Welcome to FOCUS In Sound, the podcast series from the FOCUS newsletter published by the Burroughs Wellcome Fund. I’m your host, science writer Ernie Hood.
On this edition of FOCUS In Sound, we focus on research on a devastating disease that threatens millions of people in 36 countries in sub-Saharan Africa—Human African Trypanosomiasis, better known as African Sleeping Sickness. It’s caused by a parasite transmitted to the bloodstream of mammalian hosts by the bite of an insect vector, in this case, the tsetse fly. As with malaria and other insect-borne parasitic diseases, elucidating the complex mechanisms involved in the pathogenesis and transmission of the disease is critical to understanding how to fight it. Joining us on FOCUS In Sound today is a young investigator who is doing just that, conducting basic biomolecular research on the parasites called trypanosomes. His group’s work may lead to new therapies for a condition that hasn’t seen significant progress in treatment for a long time.
Dr. Kent Hill is professor of Microbiology, Immunology and Molecular Genetics at UCLA. He received his B.S. degree from Northern Illinois University, with a double major in Chemistry and Biology. He worked as a product development scientist at Abbott Laboratories in Chicago and subsequently received a Ph.D. in Biochemistry at UCLA. He went on for postdoctoral studies at the University of Iowa, and then he returned to UCLA to set up his own lab in 2001. He was named a Burroughs Wellcome Fund Investigator in the Pathogenesis of Infectious Diseases in 2008, with the five-year award funding his research into cell-to-cell communication and social motility in the pathogenesis and development of African trypanosomes.
Kent Hill, welcome to FOCUS In Sound…
Well thanks, Ernie, for having me. I’m looking forward to a nice discussion.
To set the groundwork for your description of your group’s work, why don’t you describe for us the basics of the pathogenesis of African sleeping sickness?
African sleeping sickness is a disease that’s caused by a protozoan parasite. The parasite’s name is Trypanosoma brucei, and this is a parasite that is transmitted from one human patient to another through the bite of a tsetse fly vector. The tsetse fly can also bite animals – wild and domestic livestock – so it’s also a disease of animals besides humans. The disease is prevalent in sub-Saharan Africa, and it encompasses about 35 or 36 countries, with about 60 or 70 million people at risk. In terms of pathogenesis, the parasite is transmitted to a new person, a new patient, when an infected tsetse fly takes a blood meal. So the tsetse fly is a blood-sucking insect much like a mosquito, and when the fly is infected and it takes a blood meal, it will transmit the parasite into the bloodstream of a human host, and at that point the parasite grows and divides in the blood, and it replicates, and it does so causing fever and general malaise and achiness. People generally ascribe two stages to the disease. One is that first stage when the parasites are in the bloodstream; they grow and divide and you get sick. You get fever; the fever cycles up and down because your body is constantly battling the parasite and the parasite is constantly battling to stay alive. Ultimately, you enter stage two of the disease, when the parasite burrows out of the blood vessel and penetrates the blood-brain barrier. And at that point, it invades the central nervous system. And once it’s in the central nervous system, it causes disruption of sleep/wake cycles, extreme neurological defects, headaches and things. The disruption of sleep/wake cycles is what gives the disease its name of sleeping sickness. Ultimately, if it’s not treated, it is fatal, and the patient will usually succumb having a coma and sometimes secondary infections. In terms of transmission by the tsetse fly, then, when the parasite is in the bloodstream, if a fly takes another blood meal from that person, it can of course pick up the cycle, pick up the parasite, the parasite goes through its developmental cycle in the fly, and the process continues, and so on and so on. And again, as I mentioned, these parasites are also capable of infecting animals, so animals and humans are both susceptible to the disease.
Kent, your research focuses largely on the issue of motility in trypanosomes. Why is that particular element so important?
We focus on motility. And in terms of motility, we focus on the flagellum, and we can get into it a bit later, but another area that we’ve gotten into now is the signaling and sensing roles of the flagellum, which we think is also important. So the motility and signaling functions of the flagellum are really important for these parasites.
All of that kind of works together, doesn’t it?
