NATURE PODCAST

Podcast: An ion-drive aeroplane, and DNA rearrangement in neurons

Join Benjamin Thompson and Shamini Bundell for the latest news from the world of science.

This week, a solid-state plane engine, and ‘mosaicism’ in brain cells.

In this episode:

00:46 Solid-state flight

Powering a plane with ions. Research Article: Xu et al.; News & Views: Flying with ionic wind; Nature video: Ion drive: the first flight

06:46 Research Highlights

Ant assassins and termites’ boring debris. Research Highlight: Skull-collecting ants slay with acid; Research Highlight: Termite mounds dating back millennia can be seen from space

08:35 The brain's mosaic DNA

Researchers have shown how neurons' genes can change over time. Research Article: Lee et al.; News and Views: A mosaic mutation mechanism in the brain

16:03 News Chat

An overhaul of SI units, and prediciting volcanic eruptions. News: Largest overhaul of scientific units since 1875 wins approval; News: World’s first automated volcano forecast predicts Mount Etna’s eruptions

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Transcript

This week, a solid-state plane engine, and ‘mosaicism’ in brain cells.

Host: Shamini BundellWelcome back to the Nature Podcast. This week, we’ll be finding out about a quiet step forward in aeroplane technology.

Host: Benjamin Thompson

And hearing about the DNA differences in individual brain cells. I’m Benjamin Thompson.

Host: Shamini Bundell

And I’m Shamini Bundell.

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Interviewer: Shamini Bundell

If you watch early-1900s footage of the Wright brothers demonstrating one of the first powered, controlled aeroplanes and you listen very carefully… you probably won’t hear anything because the footage doesn’t have any sound. But the invention’s gasoline-powered engine turning two propellers presumably made pleasing rattle as it flew along just above ground level. This week, you can see footage of a new type of flying machine, one that even with the sound turned up is remarkably silent as it gently glides across a sports hall a couple of metres above the floor. And silent it just the way that one of its creators, Steven Barrett, wanted it.

Interviewee: Steven Barrett

Well, the idea is, in a way, a childhood fantasy. I used to be a big fan of Star Trek and sort of thought that the future of flight shouldn’t be things with propellers and turbines and should be more like with a kind of blue glow and something that silently glides through the air.

Interviewer: Shamini Bundell

Steven works in the Department of Aeronautics and Astronautics at the Massachusetts Institute of Technology, so he’s pretty well placed to create a glowy, futuristic-looking flying machine. But the method he’s used to power his new plane has actually been around since not long after the first commercial flights.

Interviewee: Steven Barrett

It dates back until at least the 1920s where an eccentric inventor at the time started experimenting with high-voltage electrodes and thought he had discovered antigravity, which of course was not the case, but that set some of the initial groundwork, some of the very old patents on mechanisms for creating what’s called an ionic wind.

Interviewer: Shamini Bundell

And those basic mechanisms are what lie behind the development of Steven’s new plane – a lightweight, fixed-wing aircraft with a 5-metre wingspan, powered by electricaerodynamic propulsion, or what’s sometimes known as an ion drive.

Interviewee: Steven Barrett

So, what we did for this design is to try and stick to something that looks somewhat like a conventional aircraft but under the wing, rather than conventional engines, it has a series of electrodes and those consist of an array of very thin wires at the front and then an array aerofoils at the back. Now those thin wires at the front are set at a very high voltage – +20,000 volts – and where that high field strength occurs it creates a source of ions.

Interviewer: Shamini Bundell

The ions are created when electrons are knocked off nitrogen molecules by the wires of the positive electrode at the front. The ions are therefore positive nitrogen ions, meanwhile the aerofoils at the back of the plane are negative electrodes. Opposites attract so the positive ions move towards the back of the plane.

Interviewee: Steven Barrett

And so, on that path from positive to negative, the ions collide with air molecules many, many times, transferring momentum to the air, creating a breeze or an ionic wind that’s left behind.

Interviewer: Shamini Bundell

And so, as nitrogen ions push against the air molecules, thrust is created, silently and invisibly propelling the plane forward. Well, that was the theory anyway.

Interviewee: Steven Barrett

Many attempts failed because of various things going wrong like structural failures, the power electronics frying itself. The first day that it actually worked it was about 50% power so it was a power glide but there was quite a lot of excitement and a lot of cheering when that happened.

