What the atomic structure of enamel tells us about tooth decay

Host: Nick Howe

Welcome back to the Nature Podcast. This week, drilling down into the structure of teeth…

Host: Shamini Bundell

And a mysterious planet orbiting close to a distant star. I’m Shamini Bundell.

Host: Nick Howe

And I’m Nick Howe.

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Interviewer: Nick Howe

If you look around you now, how many objects do you think will last a lifetime? I’d wager not that many. But there are about 32 in very close proximity that have a pretty good shot. I’m talking about your teeth. Despite near constant challenges from chewing, drinking and maybe grinding, if you look after them, they can last a lifetime.

Interviewee: Derk Joester

And if you think about that, there’s very few materials that have that kind of service life.

Interviewer: Nick Howe

This is Derk Joester, a researcher who specialises in the materials made by living organisms.

Interviewee: Derk Joester

What makes this even more impressive is that a lot of our other tissues and materials in the body contain cells that will sense damage and repair damage. There is very little that a body can do to maintain that material.

Interviewer: Nick Howe

This impressive durability is due to the outer layer of the visible part of teeth – the enamel. But you may be thinking that not everyone’s teeth are so durable. For instance, you may have fillings or have suffered from tooth decay or cavities, also known as caries.

Interviewee: Derk Joester

And so, caries, in a way, are sort of the Achilles’ heel of enamel. Because enamel is such a fascinating tissue, we were really interested to understand the degradation of enamel. So, what makes it weak actually against acid attack and how could we possibly help fix that problem.

Interviewer: Nick Howe

And to do this, Derk and his team have been looking right down to the nanoscale atomic composition of enamel, which they’re publishing this week in Nature. Now, enamel is made from tiny little rods, and each of these rods are composed of a mass of even smaller crystallites. It was the composition of these microscopic crystallites that Derk was particularly interested in. To find it, he used a technique called atom probe tomography. This works by taking a thin slice of enamel, then bombarding it with pulses of a laser beam. This pushes atoms – freed from their electrons – out of the structure. Different sized atoms get pushed different amounts, so as you bombard the whole structure and understand what sized atoms are where, you can find the atomic composition of a sample. So, what was the composition of the crystallites like? Well, that question can maybe be best answered if you compare the composition in human enamel to that in something else.

Interviewee: Derk Joester

We started this investigation looking at rodent enamel. What we discovered there is that the crystallites themselves are glued together with a material that contained more magnesium than the rest of the crystals, and that made it more soluble. That was part of a mechanism by which enamel dissolves, and so we wanted to see whether we had a similar effect in human enamel. We did find this material again in the human enamel, but we were then greatly surprised to see that the crystallites themselves have a much more complex inner life, if you wish.

Interviewer: Nick Howe

So, human enamel crystallites may be glued together with a similar magnesium glue, but the insides seemed a bit more complicated than their rodent comparisons. Essentially, there was on ordered arrangement of ions, such as sodium and magnesium, and the complex way in which they were arranged might give us a clue as to the strength of human enamel. For instance, there were layers rich in magnesium ions and others rich in calcium ions. The position and different sizes of these ions put pressure on the overall structure, and this means that there were mechanical stresses generated in the crystallites. Stress may sound bad but, in this way, it can actually make things resistant to cracking. This is similar to the composition of some phone screens that have been designed to not smash when you inevitably drop them on the floor. Derk thinks this could explain how enamel is so strong, but we can’t be sure.

Interviewee: Derk Joester

So, we know that in other materials, such a compressive stress in a surface layer does increase the strength, the fracture strength, and so we think that that may also apply to enamel, but because we can’t make crystallites without that feature, we can’t do an experiment to show that that is true or not.

Interviewer: Nick Howe

For Derk, the more interesting implication of the crystallite composition is that it shows that they’re more able to be dissolved by acids. In other words, they’re more liable to tooth decay.

Interviewee: Derk Joester

So, the core that has all these other ions inside, magnesium and sodium and so on, that will be more soluble, and if it is more soluble then we should be able to detect that. And indeed, we were able to show that if you take a surface of enamel and very, very carefully and briefly expose it to acid, then the crystallites dissolve in a very particular fashion. The glue dissolves between the crystallites but remarkably, in human enamel, the centre of the crystal also dissolves out, so the crystallites hollow out.

Interviewer: Nick Howe

Again, Derk is cautious in his interpretation, as whilst this does show that the crystallites are more prone to dissolving in acid, it may not be the case for the enamel overall. But these results may allow researchers insight into how to make teeth harder and less prone to decay.

Interviewee: Derk Joester

That’s clearly our hope, but I would say this is a first step. So, we are currently working on integrating our findings into a model of how enamel dissolves because enamel dissolution is fundamentally what caries are, and so our hope is that we will be able to understand the dissolution processes and that in turn then will allow us to interfere better.