Yes, absolutely. For example, one of the reasons that we think that sensing is important is because the parasite goes to specific tissues in each host, and it must somehow sense where it is in the host and know where to go, know how to change in response to the environment that it’s in. That sort of sensing is required to navigate, so that’s motility. It’s also required to change so that the parasite can survive in a new environment, and it’s required to cause the disease, because the parasite must constantly battle the host immune system, or in the tsetse fly, the various environments that it finds itself in. So sensing and motility, we suspect, are required for transmission of the parasites, survival of the parasite in any given host environment, and for disease pathogenesis. In terms of motility, as I mentioned, the way the pathogenesis goes in a human patient is that the parasites are first in the bloodstream, and when they’re in the bloodstream you’re sick, but you don’t die. You die when the parasite penetrates the blood-brain barrier and invades the central nervous system. Now these are extra-cellular organisms, so they depend on their own motility to move around in the host. And we suspect, then, that motility of the parasite and its ability to get through the blood-brain barrier and into the central nervous system is the critical, critical step in the pathogenesis of the disease. What we hope is that if we’re able to block that, we could either get rid of the parasite or at least keep it in the bloodstream where there’s different sets of drugs that are available to treat the parasite. You may or may not know, but the blood-brain barrier is a very carefully controlled access point, and a lot of drugs that are available for treating bloodstream infections do not work if a microbe is in the CNS or beyond the blood-brain barrier. So the hope is that we can prevent that.
On the tsetse fly side of things, the parasite doesn’t just sort of get sucked up and spit back out. It gets sucked up with a blood meal, it goes into the gut, it undergoes a very specific series of developmental transformations that start by movement through the gut, back up to the mouth parts. From the mouth parts it doesn’t get spit out, it goes back up and into the salivary gland. Once it gets to the salivary gland, it attaches to the host salivary gland epithelium, and then it continues through its developmental cycle to become mammalian-infectious. If you take parasites from the gut or the mouth parts or the GI tract of the tsetse fly and try to infect an animal, they’re not infectious. They have to complete that route in order to complete their developmental cycle and become infectious. So we also think that motility is important for transmission through the tsetse fly, and we would hope that that gives an opportunity to potentially intervene both in the mammalian host and in the tsetse fly vector, by targeting motility.
Kent, you’ve also made some key discoveries related to this issue of trypanosome motility, particularly the fact that they tend to be social in their movement patterns. That’s absolutely fascinating…tell us a little bit more about that, and why that is so significant.
It was kind of a surprise, and it’s been really a fun area to study. We came across this a few years ago, and when we first spotted it, we were a bit surprised, and as I tell you a little bit more about it, we looked more and more and now realize, well, gee, maybe we should have been looking for this previously. But the behavior is really fascinating, and it emphasizes that microbes themselves, although we think of them as individual cells, they have the capacity to work as groups. And that’s very well known in bacteria, in fact it’s thought to be ubiquitous in bacteria. And it’s becoming more and more evident that it occurs in fungi, and now we’re the first to sort of, not the first, but the first sort of realization that this really happens in pathogenic protozoa. The way we came across it was sort of interesting. We kind of had been looking at motility, and we know a lot about how the parasite moves when you have it in a test tube or a liquid flask, and it’s basically swimming around in suspension, or swimming around as individual cells in water. What we thought was that if you consider the life cycle of the parasite, they don’t really live that way. They don’t live in suspension. They live on and in tissues, both in the human host and in the tsetse fly vector. So we asked, what happens to these guys, these parasites, when we put them down on a surface? And people had previously developed semi-solid agarose matrices that are used for these sorts of experiments, and we adapted that to our studies, and found that when we put these parasites on surfaces, they started to assemble into little groups. And then the little groups started to assemble into bigger groups. And the big groups, the parasites within those groups could move out as an organized unit, and we could see that that organized unit could sense external signals and move away or move toward those signals, sort of as a group in a coordinated behavior. We thought that really was just fascinating, both from the standpoint of social interactions and microbial social behavior, as well as, it basically is indicating to us something different happens to these parasites when they’re on surfaces. So we think we’re looking at something that’s getting closer to what’s happening in the organism. And then lastly, the behavior itself provides us an in vitro assay, something that we can do readily to study sensing and signaling. And basically these parasites are talking to one another and coordinating their actions in response to external signals. And we now have in our hands a system with which we can dissect that.
