rectifier channels)

Question 3

Describe the nature, structure and function of delayed rectifiers

Answer 3

Nature

• activates more slowly than ‘A’ type channels and only inactivates after depolarisation after many seconds

Structure

• S1-S6 with P loop and voltage sensor

Function

• helps reset the membrane potential following an action potential
• current is always larger at positive potentials than at negative potentials
• there is a delay following membrane depolarisation before the current appears

Question 4

Describe the nature, structure and function of inward rectifiying channels (Kir)

Answer 4

Nature

• Increased with hyperpolarisation
• inward current is higher than outward current because the channel is being blocked by Mg and positively charged intracellular polyamines such as spermine and spermidine
• Strong rectifiers; will not affect AP much but can set rest potential
• Weak rectifiers; can set resting potential also decreases the length of an AP as more potassium can come out

Function

• help set resting potential
• some inhibited by ATP (KATP) and associated with a second type of protein called sulfonylurea (SURs)
• others coupled to G proteins and responsible for slowing heart rate (KACh)
• More recently: Kir KO in endothelial cell decreased endothelial dependent vasodilation

Structure

• Two membrane spanning elements (M1 and M2)
• Pore between them

Question 5

Describe the nature, structure and funcition of two pore potassium channnels

Answer 5

Nature:

• Leak type currents
• Did not activate or inactive with changing membrane potential
• Able to open at all potentials
• More current flows in the outward direction and recently this has been shown to be due to the ion permeation pathway itself
• Weakly outward rectifying
• Activated by pH changes and membrane stretch following cell volume changes
• Also activated by volatile anaesthetics
• TRESK –> mutated in inherited form of migraine

Structure

• Look like two inward rectifiers sewn together
• Four membrane spanning domains and two pore loops (M1-M2, M3-M4)
• There is also a cap region which forms the above channel pore so K+ ions have to enter under the cap from the sid
• Both P loops contribute to functional pore :.only two subunits needed to form this channel

Question 6

Describe the nature, structure and function of BKCa channels

Answer 6

Nature

• becomes more active as the membrane is depolarised or as intracellular calcium rises
• Often acts as a negative feedback element in reponse to raised intracellular calcium
• E.g. at certain synaptic terminals the channel is activated by the membrane depolarisation and subsequent influx of calcium which trigger NT release
• The activation of BKCa channels thus helps to repolarise the membrane and turn off calcium channels, ending a cycle of exocytosis

Structure

• Six TM
• Additional S0 element
• Very extended C region
• C region contains two RCK domains
• RCK2 has string of AA containining many negative charges forming a “calcium bowl” which cradles a calcium ion in the crystal structure of this region

Question 7

Describe the nature, structure and function of SKCa channels

Answer 7

Nature

• NOT activated by voltage (unlike BKCa)
• Start to open only in response to calcium

Structure

• 6 TM
• S4 NOT have all the positive charges associated with S4 region of voltage gated channels

Function

• Sometimes provide “after-hyperpolarisation” following influx of calcium that occurs during action potential
• NMDA receptor activity regulation and hence formation of memory –> Ca2+ influx through synaptic NMDA receptors can activate SK channels which then hyperpolarise the cell and thus restore Mg block of the NMDA receptor
• SK channels also appear to have very important roles in endothelial cells and in cardiac tissue (where they regulate blood pressure and cardiac rhythm)

Question 8

Describe the structure, function and nature of IKCa channels

Answer 8

Nature

• relative of SKCa (6TM with few charged residues in S4)

Function

• Causes reduction in RBC volume as potassium ions are lost from the cell along with chloride ions and water

Question 9

What is the calcium activating mechanism for IKCa and SKCa?

