Neuronal Regeneration After Dendrite Injury

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00:00:14.17 I'm Katie Thompson-Peer.
00:00:16.05 I'm a postdoctoral fellow at UCSF.
00:00:18.05 And, today, I'm gonna tell you about my work giving neurons a buzz cut:
00:00:23.28 regrowing dendrites after injury.
00:00:29.05 So, your brain defines who you are as a person, how you understand information.
00:00:36.27 You brain is attached to your spinal cord, which is in turn attached to the nerves that
00:00:43.13 extend out into the rest of your body.
00:00:47.11 These allow you to sense and touch and move.
00:00:51.21 They allow you to feel the squeeze of a good hug from your best friend and they allow you
00:00:58.04 to get your muscles to squeeze right on back.
00:01:03.04 Your nervous system is composed of cellular building blocks called neurons.
00:01:09.22 And I'm interested in asking questions about how neurons can recover after injury.
00:01:15.21 Neurons have a couple of different compartments where the different processes that are...
00:01:21.09 they're responsible for take place.
00:01:24.01 Here's a cartoon of a stereotypical neuron, with these compartments diagrammed.
00:01:30.17 The first thing that I'd like to draw your attention to are the dendrites, which are
00:01:35.00 these short projections that extend from the cell body.
00:01:38.25 And that's where a neuron gets information, where it takes information in, whether it's
00:01:44.20 taking information in from the environment or from other neurons.
00:01:49.18 Then, there's the cell body, where DNA is housed, and where a lot of the organelles
00:01:55.11 are housed.
00:01:58.23 And then, finally, there's the axon, where information is sent out to the target tissue,
00:02:03.25 be that another neuron, or a muscle, or other tissue.
00:02:07.26 So, information comes in through the dendrites, is processed in the cell body, and is sent
00:02:14.15 along the axon to the target tissue.
00:02:17.06 If this is a sensory neuron, the way that it detects light or touch or temperature...
00:02:25.10 it's doing that in the dendrites.
00:02:28.05 If this is a motor neuron that is sending information to the muscles, this motor neuron
00:02:33.08 might be housed in your spinal cord and send a long axon down your limb to drive
00:02:37.24 muscle contraction.
00:02:41.23 Unfortunately, there are a variety of conditions in which neurons can be injured in people.
00:02:49.17 Spinal cord paralysis occurs when there's damage to your spinal cord.
00:02:56.02 Stroke and traumatic brain injury are injuries to the brain.
00:03:01.04 We're hearing a lot about traumatic brain injury these days because it occurs frequently
00:03:05.20 in professional athletes and in soldiers as well.
00:03:11.07 Not only neurons in your brain and your spinal cord, but neurons out...
00:03:14.23 throughout the rest of your body can be injured as well.
00:03:18.14 I'm interested in understanding how well these neurons can regenerate or recover after these
00:03:23.23 injuries.
00:03:24.23 So, I started off this project by doing some searches through the literature, doing searches
00:03:30.23 in PubMed, which is a repository of scientific information, where you can search for scientific
00:03:36.24 articles on different topics.
00:03:39.09 And I found an interesting dichotomy.
00:03:42.12 When I looked up axon regeneration at this time -- a couple years ago -- at that time
00:03:47.22 there were 1200-plus papers on axon regeneration.
00:03:52.22 And, similarly, there was a broad understanding of how axons and dendrites develop and grow
00:04:01.04 in developing animals, in uninjured conditions.
00:04:05.00 But when I looked up dendrite regeneration, I found, strikingly, that there were only
00:04:10.19 3 papers on it.
00:04:12.21 Is there really so little known about how this part of a neuron can regenerate,
00:04:18.26 the part that is responsible for the neuron getting information in?
00:04:22.07 Or was I missing something?
00:04:24.15 So I did some more research, delved a little deeper, and what I found was that there was
00:04:30.07 some really beautiful work from Santiago Ramon y Cajal on this topic that is broadly underappreciated.
00:04:37.24 Now, Santiago Ramon y Cajal is one of the founders of modern neuroscience, for his beautiful
00:04:43.19 work on the development and the anatomy of the nervous system.
00:04:48.03 His drawings are like artwork and he's contributed to our body of knowledge in immeasurable ways.