In terms of why it’s important, as we discussed, sensing and signaling is really important for these parasites, as it is for just about any microbial pathogen. In terms of the social behavior, we don’t yet know the advantages of that for the organisms in the mammalian host or the tsetse fly, but we do note that in other microbial pathogens and bacteria, for example, a variety of social behaviors have major impacts on pathogenesis. The things that people might be familiar with are things like quorum sensing or swarming motility or biofilm formation. All of these are features of microbial physiology in which individuals come together as a group, and the behavior of the group is different than the behavior of the individual. So we’re really excited by the findings, and we’ve been spending a lot of time now following that aspect of this parasite’s physiology and biology.
It is just fascinating, and I would commend to our listeners to be sure to visit your lab’s website, and I’m sure we’ll have a link, to see some of the videos you have on there of the parasites gathering in this social movement. It’s quite interesting.
In addition, we really do think it’s important and it’s going to lead to new discoveries, but it’s just a lot of fun to do and a lot of fun to watch. We think that there’s a lot going on. The parasites can tell you a lot if you just stop and look, and it’s certainly a lot of fun to do the experiments.
Absolutely…Kent, how might this enhanced understanding of trypanosome motility and the social aspect we’ve been discussing…how might all of that lead to new treatment modalities? Might there be a way to halt or impede their movement and break this disease cycle?
I think so. Trying to go that step is obviously--that’s a big step to take., But, you know, in the last several years there’s really been a boon in the ability of scientists at research laboratories to utilize small screening facilities to look for small chemicals that inhibit various aspects of a parasite or a microbe’s biology. So we actually, at UCLA, have such a facility available, and we have collaborators that do, as well. One idea would be that now that we have assays for these types of motilities, these types of sensings, we can screen small chemical libraries to see if we can identify inhibitors that inhibit this activity. And then those inhibitors could go on to be tested as to whether or not they could be functional in vivo, etc. So that’s one way that we think this could go. Another way is really that it sort of opens up new ideas of how the parasites really move, how the parasites really behave, and that can lead to new conceptual approaches for how one might intervene. The idea of parasites communicating with one another is interesting from the standpoint of raising a new place for drugs to potentially interfere with. We hadn’t really thought about parasite-to-parasite communication as a step that we might target for chemotherapy. And then in bacteria, for example, there’s a lot of interest in trying to disrupt a microorganism’s ability to communicate with itself as a means for intervening with pathogenesis. Motility-wise, again, I think that if we can find compounds that inhibit motility, the hope is that we can find those that would be effective at preventing the parasite from getting to the tissues that it goes to when it causes disease in the animal host, or tissues that it goes to as part of its process of going through its developmental cycle and becoming infectious in the tsetse fly.
So Kent, are you optimistic that at some point in the foreseeable future we will be able to cure or effectively prevent, or at least control some of these insect vector-borne parasitic diseases such as African sleeping sickness and malaria? It seems like there’s been a real quantum leap in knowledge in this area just in the past few years…
That’s a tough hurdle to climb, but there have been a lot of breakthroughs, a lot of advances, and I’d have to say that I would be generally optimistic. I think the idea of complete eradication…one, I’m not an epidemiologist, so it’s not my area per se, but the idea of complete eradication is a little bit tough, because these are insect-borne diseases. But, there have been major advances in our understanding of the biology of the bugs, and there have been increases in our understanding of how the bug and the human host interact. That, together with this ability to screen for chemicals and, honestly, a little more public awareness of these diseases of tropical countries and developing countries, those kinds of factors combined, I think, make it possible to be optimistic about having a really significant impact and to really reduce the disease burden. And I think, again, there’s a multitude of factors that add into that, but if you look at the numbers, there’s some of these diseases that are on the decline. And for those diseases, in a lot of cases, we know a lot more information about them. So there’s kind of a nice window right now of knowledge, the right opportunity of disease decline and interest. Again, there’s a lot of interest in what have been called neglected tropical diseases. So with that in mind, I would be optimistic, and I think we can make an impact. Full eradication, I think, would be tough. But I think being able to improve the quality of life and the potential for self-sustaining economies by making a real effort, I think that potential is there.
Kent, I understand that you also use trypanosomes as a model experimental system to study flagellum and cilium biology. Flagella and cilia, of course, are tiny hair-like appendages that extend from the surface of most cells in the human body and are critical for normal development and physiology. What are you able to learn about those vital organelles in your work with trypanosomes?
