Podcast transcript

Podcast transcript

Studying brain disease using dictyostelium

 Professor Paul Fisher and Dr Sarah Annesley

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Matt Smith

Hello there. This is me, Matt Smith, welcoming you, the listener, to this particular La Trobe University podcast. Today it’s all about microbiology, and we’ll be hearing from the head of Microbiology, Professor Paul Fisher and Dr Sarah Annesley, who’s the co-leader of the Microbial Cell Biology Group at La Trobe University. They’ll be speaking about a model organism called Dictyostelium.

Paul Fisher

Dictyostelium is a cellular slime mould, so it actually goes by a variety of names. The people in the field now prefer to call is a social amoeba because it doesn’t sound quite as off-putting as slime moulds do.

Matt Smith

That’s a marketing name for a slime mould, isn’t it?

Paul Fisher

Yes, and in fact there’s good reason for it because we’ll talk about the life cycle I guess in a minute. But there is a stage in the life cycle where a large number of these cells get together and function as a social group. They also function as a true multi-cellular organism. So it belongs to a group of organisms called the amoebozoa, that are related to animals and fungi. They all split off around about the same time during evolution, after the plants had split off and so they have actually conserved a lot of those ancestral functions that are found in cells of animals and fungi yeasts and slime moulds.

It lives in soil, normally. It eats bacteria by engulfing them and digesting them and divides as any other amoeba would, and multiplies in the soil. So it’s one of the main predators of soil bacteria in fact.

Matt Smith

Tell me about its life cycle then. What did you find of interest in its life cycle?

Sarah Annesley

It starts off, like Paul said, as an amoebae, which feeds on bacteria, but then it also goes through a multi-cellular stage and that’s basically, when there’s no food left, or no bacteria left, instead of just dying, they’ve developed a way that they can avoid starvation and this starts a signalling cascade. They start signalling to each other, and that makes them come together in an aggregate of about 100,000 cells. And then this aggregate goes through a differentiation process, and at one of those stages, like Paul was saying, was the multi-cellular slug. So this slug is able to move, and it’s able to move towards the light, and also towards different temperature gradients. And the basic thing of this is that, if it’s living in the soil, it wants to get to the surface of the soil where there’s more likely to be food, and then once it’s migrated for a variable period of time, it will culminate and it will form like a stalk, with a top of sorus, containing all the spores of the amoebae, and then it will germinate and then the amoebae will come back to single cell and feed by phagocytosis again. So that gives us lots of different stages in the multi-cellular phenotypes that we can look at. So not only like I was saying that movement towards light, we can look at phototaxis or the movement towards temperature, so thermotaxis and also the fruiting body so with the stalk cells, about 20% of the cells in the aggregate sacrifice themselves essentially for the greater good of this 100,000 cells and they are basically dead cells, they’ve died through autophagi and then the rest of the cells, the 80% are in the sorus.

Matt Smith

That’s really quite an amazing little skill that it’s got. Does it become an actual one organism, or is it just like a clump ...

Paul Fisher

It’s sort of both. It’s a multi-cell organism. It’s a true multi-cell organism that is formed by the cells aggregating together, because they’ve become attractive to each other, and that forms a little multi-cellular tissue with 100,000 cells in it, and some of the cells are specialised to do some tasks, and some are specialised to do other tasks. In fact, recently, people have even found that there are specialised cells in there that act as an immune system for this little organism as well. But at the same time, the cells within it can still move relatively independently of each other, a little bit like birds in a flock, or fish in the sea. So there are single cells moving around, but they are nonetheless specialised in particular functions and they are divided up in groups, like in tissues and organs, to do particular things. So the cells at the front end, for example, are the leaders. They control where the slug goes and what it does, but they will also be the ones that sacrifice themselves later, to make the stalk. So it’s found a way to become multi-cellular that doesn’t involve cells just sticking together during division. It becomes multi-cellular by cells aggregating.

Matt Smith

One thing that I do want to know before we go much further is, how big is it, once it’s clumped together? Is it naked eye kind of ...

Sarah Annesley

You can see it with your naked eye, so you can see it on a plate, but we can see it better under a low magnification, maybe like three or four times magnification, so the slugs are probably maybe a couple of mill in length and maybe the fruiting bodies can vary, but they could be, you know, slightly bigger than that, maybe half a centimetre, something like that.

Matt Smith

What kind of things can you do with it in the lab?

