Ion channels

On-demand webinar

Summary:

Ion channels are integral membrane proteins that selectively allow ions to flow across the cellular membrane. Ion channels play an important role in various cellular and physiological processes, including cell division and migration, secretion, programmed cell death, volume regulation, and electrical signaling.

Join Hannah Bishop for this two-part webinar series to learn everything you need to know about ion channels. In part one, she outlines an introduction to ion channels, and in part two, she discusses major ion channel targets within four disease areas, such as epilepsy, pain, stroke, and cardiac arrhythmias.

Webinar objectives:

Part 1

General ion channel function and classification schemes

Ion channel function in the context of nervous system physiology

The ion channel characteristics that physiologists care about

Part 2

Examine major ion channels involved in the pathology of four specific disease areas: epilepsy, pain, stroke, and cardiac arrhythmias

Ion Channel

Video Transcript

00:00 - 00:11: Hello and thank you for joining Abcam’s webinar, An Introduction to Ion Channels, presented
00:11 - 00:12: by Hannah Bishop.
00:12 - 00:18: Hannah received her PhD in neuroscience at UC Davis, where she worked within the lab
00:18 - 00:24: of James Trimmer and studied the KV2 family voltage-gated potassium channels in mammalian
00:24 - 00:25: brains.
00:26 - 00:32: Hannah also received her postdoc in the Institute of Neuroscience at the University of Oregon.
00:32 - 00:37: Within the lab of Chris Neal, she studied neural circuits that mediate prey capture
00:37 - 00:38: behavior in mice.
00:38 - 00:41: Thank you and I hope you enjoy.
00:41 - 00:44: Hi, my name is Hannah Bishop.
00:44 - 00:49: I am a scientific support specialist here at Abcam with a background in ion channels.
00:49 - 00:53: And today I’m going to be giving two webinars on the topic of ion channels.
00:53 - 00:56: The first will be a brief introduction to ion channels.
00:56 - 01:00: So talk a bit about what are ion channels, what are the different types, what purpose
01:00 - 01:04: do they serve physiologically, and I’m going to have a special emphasis on their role in
01:04 - 01:05: the nervous system.
01:05 - 01:09: And then the second webinar, I’ll focus on a few specific ion channels that are important
01:09 - 01:12: within four different disease areas.
01:12 - 01:17: So with that, let’s get started.
01:17 - 01:21: So our objectives for this first webinar are, first of all, general ion channel function
01:21 - 01:23: and classification schemes.
01:23 - 01:28: Number two, ion channel function in the context of nervous system physiology specifically.
01:28 - 01:36: And finally, the ion channel characteristics that physiologists really care about.
01:36 - 01:38: So what are ion channels?
01:38 - 01:44: Ion channels are hydrophobic pore-forming proteins that allow the flow of inorganic ions across the membranes.
01:44 - 01:47: These are ions like sodium, potassium, calcium, chloride.
01:47 - 01:51: And they’re distinct from simple aqueous pores in a number of ways.
01:51 - 01:54: First of all, they’re selective for specific ions.
01:54 - 01:56: They’re not just letting anything through.
01:56 - 02:02: They’re selective for a specific ion or a group of ions.
02:02 - 02:04: And that’s what’s being shown here in this image.
02:04 - 02:09: The image is showing potassium and sodium ions in the vestibule of the channel pore,
02:09 - 02:13: which is what you’re seeing in A, and then in the selectivity filter of the pore as shown
02:13 - 02:18: in B. And in the vestibule, you can see the ions are hydrated.
02:18 - 02:20: They’re surrounded by water molecules.
02:21 - 02:25: And because these ions are different sizes, sodium being smaller than potassium, the exact
02:25 - 02:29: spacing of those water molecules is different between the two different ions.
02:29 - 02:33: You can see that there in A. You can see the spacing of the red water molecules is different
02:33 - 02:35: between the two.
02:35 - 02:40: And to confer selectivity, there is this beautifully evolved structure to the selectivity filter
02:40 - 02:42: inside of the channel pore.
02:42 - 02:48: The carbonyl oxygens within the selectivity filter are placed precisely to accommodate
02:48 - 02:49: a dehydrated ion.
02:49 - 02:54: So the spacing of those carbonyl oxygens perfectly mimics the spacing of water molecules in the
02:54 - 02:56: solvation shell.
02:56 - 03:01: And because the sodium ion is too small to interact with the oxygens, it could enter
03:01 - 03:05: the selectivity filter only at great energetic expense.
03:05 - 03:06: So you can see that here.
03:06 - 03:11: The sodium ion is unable to interact with all of those carbonyl oxygens.
03:11 - 03:16: And so that means that it’s much more likely that a potassium ion is going to flow through
03:16 - 03:17: this channel.
03:17 - 03:21: So it’s much more energetically favorable for a potassium ion to flow through the channel
03:21 - 03:26: than for a sodium ion to go through the channel, even though the sodium ion is a lot smaller.
03:26 - 03:31: So the filter is selecting for potassium ions with very high selectivity.
03:31 - 03:33: And we see this across ion channels.
03:33 - 03:39: They have high selectivity for one specific ion or a group of ions.
03:39 - 03:43: Number two, these channels are not continuously open.
03:43 - 03:47: They’re not just standing open, allowing anything to pass the membrane.
03:47 - 03:48: They are gated.
03:48 - 03:50: They can be opened and they can be closed.
03:50 - 03:57: And various different stimuli can cause a channel to open or close.
03:57 - 04:01: So ion channels can be classified by this gating mechanism, by the stimuli that cause
04:01 - 04:04: them to either open or close.
04:04 - 04:11: Many ion channels, most sodium channels, most potassium channels, calcium channels, some
04:11 - 04:13: chloride channels are gated by voltage.
04:13 - 04:18: So they are opened or closed by changes in the membrane potential.
04:18 - 04:26: Others, certain potassium and chloride channels, TRP channels, ryanodine receptors, IP3 receptors,
04:26 - 04:32: these ion channels are relatively voltage insensitive, but they’re gated by second messengers
04:32 - 04:35: or other intracellular or extracellular mediators.
04:35 - 04:37: And we would call these ligand-gated ion channels.
04:38 - 04:41: These are also the classic neurotransmitter receptors.
04:41 - 04:44: Many of those are ligand-gated ion channels.
04:44 - 04:48: So the neurotransmitter is binding extracellularly, that’s causing a conformational change in
04:48 - 04:54: the channel, which allows the channel pore to open and allow ions to flow.
04:54 - 04:57: And then there’s still other ion channels that are mechanically gated.
04:57 - 05:01: So it’s actual mechanical force that causes the opening of the channel and the subsequent
