Host: Benjamin Thompson

Welcome back to the Nature Podcast. This week, the roots of stress-related grey hair…

Host: Nick Howe

And the attitude needed to tackle climate change. I’m Nick Howe.

Host: Benjamin Thompson

And I’m Benjamin Thompson.

[Jingle]

Interviewer: Benjamin Thompson

First up on the show this week, we’ve got a story about a paper which looks at something that’s becoming quite close to my heart – why hair goes grey. But before we get into it, let’s look at how hair is coloured in the first place. So, within every hair follicle is a pool of cells called melanocyte stem cells, which differentiate into the pigment-producing cells that add colour to growing hair strands. But these melanocyte stem cells eventually start to dry up as humans get older, so they’re not available to top up the pigment-producing cells and hair starts to grey. But age isn’t the only reason for greying hair – stress has long been believed to play a role in it too. Here’s Ya-Chieh Hsu from Harvard University in the US.

Interviewee: Ya-Chieh Hsu

I think with regards to stress, there is definitely lots of speculation about the potential connection between stress and hair greying. I guess, however, before our work, there was no firm scientific evidence at least linking the two.

Interviewer: Benjamin Thompson

So, yes, plenty of anecdotal evidence – look at a picture of a world leader entering office and then look at one of them when they step down, for example – but now, Ya-Chieh and her colleagues report direct evidence of how stress can cause hair greying in mice, at least. In the first instance, Ya-Chieh and her colleagues wanted to show that stress was indeed capable of inducing hair greying, and they did so by subjecting groups of mice with black coats to different types of physical or psychological stresses, and these mice did indeed show signs of greying a lot faster than they normally would. The strongest effect was seen when mice were injected with a compound known as RTX – an analogue of the active ingredient in chilli peppers. To find out what was going on, Ya-Chieh looked at the hair follicles of these mice and saw that stress was having a big effect on their melanocyte stem cells.

Interviewee: Ya-Chieh Hsu

The stem cells are lost actually, in some severe cases, within five days, so that’s actually really short, which was actually a huge surprise to us.

Interviewer: Benjamin Thompson

Without the stem cells, there was no way to replenish the pigment-producing cells, and so the mice started to grey. But knowing that it happens wasn’t enough. Ya-Chieh wanted to work out exactly how stress triggers the loss of these stem cells. There have been theories that the immune system plays a role in stress-related hair greying, but Ya-Chieh showed that immunocompromised mice still went grey when injected with RTX, so their search for a culprit continued.

Interviewee: Ya-Chieh Hsu

We then thought that maybe cortisol, a stress hormone, may be involved, but that turned out to be wrong again.

Interviewer: Benjamin Thompson

In humans, cortisol levels are often raised in response to stress. However, mice that were unable to make cortisol-like hormones still went grey after being injected with RTX. So, Ya-Chieh went back to the drawing board again.

Interviewee: Ya-Chieh Hsu

So, essentially, after a lot of negative results, we finally found that stress triggers hair greying through the sympathetic nervous system.

Interviewer: Benjamin Thompson

The sympathetic nervous system is responsible for a stress response known as the ‘fight-or-flight’ response. Have you ever felt your heart start racing when you hear a sudden, loud bang? That’s your sympathetic nervous system kicking in. And it’s not just wired up to the heart.

Interviewee: Ya-Chieh Hsu

The sympathetic nerve innervates essentially all organs including the skin, and in the skin, the nerve fibres innervate each hair follicle. So, the nerve fibres are actually in close contact with the stem cells that are important in regenerating pigment.

Interviewer: Benjamin Thompson

During the fight-or-flight response, the sympathetic nervous system releases a neurotransmitter from the ends of nerve fibres. That neurotransmitter is noradrenaline, also known as norepinephrine, and it turns out that noradrenaline has quite an effect on melanocyte stem cells.

Interviewee: Ya-Chieh Hsu

What happens is that the stem cells first become hyperproliferative, which means they divide like crazy, then they start to differentiate to become these pigment-producing cells. So, once they differentiate, they start to migrate out from the hair follicle to other regions of the skin. Many hair follicles have lost all of the melanocyte stem cells so that the pigment cannot be made anymore and the hair loses its colour.