00:04:57.12 His description of axon regeneration, and the situations in which axons can and cannot regenerate,
00:05:04.19 is broadly appreciated and widely cited.
00:05:08.20 But he also did some beautiful work on dendrite regeneration
00:05:12.12 that is hardly recognized at all.
00:05:15.00 Here's one of his drawings.
00:05:17.17 What you can see is there's a variety of neurons, here, and he's used a surgical technique
00:05:24.28 to damage them.
00:05:26.07 There’s a scar at the top that's marked by A.
00:05:31.00 And what he did was to ask, after a surgical scar,
00:05:35.15 after surgical injury, could these neurons regenerate their dendrites?
00:05:40.24 Now, Santiago Ramon y Cajal was working in the late 1800s and early 1900s.
00:05:47.23 This image is from a paper that he published in 1913.
00:05:51.00 But, since then, very little has been done to ask this question of, can neurons regrow
00:05:58.09 their dendrites after injury?
00:06:00.07 So, what I'm going to tell you about today is some work that we've done using, not the
00:06:05.12 surgical techniques of Ramon y Cajal, but some more modern techniques to specifically
00:06:11.27 injure dendrites and see, can they recover?
00:06:16.08 Can they regrow?
00:06:18.21 To give you a little sense of things -- to give you a stronger sense of things -- I'm
00:06:23.10 gonna make an analogy between a neuron and a tree.
00:06:26.26 A tree sends out its branches into space, places leaves all around to collect sunlight.
00:06:37.10 Similarly, the neuron sends out dendrite branches to collect information from the environment,
00:06:46.20 from other neurons.
00:06:49.11 And it's important where these branches are put.
00:06:52.12 But what happens for our tree if all of those branches are cut off?
00:06:57.26 And, in fact, that's something that arborists do.
00:07:02.02 Here are some trees by the side of the road in the Netherlands, and they've recently had
00:07:07.27 all of their dendrite bran... all of their... not dendrite branches... they've recently
00:07:13.12 had all of their branches cut off.
00:07:17.21 We're paying attention to the trees in the foreground, not the ones in the background.
00:07:21.27 So, what happens?
00:07:23.20 This is a process that's called pollarding, where all of these branches have been removed.
00:07:29.00 How do these trees respond to it?
00:07:31.00 If you came back two years later, you would see that these trees have exuberantly regrown
00:07:38.10 new branches.
00:07:40.13 But do they look like a tree that had never been pollarded?
00:07:44.09 Do they look like a tree that had never been injured at all?
00:07:47.17 Or, to go back to our neurons, after a neuron has had its dendrites injured, can it perfectly
00:07:55.10 recreate what it once had?
00:07:57.16 Can it perfectly recreate what an uninjured neuron would look like?
00:08:02.14 Or, if not, what is the gap between how well it can recreate and where it ought to be.
00:08:11.25 So, I told you that there were some papers that had been looking at this topic before my work,
00:08:16.24 and what they had all shown was that, in the neurons that they were looking at,
00:08:20.26 they had a robust ability to regrow and regenerate after injury.
00:08:25.20 And they also showed something else that that was kind of interesting, which was that dendrite
00:08:30.04 regrowth was mechanistically distinct from axon regrowth.
00:08:34.10 So, they looked at mutants, mutant neurons, that were defective for axon regeneration
00:08:41.20 and they found that the dendrite regeneration happened just fine.
00:08:44.26 So, what that means is that we can't copy and paste our knowledge of how axons regenerate
00:08:52.00 and expect the same thing to apply to the dendrites.
00:08:55.00 Something different, something new is at play.
00:08:58.12 Okay.
00:08:59.02 So, how do we answer this question?
00:09:01.13 One way that you can address this question is by looking at cells in a dish.
00:09:06.12 You could harvest neurons from an experimental animal, put it in a dish, grow them up,
00:09:13.01 injure those dendrites, and watch them respond.
00:09:15.04 And that can be a powerful system for asking some questions.
00:09:19.22 But, for me, this wasn't the right system, because those aren't neurons that are interacting
00:09:23.25 with their local environment.
00:09:26.06 I'm interested in how neurons that are injured respond and regrow when they are interacting
00:09:31.08 with their normal tissue.