05:01 - 05:04: flow of ions across the membrane.
05:04 - 05:09: And these include the stretch receptors that are actually found in our skin that transduce
05:09 - 05:14: mechanical stimulation on the surface of our skin into the flow of ions through the channel
05:14 - 05:21: membrane and basically transducing that signal into an electrical signal that the nervous
05:21 - 05:23: system can actually understand and process.
05:23 - 05:28: This is how we sense touch.
05:28 - 05:33: Continuing in the vein of ion channel classification, this is a representation of the amino acid
05:33 - 05:38: relationships of the voltage-gated ion channel superfamily, which really should be known
05:38 - 05:43: as the voltage-gated ion channel-like superfamily.
05:43 - 05:47: They’re grouped together because of very high similarity of their primary structure.
05:47 - 05:50: These channels are thought to have evolved from a common ancestor, and therefore they
05:50 - 05:52: have been classified together.
05:52 - 05:57: But not all of the channels here in this dendrogram are actually voltage-gated, but they’re structurally
05:57 - 05:59: similar to those that are.
05:59 - 06:03: This is the third largest family of signaling proteins after G-protein coupled receptors
06:03 - 06:04: and protein kinases.
06:04 - 06:09: And you can see from the dendrogram that there’s a very wide range of diverse ion channel members
06:09 - 06:14: in just this voltage-gated ion channel superfamily, which doesn’t contain a whole other plethora
06:14 - 06:16: of ion channels that exist.
06:16 - 06:22: So there’s a ton of diversity within the realm of ion channels.
06:22 - 06:26: In addition to classification of ion channels via their primary structure, ion channels
06:26 - 06:30: can also be classified by the number of subunits they have.
06:30 - 06:35: So for example, voltage-gated potassium channels are formed by four separate polypeptides that
06:35 - 06:38: come together to form a functional channel.
06:38 - 06:43: Voltage-gated sodium channels, on the other hand, are one long polypeptide that forms
06:43 - 06:44: a functional channel.
06:44 - 06:46: This also applies to ligand-gated channels.
06:46 - 06:50: So there are ligand-gated channels that are trimeric, P2X receptors are that way.
06:50 - 06:54: There are ligand-gated channels that are tetrameric and pentameric.
06:54 - 07:00: Another mechanism by which people divide up, classify ion channels is the number of transmembrane
07:00 - 07:02: domains that they contain.
07:02 - 07:06: So this is the number of times the protein actually passes through the membrane.
07:06 - 07:10: And so when thinking about potassium channels, there are two transmembrane domain or two
07:10 - 07:16: transmembrane channels that include the inward rectifier potassium channels, four transmembrane
07:16 - 07:20: domain channels that include tandem pore domain channels, as they’re called.
07:20 - 07:26: These are the channels that underlie the leak current in most cells, and six transmembrane
07:26 - 07:34: domain channels, which are the voltage-gated and calcium-activated potassium channels.
07:34 - 07:39: With all this channel diversity, what role are ion channels actually playing physiologically?
07:39 - 07:45: Well, the ability to control ion fluxes through these channels is essential for many, many,
07:45 - 07:46: many, many cell functions.
07:46 - 07:50: You’re going to see ion channels throughout the body and cells everywhere.
07:50 - 07:54: We’re going to, again, focus today more on the nervous system, but ion channels are incredibly
07:54 - 07:57: important for a lot of cellular functions.
07:57 - 08:01: They’re important for cell cycle regulation and therefore cell proliferation.
08:01 - 08:04: They’re important in the context of cell migration or motility, which is actually the example
08:04 - 08:06: you’re seeing here.
08:06 - 08:11: This is the signal transduction cascade that allows sea urchin sperm to actually change
08:11 - 08:15: direction and perform chemotaxis towards the egg.
08:15 - 08:20: And so you can see this is essentially a cascade activating a number of ion channels that
08:20 - 08:27: actually allows the flagellum of the sperm to turn, allowing the sperm to turn and change
08:27 - 08:30: direction and move towards the egg.
08:30 - 08:35: So cell migration and motility, fluid transport, and therefore cellular volume regulation,
08:35 - 08:42: programmed cell death, secretion, immune cell activation, muscle contraction, so excitation
08:42 - 08:47: contraction coupling that allows us to move our muscles, and in the nervous system, signaling
08:47 - 08:51: in the form of action potential propagation and neurotransmitter release.
08:51 - 08:57: All of these things are mediated by and depend on ion fluxes through ion channels.
08:57 - 09:03: They’re incredibly important in cells throughout the body.
09:03 - 09:06: As most of what I’m going to discuss in the second webinar is going to focus on the role
09:06 - 09:11: of ion channels in the context of the nervous system, I’m going to give a brief refresher
09:11 - 09:15: on how neurons use ion channels to process signals.
09:15 - 09:19: So neurons signal via both electrical and chemical signals.
09:19 - 09:23: Within neurons, signaling is primarily electrical.
09:23 - 09:27: And then between neurons, signaling is primarily chemical.
09:27 - 09:33: The electrical signals that flow within neurons are mediated by transient changes in the neuron’s
09:33 - 09:38: membrane potential, which is the electrical potential difference across the cell’s membrane.
09:38 - 09:43: And remember, neurons are, all of our cells are sitting at rest around typically around
09:43 - 09:48: negative 70 millivolts, so the inside of the cell is negatively charged with respect to
09:48 - 09:54: the outside of the cell, and that’s considered rest for cells and for neurons.
09:54 - 09:58: And so these neurons receive signals from other cells in the form of neurotransmitters
09:58 - 10:01: that are depicted here in yellow.
10:01 - 10:04: These are chemicals like dopamine, serotonin, glutamate.
10:04 - 10:08: The cell receives these chemical signals, usually out in these processes known as the
10:08 - 10:12: dendrites, and it converts them into an electrical signal.
10:12 - 10:16: And so, you know, the neurotransmitter gets released, and I mentioned that many neurotransmitter
10:16 - 10:19: receptors are themselves ligand-gated ion channels.
10:19 - 10:26: So that neurotransmitter, glutamate, serotonin, dopamine, whatever it is, binds to a receptor,
10:26 - 10:29: and that causes a conformational change that opens an ion channel.
10:29 - 10:35: And so now you’ve got the flow of ions into the cell, and most of these channels are allowing
10:35 - 10:39: the flow of sodium into the cell, and that’s going to make the inside of the cell slightly
10:39 - 10:40: more positive.
10:40 - 10:44: It’s going to raise the membrane potential.