Interviewer: Nick Howe

That was Derk Joester from Northwestern University in the US. We’ll pop a link to his paper in the show notes.

Host: Shamini Bundell

Later on, we’ll be finding out about a Neptune-sized planet in an unexpected place. Before that, though, it’s time for the Research Highlights read by Dan Fox.

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Dan Fox

A 3D bioprinted ‘cardiac patch’ could repair the damage caused by a heart attack. Myocardial infarctions or heart attacks cause permanent damage to heart muscle cells called cardiomyocytes. Researchers are interested in trying to repair this damage using stem cells, but integrating these has proved challenging. Now, a team of researchers have used 3D bioprinting to create patches of stretchable gel scaffolding that matches the curvature of the heart and can even expand and contract as the heart beats. These patches were loaded with cardiomyocytes and attached to the hearts of mice that that had survived an experimental form of myocardial infarction. After four months, the patches had acquired a blood supply and had stimulated heart muscle formation, providing a potential avenue for future therapies. Take that research to heart at Science Advances.

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Dan Fox

Scientists have developed a new technique to reveal the structure of superlattices into superb detail. Superlattices are made of two carbon sheets, each only one atom thick, stacked on top of each other and slightly misaligned. The resulting structure can have surprising properties, like super conductivity, making them of great interest to physicists. To reveal the details of these structures, a team of researchers used the technique that applies alternating electric fields to a material, creating slight changes in the material’s surface. A fine probe then detects these changes to reveal the material’s topography. The technique showed that the superlattices contain tile-sharped regions, and interactions between these and the materials’ electrons gave rise to the interesting physical properties. Examine the structure of that research more closely in Nature Nanotechnology.

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Host: Shamini Bundell

Now, it’s time for a space mystery. A team of researchers have disovered a planet in a very unexpected place – the ‘hot Neptune desert’. Reporter Ali Jennings called up lead researcher David Armstrong from the University of Warwick in the UK looking for answers, the and the first question: what actually is the hot Neptune desert?

Interviewer: Ali Jennings

The hot Neptune desert – it’s not a sort of dry, sandy, rocky…

Interviewee: David Armstrong

No, I mean it’s a desert in parameter space, so a bit more of an abstract concept. So, if you look at all the planets we know of and sort of separate them by how big they are and how far away from the star they are, there are certain bits where planets just don’t tend to appear. So, the Neptune desert is one of those. There’s a kind of region very close to the star where Neptune-sized planets are just extremely rare, if not completely absent. We see a lot of Jupiters very close to the star. They’re called the hot Jupiters. And there’s a few sort of Earth-sized planets very close to the star as well, very hot lava worlds. But in between, we just don’t tend to find planets at all.

Interviewer: Ali Jennings

So, what were you originally looking for in this region of space?

Interviewee: David Armstrong

We used candidates found by the TESS NASA mission, which observes many, many stars for about a month at a time, looking for small dips in their brightness caused by planetary transits. And then in this case, we looked at a very interesting candidate and thought, ‘Okay, this is an unusual size for where it is.’ We started observing it and the signal turned out to be very, very big compared to what we expect. Initially, we thought it was probably going to turn out to be a binary star or something like that, but after a few more data points it became clear that this was still a planet, just a very rare one.

Interviewer: Ali Jennings

So, normally, you would expect to see something that’s either sort of Earth-sized or smaller, or you would expect to see something that’s Jupiter-sized, so really quite big. But what did you find?

Interviewee: David Armstrong

So, we found the planet TOI-849b, which is very slightly smaller than Neptune, actually, but despite being slightly smaller, it’s two and a half times more massive.

Interviewer: Ali Jennings

So, it’s a sort of solid version of Neptune.

Interviewee: David Armstrong

Yes. It’s got a similar density to the Earth, which is unheard of for a planet the size of Neptune or anything close to the size of Neptune. We think this planet, TOI-849b, has, at most, a few percent light gases, and the rest is just heavy elements. So, under the standard theories, it really should have accreted a very large envelope of gas, and the question is why it hasn’t.

Interviewer: Ali Jennings

And do you have any possible ideas?

Interviewee: David Armstrong

Maybe it started forming as a planetary core but somehow just never collected all the atmosphere and gas that we would expect a core of that size to start accreting. When the planets form, there’s a protoplanetary disk of just dust and gas surrounding the star, and the planets form out of that disk. You can actually open up gaps in the disk, and if you open these gaps up, maybe there’ll be less gas around for the core to accrete, and so it would be able to start forming the core and then just skip out on the next phase. The other angle is that say it actually formed like a normal gas giant, and so really it was something close to the size of Jupiter or at least approaching it. Then the question comes: how can you remove all that gas to leave what we see today? And there’s a few ways you can do that, and so one of them is if it collided with another forming planet quite late in the formation stages. When two planets collide, roughly when there’s no gas left for them to accrete afterwards, they blow off the envelope that they have and you’re left with a very high-mass core like this.