Interviewer: Shamini Bundell

From that first glide, the team were soon able to make the first fully powered flight, and it’s no surprise they were so excited about it – it’s taken decades to put this technology into practice in this way. For example, spacecraft have been using ion thrusters for decades but with a design that only works in a vacuum. Here on Earth, it’s relatively simple to create a little ion-driven lifter that jumps off a table, but that requires the craft to be attached by wires to a large power source nearby. The new plane has on-board batteries and is remote-controlled.

Interviewee: Steven Barrett

So, what we achieved was the first ever sustained flight of an aeroplane that is propelled by electroaerodynamic propulsion, and that’s also by many definitions the first ever solid-state flight, meaning no moving parts.

Interviewer: Shamini Bundell

This achievement has been made possible with modern technology such as lightweight batteries, and it’s an impressive feat of engineering to get it to work. Here’s Kris Pister of the University of California, Berkeley.

Interviewee: Kris PisterNo one has ever been able to do this before and plenty of people would have said, ‘No, that’s not possible, that will never work.’

Interviewer: Shamini Bundell

Kris works in this area himself and is optimistic about some of the applications, though perhaps not the ones involving futuristic, glowing flying machines or ion-powered passenger planes.

Interviewee: Kris Pister

I’m sceptical of whether it will have practical application at large scale in the atmosphere. I think that it’s a technology that scales well, so for me as a micro-robot person, propellers don’t work well at a millimetre scale whereas this technology has the same performance kind of independent of scale. So, at a small scale, this may end up being the best game in town.

Interviewer: Shamini Bundell

Potential applications include creating silent drones which could be used to observe wildlife or monitor traffic in urban areas without creating noise pollution. Hopefully this is just the first step in developing useful flying ionocraft, and the sight of this silently gliding machine with no visible power source or propulsion may well inspire future researchers to explore new uses for this strange technology.

Interviewee: Kris Pister

It’s cool because the physics is so different from the physics of flight that we’re used to and you don’t have to be a physicist to appreciate that.

Interviewer: Shamini Bundell

That was Kris Pister and we also heard from Steven Barrett, whose paper is published in Nature today. Find more coverage of the work at nature.com/news, where you’ll also find the video of the plane in action.

Host: Benjamin Thompson

Later in the show, Lizzie Gibney will be joining us to talk about a momentous day for scientific measurement – that’s in the News Chat. Up next though, it’s time for the Research Highlights, read this week by Anna Nagle.

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Interviewer: Anna Nagle

The nests of Formica archboldi ants are filled with a rather macabre collection: the scattered body parts of their prey, ants from the Odontomachus genus. Until now, the way the Formica ants were able to take down their targets had been something of a mystery, as their Odontomachus prey are much larger and fiercer with spring-loaded jaws. Researchers discovered that the Formica ants are coated in chemical waxes that mimic that of their prey. These waxes disguise their odour, allowing the ants to get up close to their victims before delivering a precise stream of paralysing formic acid. Head over to Insectes Sociaux to find out more.

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Interviewer: Anna Nagle

In the semi-arid forests of northeastern Brazil, there stand tens of millions of huge earthen mounds. Equally spaced and up to four metres tall, the dirt hillocks cover an area the size of Great Britain and are thousands of years old. To find out more about them, a team of researchers cut into hundreds of the mounds. Most were solid dirt, but many contained entrances to an underground network of tunnels created by the subterranean termite Syntermes dirus. The team suggest that the discarded detritus is the result of the termites tunnelling travails. It’s thought that by dumping dirt at regularly spaced intervals, the termites minimise the time taken to reach a disposal zone from anywhere in their underground network. To dig that research out, go to Current Biology.

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Host: Shamini Bundell

Next up in the podcast, Ali Jennings has been learning about how neurons change their DNA.

Interviewer: Ali Jennings

Most cells in your body carry a copy of your genome – information written in DNA. And for the most part, the DNA sequence doesn’t change. However, things are a little different when you look at the brain. It’s been known for a while that cells in the brain can have different DNA sequences from each other, and not just small mutations – large chunks of genome can be copied and moved around. Having different DNA sequences in different cells is called genomic mosaicism, and it can have profound effects on gene function and cell survival, both positive and negative. But despite its importance, researchers have struggled to explain how it happens. Now, this might be about to change because a group of researchers have uncovered a possible mechanism for mosaicism. They wanted to understand how a single gene could vary between cells, so they chose a gene that is known to show many mosaic changes – the amyloid precursor protein gene, or APP for short. The researchers examined APP in neurons taken from human brains. But instead of reading the DNA sequence directly, they looked first at RNA which is transcribed based on the DNA. This let them spot possible changes in the APP gene in different cells. Here’s Jerold Chun, from the Sanford Burnham Prebys Medical Discovery Institute in California.