Thanks, Ernie. That’s actually a big part of what we do, and so I kind of talk about our lab as having two motivations behind it. One is understanding the biology of the flagellum, or as you call it, the cilium, it’s an analogous term for the same organelle, so an understanding of the flagellum in the context of these parasites as pathogens. And that’s really a big motivation. The other motivation in the lab is to use these organisms to understand fundamental biology. And as you said, the cilium in invertebrate systems and humans plays incredibly important roles. People think about cilia or flagella in motility – sperm motility, the motility of cilia in your respiratory airway, but it turns out there are cilia in almost all tissues of your body, and for the most part, those cilia perform sensing functions. Some sensing or signaling functions that people might be aware of are things like photo perception in your eyes. Your photoreceptors in your eyes are actually modified cilia. The olfactory receptors in your nose that sense odorants and allow you to smell, those receptors are localized to cilia. The sensory patches in your ear that allow you to sense gravity and hear and sense movement, those sensory patches require cilia for assembly. And elsewhere in the body there are a number of sensory functions of cilia that have really come to prominence in the last ten years.
So we’ve focused on this aspect of trypanosomes’ flagella for a while now, and in terms of what we think we might be able to provide, I think we can actually advance efforts to understand these organelles both from the motility standpoint and the sensing standpoint. From the motility standpoint, one example was a protein we discovered in Trypanosoma brucei. The protein was called trypanin, and we showed that it was required for motility. A student in the lab decided that she wanted to know whether or not that protein functions in motility in vertebrate systems. To make a long story short, she engaged in a collaborative effort with folks here at UCLA and Cal Tech and was able to discover that this protein is required for movement of cilia in the inner ear of developing zebrafish, and if you disrupted that, that led to a defect in development of the sensory patches that are required for sensing sound. So that’s one example of how we can take our studies and advance it into a vertebrate system. In terms of the sensing functions and the signaling functions, the ability of an organism to sense its extracellular environment requires proteins on the cell surface, and for us, that ’s protein on the surface of the flagellum or the cilium. And so we’ve made a big effort to identify the proteins that are on the flagellum of T. brucei. We actually think those will be a bit different in many cases than in humans and vertebrates, because it’s dictated by what the parasite’s needs are. But what we can do now is use our system, which is genetically tractable, meaning that we can do molecular genetics, etc., knockouts, knock-ins, RNAi, to study the function of genes, and particularly study the systems that deliver proteins into and out of the flagellum membrane, so the surface of the flagellum. And we think that those mechanisms - and we have evidence that those mechanisms are conserved between trypanosomes and humans. So by identifying some of the mechanistic aspects of flagellum membrane protein targeting in trypanosomes, we think there’s a way to apply that information to understand flagellum membrane protein trafficking in humans and other organisms.
And that’s important, I’m sure, because defects in cilia operation often lead to diseases in humans, isn’t that correct?
Oh yeah, absolutely. There is, in fact, an entire new class of diseases called ciliopathies. The cilium or flagellum diseases that people may be familiar with are infertility, respiratory infection; there’s actually an interesting disease called situs inversus where the visceral organs are reversed and that’s due to a defect in motility. But now there’s a variety of inherited genetic diseases that lead to major developmental abnormalities such as polydactyly, in which people have more than the number of digits than they’re supposed to have; there’s hydrocephaly and brain malfunction; there’s also links to obesity and diabetes, and that’s related to the sensing functions of these cilia. A big one that people have been studying for a while now is polycystic kidney disease. It turns out to be one of the major reasons why people are on dialysis, and the disease is associated with a defect in cilium function and sensing cilium function within the kidneys. So many aspects of human physiology, many aspects of human development require cilia, and the list of diseases that come up when you disrupt cilium function is growing day by day.
Kent, obviously your work is going to have major implications for those diseases along with the neglected tropical diseases that you’re focusing particularly on.
We hope so. That’s our hope.
Kent, it’s been great to have the opportunity to get to know you and your group’s fascinating work, which is making a major contribution to improvement in world health. We wish you the best of luck for continuing success, and thank you so much for joining us today on FOCUS In Sound…
Thank you very much, Ernie, I appreciate you having me on the show.
We hope you’ve enjoyed this edition of the FOCUS In Sound podcast. Until next time, this is Ernie Hood. Thanks for listening!