10:44 - 10:50: And these electrical signals on the dendritic side, they propagate passively through the
10:50 - 10:51: neuron.
10:51 - 10:57: So from the site of where the neurotransmitter was released and activated those receptors,
10:57 - 11:01: you get a slight depolarization, and those slight depolarizations flow passively.
11:01 - 11:04: So they actually attenuate in space and time through the dendrites.
11:04 - 11:08: You could think of it as kind of ripples on a pond.
11:08 - 11:16: So there they are moving through the dendrites towards the cell body, the soma of the neuron,
11:16 - 11:21: and towards, importantly, the axon hillock or the axon initial segment.
11:21 - 11:27: And if those inputs are timed correctly, if they’re large enough, when they reach the
11:27 - 11:33: axon initial segment, and if they can force the membrane at that part of the cell to reach
11:33 - 11:38: a certain threshold, then this neuron will fire its own action potential.
11:38 - 11:43: So this will be an all-or-nothing electrical signal that it’s going to send down its axon, and
11:43 - 11:45: this is an active process.
11:45 - 11:49: So the action potential does not attenuate in space and time.
11:49 - 11:53: It stays the same size from the axon initial segment all the way to the end of the axon
11:53 - 11:54: terminal.
11:54 - 12:01: And so this electrical signal will propagate very quickly to the end of the axon terminal.
12:01 - 12:08: And the whole point of this action potential, really, is to result in the secretion of neurotransmitter
12:08 - 12:10: at the end of the axon terminal.
12:10 - 12:16: So we’re going to turn this electrical signal back into a chemical signal that can then
12:16 - 12:22: direct receptors on the next neuron down the line.
12:22 - 12:30: So we’ve got chemical signaling between neurons, electrical signaling primarily within neurons.
12:30 - 12:37: The collective activity of the ion channels that are present on a neuron results in the
12:37 - 12:40: initiation and the propagation of electrical signals within the cell.
12:40 - 12:46: So since they are the mediators of these electrical signals in the cell, changes to
12:46 - 12:48: the expression level of the proteins.
12:48 - 12:52: How many of a given type of ion channel does the cell have?
12:52 - 12:54: Changes to the function of the protein.
12:54 - 12:58: So what kinds of stimuli will cause the channel to open or to close?
12:58 - 13:03: And changes to the localization of the protein on the cell.
13:03 - 13:07: So where are they found in the membrane of the cell?
13:07 - 13:09: When we’re thinking about neurons, are they out on the dendrites?
13:09 - 13:10: Are they in the axon?
13:10 - 13:12: Are they at the axon initial segment?
13:12 - 13:14: Where are they located?
13:14 - 13:21: Changes to these three features of ion channels will lead to altered propagation and altered
13:21 - 13:27: integration of electrical signals, and therefore, as we might say, altered excitability.
13:27 - 13:33: So how readily this cell can fire its own action potential in response to neurotransmitter
13:33 - 13:36: input onto its dendrites.
13:36 - 13:41: It’s going to depend on the kinds of channels it’s expressing, the function of those channels,
13:41 - 13:45: and where on the cell those channels are actually located.
13:45 - 13:50: So in order to understand how information is processed within individual neurons, and
13:50 - 13:54: therefore, in order to understand neuronal function and brain function, as researchers,
13:54 - 14:04: we need a detailed map of the abundance, the distribution, and the function of ion channels.
14:04 - 14:09: So to give a specific real-world example of how ion channel biologists study the expression,
14:09 - 14:13: localization, and function of ion channels, I’m going to talk a bit about my favorite
14:13 - 14:14: channel, KV2.1.
14:14 - 14:19: This is the channel that I studied as a graduate student at UC Davis in the lab of Dr. James
14:19 - 14:20: Trimmer.
14:20 - 14:24: So KV2.1, it’s a voltage-gated potassium channel.
14:24 - 14:29: So this channel opens in response to changes in the membrane potential.
14:29 - 14:32: And as you can see, it’s expressed in most neurons throughout the brain.
14:32 - 14:36: This is immunohistochemical stains of mouse, and then underneath we’ve got rat, ferret,
14:36 - 14:37: macaque, and human.
14:37 - 14:42: It’s very highly conserved, expressed throughout mammals, also expressed in invertebrates,
14:42 - 14:43: Aplysia.
14:43 - 14:47: And it’s very highly expressed in the majority of neurons in the brain.
14:47 - 14:53: Almost every neuron in the brain has at least KV2.1, and the neurons that have it express
14:53 - 14:56: it to a very high degree.
14:56 - 15:01: So it’s expressed in most neurons in the brain, and it’s a major generator of what we call
15:01 - 15:03: the delayed rectifier current.
15:03 - 15:06: So this is probably the best-known potassium current.
15:06 - 15:11: This is the outward flow of potassium following the depolarization of the cell.
15:11 - 15:17: So this is the classic, your cell is depolarized, you open sodium channels, sodium rushes in,
15:17 - 15:20: causes the membrane potential to become a little more positive.
15:20 - 15:25: And then we have a delayed rectification of that change in the membrane potential.
15:25 - 15:30: So there’s a delay in the opening of these potassium channels.
15:30 - 15:35: And once they open, potassium flows out of the cell, and that positive charge leaving
15:35 - 15:40: causes the inside of the cell to become more negative, and it drives the cell back down
15:40 - 15:41: towards rest.
15:41 - 15:47: So these are the delayed rectifier currents, and KV2s, KV2.1 in particular, is a major
15:47 - 15:54: generator of the delayed rectifier current.
15:54 - 15:59: Its expression is restricted to the soma, the proximal dendrites, and the axon initial
15:59 - 16:02: segment of the cells in which it’s expressed.
16:02 - 16:06: And it forms these beautiful, large micron-sized clusters on the cell surface, which you can
16:06 - 16:07: see here in the stain.
16:07 - 16:13: So the channel is actually preferentially sort of localized to these discrete clusters
16:13 - 16:16: on the cell surface.
16:16 - 16:22: Now, interestingly, work from Dr. Trimmer’s lab over the years demonstrated that KV2.1
16:22 - 16:28: channel localization to these clusters can be altered both in vivo and in vitro.
16:28 - 16:36: So in vivo, stimuli such as kainate-induced seizures in animals, hypoxia via CO2 exposure,
16:36 - 16:43: or global ischemia, all of these stimuli result in the lateral translocation of the channel
16:43 - 16:49: on the cell surface to this more diffuse expression pattern, localization pattern.
16:49 - 16:50: And it’s not…
16:50 - 16:54: The channel is still on the cell surface, so it’s present on the cell surface.
16:54 - 16:56: It’s just changing its localization.