Answer 9

• Mediated by a calmodulin molecule constituvely bound to the intracellular C-terminal segment of the protein
• Sometimes referred as beta subunit

Question 10

Describe the nature, structure and function of sodium activaed potassium channels

Answer 10

Nature

• Slack and Slick form subunit for this channel
• Activated by increased intracellular levels of sodium
• Closest relative is BKCa
• Large conductance

Structure

• Two RCK domains at C terminal
• NO calcium bowl
• S1-S6

Function

• Protective role in event of ischaemia
• Intracellular sodium concentration increases as a result of inhibition of the Na/K ATPase
• Current produced tends to be inwardly rectifying

Question 12

Name and describe KATP channel openers

Answer 12

1. Diazoxide - hypertension
2. Nicorandil - prevention and treatment of angina
3. Glibenclamide - target K_NDP in smooth muscle
4. Cromakalim - target K_NDP in smooth muscle - lowers blood pressure w/o big effect on blood glucose (unlike diazoxide)
5. Minoxidil - hypertension –> potent, long-lasting vasodilator –> causes reflex tachycardia and fluid retention :. administered with beta blocker and diuretic :.used in patients with severe hypertension resistant to other drugs –> used in male pattern baldness

SE: headache

Question 13

Describe K_ATP channel blockers

Answer 13

• Glibenclamide and tolbutamide are SURs and act via the suplhonylurea receptor subunt of K_ATP channels
• Block pancreatic B-cell and produce similar effects of raised intracellular ATP
• Leads to increase in cell excitability and promotes secretionof insulin
• Treatment for mild forms of type 2 diabetes

Question 14

Describe some BKCa openers

Answer 14

1. Soyasaponins activate BKCa channels
2. Dehyrdrosoyasaponins I require presence of BKCa channel beta subunits for activity
3. Leads to increased channel open probability
4. Other BKCa openers: MaxiKdiol
5. Developed with intention of treating conditions such as urinary incontinence, overactive bladder, asthma and stroke
6. None have reached beyond phase III

Question 15

Describe some SKCa enhancers

Answer 15

In some rat models of epilepsy drugs that enhance the activity of SKCa channels have proved effective in stopping epileptic discharge patterns

• they do not open channels
• prolong/enhance activation by calcium
• appear to bind at the interface between calmodulin and the channels :. difficulty in getting a selective drug due to sequence similiarity of this region

Treatment: hypertension as modulation of SKCa expression level in mice affects both arterial tone and blood pressure

Question 16

Describe some SKCa blockers

Answer 16

• Firing rate of dopaminergic neurones controlled by SKCa channels
• SKCa channels blockers can enhance firing rate :.increase dopamine release –> useful in treatment for Parkinson’s
• SK channels are present in a number of neurones and blocking them could risk inducing epileptic behaviour
• Some SK channels blockers can prevent or reduce arrhythmic activity in the heart
• genome wide association studies have linked a type of atrial fibtrillation (lone atrial fibrillation) to SK channels
• SK channels also modulate artial tone and blood pressure –> SK based treatment for arrhytmia can lead to hypertension

Question 17

Describe some Eag channel blockers

Answer 17

• Kv10.1 channels –> expressued in human tumours e.g. colon, breast, renal, etc whilst healthy tissue show much reduced expression levels
• Blocking Eag channel :.lead to apoptosis
• Overexpression of Eag can lead to cancer progression while inhibition reduces progression
• Eag2 –> upregulated in medulloblastoma
• Eag channels are translocated to the trailing edge of tumour cells where they facilitate migration and metastasis
• Antipsychotic thioridazine: inhibits Eag2 and slows medulloblastoma growth and metatasisin in a mouse model
• In a single human study –> treatment with thioridazine reduced size of original brain tumour
• Adverse side effects

Question 18

Why are Kv1.3 channels important?