Paul Fisher

It’s recognised as a model organism by the National Institutes of Health in the US and the reason for that is that, having evolved from a common ancestor with us and other animals and fungi, it still has those ancestral properties that are conserved in all cells, and so you can study those ancestral features, the way cells actually function, at the cellular and molecular level in this organism, and it has the great advantage that you can grow it cheaply and easily in the lab. You can manipulate it easily, you can do nasty things to it and nobody cares. You don’t have to fill out ethics application forms for killing slime moulds. So all of those things mean that you can do experiments with it that would be not possible in more complicated organisms, and those experiments can then tell you how things work at the cellular level, not just in dictyostelium, but also in animal cells and in fungal cells, and so on.

Matt Smith

What kind of experiments have you been doing?

Sarah Annesley

We started off being interested in mitochondrial diseases, so the mitochondria are the things that generate the energy for the cells, and we created various mitochondrial diseases and then wanted to see what sort of phenotypes were occurring due to that. The reason we were interested in doing it in dictyostelium was because mitochondrial diseases are very complicated in humans, but more and more there seems to be with the neurological diseases, they also have a mitochondrial disfunction, so they’re becoming increasing in importance, how these organelles work and what they interact with. Whereas in humans it seems that each tissue in the body has mitochondria, but say some tissues that require more energy will have more mitochondria, so often your muscles or your brain, things like that.

Paul Fisher

I should impose there Sarah and just point out to the listeners that the mitochondria are the little compartments inside cells that produce the energy for the cells, so that’s why Sarah was referring to the energy needs.

Matt Smith

I seem to recall from high school something called ATP.

Sarah Annesley

Yes, that’s correct.

Matt Smith

Okay there we go, something’s still in my head.

Sarah Annesley

So basically in humans though, it’s very hard because each tissue has a different number of mitochondria and it’s not that every cell has many mitochondria within the one cell, and it might be that some of the mitochondria within that cell have a mutation, and some don’t. So when you go through a division process, it will be that some cells have more mutated mitochondria than another cell, and then it depends where they end up into what tissue of the body. So basically that means that you could have a mutation in a mitochondrial protein, but depending on how many of those mutated mitochondria you have in one cell, and which cell, tissue in the body, so it’s very complicated. You can get one person with the same mutation, but they can have a very different phenotype to another person, which has the same mitochondrial mutation, but it’s ended up in a different tissue of the body. Whereas when we use a simple model like dictyostelium, it’s a haploid clonal organism, so every cell is basically the same. So we can avoid all these complexities that have worked out in humans and higher organisms. So what we’ve found when we make mitochondrial mutation, no matter what sort of mutation it is, it always results in the same sort of pathways resulting, and the same phenotypes that result from these disfunction.

Matt Smith

Okay, and it’s the sort of thing that you can extrapolate to humans because the signalling, the functions on that level work the same.

Sarah Annesley

Yes, the proteins and pathways.

Paul Fisher

So, one example is that the mitochondria inside human cells are also responsible, obviously, for producing energy for the cells, they produce more than 90% of the energy of the cell, and in humans, you have these pathways that are listening to how much energy the cell has, and then taking remedial action accordingly. So if a cell has an energy crisis, for example, because the mitochondria are not functioning properly, that activates a protein called AMPK, it’s like an alarm protein in cells, dictyostelium has exactly the same protein. It has mitochondria producing the energy, it has this protein AMPK listening to the energy needs of the cell. What AMPK does for the cell, it conserves energy by shutting down some of the cell’s energy-consuming processes, but it also boosts energy production by stimulating production of more mitochondria, so more energy powerhouses for the cell. And you combine those two things, it can bring the energy levels, the ATP levels of the cell, back to normal, but only as long as this protein, AMPK, is active. So, I like to use the analogy of, in winter time, setting your thermostat to 20 degrees, and then opening up all the doors and windows in the house, and letting all the heat escape. So, what would happen is that probably the house will still stay at 20 degrees, but only because the heat is on all the time. And in this case, the energy levels for the cell get brought back to normal and they stay normal, but only because this protein is active, all the time. That means that the things that had shut down in the cell are permanently shut down and that’s what causes the clinical for dictyostelium, we call them phenotypes in the lab, and for humans it would lead to disease outcomes.

Matt Smith

Besides giving you an easier to study this process in action, are you learning anything from it? Is there new outcomes besides watching the process?

Paul Fisher

Yes, well, what I was just describing to you with AMPK was the discovery that we made in dictyostelium, using our genetic approaches in this organism. Because we’re able to manipulate the organism genetically, we could change the amount of this alarm protein in the cell in its active form and we could reduce the amount of the protein as well. So by changing the amount of this protein by increasing it, we could for example, make the cells behave as though they had a mitochondrial disease when they didn’t. So they had all the symptoms of mitochondrial disease in that it didn’t have a mitochondrial disease when we made this protein more active in the cells. If we made it less active in cells that did have a mitochondrial disease, we could cure all of the symptoms. So those two things together tell you that this protein was actually responsible for the symptoms in the first place – its ongoing high level of activity.