16:56 - 16:58: It’s no longer in these discrete clusters.
16:58 - 17:01: And in vitro, it could induce the same kind of changes.
17:01 - 17:06: So in cultured hippocampal neurons, glutamate stimulation of cultured hippocampal neurons,
17:06 - 17:13: chemical ischemia, and stimulation of M1 muscarinic acetylcholine receptors, those neurons, all
17:13 - 17:18: result in the same lateral translocation of the channel across the surface of the cell
17:18 - 17:21: to this more diffuse expression pattern.
17:21 - 17:26: You’ll notice here in this particular staining, which will be important on the next slide,
17:26 - 17:31: here we’ve stained for total KV2.1 and general KV2.1 stain.
17:31 - 17:37: And then we’ve also stained using a phospho-specific antibody against the serine 603 site.
17:37 - 17:43: And so we’ve got this phospho-specific antibody, and you can see after CO2 exposure, this phospho-specific
17:43 - 17:45: signal goes away.
17:45 - 17:50: And this is because the channel has actually become dephosphorylated at that serine 603
17:50 - 17:55: site in response to these various forms of stimulation.
17:55 - 18:02: And the channel is actually covered in phosphorylation sites, primarily in particular on its very
18:02 - 18:05: long C-terminal tail, which is depicted here.
18:05 - 18:10: And so this change in channel localization, with this change in channel localization,
18:10 - 18:13: we also observe changes to the phosphorylation state of the protein.
18:13 - 18:18: And specifically, that’s dephosphorylation at a number of sites.
18:18 - 18:24: Additionally, those changes in the localization and in the phosphorylation state of the protein
18:24 - 18:29: occur alongside alterations in the voltage dependence of the channel’s gating.
18:29 - 18:32: So remember, this is a voltage-gated ion channel.
18:32 - 18:36: Its opening and closing depends on the membrane voltage.
18:36 - 18:41: And so when with these stimuli, so let’s say, you know, kainate-induced seizures or ischemia,
18:41 - 18:45: or if we’re thinking about cultured hippocampal neurons, which is in the case of this particular
18:45 - 18:51: graph, if you stimulate cultured hippocampal neurons, you get this left shift in the voltage
18:51 - 18:54: dependence of gating, which is what we’re seeing here.
18:54 - 18:56: And what does this left shift mean?
18:56 - 19:03: Well, we’re seeing the conductance as a function of the voltage, and we get a left shift.
19:03 - 19:07: And so our channel actually has some conductance.
19:07 - 19:12: It’s capable of conducting ions at more negative membrane potentials.
19:12 - 19:18: And so you could think of this as the channel is actually able to open earlier or faster.
19:18 - 19:19: It’s quicker in its opening.
19:19 - 19:25: So it’s opening sooner, and if we’re opening a potassium channel sooner, that means potassium
19:25 - 19:30: ions are going to be flowing out, and we’re going to be able to prevent the cell from firing.
19:30 - 19:36: Okay, so this shift in the gating ultimately leads to what we would call a homeostatic
19:36 - 19:40: suppression of excitability, and that’s what you’re seeing here in this figure here in D.
19:41 - 19:47: These traces depict firing of cultured rat hippocampal neurons either before or after
19:47 - 19:54: application of 5-micromolar glutamate during either a 20-picoamp, as in the top, or a 100-picoamp
19:54 - 19:55: current injection.
19:55 - 20:01: And as you can see, following 5-micromolar glutamate stimulation of these cultured hippocampal
20:01 - 20:05: neurons, we actually see a suppression of firing.
20:05 - 20:09: This firing of the cell decreases, which is not what you would expect.
20:09 - 20:11: Glutamate is an excitatory neurotransmitter.
20:11 - 20:15: You would expect throwing glutamate on some cultured hippocampal neurons should make them
20:15 - 20:16: fire really strongly.
20:17 - 20:22: Actually, we see the suppression of firing, and this effect was shown to be dependent
20:22 - 20:24: upon KV2.1, KV2s.
20:25 - 20:32: By applying the KV2 blocker Hanatoxin, we can see that there’s actually an increase in firing
20:32 - 20:33: following glutamate stimulation.
20:33 - 20:38: So if you include Hanatoxin in the bath now during your recording, and you apply that
20:38 - 20:43: 5-micromolar glutamate, now we actually see an increase in the firing activity.
20:43 - 20:49: So this suppression of firing that we were seeing before was due to the flow of potassium
20:49 - 20:54: ions through these KV2 channels, which are blocked here when Hanatoxin is included.
20:54 - 21:00: So there appears to be a relationship between the phosphorylation state, the localization,
21:00 - 21:02: and the function of this channel.
21:02 - 21:09: And KV2.1 is therefore capable of suppressing firing in the context of excitotoxicity.
21:09 - 21:13: So this is in the context of extremely high levels of neural activity that could actually
21:13 - 21:14: damage the brain.
21:14 - 21:17: So think about seizure activity or ischemia.
21:21 - 21:26: So what are these ion channel characteristics that physiologists care about?
21:26 - 21:27: Expression.
21:27 - 21:30: What channel types are expressed?
21:30 - 21:32: What are their sequence, and what is their structure?
21:32 - 21:37: There are several de novo KV2.1 mutations that have been identified as rare genetic
21:37 - 21:39: causes of infantile epilepsy.
21:40 - 21:43: Questions that we’re interested in are, you know, are these mutants expressed?
21:43 - 21:44: Do mutations affect function?
21:46 - 21:47: Then the function, obviously.
21:48 - 21:52: Do mutations or stimuli, such as high levels of glutamate stimulation, as in the culture
21:52 - 21:56: type of hippocampal neurons here, does that actually affect the gating of the channel?
21:57 - 21:58: And then finally, localization.
21:58 - 22:01: Where in the cell is the channel normally found?
22:01 - 22:06: Do mutations or other stimuli, such as excitotoxicity in the context of, say, ischemia,
22:06 - 22:08: affect this particular localization?
22:11 - 22:17: Now to zoom back out and remind you, KV2.1 is only one of hundreds of ion channels that
22:17 - 22:19: are present on neurons and other excitable cells.
22:19 - 22:24: And so therefore, as ion channel biologists and physiologists, there’s an incredible
22:24 - 22:29: amount of work to be done in order to understand neuronal function in the context of both
22:29 - 22:34: normal physiological conditions and as well as nervous system pathologies.
22:34 - 22:38: We need a detailed map of the expression, the localization, and the function of ion
22:38 - 22:39: channels.
22:41 - 22:43: So with that, thank you for your attention.
22:43 - 22:48: And in the next webinar, I’m going to discuss major ion channel targets within four major
22:48 - 22:48: disease areas.
22:49 - 22:54: And also be sure to check out our other neuroscience and ion channel resources on this
22:54 - 22:54: page.
22:54 - 22:55: Thanks.