Interviewer: Benjamin Thompson

And this process was not reversible. Once the stem cells were gone, no pigment could be made. However, Ya-Chieh and her colleagues showed that suppressing the stress-induced proliferation of the stem cells prevented hair greying. And they also showed that blocking the release of noradrenaline or chemically disabling the sympathetic nervous system meant that stress had no effect on the mice’s hair colour. Christopher Deppmann, from University of Virginia in the US, researches nervous system development and has written a News and Views article about the new work. He was impressed with the thoroughness of the research.

Interviewee: Christopher Deppmann

So, this is really remarkable. It’s been almost anecdotal that stress can cause premature greying, but nobody has really understood the mechanism by which that happens. The Hsu Lab did this really beautiful series of experiments kind of basically saying, okay, well, here are all the different things that happen during stress – there’s an endocrine reaction, there’s a neuronal reaction, there’s an immune reaction – and we’re just going to one by one take them out. And by doing that molecular dissection essentially, they found that it’s really the neuronal component or the ‘fight-or-flight’ component that causes the depletion of these stem cells.

Interviewer: Benjamin Thompson

Of course, this work was done in mice, and humans may react differently. Ya-Chieh does have some evidence to show that noradrenaline does have an effect on human melanocyte stem cells in vitro, but she emphasised that it’s early work. Christopher says that this research throws up other things to look at too.

Interviewee: Christopher Deppmann

I think maybe the most obvious question is whether or not this mechanism of stress-induced greying is the same as age-related greying. It would be interesting, in the future, to age mice and just kind of modulate what’s going on with their sympathetic nervous system to see if you can slow down greying or speed it up. I think that there are maybe some more esoteric questions, like is there an evolutionary component to this, and then I think that there are therapeutic implications like, could this be used to have anti-greying therapies?

Interviewer: Benjamin Thompson

Any therapies would be a long way off as this new work is just the start of the story. It does appear to answer how stress can cause hair greying, but why does the system exist at all?

Interviewee: Ya-Chieh Hsu

The real answer is I don’t know. I think we do have speculation of why this might be important and my lab is working hard to think about if we can essentially test whether there are specific reasons why this is happening in the first place. Maybe what’s important is that the sympathetic nerve provides a way to modulate the behaviour of melanocyte stem cells so that the stem cells potentially can react very quickly to other things, for example, light, for example, UV – something that you do want to produce pigment very quickly.

Interviewer: Benjamin Thompson

That was Ya-Chieh Hsu from Harvard University. You also heard from Christopher Deppmann from the University of Virginia. You can read Ya-Chieh’s paper and Christopher’s News and Views over at nature.com.

Host: Nick Howe

Later in the show, we’ll be hearing about the latest on an outbreak of a new virus in Asia that’s got some researchers concerned. But first, here’s this week’s Research Highlights, read by Dan Fox.

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Dan Fox

The bones of 22 giant ground sloths found in Ecuador suggest that these elephant-sized herbivores may once have lived in herds. Researchers have analysed 575 bones from a fossil site in Ecuador and found the remains of both adult and juvenile giant ground sloths that seem to have died en masse. The fossils were found in what may have once been a marshy environment containing a lot of digested plant matter in the form of fossilised dung. This suggests that these sloths may have been wallowing in mud – a similar behaviour to modern animals such as hippopotamuses. Given the range of ages observed and that the remains were found in a single fossiliferous layer, the team hypothesised the sloths lived in a group and all perished at the same time, possibly of thirst or of a disease that spread through their wallow. Read that paper in full at Palaeogeography, Palaeoclimatology, Palaeoecology.

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Dan Fox

The fallout from atom bombs could be used to put an end to counterfeit whisky scams. Vintage whisky is big business, with one bottle fetching over US$1 million in 2018. Now, researchers at the University of Glasgow have found a way to accurately date different samples of Scotch and they do it using the large amount of carbon-14 deposited in the atmosphere by the atom bomb tests of the twentieth century. The new technique compares the ratio of carbon-14, -13 and -12 in a sample against the values of a whisky with known production dates. They’ve already found multiple imposters, including one drink, reportedly from 1863, that was actually made after 2007. Drink up that research at Radiocarbon.