00:09:32.25 So, instead, I decided to look at how neurons would respond to injury in an intact animal,
00:09:39.22 specifically looking at the experimental organism Drosophila melanogaster, a fruit fly, which
00:09:46.16 has some specific and technical advantages that make these experiments possible.
00:09:50.21 But I'm not looking at adult animals.
00:09:53.27 I looked at fruit fly larvae, because they're transparent and everything is easy to see.
00:10:00.01 Here's an image of the side view of a fruit fly larva.
00:10:05.13 And what we've done is to use a genetic tool to exclusively drive GFP, a green fluorescent protein,
00:10:14.04 that's membrane tagged, in one small subset of neurons.
00:10:18.22 So, the animal is transparent, and it's expressing this fluorophore, this fluorescent protein,
00:10:25.18 only in one class of neurons, so that's all you see.
00:10:29.02 And the benefit of this is that we can find a particular neuron, injure it, and come back
00:10:36.23 time and again and find that same neuron.
00:10:40.28 This allows us to precisely follow the response of a single neuron to a defined injury, like
00:10:49.02 injury to its dendrites.
00:10:52.02 In addition, each of these neurons is responsible for a particular territory on the fruit fly
00:11:03.04 larva.
00:11:04.04 So, this class of neuron is sensory.
00:11:08.05 It's important for detecting temperature, and touch, and light for these animals.
00:11:16.08 Each neuron extends a dendrite arbor and has its own territory in the body wall that it's
00:11:22.20 responsible for.
00:11:23.20 So, I've got three clouds on the three different neurons of this class in that one segment
00:11:30.15 of the animal.
00:11:32.17 And what you can see is that the territory that each neuron is responsible for is unique
00:11:37.24 and non-overlapping.
00:11:38.26 It doesn't crossover with the next... adjacent neuron.
00:11:43.28 Finally, these neurons are interesting and useful to study for this question because
00:11:49.19 the normal development of these neurons has been extensively analyzed.
00:11:54.28 We have a large panel of markers and tools that we can use to manipulate these neurons.
00:12:01.08 And we have a good baseline understanding of how these neurons develop, and grow, and
00:12:06.17 shape themselves in uninjured animals, which is a great point of comparison for us when
00:12:11.28 we're looking after injury.
00:12:15.10 What do they look like a little bit closer up?
00:12:18.06 Here's one.
00:12:20.20 There's the cell body, there.
00:12:21.26 The axon is going down.
00:12:24.17 It's unbranched and if I was showing this to you in three dimensions you would see that
00:12:28.06 it's at a little bit of a deeper level than the rest of what you're seeing.
00:12:31.22 And the rest of it are the dendrites, all around.
00:12:34.24 And here I'm showing you an image where I've overlaid a skeleton drawing of the dendrite branches
00:12:39.25 of this one neuron onto the image as a whole so you can see what one neuron
00:12:44.04 looks like.
00:12:45.04 So, this is an uninjured neuron.
00:12:48.12 How do we injure these neurons?
00:12:51.09 What I'm going to tell you about is some work where we're using a two-photon laser -- so,
00:12:55.26 that's a specific type of optical tool -- to damage these dendrites.
00:12:59.28 What we do is we set it at a very high power setting and focus it on the first branch point
00:13:04.25 that's closest to the cell body, set it for a very high power setting for a short period of time...
00:13:11.01 zap zap... we zap those dendrites.
00:13:15.02 And that cuts off the dendrites adjacent to the cell body.
00:13:19.09 So, what happens next?
00:13:21.07 Here, again, is our uninjured neuron.
00:13:25.05 This would be 24 hours after it would have been injured, or 72 hours after it would have
00:13:30.25 been injured if it was one that was for that purpose, but this is our uninjured control.
00:13:36.03 And what you can see is that, between these time points, the neuron is extending terminal branches
00:13:42.15 and scaling with the growth of the whole animal, because, remember, we're looking
00:13:45.25 at a fruit fly larva, so it's kind of growing the... during this whole to... period of time.
00:13:49.24 But the overall architecture of this neuron is stable and remains the same.
00:13:55.00 What happens to our neuron that had been injured?
00:13:57.21 Here's the cell body, and the axon going down, and you can see it's starting to sprout some
00:14:04.04 dendrites all around.
00:14:05.19 But, for the most part, that territory that that neuron is responsible for is uncovered.
