AudioHelicase: Whitehead’s Sebastian Lourido on Toxoplasma, malaria parasites, and global health

Whitehead Institute Member Sebastian Lourido studies a group of parasites called the Apicomplexa. These single-celled organisms are among the most common pathogens and are capable of causing devastating diseases in humans and animals, including toxoplasmosis, malaria, and infant diarrhea. Lourido's laboratory is investigating in particular how the Apicomplexan Toxoplasma gondii  invades host cells and establishes its site of replication. The work holds great promise for exposing treatable vulnerabilities in the parasite—and in the closely related Plasmodium parasites, which cause malaria and contribute to more than half a million deaths each year.

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EDITED TRANSCRIPT

Lisa Girard: I’m Lisa Girard, Director of Strategic Communications at Whitehead Institute. Welcome to AudioHelicase, the podcast of Whitehead Institute, unwinding the science and the people behind some of the Institute’s most exciting discoveries.

In this episode, I am talking with Sebastian Lourido,Whitehead Member and an assistant professor of biology at MIT.

Lourido studies a group of parasites called the Apicomplexa. These single-celled organisms are among the most common pathogens and are capable of causing devastating diseases in humans and animals, including toxoplasmosis, malaria, and infant diarrhea. Lourido is focused on the Apicomplexan Toxoplasma gondii, which is responsible for the disease toxoplasmosis. His laboratory is investigating in particular how Toxoplasma gondii  invades host cells and establishes its site of replication. The work holds great promise for exposing treatable vulnerabilities in the parasite—and in the closely related Plasmodium parasites, which cause malaria and contribute to more than half a million deaths each year.

To begin, I asked Sebastian to tell me more about the parasites he studies.

Sebastian Lourido: My laboratory works on Toxoplasma gondii, which is an organism that belongs to this group of parasites called the Apicomplexa. This is a very ancient group of organisms that has been devoted to parasitism for what we estimate to be nearly 500 million years. By some estimates, this predates the emergence of animals as a group. And in fact in the early days, they were likely parasitizing other single-celled eukaryotes. And so, fast forward 500 million years, and these organisms have taken a foothold in many different niches, parasitizing many different animals throughout the world. And you can basically find an Apicomplexan infecting pretty much any group of animals that you can think of – from insects all the way to us humans.

Girard: What aspects about these parasites interest you?

Lourido: We work on trying to identify some of the core molecular mechanisms that allow these organisms to infect the cells of the animals that they infect and to be able to replicate within those cells, and ultimately to be able to cause disease. We work specifically on Toxoplasma because it has this remarkable ability to infect a wide range of cells throughout the human body and has actually taken a foothold on a large portion of the human population. Sometimes it’s regarded as one of the most successful parasites in the world because of its ability to infect such a broad range of individuals.

We can see the same sorts of behaviors occurring in a really broad swath of the Apicomplexan phylum. We see malarial parasites interacting with red blood cells and entering them in the same way that Toxoplasma enters human fibroblasts, for instance.  Or another of these infections is that from Cryptosporidium parvum, which can infect the cells of the intestine, and it does so in exactly the same manner, and there’s shared machinery at the molecular level that all of these different parasites are utilizing for these processes.  As we’ve learned some of the details behind these interactions, we can see and identify in all of these organisms the same sort of molecules playing comparable roles. So even though they’ve undergone tremendous specialization, in some regards, some of the core machinery underlying their ability to infect cells and cause disease has been maintained throughout their evolution and is key to why they’ve been able to infect such a diverse number of animals.

Girard: As a parasite, Toxoplasma has to adapt quickly to different environments, and those environments can change rapidly, like when the immune system is beginning to attack, the parasite is being transmitted from one organism to another, or it finds itself in the intestine of the infected host.  What enables Toxoplasma and the other Apicomplexans to respond so fast?

Lourido: Each one of these parasites is comprised of a single cell.  It’s one cell that has to react to its environment. In a sense, to personify it, it has to make decisions about whether it’s going to replicate or move or invade the cell of the host.  And these decisions, which to us appear to be the product of an almost sentient being, are in fact chemical reactions that are occurring within these cells.  And that is what as biologists we call cell signaling. It’s how cells integrate chemical cues and make decisions about what cellular processes to regulate in different ways.  The most ubiquitous message that is used within cells to regulate such rapid responses is calcium. Changing the concentration of calcium within these cells, for these parasite cells as much as our neurons, has the ability to activate these cells in different ways. A key element in the signals that are being transformed into biological processes is taking those calcium concentrations that are shifting or that are increasing and transforming them into different types of cellular responses or different types of biochemical responses.

One of the key discoveries early in my work as a graduate student was that a particular member of this family of responsive signaling proteins, called calcium dependent protein kinases, was in fact involved in the ability of parasites to respond to these signals and to release the adhesins that they use to attach to host cells and to migrate within organisms. And this was one of the key discoveries that in fact laid the foundation for some of the work that I started once I became a Fellow at the Whitehead Institute.