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Interviewer: Nick Howe

Let me take you back to just over four years ago. On 12 December 2015, at the annual United Nations Climate Change Conference, the Paris Agreement was adopted. After the disappointment of previous COP meetings, the Paris Agreement was widely considered to be a success. Now, finally, states were committed to take substantive action on climate change. Fast-forward to the present day and we’ve seen temperature records broken, heatwaves, droughts and wildfires, and whilst you can argue whether or not these are direct consequences of climate change, it’s certainly the case that they are at least made more likely by it. And it seems clear that most countries aren’t doing enough to change things. The Climate Action Tracker, an independent tracker of government climate action, suggests that the majority of the biggest emitters are taking insufficient action on climate change. The UN also recently released the emissions gap report which suggests that the current pledges will only limit warming to 3.2 ˚C – a way away from the 1.5 ˚C that the International Panel on Climate Change suggests will allow us to avoid the worst impacts of climate change. This year – 2020 – is a critical one, as states need to renew and improve upon the pledges made in the Paris Agreement. There is no room to backpedal. Christiana Figueres was executive secretary of the UN Framework Convention on Climate Change when the Paris Agreement was adopted. She’s written a Comment in this week’s Nature about what is necessary to tackle climate change. So, what is necessary?

Interviewee: Christiana Figueres

There is a mythology here about climate change, composed of several misunderstandings. The first is that climate change is too complex and too overwhelming and too daunting to undertake. It is understandable that people feel that way, but actually, when you break it down and you understand what each of us has contributed negatively and what we can contribute positively to the solution, then you see that this is actually in our hands. And the second mythology is that we have a lot of time to deal with this, that climate change is somewhere else other than where we live and certainly not within our lifetime. Both of those are untrue.

Interviewer: Nick Howe

So, how to dispel these myths? According to Christiana, in a word, optimism.

Interviewee: Christiana Figueres

Well, let me just explain what I mean by optimism. I don’t mean closing my eyes to the challenges – I mean quite the opposite. When I speak about optimism, I mean opening your eyes, understanding, doing the reading, asking the questions, so that you really understand what it is that we’re up against and then precisely because it is tough, then go at it with an absolute determination to collectively address and solve the issues. That’s what I mean by optimism. I mean gritty determination because, frankly, we don’t have any other option.

Interviewer: Nick Howe

Now, optimism is all well and good, but to meet a 1.5˚C target requires reaching net zero emissions by 2050. Such a goal will require greater collaboration between states and further pledges to cut emissions. I reached out to Lavanya Rajamani, an expert on international environmental law and an author for an upcoming IPCC report. I wanted to know – when it comes to garnering the type of international agreements required to face this huge challenge – is optimism enough?

Interviewee: Lavanya Rajamani

I think it’s a very important element of what we need to be doing and thinking and feeling right now, and what is the alternative, in any case? The fatalism would really be toxic for politics and also for our emotional health. So, we certainly need it, but it is not all we need, of course.

Interviewer: Nick Howe

So, what else do we need? Lavanya explained to me that an important reason that Paris was so successful was that much of the work to reach an agreement had happened beforehand – a lot of it by Christiana herself. Countries were determined to work towards it and there was political will to push for agreement – it’s something their constituents wanted. Now, going forward to the new pledges in 2020, Lavanya thinks there are several things that can continue to push states towards more action.

Interviewee: Lavanya Rajamani

Climate litigation is now very much on the upswing. There are numerous national courts that are finding ways of pushing their states to do more. There are non-state actors that are pushing for greater action. We have a lot of activism – the Extinction Rebellion, the climate school strikes. There is so much that is a part of the push factor for ambition and all of that plays a role in creating the optimum conditions for states to make more ambitious targets.

Interviewer: Nick Howe

In the end, Lavanya and Christiana agree that it’s everyone’s responsibility to push for action on climate change. Here’s Christiana again.