00:14:11.28 Those dendrites that were... we focused on were cut away, they were fragmented and engulfed
00:14:17.14 by adjacent engulfing cells.
00:14:21.02 And now that territory is uncovered.
00:14:23.11 So, we come back two days later to see what happens.
00:14:28.01 And this is what the neurons look like.
00:14:30.11 And what we found is that they have a robust ability to regrow dendrites after all of their
00:14:35.03 dendrites have been cut off, in agreement with a prior publication.
00:14:38.10 But, when you look a little bit closer at these uninjured and regenerated neurons,
00:14:46.14 what you can see is that there are gonna be some striking differences between where an uninjured neuron
00:14:52.15 looks like -- what its architecture is -- and where the regenerated neuron is.
00:14:57.00 So, today, what I'm going to tell you about are three ways where the neuron is doing a
00:15:01.28 good job of regenerating and recovering after injury.
00:15:06.18 And then we'll delve a little more deeply into three ways where it's falling short,
00:15:11.16 not quite hitting the mark.
00:15:13.24 Okay.
00:15:14.24 So, where do we start?
00:15:18.17 First of all, this neuron... it's supposed to have lots of projections, it regrows lots
00:15:25.10 of projections.
00:15:27.04 If we go ahead and count the number of branches, what we find is that these neurons are actually
00:15:33.14 doing a remarkably good job of regrowing the... the right number of dendrite branches.
00:15:40.22 A neuron that, on average, should have 450 branches regrows, on average, 450 branches.
00:15:50.06 Not bad.
00:15:52.09 Furthermore, we found no evidence that these neurons changed their cell type identity after injury.
00:15:59.15 So, if we... it looked to us like a ddaC neuron before, because of the panel of markers and
00:16:05.07 genes that it expressed, it looks like a ddaC neuron to us afterwards, because it's still
00:16:10.04 expressing those same genes.
00:16:14.02 Secondly, I've told you that it's extending all these projections, these branches.
00:16:18.25 Morphologically, they... they look like dendrites, but that doesn't mean that they are dendrites,
00:16:25.19 so we wanted to look and see whether they expressed... these projections that the neurons
00:16:29.28 were extending expressed the proteins, localized the proteins, that we would expect dendrites
00:16:36.28 to have.
00:16:38.13 So, we looked for the localization of mechanoreceptors that are trafficked out into dendrites,
00:16:45.14 that should be trafficked out into dendrites, to see, did our regenerated dendrites have them?
00:16:50.13 And, lo and behold, it looked good.
00:16:54.10 By protein localization, these structures look like dendrites to us as well, which,
00:16:59.14 again, is an agreement with a prior publication.
00:17:02.08 Okay, so, right number of branches, those things by protein localization look like dendrites...
00:17:09.00 do they function like dendrites?
00:17:11.02 Can that neuron receive the information through those structures that it's supposed to?
00:17:17.10 We can record the electrical activity of these neurons in response to stimuli and directly
00:17:23.19 test that.
00:17:24.22 So, what you do is you stick a recording electrode, you suck up the cell body of that neuron,
00:17:30.16 and then you poke that fly.
00:17:33.21 These neurons are mechanoreceptive, so you can just see the number of times it fires
00:17:40.08 in response to poking it.
00:17:42.02 Okay?
00:17:43.02 So, here's a control neuron.
00:17:44.02 It's relatively silent, up until we give it a poke, which is that
00:17:49.18 carrot/arrowhead/triangle thing.
00:17:52.06 Now, what you can see is that that neuron fires in response to that stimulus.
00:17:57.00 Okay?
00:17:58.00 So, this is our control neuron, looking pretty good.
00:18:01.20 What happens to a neuron that doesn't have any dendrites?
00:18:04.08 When we looked at a neuron where we had cut off all of the dendrites 4 hours previously,
00:18:11.09 it didn't have any response.
00:18:13.04 It can't tell that it's getting... that that larva is getting poked,
00:18:15.24 that that fly is getting poked.
00:18:19.06 But what about a neuron that has had 3 days to regenerate and recover and regrow
00:18:24.24 after injury?
00:18:28.02 Well, those neurons, they do respond.
00:18:30.14 So, that neuron is relatively quiet, and then gets poked, and then responds afterwards.