Ion channels

Video Transcript

00:00 - 00:12: Hello and thank you for joining Abcam’s webinar, Ion Channels and Disease, presented by Hannah
00:12 - 00:13: Bishop.
00:13 - 00:19: Hannah received her Ph.D. in neuroscience at UC Davis, where she worked within the lab
00:19 - 00:25: of James Trimmer and studied the KV2 family voltage-gated potassium channels in mammalian
00:25 - 00:26: brains.
00:27 - 00:32: Hannah also received her postdoc in the Institute of Neuroscience at the University of Oregon.
00:32 - 00:37: Within the lab of Chris Neal, she studied neural circuits that mediate prey capture
00:37 - 00:38: behavior in mice.
00:38 - 00:42: Thank you and I hope you enjoy.
00:42 - 00:49: Hi, my name is Hannah Bishop and I am a scientific support specialist here at Abcam with a background
00:49 - 00:51: in Ion Channels.
00:51 - 00:55: And in this webinar, I’m going to highlight a few important Ion Channel targets within
00:55 - 00:58: four specific disease areas.
00:58 - 01:04: So I’m going to cover major Ion Channels that are involved in the pathology of epilepsy,
01:04 - 01:10: pain, stroke, and cardiac arrhythmias.
01:10 - 01:12: So I’m going to start with epilepsy.
01:12 - 01:16: Epilepsy is defined as a group of chronic brain disorders that are characterized by
01:16 - 01:17: recurrent seizures.
01:17 - 01:23: And these seizures are caused by excessive synchronized discharges of brain neurons.
01:23 - 01:26: And the lifetime prevalence of epilepsy is approximately 3%.
01:26 - 01:30: It affects almost 70 million people worldwide.
01:30 - 01:33: Seizures can be roughly divided into two different classes.
01:33 - 01:36: Generalized seizures affect both sides of the brain.
01:36 - 01:39: This accounts for about 40% of epilepsy patients.
01:39 - 01:43: And these are mostly genetic in origin.
01:43 - 01:47: Partial or focal seizures affect one part of the brain.
01:47 - 01:50: And these account for about 60% of epilepsy patients.
01:50 - 01:55: And these types of seizures can be caused by brain lesions, developmental malformations,
01:55 - 01:59: tumors, stroke, or genetic causes.
01:59 - 02:03: Hundreds of genes have been identified as contributing to epilepsy.
02:03 - 02:08: And mutations in Ion Channel genes account for a significant proportion of known genetic
02:08 - 02:10: epilepsies.
02:10 - 02:15: This pie chart here is from this excellent Pharmacological Reviews article by Julia Oyer
02:15 - 02:19: and colleagues on Ion Channels and genetic epilepsies.
02:19 - 02:27: And as it shows, mutations in Ion Channels contribute to about 27% of genetic epilepsies.
02:27 - 02:31: So what are a few of the most important Ion Channel targets in epilepsies?
02:31 - 02:36: Well, I have to start with NAV1.1 or SCN1A.
02:36 - 02:39: This is the most studied epilepsy gene.
02:39 - 02:44: Perhaps the most studied sodium channel mutations causing epilepsy are the SCN1A loss of function
02:44 - 02:48: mutations that cause either GEFS+ or Dravet Syndrome.
02:48 - 02:52: So GEFS+ is generalized epilepsy with febrile seizures.
02:52 - 02:56: So these are febrile seizures, seizures that are caused by fevers, and they tend to resolve
02:56 - 02:58: in adolescence.
02:58 - 03:00: And Dravet Syndrome is much more severe.
03:00 - 03:04: This is caused by missense mutations that lead to truncations of the channel.
03:04 - 03:09: And they’re characterized by multiple seizure types and developmental regression.
03:09 - 03:16: And as you can see here, NAV1.1 is expressed predominantly in GABAergic interneurons where
03:16 - 03:19: it’s enriched within axon initial segments.
03:19 - 03:23: And so it’s very important for the initiation and the propagation of action potentials within
03:23 - 03:24: these cells.
03:24 - 03:29: And as you can imagine, the impaired ability of GABAergic interneurons to actually fire
03:29 - 03:35: action potentials is going to reduce overall inhibitory drive within the brain, and that’s
03:35 - 03:41: going to allow excitatory cells to fire too much, which will lead to seizures.
03:41 - 03:46: Next up, I’d like to highlight NAV1.2 or SCN2A.
03:46 - 03:52: Now, interestingly, both gain-of-function and loss-of-function mutations have been described,
03:52 - 03:57: both resulting in epilepsy and a wide spectrum of seizure disorders.
03:57 - 04:02: And so since variants in the same gene can affect function in opposite directions, either
04:02 - 04:07: causing the channel to be overactive in the case of gain-of-function or underactive in
04:07 - 04:11: the case of loss-of-function, because of this, an accurate functional diagnosis is
04:11 - 04:15: necessary to determine the best treatment for patients.
04:15 - 04:21: So, for example, NAV or voltage-gated sodium channel blockers would actually exacerbate
04:21 - 04:25: the disease in someone with a loss-of-function mutation, but would be helpful in someone
04:25 - 04:28: with a gain-of-function mutation.
04:28 - 04:32: This is also one of the most prominent genes associated with neurodevelopmental disorders,
04:32 - 04:36: including autism spectrum disorders, intellectual disabilities, schizophrenia.
04:36 - 04:41: And it’s expressed predominantly in the axon-initial segment of excitatory neurons, and it’s been
04:41 - 04:45: proposed to play an important role in the backpropagation of action potentials back
04:45 - 04:49: into the soma and the dendrites.
04:49 - 04:53: Next up is NAV1.6 or SCN8A.
04:53 - 04:59: This is predominantly expressed in the axon-initial segment and nodes of Ranvier, and most epilepsy-causing
04:59 - 05:05: mutations are missense gain-of-function mutations, and this leads to an increased persistent
05:05 - 05:08: sodium current, and therefore, increased excitability.
05:08 - 05:14: If you’ve got sodium channels standing open and sodium ions are leaking in, that’s going
05:14 - 05:19: to depolarize your cell and maintain your cell in a more depolarized state, increasing
05:19 - 05:22: the likelihood that the cell will fire.
05:22 - 05:31: And then finally, I want to highlight KCNQ2 and KCNQ3, which encode the KV7.2 and KV7.3 channels.
05:31 - 05:34: These are voltage-gated potassium channels, and remember, voltage-gated potassium channels
05:34 - 05:39: are formed by four separate polypeptides that come together to form a functional channel,
05:39 - 05:48: and therefore, these KV7.2 and KV7.3 subunits can form HOMO and heterotetramers, and they’re
05:48 - 05:53: responsible for a current called the M current, and this is a potassium current that regulates
05:53 - 06:00: membrane potentials at subthreshold voltages, and that allows these potassium channels to
06:00 - 06:02: constrain repetitive firing.
06:02 - 06:08: And it’s called the M current because it is regulated by muscarinic acetylcholine receptor
06:08 - 06:10: activation.
06:10 - 06:19: And these KCNQ2 and KCNQ3, KV7.2 and KV7.3, co-localize at axon initial segments and nodes of Ranvier,
06:19 - 06:24: and mutations cause an autosomal dominant epilepsy called benign familial neonatal epilepsy
06:24 - 06:26: or BFNE.
06:26 - 06:28: It’s not a particularly severe disorder.
06:28 - 06:33: Mutations usually spontaneously remit in most patients within a few weeks to months
06:33 - 06:40: after birth, but also de novo missense KV7.2 mutations have been described, and they have
06:40 - 06:46: been shown to be one of the most common causes of early-onset epileptic encephalopathy, and
06:46 - 06:52: that includes Ohtahara syndrome, which is the earliest-appearing epileptic encephalopathy,