Interviewee: Jerold Chun

The first thing we did is we looked at RNA and what that revealed was a whole menagerie of previously unseen variants of APP. There were truncated forms that had lost the central portions of the gene, and they had been stuck back together to form new molecules.

Interviewer: Ali Jennings

This suggested that the neuron’s DNA contained different versions of the APP gene along with the original. And indeed, looking through the genome of the neurons revealed a whole host of APP gene variants. To visualise what they’d found, the team ran the samples through a gel to separate the differing DNA segments into different bands, depending on their size.

Interviewee: Jerold Chun

Sure enough, we pulled up a whole bunch of bands – not just a few, just a ton of bands. In fact, so many bands that they were smearing, they formed a smear within our gel.

Interviewer: Ali Jennings

So, how did each cell end up with multiple differing versions of the APP gene? And why did different neurons have different gene variants? Jerold and his team think that something unusual is happening to the RNA from the APP gene.

Interviewee: Jerold Chun

We have APP. It’s transcribed to produce an RNA, and then it can be copied in its entirety back into the genome as a genomic cDNA.

Interviewer: Ali Jennings

cDNA, or complimentary DNA, is formed when you copy an RNA sequence back into DNA, reversing the usual route of DNA being transcribed into RNA, which then produces protein. This reverse transcription could explain all of the APP gene variation.

Interviewee: Jerold Chun

It appears that they are reverse transcribed through an enzyme called reverse transcriptase. Reverse transcriptase is kind of a sloppy copier, and so it starts to produce errors as it copies.

Interviewer: Ali Jennings

So, neurons gain multiple, badly-copied versions of the APP gene thanks to the enzyme reverse transcriptase. This leads to the mosaicism of APP – the changes in its DNA sequence. Mosaic changes in the genome seem to occur as a part of normal neuronal activity, but there’s another side to the story. One of the reasons that Jerold’s team chose to look at APP is that amyloid precursor protein is linked to Alzheimer’s disease. One element of this is that people with Alzheimer’s show more mosaicism in their APP gene. The team wanted to see if their newly discovered mechanism – the reverse transcription of APP – was also responsible for the mosaicism in Alzheimer’s brains. So, they compared neurons from people with Alzheimer’s to neurons from healthy people.

Interviewee: Jerold Chun

And so, in a normal case, we don’t see nearly as many of these variants. In Alzheimer’s disease, we have more variants, variants that have more mutations or variations within them, and just plain old more variants inserting within the neuronal genome that may cause them to die. So, you might think of this in the case of Alzheimer’s disease as something gone awry with the normal process.

Interviewer: Ali Jennings

This work offers new insight into a potential mechanism underlying some forms of Alzheimer’s disease. But what does it mean for future treatments?

Interviewee: Jerold Chun

What was very, very interesting about that conclusion was that it suggested that maybe if you were able to inhibit reverse transcriptase, you might have an effect on the course of Alzheimer’s disease.

Interviewer: Ali Jennings

So, by stopping the sloppy reverse transcriptase from inserting lots of incorrect versions of APP back into the neurons’ genomes, you might be able to tackle the disease. We asked Pierluigi Nicotera, the Scientific Director of the German Centre for Neurodegenerative Diseases, for his thoughts on the new paper.

Interviewee: Pierluigi Nicotera

So, I must say when I started reading the paper I was initially a bit sceptical because it seemed a bit far-fetched. As I read over it I got more and more enthusiastic about it. To see the extent to which they went into making sure that there was absolutely a very thorough validation of their own observations and results I think is beyond many other papers that I’ve seen in my entire career. I think this is really a breakthrough.

Interviewer: Ali Jennings

But could targeting reverse transcriptase really offer a new root to treating Alzheimer's disease?

Interviewee: Pierluigi Nicotera

In principle, it’s absolutely plausible to me. If I had the possibility to run a clinical trial like this in a group of patients with an early stage of Alzheimer’s disease and without too much side effects, I think I would go for it.