00:18:37.04 So, these dendrites are functioning as dendrites should.
00:18:41.09 There's information coming into that neuron because these dendrites are regenerating.
00:18:45.21 Okay.
00:18:46.21 So, after injury, all is not lost.
00:18:49.25 In some ways, that neuron is doing really well.
00:18:52.22 It's regenerating the right number of branches, it's still the same type of neuron that it
00:18:58.00 was before, those branches that it's regrowing do look like dendrites in terms of the localization
00:19:05.17 of ion channel mechanoreceptors that should get trafficked out into those projections,
00:19:11.09 and that neuron is functional.
00:19:12.24 It is receptive to the stimulus that it's supposed to be sensing.
00:19:16.19 It is getting information in.
00:19:18.00 Okay.
00:19:20.12 So far, so good.
00:19:21.27 But, going back to our picture of uninjured and regenerated dendrites, that's not the
00:19:27.26 whole story.
00:19:29.13 If I were to remove these labels and shuffle these images, you could probably still tell
00:19:36.08 the difference.
00:19:37.08 Well, part of that is that one of these neurons is a lot smaller than the other one.
00:19:42.06 So, we went ahead and we measured the territory that these dendrites covered,
00:19:47.16 the dendrite arbor as a whole covers.
00:19:50.01 What we found was that, in fact, after injury the regenerated dendrites only covered about
00:19:55.27 half the territory that an uninjured neuron would cover.
00:19:59.23 Okay?
00:20:00.23 Same number of branches, half the territory, a whole lot denser.
00:20:04.26 Part of how it does that is it has a whole lot more branches right in close to the cell body,
00:20:09.26 which is something that we almost never see in the uninjured neurons.
00:20:13.07 Okay?
00:20:14.12 How does it pack all these dendrite branches in into the smaller territory?
00:20:18.18 Does it do a good job of that?
00:20:20.28 So, I mentioned before that each of these neurons is responsible for covering a distinct
00:20:27.09 and non-overlapping territory on the body wall.
00:20:30.21 And, within that territory, also, the dendrite branches of a single neuron generally don't
00:20:37.10 cross over one another.
00:20:39.06 This is kind of like a tree, where the tree's branches stretch out, grow out, and it's important
00:20:48.27 that they expand and cover that territory so that the leaves are catching as much sunlight
00:20:54.13 as possible, as opposed to all being on top of one another to block each other out.
00:20:59.26 Same thing with the neuron.
00:21:02.05 It's responsible for covering that territory maximally, as opposed to redundantly.
00:21:09.21 So, that's our uninjured neuron, doing pretty well.
00:21:12.25 Those orange carrots show you a few places where it does have some dendrites crossing over,
00:21:17.23 but not many.
00:21:19.22 What happens to our neurons after injury?
00:21:23.16 Well, they're not so hot.
00:21:25.13 These neurons have many examples in which dendrites are crossing over one another.
00:21:30.25 And what we found when we looked more closely is they have many examples of both dendrites
00:21:36.00 that are crossing over one another and touching, and also many examples where dendrites are
00:21:42.08 crossing over one another and not touching.
00:21:45.25 And there are in fact failures in multiple pathways that are responsible for preventing
00:21:51.13 these kinds of things.
00:21:53.08 There are failures in the pathways that are supposed to prevent touching crossings and
00:21:57.20 there are failures in the pathways that are supposed to prevent not touching crossings.
00:22:01.23 Okay?
00:22:02.23 So, these dendrites are ignoring multiple cues from their environment that are supposed
00:22:08.15 to determine where they place their dendrites in space after injury.
00:22:15.12 What kind of consequences could this have for the neuron, right?
00:22:19.04 A denser arbor, more compact, dendrite branches crossing over one another...
00:22:25.13 does the neuron care?
00:22:29.07 I mentioned before that we can record the electrical activity of these neurons.
00:22:32.08 This is the same trace that I was showing you before, where you're showing...
00:22:36.10 you're seeing how it's spiking in response to a poke, but when we count the number of spikes,
00:22:42.28 does it look the same?
00:22:45.27 So, here, I'm graphing the number of times it fires in response to a poke.
00:22:53.04 And we've got pokes of 3 different strengths, weak to strong.
