Yeast's Sense of Direction

Alex -  Candida albicans costs a lot of money to the UK, and in people's personal lives, because it's the cause of thrush - but it also kills people in a hospital who are undergoing very sophisticated treatments.  Clinicians that I meet say that it's a very sad experience for them because they manage to save peoples' lives through very complicated treatments, but then they lose them to this terrible infection.  I'm studying the organism because it penetrates into our tissues and it forms big balls and lesions in our kidneys, and our brains and lungs, and then the immune system kicks in and fights it, and the battle between the fungus and the immune system is what kills us.

Chris -  And how are you trying to understand that process, and then combat it?

Alex -  Candida albicans grows in two major shapes - rugby ball-shapes (that's how it lives in us as a commensal organism in our gut).  If it escapes from our gut and goes into the bloodstream, it gets a signal from the blood itself that triggers a complete change in the shape of the organism.  It sends out a long filament known as a hypha, and these long filaments are very sticky and contain extra molecules on the surface which allow them to stick to especially small capillaries, like in our kidneys.  Interestingly, they also help to form very drug resistant layers on medical plastics - on catheters and tubes that are put into people in their intensive care units.  So, the hyphae of this organism seem to be a very important point for virulence.  If they can't form hyphae then they can't invade tissues and then the immune system kicks in and we can clear them from the blood quite easily.  So I'm looking at hyphae.  Hyphae are very interesting because they always grow in a particular direction, and the direction that they grow in is programmed for the specific organism.  So, for example, hyphae that grow in wood in trees know the signals that they need to find the nutrients in the tree.  In Candida albicans, the hyphae have the same kinds of responses built in - they know how to penetrate the surface and so they get down out of the blood supply and into the tissue underneath.

Chris -  Do we know how they do that?

Alex -  Well, we have great difficulty in studying this organism in humans, so we've developed some in vitro systems here.  We do a lot of live cell imaging so we can watch the hyphae as they grow, and we can try to think of ways of making them grow in directions that we want them to grow.  We have two main ways of doing this.  First of all, we can microfabricate special little surfaces which have got tiny obstacles in them at the nanometre level, and we can make the fungi grow around obstacles and ask whether they want to always grow in the same direction, or whether they want to follow particular shapes.  And because we know that they escape into the blood supply, we want to find out whether they are seeing particular cues on the blood vessels and know that that is a point of penetration.  We don't know the answer to that yet because we have to design the shapes and the obstacles in vitro and then see, if we can make a mutant that doesn't recognise these shapes, do they then cause disease?  The other thing we're doing is looking to see whether electric fields might play a part.  Electric fields play a big role in wound healing - they act as signals for the growth of new cells to fill the wound.

Chris -  That was shown here in the University of Aberdeen, wasn't it?

Alex -  It was, yes.  Min Zhao showed that phenomenon.  So we use electric fields because nearly all filamentous fungi grow towards a cathode in an electric field.  There are several theories as to why this should be, but no one's really come up with an answer.  In fungi, we can delete genes, we can manipulate genes, and overexpress them and change their expression.  We've now been able to make a mutant which actually grows in the opposite direction, so instead of growing towards the cathode it now grows towards the anode.  Now, we have to find out exactly how it does this.  But because you can easily knock genes out and you can easily label proteins in the cell with fluorescent signals, you can turn the electric field on and off, you can make them try to grow around obstacles or make them grow straight.  So now, we're beginning to control these things and then hopefully, we can find out the molecules involved.  We need to know what the signal is, how it's sensed, and then we need to know how that is then translated into steering the whole of the hyphal tip around to a new growth direction.

Chris -  So do you know what's making these yeasts grow in the wrong direction yet?  Because you can see how you could take that understanding and then just throw drugs at it in order to try and combat this problem in vivo?

Alex -  We don't really need to know how to make cells grow in the wrong direction for curing disease.  What we need to do in curing disease is to take away control of the hyphal tip from the fungus.  We've got a mutant that has very curly tips, so it can't grow in a sustained direction, and if it can't do that then it can't puncture human cells.  So what it does is it just grows round and round in a more and more aberrant sort of spiral way, and it means that it just sits on the surface and can't actually go anywhere.  If we can find a drug that will remove that kind of control from the hyphal tip, then we won't be killing the organism but we'll stop it invading tissue.