Interviewer: Ali Jennings

This study has revealed a new way for neurons, although born with identical genomes, to become unique through modification of their own DNA. The paper shows how this mechanism is important for APP and Alzheimer’s disease, but both Pierluigi and Jerold wonder if this kind of DNA rewriting is also happening with other genes, perhaps contributing to the huge variety of neurons that we see across the brain.

Host: Shamini Bundell

That was Ali Jennings talking to Pierluigi Nicotera. You also heard from Jerold Chun, whose paper you can read over at nature.com/nature.

Interviewer: Benjamin Thompson

Finally this week, it’s time for the News Chat and joining me in the studio is Lizzie Gibney, Senior Reporter here at Nature. Lizzie, thanks for stopping by.

Interviewee: Lizzie Gibney

Hello Ben.

Interviewer: Benjamin Thompson

Well, for our first story today then Lizzie, we’re going to talk a bit about science history and perhaps history in the making, and you were there in the room. What’s been going on?

Interviewee: Lizzie Gibney

That’s right. So, I went to Versailles which is just outside Paris, for the official event that defined the redefinition of some of the units of measurement that we use, both in science and in the rest of the world.

Interviewer: Benjamin Thompson

So, what sort of measurements are we talking about then?

Interviewee: Lizzie Gibney

So, the unit of mass – the kilogram, the unit of current – the ampere, the unit of temperature which is the kelvin, and the unit of substance – the mole.

Interviewer: Benjamin Thompson

Well, before we talk about maybe what they’ve been redefined as, maybe we should talk about what they were defined from.

Interviewee: Lizzie Gibney

Well, the most famous one is the kilogram so that is the very last unit that’s defined according to a physical thing. So, this is like about a palm-sized block of metal, platinum iridium, and since 1889 that has been the kilogram. So, every time any kilogram measurement has been made in the world, that effectively has been calibrated at some point traced back to this one kilogram that sits in Paris. Now, that is clearly a bit of a ludicrous situation to be in because although it’s kept under lock and key in very careful conditions, someone could lose it, it might gain or lose a few atoms here or there, and that would be a big problem for the world.

Interviewer: Benjamin Thompson

So that’s where we were then, so how much does a kilogram weigh now then and how do you define it?

Interviewee: Lizzie Gibney

A kilogram still weighs a kilogram, although the former kilogram may not weigh exactly a kilogram anymore because the definition is changing. So, the definition is now going to be based on fundamental constants of nature so for mass that’s Planck’s constant. Now, it might not seem easy to see the relationship between mass at Planck’s constant. This constant relates frequency to the energy of a particle. It’s a quantum number. But the way that physicists have done this is a very, very clever system. They essentially put a mass on one side of a balance, and on the other side of the balance produced an electromagnetic force, and if you plug in the current and the magnetic field that you’re using to create that, you can relate these quantum constants to the mass that you have on the other side. Now, for years they’ve been doing experiments like these to come up with very, very, very precise measurements for Planck’s constant, measuring it against the kilogram. Now, in the future, what they’re going to do is they’re going to flip that experiment and they put in Planck’s constant and that they can use to measure any mass that they want on the other side of the balance.

Interviewer: Benjamin Thompson

Right, well that seems quite clever, but it also seems kind of complicated as well though. Why would we want to do that compared to this block that we have, and have been using?

Interviewee: Lizzie Gibney

Well for one reason as I said, that block is vulnerable because by definition it always has to weigh a kilogram. There was something very nice said at the conference which was if you left a fingerprint on that metal block, it would still weigh a kilogram, but the whole rest of the world would weigh less. That is clearly a problem. Other than that, the idea of defining mass by these experiments and fundamental constants means that you can actually do that anywhere – it democratises the system. So, if you have the right setup – which is very precise but they’re trying to make it easier and cheaper – then you can create an exact a kilogram as possible anywhere in the world.

Interviewer: Benjamin Thompson

Brilliant, well if that’s the kilogram then, you mentioned some others too, how have they changed then?

Interviewee: Lizzie Gibney

So that’s a little bit more subtle. Some of them involved the kilogram in their definition so they’ve shifted as a result of the change in the kilogram. One of the other more significant ones is the ampere, which used to be to do with two infinite wires and the force between them. Now, two infinite wires clearly can’t exist, so this was a hypothetical, abstract concept which wasn’t very satisfying for metrologists – the scientists who study measurement – so instead, it’s going to be defined in terms of the flow of individual electrons, so the actual charge on a single electron, which is just a wonderfully precise way to measure current.