00:22:59.11 And we're gonna be looking at neurons that were control neurons, neurons that had 3 days
00:23:03.19 to regenerate or regrow after injury, and neurons that don't have any dendrites because
00:23:08.11 they've only had 4 hours since the injury.
00:23:11.25 And when we look at that quantification, we find that our control neurons are responding
00:23:16.28 strongly to that sensory stimulus that they're supposed to be responding to, our neurons
00:23:22.23 without any dendrites don't care, and our neurons that have regenerated and regrown
00:23:27.28 have recovered the ability to sense that stimulus, but it's not all the way.
00:23:33.00 Right?
00:23:34.00 They're not looking... regenerating quite as well.
00:23:36.24 They're not responding quite as well as an uninjured neuron would.
00:23:43.02 What other kinds of consequences could these changes in regeneration have for the neuron?
00:23:49.11 So, it's been known for a long time that these neurons change their dendrite shape as these
00:23:56.13 animals go through metamorphosis.
00:23:58.02 So, I mentioned before, we're looking at fruit fly larvae.
00:24:02.05 At some point, they undergo metamorphosis to become fruit fly adults.
00:24:06.17 And, during that process, these neurons don't die -- they survive -- but they need a different
00:24:13.08 shape of arbor for their adult function.
00:24:16.27 So what they do is they cut off all of the dendrite branches, those are engulfed by other cells,
00:24:23.11 and then they regrow new dendrites during metamorphosis.
00:24:28.19 And that all happens at very precise points in time, in terms that... which can be measured
00:24:34.13 in terms of hours after metamorphosis began.
00:24:37.28 So, at one point, X number of hours after metamorphosis begins, they cut off all those
00:24:44.18 dendrite branches.
00:24:45.18 And at a later point they regrow new ones.
00:24:49.12 What happens to our neurons that went through the process already of having their branches
00:24:54.03 cutting... cut off during the larval phase and that already had to regenerate their branches?
00:24:59.20 So, here's a control neuron.
00:25:02.06 This is 7 hours after the beginning of metamorphosis and this is what the neuron is supposed to
00:25:07.22 look like at this time.
00:25:09.17 There's the cell body in the middle, and the axon going down, and the dendrites all around.
00:25:15.05 And this neuron, other than the dendrites looking a little wobbly, is beginning to
00:25:21.03 cut off some of those dendrite branches, but it's still at very early stages of this process
00:25:26.25 of changing its dendrite arbor during metamorphosis.
00:25:31.11 What happens to the neurons that had to regenerate already?
00:25:36.21 When we look at the same point in development, 7 hours after metamorphosis began,
00:25:41.23 they're a little bit ahead of the game.
00:25:44.03 So, this neuron has pruned away its dendrites far too early.
00:25:52.11 That's a little bit like a tree that has to change its color in the fall.
00:25:58.07 Most of the trees are waiting for autumn.
00:26:01.15 Most of the trees are going to wait until October to turn the glorious reds and yellows
00:26:06.00 that you would expect to see.
00:26:08.08 But our tree that had been pollarded, our tree that had been injured and had to regrow,
00:26:14.04 instead of waiting for October to come around, this is it in mid-July.
00:26:18.24 It's turning yellow already.
00:26:20.25 It is ignoring the cues from its environment that determine when processes are supposed
00:26:26.20 to take place.
00:26:27.21 And, instead, it's going way too early.
00:26:30.11 It's out of sync with the rest of the trees.
00:26:33.01 So, to take us back to the diagram of a neuron, our stereotypical neuron, we've got our
00:26:40.23 red arrows that are the information flow in from the dendrites, process in the cell body, out
00:26:46.01 through the axon.
00:26:48.15 What you might notice is that that red arrow, showing the direction of communication between
00:26:53.24 the dendrites and the cell body, with regards to the flow of information, with regards to
00:26:59.01 the flow of electricity from the dendrites to the cell body, is in the right place for
00:27:06.23 communication between dendrites and the cell body.
00:27:09.13 So, what we wondered was, what would happen if we blocked that?
00:27:15.00 Could that have any effect on how these neurons regenerate or recover?
00:27:20.21 So, what we did is we expressed a transgene, that's a potassium channel, that hyperpolarizes
00:27:27.18 these neurons, changes the way the act, electrically, and that basically electrically silences them.
00:27:34.21 So, they're no longer going to sense stimuli.