06:52 - 06:56: with seizure onset that occurs in the first three months of life and often in the first
06:56 - 06:58: 10 days of life.
06:58 - 07:04: So many of those have been traced to de novo KV7.2 missense mutations.
07:04 - 07:08: And for more information about these and other ion channel mutations that cause epilepsy,
07:08 - 07:16: check out this excellent Pharmacological Reviews article by Julia Oyer and colleagues.
07:16 - 07:18: So next up is pain.
07:18 - 07:23: So pain can be divided into three broad classes.
07:23 - 07:25: First is acute nociceptive pain.
07:25 - 07:30: So acute nociceptive pain is caused by stimulation of nociceptors, so these are pain-sensing
07:30 - 07:36: neurons, due to tissue injury or inflammation, and it is defined as having a short duration.
07:36 - 07:42: So this is pain associated with a skin cut or a scrape or a burn, something like that.
07:42 - 07:46: Then nociceptive pain can also move into chronic territory.
07:46 - 07:51: So again, this is caused by stimulation of nociceptors, these pain-sensing neurons, due
07:51 - 07:57: to tissue, often tissue inflammation, but it’s long in duration, so it is chronic.
07:57 - 08:01: And some clinical examples of this include inflammatory pain like joint pain, including
08:01 - 08:03: arthritis.
08:03 - 08:08: And then finally, chronic neuropathic pain is a third class.
08:08 - 08:13: This is pain that’s caused by damage to or dysfunction of the nervous system and specifically
08:13 - 08:16: the somatosensory system, and it’s long duration.
08:16 - 08:18: Again, this is chronic.
08:18 - 08:23: Some clinical examples here include diabetic neuropathy, shingles pain, pain associated
08:23 - 08:28: with CNS disorders, including multiple sclerosis and Parkinson’s disease.
08:28 - 08:34: And chronic neuropathic pain affects about 7 to 10% of the population, while chronic
08:34 - 08:38: pain in general affects one in five adults worldwide.
08:38 - 08:44: So clearly, this is an incredibly important research area, and we’d like to better understand
08:44 - 08:48: the underlying causes of chronic pain in order to create better and more targeted treatments
08:48 - 08:52: that have fewer side effects.
08:52 - 08:56: So to highlight a few ion channel targets that are known to be important in the context
08:56 - 09:00: of chronic pain, I have to begin with NAV1.7.
09:00 - 09:06: NAV1.7 is widely acknowledged as the gatekeeper of pain.
09:06 - 09:11: It serves to initiate firing of pain-signaling neurons, those nociceptive neurons, within
09:11 - 09:14: the dorsal root ganglia of the spinal cord.
09:14 - 09:20: And so it plays roles in both inflammatory and neuropathic pain, and because it’s sort
09:20 - 09:24: of considered the gatekeeper of pain, it has obviously been the focus of very intense study.
09:24 - 09:31: However, to date, no NAV1.7 selective drugs have actually reached the clinic.
09:31 - 09:32: Next I have to talk about TRP channels.
09:32 - 09:34: These are non-selective cation channels.
09:34 - 09:40: They detect a wide variety of environmental somatosensory stimuli at nerve endings.
09:40 - 09:45: TRPV1 is considered to be the archetypal nociceptive ion channel.
09:45 - 09:51: It’s expressed by practically all nociceptive primary sensory neurons, and it modulates
09:51 - 09:55: pain-signaling at the first sensory synapse within the spinal cord.
09:55 - 09:56: It’s a polymodal sensor.
09:56 - 10:01: It’s activated by heat, low pH, and other compounds, including capsaicin, and it’s also
10:01 - 10:04: been implicated in headache.
10:04 - 10:09: TRPA1 plays an important role in inflammatory pain.
10:09 - 10:11: It’s very sensitive to the redox state around it.
10:11 - 10:17: So reactive oxygen species cause channel activation, and reactive oxygen species are markedly increased
10:17 - 10:24: at sites of tissue inflammation, and so through TRPA1, these ROS can stimulate peripheral
10:24 - 10:27: terminals of nociceptors of these pain neurons.
10:27 - 10:32: Gain-of-function mutations to TRPA1 are associated with familial episodic pain syndrome.
10:32 - 10:37: This is an autosomal dominant neurological disorder that’s characterized by onset in
10:37 - 10:43: infancy of debilitating upper body pain that can be triggered by cold, that can be triggered
10:43 - 10:48: by physical stress, it can be triggered by fasting.
10:48 - 10:56: There are a number of different stimuli that can trigger this very intense upper body pain.
10:56 - 11:01: TRPM1 receptors are implicated in neuropathic pain and in migraine.
11:01 - 11:02: These are thermoreceptors.
11:02 - 11:10: They’re activated by cold temperatures and different cooling compounds, including menthol and isolin.
11:10 - 11:12: And then finally, I want to highlight the P2X receptors.
11:12 - 11:15: These are ATP-sensing channels.
11:15 - 11:16: They’re cationic.
11:16 - 11:19: They’re found in nociceptive neurons, and they’re thought to play a role particularly
11:19 - 11:22: in inflammatory pain.
11:22 - 11:27: They participate in very complex interactions with ASICs, which are the acid-sensing ion
11:27 - 11:29: channels, and TRPA1 and TRPV1 channels.
11:29 - 11:35: So when those are all found co-localized together, they have some pretty complex interactions.
11:35 - 11:40: To specifically highlight, I’d like to look at the P2X7 receptor.
11:40 - 11:45: This is also implicated in inflammatory pain, and it’s a major driver of inflammation.
11:45 - 11:51: The P2X receptor is predominantly expressed on microglia, astrocytes, and oligodendrocytes,
11:51 - 11:57: and it’s been shown to modulate behavioral responses to painful stimuli, and it causes
11:57 - 12:01: microglial release of the inflammatory cytokine interleukin-1 beta.
12:01 - 12:08: So it’s actually driving inflammation in these areas when it’s activated.
12:08 - 12:12: Okay, so next up is stroke.
12:12 - 12:17: So stroke is a neurological disorder that’s caused by inadequate oxygen, blood, and nutrient
12:17 - 12:18: supply to the brain.
12:18 - 12:21: It’s the second leading cause of death globally.
12:21 - 12:24: And stroke can be divided into two general classes.
12:24 - 12:25: First of all, ischemic stroke.
12:25 - 12:30: This is due to the blockage of a blood vessel, which leads to reduced blood flow to an area
12:30 - 12:34: of the brain, and this accounts for 85% of stroke cases.
12:34 - 12:38: And then hemorrhagic stroke, which is where there’s a blood vessel rupture that leads
12:38 - 12:46: to bleeding within the brain, and this accounts for about 15% of stroke cases.
12:46 - 12:50: So in thinking about ion channels implicated in stroke, I’m going to focus on ischemic
12:50 - 12:51: stroke.
12:51 - 12:56: Ischemic stroke is thought to damage brain tissue primarily through excitotoxicity, and
12:56 - 13:01: this is a term that’s used to describe cell death that’s induced by high levels of glutamate
13:01 - 13:03: within the synaptic cleft.
13:03 - 13:10: So how do we get from ischemia, you know, low blood flow, low oxygen levels, to excitotoxicity?
13:10 - 13:15: Well, in ischemic conditions, there’s very low oxygen.
13:15 - 13:20: Ischemia causes ATP levels, therefore, to decrease because cellular respiration is compromised
13:20 - 13:21: when there’s less oxygen.
13:21 - 13:24: So we’ve got lowering ATP levels to start.
13:24 - 13:31: This decreased ATP production impairs glutamate transporters, and this ends up resulting in
13:31 - 13:37: neuronal depolarization, and that leads to unregulated accumulation of glutamate within
13:37 - 13:38: the synaptic cleft.