Interviewee: Christiana Figueres

I think the one thing that we need to realise is that we can all contribute because we all think that it is far away, that it’s complex, that we can’t do anything, and every single one of us can contribute. That’s the real a-ha. The a-ha is it is we humans who have caused climate change – it is no one else. It is one species on the planet – that’s the human species – that has caused climate change, and it is that one species that can actually solve climate change, and we have to understand that. And every single one of us being a member of that human species can actually contribute to the solution. That doesn’t mean that one individual can solve it, but it does mean that the collectivity of all of us individuals can solve it if we understand the urgency and we understand the solutions that we can bring.

Interviewer: Nick Howe

This week, world leaders are gathering in Davos, Switzerland for the World Economic Forum and here, the message is clear: climate change is one of the biggest risks for economies in the future, and states need to act. So, will they?

Interviewee: Christiana Figueres

Yeah, that remains to be seen at the end of this year, and how much confidence the private sector that is moving forward on decarbonising, how much confidence they can give to the governments that then will have to increase their ambition at the end of the year remains to be seen. It is, frankly, a really difficult and challenging test to the world to see if we are at that point where not only the private sector but our governments also understanding the consequences here and willing to increase their ambition.

Interviewer: Nick Howe

That was Christiana Figueres, former executive secretary of the UN Framework Convention on Climate Change. You also heard from Lavanya Rajamani from Oxford University, right here in the UK. You can read Christiana’s Comment over at nature.com/opinion.

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Interviewer: Nick Howe

Last up on the show, it’s time for the News Chat, and I’m joined in the studio by Ewen Callaway, senior reporter here at Nature. Ewen, hi.

Interviewee: Ewen Callaway

Hello.

Interviewer: Nick Howe

Thanks for joining me, Ewen, and first up this time, there’s a new virus that’s been found spreading through Asia, and this is a developing story, so some of the things we say may have changed by the time this goes out, but Ewen, what do we know at the moment?

Interviewee: Ewen Callaway

What we know at the moment is that this outbreak is centred in a city in southern China called Wuhan. It’s an outbreak of a respiratory disease caused by a virus that’s called a coronavirus, which is a large family of viruses that includes the common cold but includes the SARS virus that caused an outbreak about a decade ago in China and Hong Kong and spread around the world. So, people are really on guard for another SARS-like outbreak that spreads all over the world, kills hundreds of people and has a high death rate. And at this point, we don’t really know. It’s changing by the hour.

Interviewer: Nick Howe

So, we know that it may have originated in Wuhan, but where has the virus itself actually come from? Do we know anything about that?

Interviewee: Ewen Callaway

So, the working hypothesis right now is that the virus emerged in a seafood market in Wuhan. Of the initial forty or so cases, most of those people had been to this seafood market and so investigators are working on the hypothesis that some unknown animal transmitted the virus to people. What is not certain is the extent to which humans are transmitting the virus between them themselves, which would be a lot more cause for concern. Just yesterday, Chinese health authorities reported that their health workers in Guangdong province, so not in Wuhan, had become sick with this illness or acquired this virus, which strongly suggests that there is some amount of human-to-human transmission going on. And so, the real question going forward is to determine how extensive this human-to-human transmission is.

Interviewer: Nick Howe

There have been all these cases in China, but I understand it’s spread beyond China as well.

Interviewee: Ewen Callaway

Yeah, that have been a number of cases – I think four confirmed in Japan, South Korea and Thailand – and these are people, some of whom, were at this seafood market but some of whom were not, who had acquired this virus and sometimes had very serious respiratory illness. Some cases have been quite mild. But yes, there have been some international cases.

Interviewer: Nick Howe

So, it’s obviously still early stages, but have we got a sequence for this virus? Do we know anything about its genetics?