Interviewer: Ali Jennings

You have these different possible explanations. How are you going to find out which one it is?

Interviewee: David Armstrong

So, there’s a few sort of follow-up observations that we’re currently trying to pursue that can help with that. So, something called the Rossiter–McLaughlin effect, for example, is where we study whether the planets aligned with the host star’s spin axis. So, different formation mechanisms would lead to different sort of end states that we can try and explore. If it sort of formed quite, I don’t want to say calmly because this is anything but a calm sort of situation, but it migrated through the disk and just opened a gap but didn’t collect that gas, for example, we’d expect it to still be fairly well aligned with the host star’s spin axis. Whereas if it underwent something more aggressive like colliding with another planet, it’s most likely that it would be very misaligned and we’d see that in the outcome of this Rossiter–McLaughlin effect.

Interviewer: Ali Jennings

So, that’s trying to untangle the mystery of how it got to where it is, but what can you learn about planetary cores in general from being able to see this one so well?

Interviewee: David Armstrong

So, we’ve talked a lot about how unusual it is to find a planet this dense with so little atmosphere, but you can actually turn the question around. So, when it’s got such a small atmosphere and it’s sitting where it is so close to the star, any atmosphere that was left should have been burned away very quickly. Then it sort raises another question of why does it still have this atmosphere? And so, we think one possible reason for that is that the core that’s left is essentially being heated up by the radiation from the star and evaporating certain amounts of materials to sustain a small atmosphere, something that we could then try and measure. And if we can detect any elements in it then we can say that we’re actually potentially detecting elements that have some from the planetary core itself, and that lets us study its composition from another angle.

Interviewer: Ali Jennings

Can it tell us anything about the cores of the planets in our Solar System, for example?

Interviewee: David Armstrong

It’s not exactly like the planets in our Solar System, so the core is likely larger even than Jupiter’s core from what we understand. What it can tell us about is the sort of background context of planet formation. So, working out how the planets in our Solar System formed is kind of like working out our own history, really, if you go back far enough. And really, with only our own Solar System to study, we just don’t have the sample to really understand the whole proper context of that. So, looking at how other planets form and how it works in different exoplanetary systems can really teach us about the wider galaxy and background of planets of which our Solar System is a part.

Host: Shamini Bundell

That was David Armstrong from the University of Warwick. You can find the full paper in the show notes.

Host: Nick Howe

Last up, it’s that time again. We’re having a look at some other non-corona science news which has been highlighted in the Nature Briefing – that’s Nature’s daily pick of science news and stories. Shamini, what’s caught your eye this week?

Host: Shamini Bundell

So, this week, I have seen an article in Nature which isn’t exactly about CRISPR babies but it’s about why CRISPR babies might actually be kind of a bad idea.

Host: Nick Howe

And pray, tell, why might they be a bad idea, ignoring all the other ethical and legal things that we’ve talked about previously on the show?

Host: Shamini Bundell

Well, I mean in many ways it is all tied down to the problems that people have anticipated. So, quick catch up. CRISPR – really cool gene-editing technique. It lets you snip DNA and then you can do things like rewriting bits of it, so really useful and really widely researched. The controversial thing is using it in human embryos or germline cells. The reason being, if you do DNA rewriting in a cell that could go on to form a person and then form that person’s children, any change you make is potentially there forever, right? It’s going to get carried on via everyone’s descendants so you have to be really sure that what you’re doing doesn’t have any unwanted side effects. So, obviously, people have been looking into this and there have been some papers, that haven’t been officially published in a journal yet but they’ve been published in a preprint server, that have found really quite bad side effects from human embryo CRISPR–Cas9 editing.

Host: Nick Howe

Right, and so what are these bad side effects? Are they mutations where they don’t want them and things like that?

Host: Shamini Bundell

Yeah, so in general, when you’re doing CRISPR editing, you want one thing to happen, right, so maybe you want to fix a gene that causes a heritable disease. And what these three papers have found in some sort of experimental embryo editing just for research is a whole range of other effects, including DNA rearrangement, big chunks of the DNA getting accidentally cut out that isn’t supposed to be there. And in particular, these studies have found these effects quite close to the target site, so they’re close to the site where the CRISPR editing was aiming rather than sort of off-target effects miles away. And apparently, this location of them being really close makes them quite hard to spot, so maybe that’s why this hasn’t been noticed before.

Host: Nick Howe

Right, so it’s not quite precise enough to make the effects you want, and they’re hiding because they’re close to where you do want them.