And we’ve gone on from there, and we’re now looking at other things. So, we’re looking at other newer degenerative diseases, because mitochondrial disease mainly affects the brain and the central nervous system, because that is one of the big energy-consuming organs. So we’re now looking at Parkinson’s Disease, and Alzheimer’s Disease, and another set of neuro-degenerative diseases called Batten Disease and another one called Mucolipidosis. And we’re asking dictyostelium to tell us, through the experiments that we do on it, what are the pathways that are involved in these diseases that damage cells, and if we know that, we’ll understand the human disease better.

Matt Smith

Besides on a cellular level, I'm getting the mental image of dictyostelium with Alzheimer’s Disease. Is that what’s happening? It wouldn’t manifest in quite the same way, would it?

Paul Fisher

It won’t manifest in the same way.

Sarah Annesley

There can be proteins involved, but the actual outcomes could be slightly different. Say for example, I was looking at a protein that’s involved in helping a cell move. We showed that that protein was actually important in the slug’s ability to move towards the light, but in humans that protein is known actually to be involved in the development of cells, so people with defects in this protein, have severe developmental defects in the development of bone and other tissues like that. But the proteins that are actually involved in that pathway are actually the same proteins that are involved in this other pathway in dicty So it seems that the actual partners are the same and the interactions are still the same, it’s just the actual phenotipic outcome or the clinical outcome is actually the end result can be slightly different.

Paul Fisher

So the end result is different because the biology of the organism is different. But when you get down to the level of the single cell, you can have the same underlying process going on. So, for example, Parkinson’s Disease – all the different brain diseases are defined by the clinical outcomes, and as we find out more about them, we find out for example in the case of Parkinson’s Disease, that the neurons are accumulating aggregated insoluble forms of a protein called alpha-synuclein. And it turns out that in other brain diseases, you also get this same thing happening, but it’s happening in different brain cells. Because it’s in different parts of the brain, it gives you a different set of symptoms but the molecular processes underlying it are probably the same.

What that means is, at the cellular molecular level you can have the same process. How it looks superficially in the organism, be it a human or a slime mould, is maybe different, but the underlying process at the cellular molecular level can be the same.

Matt Smith

What’s the next step for this sort of research then? If you find out how this is working and how it’s applied in the dictyostelium and can extrapolate that to how it’s working in humans?

Sarah Annesley

Well, we do start with the simple model like dictyostelium, but then it does have to be proven and taken further in a mammalian model or a human system. But it can lead us to a quicker way of identifying proteins and identifying novel proteins and things that are involved in these pathways and then that gives us a target of which to work on in a mammalian system. Instead of trying to find the unknown, we work on a known, and then confirm what these things do in that system.

Paul Fisher

And those proteins can then potentially be the targets for drugs that might allow you to treat these diseases, so if you know that a particular protein is responsible for the clinical result, you may be able to develop a treatment by designing a drug that will target that protein and not other proteins.

Matt Smith

Dictyostelium sounds like a quite useful little beastie to be working on. Is there a lot of people working with that around the world?

Sarah Annesley

Around the world there’s quite a few. In Australia I think we’re one of the only ones. There’s another lab in a microbiology department which work with the mitochondrial genetics using dictyostelium and other amoebae. Paul, there’s 150 ...

Paul Fisher

A hundred, 150 labs around the world.

Matt Smith

But it really speeds up this sort of research is the impression that I'm getting.

Paul Fisher

Yes, that’s right. In fact, dictyostelium as we said before, is one of this elite group of model organisms recognised by the National Institutes of Health and I did some backgrounding research recently to look at how much bang for your scientific buck you get out of these organisms, because if you look at Nobel prize winners, more than half of the Nobel Prizes in physiology and medicine or in chemistry, go to people for work on model organisms, and if you work out how many publications are generated that are significant enough to get in the top two science journals in the world, namely Nature and Science, it’s about four times more likely if it’s a model organism than if it’s anything else. And if you look across at the Australian Research Council and National Health and Medical Research Council grants, you can work out also the cost per publication, relatively speaking, and it costs about four times less per publication. So the scientific bang for the buck is about fifteen times higher for model organisms than it is for other kinds of research. Australia doesn’t fund it very well, I have to say. Model organism research in Australia for any of the NIH model organisms is not strongly supported in Australia, so there is probably a case for Australia putting aside a special category of research funding just for model organisms, to make up that deficit.

Matt Smith

That was Professor Paul Fisher and Dr Sarah Annesley, from the Department of Microbiology at La Trobe University. And that’s all the time we have today for the La Trobe University podcast. If you have any questions, comments or feedback, then send us an email at podcast@latrobe.edu.au.

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