Interviewee: Ewen Callaway

Yes, we do. The different health authorities that have been dealing with cases have been quite good in making public genetic data. So, I think we now have more than a dozen sequences from patients both in China and internationally, and there are a few things we can say from these sequences. Number one is that it looks like it’s most closely related to what they call SARS coronaviruses. It’s not identical by any means to SARS, but it’s most similar to SARS and these coronaviruses that circulate in bats. The other thing, and this is really a moving target because people don’t have a lot of data and the data is quite rough and ready and it may contain errors, but so far, people are finding that there’s not a lot of genetic diversity in the viruses found in different patients, and that could mean a couple of things. It could mean that the virus isn’t changing much. It could mean that the people who got infected got infected from a similar animal source carrying very similar viruses, and it suggests to them, based on this limited analysis, that the virus made the jump from animals to humans relatively recently and around the time that the first cases were detected in December. But again, this is really a moving target right now and each new piece of data changes things.

Interviewer: Nick Howe

So, in terms of trying to contain this or control this, is there anything that can be done?

Interviewee: Ewen Callaway

Not really. I think right now, the focus is really on understanding the origins of this virus and whether it’s spreading before you can really do anything else, so I think people are in, I wouldn’t say frantic, but people are trying to gather information about this outbreak so they can maybe do something about it, but there’s not a vaccine. It’s not clear how exactly it’s spreading between humans. It’s not clear how long people are infectious for. There really are a lot of unknowns and there’s a real urgency to answer questions, not least because this weekend, the Chinese New Year holiday begins and there’s a period of a lot of travel around the country and around the region, and so this is an event that could really fan the flames of this kind of outbreak. So, there’s a lot of work to be done and it’s really quite urgent.

Interviewer: Nick Howe

Well, as we said, that is an emerging story so I’m sure we’ll get more details in the next few days. Moving on to our second story, there’s been some new research published about ancient African population genetics. Ewen, what can you tell me about this?

Interviewee: Ewen Callaway

Yeah, this is a paper from some researchers led by David Reich at Harvard Medical School, and it is four ancient genomes from West Africa, from Cameroon, and it’s hugely exciting to be able to get more ancient genomes from Africa. We really have a paucity of ancient human data from this region because the climate there is not necessarily as conducive to preservation, and maybe the archaeology that’s been done there isn’t as extensive as in other parts of the world, so any ancient human genome from Africa is exciting. And this, in particular, is looking at the site called the Shum Laka rock shelter in Cameroon, which is a really important site because it spans quite a long period, I think going back over 30,000 years right up to a few thousand years ago, and it covers a lot of major transitions in early African history – development of agriculture, new technologies and the potential emergence and spread of a language family that’s spoken widely across Africa – so it’s a really important and interesting region and it’s interesting to get ancient DNA from individuals from this area.

Interviewer: Nick Howe

So, what have these four individuals told us? What information have we got from it?

Interviewee: Ewen Callaway

Yeah, so, the paper describes genome data from four children, two pairs of children, who lived 8,000 and 3,000 years ago, and I think one of the top lines from this is that these pairs of individuals, they’re pretty closely related to one another. So, for 5,000 years, maybe at least in this population, there wasn’t a whole lot of genetic change. There was a fair amount of stability. But then when the researchers looked at whom they were most closely related to amongst contemporary Africans, they found that there was a disconnect between these ancient individuals and modern inhabitants of Cameroon and these parts of West Africa, and in fact, these individuals are most closely related to hunter-gatherers in western Central Africa, sometimes known as pigmies. So, that’s a bit of a surprise, suggesting that there maybe has been a population shift in this part of time. It’s really hard to know when you only have four samples, so it’s raising a lot of interesting questions, but yeah, I think this disconnect between modern and ancient is something that jumps out.

Interviewer: Nick Howe

So, I’m sure there’ll be a lot more debate and analysis of this, and we’ll find out more about it in the future, but thank you so much for joining me, Ewen.

Interviewee: Ewen Callaway

You’re welcome.

Interviewer: Nick Howe

And listeners, for more on those stories, head over to nature.com/news.

Host: Benjamin Thompson

That’s all we’ve got time for this week. But, if you haven’t had enough science, don’t forget you can check out our sister podcast, Science Talk by Scientific American. Find that wherever you get your shows. I’m Benjamin Thompson.

Host: Nick Howe

And I’m Nick Howe. Thanks for listening.