00:27:38.27 And we ask the question, what happens to those neurons that are electrically silent.
00:27:44.08 Can they regenerate just as well?
00:27:47.10 Here's our control neuron.
00:27:48.21 This is a control injured neuron, but it's wild-type.
00:27:53.02 There's the cell body, axon going down, dendrites all around, robust regrowth after injury.
00:27:59.16 What about our neuron that's expressing a transgene that's electrically silent?
00:28:05.04 Not so hot.
00:28:07.01 There's the cell body, axon going down, some dendrite regrowth but not much.
00:28:14.15 So in... or... by inhibiting the electrical activity of these neurons, we've significantly
00:28:20.12 blocked their ability to regrow after injury.
00:28:23.04 Okay.
00:28:24.04 So, you... you can't go home again.
00:28:26.26 After injury, there are some things that you're not going to get back.
00:28:31.05 The dendrite arbor is smaller, which, given the fact that we have the same number of branches,
00:28:38.00 gives us a dendrite... denser dendrite arbor.
00:28:41.03 And those dendrite branches are crossing over one another.
00:28:45.20 In terms of the neuron's ability to receive information, which is the function of those dendrites,
00:28:50.25 it's recovered that ability, but only partially.
00:28:54.16 It's not sensing that stimulus, the poke, as well as it normally should.
00:29:00.08 And later changes that are supposed to happen at precise times in metamorphosis are
00:29:05.22 out of whack -- they're out of sync.
00:29:08.15 And finally, when we silence the electrical activity of that neuron, that significantly
00:29:14.04 blocks its ability to recover and regrow from this injury.
00:29:19.02 And what I've shown you is true...
00:29:20.06 I've shown you what's true for the ddaC neuron, but this is also true for a couple of other
00:29:26.14 classes of neuron that we've looked at so far.
00:29:30.04 So, to get back to our first question, can neurons
00:29:34.20 perfectly recreate what they had after injury?
00:29:38.01 Can they perfectly regenerate dendrites?
00:29:40.19 The answer is clearly both yes and no.
00:29:45.06 They do well at some things.
00:29:47.06 They regrow dendrites.
00:29:48.09 Those... the structures that they regrow are in fact dendrites and they regrow the right
00:29:53.08 number of them.
00:29:56.03 And they have some ability to get information from their source.
00:30:01.20 But there are consequences for the neuron in terms of the... the density, the shape
00:30:06.21 of that dendrite arbor, their ability to get information at the same level that they were supposed to,
00:30:13.04 and other features as well.
00:30:15.08 So, where does this take us?
00:30:17.21 Where do we go from here?
00:30:19.22 Remember, I said, axon regeneration had many, many, many papers on it -- a broad understanding
00:30:28.11 of the situations in which an axon can or cannot be regrown by a neuron.
00:30:34.01 By contrast, we're only starting to figure out the very basic principles that define
00:30:40.03 whether a neuron can regrow its dendrites or not.
00:30:43.12 We're only at the beginning of building a foundation for this field of scientific research.
00:30:48.23 So, what kind of questions do we want to ask?
00:30:53.02 Back to our cartoon of a neuron, after it's been injured, how does it know that it's been injured?
00:30:59.23 How does it know that it doesn't have any more dendrites?
00:31:04.00 How does it know that it needs to regrow them and needs to respond to this kind of injury?
00:31:09.27 One hypothesis that we have is that it is the lack of information coming from those
00:31:15.11 dendrites after injury that tells it it needs to regrow new dendrites.
00:31:22.09 But is that actually the case?
00:31:24.23 It remains to be seen.
00:31:28.12 Also, how does dendrite regeneration compare to the normal behavior of neurons, to the
00:31:37.14 normal mechanisms that are responsible for maintaining dendrites in uninjured situations?
00:31:44.05 Because dendrites and neurons have machinery that makes sure that they have the right number
00:31:50.15 of dendrites for their situation.
00:31:53.08 So, is what we're seeing after injury merely an extreme example of that dendrite maintenance process,
00:32:01.22 or is it something else entirely?
00:32:05.18 To take you back to the tree analogies, trees have a normal process of losing their leaves,
00:32:13.26 of losing their branches during the winter time, and regrowing them when summer comes around.