Interviewer: Benjamin Thompson

Alright then, well let’s think about this then, so we’ve had these hundreds of years, these old standards, and now we’ve got these new standards. Is it a flip of a switch from old to new? When are they going to get brought in?

Interviewee: Lizzie Gibney

So, this was the official green light and people have been working on it for decades now, but it will come into force on the 20th May next year, so mark your diaries.

Interviewer: Benjamin Thompson

Well, you talked about kelvin there, what was the temperature in the room? How excited were people to be there and do this vote?

Interviewee: Lizzie Gibney

It was an absolutely wonderful atmosphere. As I say, there’s been a huge amount of work that’s gone into this and they were certain the vote would go through. They knew that it would be given this green light, but there was a standing ovation, there was champagne afterwards. They were all just wonderful quotes from people saying this is a dream come true, this is a thrill ride. Yeah, this is the biggest day in metrology for probably hundreds of years, since the founding of the SI system and even since the introduction of the metric system during the French Revolution.

Interviewer: Benjamin Thompson

Fantastic. Well, Lizzie, I know you were tweeting up a storm during the event. Where can people find your thread?

Interviewee: Lizzie Gibney

@LizzieGibney on Twitter.

Interviewer: Benjamin Thompson

Perfect. Alright, well let’s move on to our second story, and it also involves measuring something, but my goodness, I think the scales couldn’t be more different if we tried. What have we got for this one?

Interviewee: Lizzie Gibney

That’s right, so this is the first automated system for getting an early warning about volcano eruptions, at least at this stage a particular eruption which is of Mount Etna which is a volcano in Sicily.

Interviewer: Benjamin Thompson

I mean I know on the podcast before we’ve talked about volcanic eruptions. What’s been going on that’s allowed them to kind of do this?

Interviewee: Lizzie Gibney

This has been a study looking at the very low-frequency sound waves, infrasound, so these are waves that people can’t hear but that travel for thousands and thousands of kilometres, and this is something that scientists have realised that they can actually use in order to not only detect eruptions but to predict them.

Interviewer: Benjamin Thompson

Okay, and how does that work?

Interviewee: Lizzie Gibney

The idea is that as gas comes out of the magma, of the lava, ahead of an eruption, it causes the air within the crater to kind of move back and forth, to slosh, in a way that creates sound waves a bit like they would in an instrument, and just like in an instrument you can use those waves to figure out the geometry of the space inside which they’re moving. So, what scientists did was, starting back in 2010, they started to look at eruptions and listen out for these infrasound signals, and to figure out whether they could actually use that infrasound in order to predict when an eruption was going to happen.

Interviewer: Benjamin Thompson

And did they manage it?

Interviewee: Lizzie Gibney

They did. So, in a period of about eight years, the system was successfully able to predict 57 out of 59 events, which means it actually sent messages to the scientists an hour before the eruption took place.

Interviewer: Benjamin Thompson

I mean that’s really clever, and I know with these kind of disastrous events, I mean an hour is so useful, right?

Interviewee: Lizzie Gibney

Exactly, so often you need experts to vet information if they see an eruption might be on the cards – that takes time. This is an automated system that can work faster than that, which is really important in all the situations where time is really of the essence, so if you’re a community that lives near a volcano or maybe you’re a plane that’s flying into a region near a volcano, this is exactly what you need to know and fast.

Interviewer: Benjamin Thompson

I know that all volcanoes aren’t necessarily created equal and there’s different sorts. Is this something that could be used in other locations?

Interviewee: Lizzie Gibney

So, scientists hope that the system will work in other kinds of open vent volcanoes – a particular kind that exists. So for instance, one is Mount Pavlov in Alaska, so that might be another test in the future for this kind of early warning system, and they’re also actually already employing sensors in order to see if this will work in Iceland.

Interviewer: Benjamin Thompson

Great stuff, thank you, Lizzie. That’s it for this week’s News Chat and listeners, that’s it for this week’s show. As always, if you’d like to find out the latest news from the world of science, head over to nature.com/news.

Host: Shamini Bundell

And don’t forget to check out our video of the silent ion plane. You can go to nature.com/news or youtube.com/NatureVideoChannel to see it in action. I’m Shamini Bundell.

Host: Benjamin Thompson

And I’m Benjamin Thompson. See you next time.

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