13:38 - 13:41: So we get this buildup of glutamate within the synapse.
13:41 - 13:48: And this excess glutamate is now available to overactivate, in particular, NMDA receptors.
13:48 - 13:51: And importantly, NMDA receptors are permeable to calcium.
13:51 - 13:59: So the NMDA receptor is bound by glutamate, causes the channel to open, and it is a calcium-permeable
13:59 - 14:01: channel, so you get this influx of calcium.
14:01 - 14:07: This leads to mitochondrial dysfunction, the generation and production of reactive nitrogen
14:07 - 14:13: species as well as reactive oxygen species, and all of these contribute to neuronal toxicity
14:13 - 14:16: and eventually to cell death.
14:16 - 14:22: So to highlight a few important channels to this process, as I pointed out earlier, NMDA
14:22 - 14:24: receptors, all subunit types are important here.
14:24 - 14:29: They’re activated by extremely high levels of glutamate in the synaptic cleft that lead
14:29 - 14:34: to that initial calcium influx, and that really kicks off the cascade that ends ultimately
14:34 - 14:35: in cell death.
14:35 - 14:42: Then also, ASIC1A channels, so these are the acid-sensing ion channels.
14:42 - 14:47: This oxygen deprivation in the cell, due to lack of blood flow, causes the cells to
14:47 - 14:53: switch to anaerobic glycolysis, and that results in the production of lactic acid, which lowers
14:53 - 14:58: the pH and ends up stimulating ASICs, in particular, ASIC1A.
14:58 - 15:04: ASIC1A, at that point, once it’s stimulated, it causes both sodium and calcium influx in
15:04 - 15:06: response to that acidification.
15:06 - 15:10: So now we’ve got further calcium influx into the cell.
15:10 - 15:16: TRPM2 receptors are activated in response to oxidative stress by these reactive oxygen
15:16 - 15:22: species and elevated intracellular calcium, and they facilitate further calcium entry.
15:22 - 15:27: So as calcium levels rise and those reactive oxygen species increase, we get activation
15:27 - 15:32: of TRPM2, which is going to let even more calcium flood into the cell.
15:32 - 15:39: Then the P2X receptor, so P2X7 specifically here, is activated by ATP, and specifically
15:39 - 15:41: extracellular ATP.
15:41 - 15:44: So I told you that ATP levels inside the cell were decreasing.
15:44 - 15:52: ATP levels outside of the cell actually increase, and this is because panx1 activation here
15:52 - 15:58: ends up permitting the rapid efflux of ATP from the cytosol into the extracellular space,
15:58 - 16:03: and there are other transporters as well that are known to shuttle ATP out into the
16:03 - 16:04: extracellular space.
16:04 - 16:07: So we’ve got a ton of ATP outside of the cell.
16:07 - 16:12: That ATP is now available to activate the P2X7 channel.
16:12 - 16:22: And after, in particular, a prolonged activation of the P2X7 channel by ATP, it can actually
16:22 - 16:30: join forces with the panx1 channel and form a larger nonspecific pore.
16:30 - 16:37: And that allows, it basically goes from a smaller cation permeable channel to one with
16:37 - 16:41: a significantly larger and very nonspecific pore.
16:41 - 16:47: And this ends up contributing to a massive ion influx that really leads straight to cell
16:47 - 16:48: death.
16:48 - 16:55: And so, you know, all of these channels ultimately are, you know, increasing calcium initially
16:55 - 17:01: in the cell, and that is, you know, leading, everything is leading to a path of apoptosis
17:01 - 17:05: and necrosis and ultimately cell destruction.
17:05 - 17:11: Okay, moving outside of the brain, the final disease area that I’d like to highlight are
17:11 - 17:13: the cardiac arrhythmias.
17:13 - 17:19: So the heartbeat provides the mechanical force that’s needed to pump blood through the body.
17:19 - 17:24: And the contraction of the heart muscle depends on the sequential activation of ion channels
17:24 - 17:26: in the heart.
17:26 - 17:30: And if there’s any kind of disruption to that orderly activation of the channels, this can
17:30 - 17:38: lead to an arrhythmia, and this can cause the heart to inadequately pump and inadequately
17:38 - 17:42: be able to move blood through the body.
17:42 - 17:47: And so in older patients, most arrhythmia-related sudden deaths are caused by acute ischemia
17:47 - 17:49: or coronary artery disease.
17:49 - 17:54: In younger patients, sudden arrhythmic deaths usually have an underlying genetic cause,
17:54 - 17:58: and many of these arrhythmias are caused by mutations to ion channels.
17:58 - 18:06: So these cardiac arrhythmias can be classified based on where they originate within the
18:06 - 18:07: heart.
18:07 - 18:09: So there are supraventricular arrhythmias.
18:09 - 18:14: These are arrhythmias that begin in the atria, which are the heart’s upper chambers.
18:14 - 18:19: They’re known by their fast heart rates, or tachycardia, and this occurs when the heart
18:19 - 18:22: at rest is beating at over 100 beats per minute.
18:22 - 18:26: And the fast pace is sometimes paired with an uneven heart rhythm.
18:26 - 18:31: So sometimes the upper and lower chambers will actually beat at different rates.
18:31 - 18:35: And types of supraventricular arrhythmias include atrial fibrillation.
18:35 - 18:38: So this is one of the most common types of arrhythmias.
18:38 - 18:41: The heart can race at more than 400 beats per minute.
18:41 - 18:42: Atrial flutter.
18:42 - 18:47: This can cause the upper chambers to beat at 250 to 350 times per minute.
18:47 - 18:54: And in the case of both of these, atrial fibrillation and atrial flutter, some but not all of these
18:54 - 18:58: signals can travel to the lower chambers, and as a result, the upper and lower chambers
18:58 - 19:02: can actually beat at different rates.
19:02 - 19:05: Ventricular arrhythmias are arrhythmias that begin within the ventricles, which are the
19:05 - 19:07: heart’s lower chambers.
19:08 - 19:12: And they can be very dangerous and usually require medical attention right away.
19:12 - 19:19: And ventricular tachycardia is a fast, regular beating of the ventricles that can last for
19:19 - 19:22: only a few seconds or for much longer.
19:22 - 19:28: A few beats of ventricular tachycardia often doesn’t really cause any problems, but episodes
19:28 - 19:32: that last for more than a few seconds can be extremely dangerous.
19:32 - 19:35: Ventricular tachycardia can turn into other more serious arrhythmias, such as ventricular
19:35 - 19:38: fibrillation, also known as V-fib.
19:38 - 19:44: And V-fib occurs if disorganized electrical signals make the ventricles quiver instead
19:44 - 19:45: of pump.
19:45 - 19:50: And so without the ventricles pumping, you’re not going to get blood pumped to the body,
19:50 - 19:55: and sudden cardiac arrest and death can occur within a few minutes of that happening.
19:55 - 20:00: And finally, bradyarrhythmias are slow heart rhythms that can be caused by disease in the
20:00 - 20:02: heart’s conduction system.
20:02 - 20:08: So this is going to be disease to the sinoatrial node or the atrioventricular node.
20:08 - 20:14: Types of bradyarrhythmias include sinus node dysfunction, slow heart rhythms that are due
20:14 - 20:19: to abnormal SA, the sinoatrial node, heart block, which is a delay or complete block
20:19 - 20:24: of the electrical impulses that travels from the sinoatrial node where it starts down into
20:24 - 20:25: the ventricles.
20:25 - 20:31: And this can cause the heartbeat to be irregular and slow.
20:32 - 20:38: So the main inherited cardiac arrhythmias are long QT syndrome, short QT syndrome, catecholaminergic