00:32:19.15 They respond to cues in their environment that allow them to determine... we have the
00:32:24.18 right number of branches, this is how much we need to grow.
00:32:28.13 By contrast, the trees that have been pollarded, the trees that have all of their branches
00:32:34.14 cut off, they need to regrow branches too.
00:32:39.04 But is the regrowth that happens after injury just an extreme example of what a tree needs
00:32:44.15 to do after wintertime?
00:32:46.27 Or is it something else entirely?
00:32:50.08 Also, we'd like to take a closer look at what's going on in these neurons as they're regrowing
00:32:57.11 new dendrite processes.
00:33:00.13 What are the cell biological processes that are allowing these neurons to extend out new dendrites?
00:33:06.26 What are the molecular motors that are pushing those growing projections?
00:33:13.24 What are the changes in gene expression that the neuron might have after injury?
00:33:18.25 And how are these things similar or different to what goes on in uninjured neurons, in neurons
00:33:24.08 that haven't been damaged?
00:33:28.15 We started off by talking about the nervous system as a whole and, so far, I've been telling
00:33:32.28 you a lot about neurons in the body of fruit flies.
00:33:38.11 But how much of what we've learned, how much of these general principles, applies broadly?
00:33:45.21 What happens to other neurons?
00:33:47.26 So, we've been talking about sensory neurons that detect cues from the environment,
00:33:54.01 like the poke.
00:33:55.17 But what about neurons in your brain or your spinal cord that receive information from
00:34:01.23 other neurons?
00:34:03.25 Those dendrites don't receive information from a poke.
00:34:08.06 Those dendrites receive information at synaptic connections with other neurons.
00:34:14.11 For those kinds of neurons, after they've been injured, if they can regrow new dendrites,
00:34:19.24 can those dendrites form synapses, form synaptic connections with other neurons.
00:34:26.10 Can they form the synaptic connections with the right neuron, with the neurons that they're
00:34:30.05 supposed to be connected to?
00:34:32.18 Finally, we're not only interested in studying neurons in fruit flies.
00:34:39.19 There have been some recent publications that show that the themes that we found, at least
00:34:45.24 in part, in fruit flies apply to other invertebrate systems as well, C elegans.
00:34:52.22 But has what we learned applied to other types of animals?
00:34:56.18 What about mammals?
00:34:58.07 What about people?
00:34:59.22 Can regen... can their dendrites regenerate after injury?
00:35:03.12 It's only with a deep understanding of the principles that control when neurons can respond
00:35:10.09 to injury to their dendrites that we can hope to design intelligent interventions for clinical
00:35:17.09 conditions in which dendrites are injured, that we can hope to figure out how well dendrites
00:35:23.20 in people can regenerate after injury, and where they ought to be and how we can help
00:35:29.14 them get there.
00:35:30.22 So, with that I'd like to say thank you to my mentors, Yuh-Nung and Lily Jan, to my collaborators
00:35:37.20 in the Jan lab, especially Laura DeVault, a graduate student in the lab who's been
00:35:42.20 my close scientific collaborator, and who also was the one to first come up with the pollarded
00:35:47.12 tree analogy, to my funding sources, and to iBiology.
00:35:52.23 Have a nice day.

This Talk

Speaker: Katherine Thompson-Peer

Audience:

Educators of H. School / Intro Undergrad
Educators of Adv. Undergrad / Grad
Researcher
Educators

Recorded: May 2017

Talk Overview

When it comes to neuronal regeneration, very little is known about the ability of dendrites to regrow following trauma. In this lecture, Dr. Thompson-Peer describes her discovery of fundamental principles of neuronal regeneration, particularly of how neurons respond to and recover from dendrite injury. She also describes how this work is laying the foundation for a vastly understudied field of science.

This talk is part of the Young Scientist Seminars, a video series produced that features young scientists giving talks about their research and discoveries.

Speaker Bio

Katherine Thompson-Peer

Dr. Katherine (Katie) Thompson-Peer earned her bachelor’s degree in Biology from the University of Pennsylvania, followed by a two-year stint at the Johns Hopkins University. She earned her PhD from Harvard University, studying the timing of neuronal development with Joshua Kaplan. She began her postdoctoral fellowship, working with Yuh-Nung Jan at the University of California,… Continue Reading