Host: Shamini Bundell

Yeah, so they’ve had unintended side effects and the fact that these three studies have all found sort of not identical – there are definitely differences and they interpreted the data differently – but they’ve all found problems with this gene editing in embryos. This is kind of a really bad sign for the progress of CRISPR to hopefully one day be able to be used to permanently get rid of, let’s say, hereditary diseases. But this research is very much ongoing so I have no doubt that we’ll be hearing a lot more about that. And what story have you found today, Nick?

Host: Nick Howe

Well, I don’t know if you’re familiar with the film Iron Sky at all?

Host: Shamini Bundell

No, what’s that one?

Host: Nick Howe

So, this is a film about how after the end of World War II, some Nazis actually escaped and they set up a base on the Moon so they could then reinvade the Earth in 2018.

Host: Shamini Bundell

It sounds brilliant, not the usual kind of stuff that we’d cover on the Nature Podcast but that’s fine. I’m willing to branch out.

Host: Nick HoweWell, the reason I bring it up is that in reality, some Nazis did actually make it to the Moon in the form of craters.

Host: Shamini Bundell

Nazis turned into craters and went to the Moon. You’ve not got me convinced yet, Nick.

Host: Nick Howe

Okay, to be fair, I’m stretching the truth somewhat. So, what this story is about is that on the Moon, there are two craters that are named after prominent scientists, but those scientists actually turned out to be Nazis, and they were unintentionally named after them by the International Astronomical Union.

Host: Shamini Bundell

Okay, how do you accidentally name something after a Nazi? So, these were named, presumably, well after the Second World War?

Host: Nick Howe

Yeah, so one was named in 1970 after Johannes Stark, who won the Nobel Prize in 1919, and another one was named in 2005 after Philipp Lenard, who won a Nobel Prize in 1905, and the union say that they just didn’t know that they were Nazis and the references they were using when deciding never mentioned the fact that they were.

Host: Shamini Bundell

And do you mean that they were just part of society in Nazi Germany or they personally shared a lot of those political viewpoints?

Host: Nick Howe

Yeah, so that’s a fair question because a lot of scientists at the time went along with it because it was quite a hard thing to not go along with, shall we say, but these guys were outspoken with anti-Semitic views. They were proponents of this theory of Aryan physics, which is basically the idea that people who were descended from the Aryan race are much better at physics, they just have a natural capacity for it. And they also attacked, in the press, Albert Einstein quite a lot because he was a Jew.

Host: Shamini Bundell

Oh dear, and now they’re on the Moon. Oh no, okay, I see how we’ve got to the Nazis on the Moon situation.

Host: Nick Howe

Yeah, and so a quantum physicist, Mario Krenn, noticed this after reading a book that was written about them, and contacted the union and said, ‘Hey, should we have craters named after Nazis?’ In fairness, they’re on the far side of the Moon so you never see them, but still, that doesn’t seem like a good move. And the union has very quickly responded and they’re going to change them now that they know, but it’s sort of an interesting case in we remember some of the great things that scientists do but we may not remember like everything that happens, especially a long time after the fact.

Host: Shamini Bundell

And has the International Astronomical Union got sort of like replacement names for people who they think are more deserving to have a Moon crater named after them?

Host: Nick Howe

So, there’s nothing yet. They have moved very quickly to say they’re going to get rid of the names and come up with some new ones, but there’s been no suggestions as far as has been reported, but hopefully soon we’ll see Nazis not on the Moon.

Host: Shamini Bundell

Hopefully we’ll see some new and exciting scientists on the Moon. That would be great.

Host: Nick Howe

Yeah, definitely, and it’s just part of an ongoing and quite interesting conversation about who and how we remember people in science. Like these ones are quite clearly, you probably don’t want to be naming things after them, but there are other scientists where things are a little bit more grey, and I think this is something that we’ll be continuing to talk about as a science community for a long time.

Host: Shamini Bundell

Yeah, it’s very relevant to decisions society in general is taking right now.

Host: Nick Howe

Well, thanks for chatting to me, Shamini, and listeners, we’ll put links to everything we’ve just discussed in the show notes. And if you’re interested in more, but instead as an email delivered daily, then make sure you check out the Nature Briefing. We’ll also put a link to that in the show notes.

Host: Shamini Bundell

Oh, and I’ve got one more thing to mention before we wrap up and that is flying snakes, obviously. If you didn’t know, snakes can fly. Don’t worry, only certain snakes can fly, but it is a very impressive feat, and we’ve got a video about it on our YouTube channel. We’ll put a link to that in the show notes so check that out if you want to find out how wiggling from side to side can help snakes glide further.

Host: Nick Howe

I wonder if it could help me glide further as well? But that’s all for this week. If you want to get in touch with us then you can reach us on Twitter – we’re @NaturePodcast – or send us an email – we’re podcast@nature.com. I’m Nick Howe.

Host: Shamini Bundell

And I’m Shamini Bundell. Thanks for listening.