20:38 - 20:43: polymorphic ventricular tachycardia, or CPVT, and Brugada syndrome.
20:43 - 20:48: Long QT syndrome is the most common cardiac channelopathy.
20:48 - 20:51: It occurs in 1 in 2,500 people.
20:51 - 20:54: And there are 14 different types of long QT syndrome.
20:54 - 20:59: The majority of cases, however, can be ascribed to three major genes, mutations to three major
20:59 - 21:00: genes.
21:00 - 21:06: Long QT syndrome 1 can be ascribed to mutations to KCNQ1 or KV7.1.
21:06 - 21:11: This accounts for 35% of long QT syndrome cases.
21:11 - 21:20: Long QT syndrome 2 can be traced to mutations in KCNH2 or the HERG-KV11.1 channel.
21:20 - 21:26: And mutations to both of these, KCNQ1 and KCNH2, are loss of function.
21:26 - 21:30: So basically, the repolarizing potassium currents are reduced.
21:30 - 21:36: So potassium is going to bring the cardiomyocyte back down to rest.
21:36 - 21:42: And so we can’t repolarize now that we have these impaired potassium currents.
21:42 - 21:45: And so that’s going to end up prolonging the repolarization phase.
21:45 - 21:49: It’s going to make repolarization take longer.
21:49 - 21:56: Long QT syndrome 3, this is caused by mutations to SCN5A or NAV1.5.
21:56 - 22:01: This accounts for about 10% of long QT syndrome cases.
22:01 - 22:03: And this is caused by gain-of-function mutations.
22:03 - 22:08: So this is going to result in enhanced sodium entry into the cardiomyocyte.
22:08 - 22:14: And again, this is going to disrupt that delicate balance between depolarization and repolarization
22:14 - 22:18: during that plateau phase of the action potential.
22:18 - 22:23: And that’s going to delay repolarization and could lead to or could increase the risk
22:23 - 22:27: of lethal ventricular arrhythmias.
22:27 - 22:34: So loss of function mutations to SCN5A and NAV1.5 can cause Brugada syndrome.
22:34 - 22:38: So Brugada syndrome accounts for 4 to 12% of studying cardiac death.
22:38 - 22:42: There are more than 300 mutations associated with Brugada syndrome, but most of them are
22:42 - 22:46: in the SCN5A gene.
22:46 - 22:51: Mutations to SCN5A have also been linked to other types of inherited channelopathies,
22:51 - 22:57: including but not limited to progressive familial heart block type 1, sick sinus syndrome type
22:57 - 23:01: 1, idiopathic ventricular fibrillation, and sudden infant death syndromes.
23:01 - 23:09: All of these have mutations that are linked to, that are in NAV1.5.
23:09 - 23:13: Gain-of-function mutations in three potassium channel genes have been associated with short
23:13 - 23:14: QT syndrome.
23:14 - 23:21: So it can be caused by mutations in the HERG KV11.1 that I mentioned before, and also in
23:21 - 23:25: KCNQ1, KV7.1 that I mentioned before.
23:25 - 23:29: So gain-of-function mutations there can cause short QT syndrome, makes sense if you have
23:29 - 23:35: more potassium efflux, you’re going to shorten that depolarization, you’re going to repolarize
23:35 - 23:37: faster.
23:37 - 23:45: And then it can also be caused by mutations in KCNJ2, or the KIR2.1 channel.
23:45 - 23:52: Short QT syndrome has cardiac arrest and sudden cardiac death as the most common presentations.
23:52 - 23:56: This is the most severe cardiac channelopathy.
23:56 - 23:57: Then the ryanodine receptor.
23:57 - 24:03: So I mentioned the catecholaminergic polymorphic ventricular tachycardia, or CPVT.
24:03 - 24:09: This is a significant cause of sudden unexplained death, especially in young people.
24:09 - 24:15: The arrhythmias that are typically seen in CPVT occur during exercise or at times of
24:15 - 24:20: emotional stress, and these are times when there’s increased catecholamines in the body,
24:20 - 24:21: so like adrenaline.
24:21 - 24:27: So there’s increased adrenaline release when you’re exercising, when you’re stressed out.
24:27 - 24:33: So autosomal dominant mutations in RYR2, ryanodine receptor type 2, account for 50
24:33 - 24:37: to 65% of CPVT cases.
24:37 - 24:43: And RYR2, what it’s doing is it’s regulating the release of calcium from the sarcoplasmic
24:43 - 24:46: reticulum during the plateau of action potential.
24:46 - 24:49: So we have this cardiac action potential, and during that plateau, you’ve got release
24:49 - 24:55: of calcium from this intracellular sarcoplasmic reticular calcium store, and it’s that calcium
24:55 - 25:02: release that actually drives excitation-contraction coupling and allows the muscle cells to contract.
25:02 - 25:10: But these mutations in the RYR2 channel lead to uncontrolled release of calcium from the
25:10 - 25:16: sarcoplasmic reticulum, and in particular, during stimulation by catecholamines.
25:16 - 25:21: So the sarcoplasmic reticulum calcium content increases in response to stimulation from
25:21 - 25:26: catecholamines, and so you’ve got more calcium inside the sarcoplasmic reticulum, and now
25:26 - 25:29: when you have these random release events, there’s more driving force for calcium to
25:29 - 25:36: come out, more calcium is going to flow out, and you can get this irregular activation
25:36 - 25:38: of excitation-contraction coupling.
25:38 - 25:45: And so this is going to lead to an arrhythmia, and typically arrhythmias that are very serious.
25:45 - 25:51: Okay, so to zoom back out and remind you, these channels that I’ve highlighted are just
25:51 - 25:57: a few players in these very complex and dynamic systems that regulate both neuronal and cardiac
25:57 - 26:04: excitability, and so in order to understand how information is processed, say, in a nociceptive
26:04 - 26:10: neuron to convey information regarding the sensation of pain, or to understand the basis
26:10 - 26:16: of cardiac arrhythmias that cause sudden cardiac death, we need to have a detailed knowledge
26:16 - 26:22: of the ion channels that are present within a given system.
26:22 - 26:27: And so these ion channels are playing crucial roles in physiology, and in order to understand
26:27 - 26:31: the function of excitable cells in the context of both normal physiological conditions as
26:31 - 26:37: well as nervous and cardiovascular system pathologies, as researchers, we need a detailed
26:37 - 26:42: map of the expression, the localization, and the function of each of these ion channels,
26:42 - 26:47: because remember, changes to the expression, the function, and the localization is going
26:47 - 26:52: to lead to altered propagation and integration of these electrical signals, and therefore
26:52 - 26:58: altered excitability and overall, you know, changes, huge changes in the overall activity
26:58 - 27:02: of these cells and the overall function.
27:02 - 27:06: So again, thank you for your attention, and be sure to check out our other neuroscience
27:06 - 27:10: and ion channel resources below and elsewhere on this page.
27:12 - 27:18: www.abcam.com

About the presenter:

Hannah Bishop, Scientific Support Specialist at Abcam, received her PhD in Neuroscience from UC Davis. She worked within the lab of James Trimmer and studied the Kv2 family of voltage-gated potassium channels in the mammalian brain. She then completed a Postdoc in the Institute for Neuroscience at the University of Oregon. There she worked in the lab of Cris Niell where she studied neural circuits that mediate prey capture behavior in mice. She currently works as a Scientific Support Specialist here at Abcam and an instructor of Biology at the University of Oregon.

Ion channels

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