Answer 18

• along with IKCa control efflux of K+ from human T-lymphocytes
• overexpression of Kv1.3 or IK channels restored anti-tumour activity
• shown to increase survival of tumour bearing mice
• Kv1.3 –> mitochondria in cancer cells
• mitochondria are involved in apoptotic pathway
• mitochondria are negatively charged :. accumulate inhibitors of mitoKv1.3 (TPP+) due to +ve charge
• Inhibition of mitoKv1.3 has been shown to selectively kill cancerous cells that overexpress mitoKv1.3 :. sparing non-cancerous cells

Question 19

Describe M-current openers

Answer 19

• M channels belong to Kv family (7.x) and are encoded by KCNQ2/3 genes
• Flupirtine: M-channel opener :. used as anelgesia
• Retigabine: opener :. epilepsy and neuropathic pain

–> was evaluated for postherpic neuralgia

–> binds to site formed at interface beween M-channel subunits –>acts to stabilise the open-channel conformation

Question 20

Describe M current blockers

Answer 20

1. Cognitive enhancers that may act via inhibition of potassim channels
2. In rodent lesion models of Alzheimer’s disease these drugs improve performance in animal water maze tests
3. In vitro - increase NT release
4. May interact with M-channel blockers

Question 21

What can be used to treat multiple sclerosis?

Answer 21

• Dalfampridine reported sig. improvements in ability of patients suffering MS to walk
• active ingredients: 4-aminopyridine (4-AP)
• Blocks many potassium channels

Question 22

Class III antidysrhymtics (hERG blockers)

Answer 22

• K+ channels subtypes expressed in cardiac muscle
• Together with Na+ and Ca2+ channels determine heart rate and AP shape
• Methanesulphonanilide group of III:
• block Kv11.1 channels (HERG)
• leads to AP prolongation, an increased refractory period and to a reduced likelihood of re-entrant arrhytmias
• Increase in AP by allowin more entry of more calcium can also have usefu inotropic action
• SE: cardiac toxicity of antihistamines terfenidine/astemizole has been attributed to block of thee cehhanl
• Block of inward rectifier channels –> important in controlling resting potential could play a part in antidysrhytmic action of class III drugs
• Proposed that K+ channel openers (acting on cardiac KATP) might have a useful role in cardiac dysrhytmia –> reduce AP duration :. treat long QT syndrome
• In other cases, potassium channel openers might be expected to be pro-arrhytmicy by reducing refractor period

Question 23

Immunomodulation

Answer 23

• Human T cells express Kv1.2 potassium channel
• Mice blockers of this channel inhibit T cell proliferation
• However mouse T cell may rely on a different repertoire of K+ channels

Question 24

Obesity, insulin resistance and more on pain

Answer 24

• Sh6K-186, a blocker of Kv1.3 channels –> reduced weight gain and adiposity, decreased blood levels of cholesterol –> also enhances peripheral insulin sensivity
• Changes are similar to effects seen when knocking out Kv1.3 gene
• However treatment of obesity with this type of blocker in man could have immunosupressive effects

Question 25

K2P channel openers

Answer 25

• Volatile anaesthetics activate members of the two pore potassium channel family
• Halothane is a potent activator of the TREK1 and TASK1 potassium channel while other more unusual anaesthetics such as xenon, NOS and cyclopropane activate only TREK1
• TREK1 KO mice challenged with some anaesthetic agents have been shown to have reduced senstivity (while there is no change in sensitivty to IV)
• TRESK mutation produced reduced channel activity and linked with familial form of migraina with aura
• suggests the possibility that activation of TRESK channels could be beneficial for migraine sufferes
• Loss of function mutations in TASK1 channel have been shown to cause human pulmonary hypertension

Question 26

K2P channel blockers

Answer 26

• Transgenic mice lacking gene responsible for TREK1 channels show depression resistant phenotype similar to mice treated with Prozac
• Fluoxetine –> SSRI –> blocks TREK1 channel but not at clinically relevant ranges
• TREK1 involved in suppressing the firing in serotonergic neurons, block or elimination of TREK1 current would upregulate serotonin release

Question 27

Sodium activated potassium channels

Answer 27

• High expression levels seen in sensory pain neurones so targeting these channels may be useful