Girard: Apicomplexans are so distant from other commonly studied organisms, like yeast and fruit flies.  How has this affected your research?

Lourido: One of the key challenges in studying organisms like Apicomplexan parasites, or Toxoplasma gondii to be specific, has been their evolutionary distance from model organisms. So many of the lessons from molecular biology in yeast and mammalian cells can’t be easily drawn into inferences about the biology of Apicomplexan parasites. And it’s therefore particularly important for us to be investigating these processes directly on these organisms to try to understand them.

It has been very labor intensive to dissect their genomes and to change them in ways that allow us to investigate them. In fact as a graduate student, it took me almost two years to generate a single disruption of a particular gene. And so it was clear when I started working as a Fellow at Whitehead, that we needed to improve these technologies if we were in fact to investigate in any form of throughput the genome of these parasites. And so we basically followed the path laid forth by the manipulation of mammalian cells using CRISPR, and we were one of two who effectively adapted the technology to modify the genome of Toxoplasma.

Girard: CRISPR has transformed genetic research in the past few years.  How has it affected your work?

Lourido: Based on some of our early results that CRISPR was incredibly efficient in Toxoplasma, we wondered whether in fact we could take that and develop it into a platform that would allow us to manipulate every single gene within the Toxoplasma genome. In Toxoplasma there are about 8000 of these genes. By disrupting in a large population of parasites each one of these genes and then testing the consequences of that disruption, we could in fact infer what each gene in the parasite was doing in a way. This ends up being a very powerful approach in order to get essentially a map of the genetic contributions to a particular cellular process. The one that we’ve been most interested in in our lab has been how parasites are able to infect and replicate within human cells. This has many other potential applications. We can look at how drugs that are being used to treat these parasites are interacting with the parasite genome. But in this first scenario, what we ended up discovering was a whole of class of conserved proteins among Apicomplexans that appear to be really important for this process that we had been studying.

In particular I’d like to highlight one of them, which we call CLAMP, which ended up being incredibly important for these parasites to even start the infectious process by invading cells. At a first pass, CLAMP would’ve looked like any of the thousands of proteins in the Toxoplasma genome that don’t appear to have much similarity to anything else. But when you looked at the secondary structure, that is how we would predict that sequence to fold, then you start seeing distant similarity to other proteins and among them the proteins that form the tight junctions between our own cells that hold our cells together.  This was interesting because the process through which Toxoplasma enters host cells generates similar interactions between the host cell and the parasite cell.  So there’s the potential that Toxoplasma is using is similar of set of proteins in the form of CLAMP and related proteins in order to form that tight apposition between the parasite’s cell and the host cell.  

Girard: Does CLAMP play a similarly critical role in other Apicomplexans?

Lourido: One of the earliest indications that we’re on the right track was when we asked collaborators to manipulate CLAMP in Plasmodium falciparum, which is the causative agent of some of the most severe forms of malaria in the world. And the consequence of that was crystal clear.  As soon as they removed CLAMP from these parasites, the parasite population absolutely crashed, showing that CLAMP in malaria was as important as we had shown it was for Toxoplasma gondii. And it really validated some of our efforts to try to use Toxoplasma as a model for this phylum.

Girard: CLAMP sounds like it could be a way to foil the malaria parasite.  How do you think your research could affect the battle against malaria?

Lourido: We hope that our work will continue to shed light on some of the core processes of these parasites into the future.  And that’s important because malaria continues to be a really important disease in human populations throughout the world, in Asia, in Africa.  There are an estimated 200 million infections of malaria each year and probably about half a million deaths or so. I’ve heard many people that say the solution to malaria is bed nets and vector control, and these are of course important avenues to restrict the spread of the disease.  But if we have to address this disease in its current context, and we have to reflect the current reality of some of these very poor populations, then we have to admit that the way in which we concurrently address the disease is through drugs and through antimalarial therapies and that requires continuing to push forward on understanding the basic biology of these organisms and the way in which these drugs interact with that biology to block it. From basic cell biology, we’ve learned so much about our own cells, and many of the cutting edge treatments against cancer have evolved from our understanding of cancer cells. This enormous problem with Apicomplexan organisms that is reflected in the case of malarial infections, but also reflected in infant diarrhea that is caused by Crytosporidium and babesiosis caused parasites that are transmitted by ticks in Cape Cod, all of these different infections really demand the same level of attention and specificity and accuracy that our cancer treatments demand.  To do that, we really need to development sophisticated models where we can quantitatively and very precisely look at the biochemistry of these organisms and look at what these different signaling pathways are doing and how they’re all integrated into the kinds of cellular behaviors that allow these parasites to infect and cause disease.

Girard: That was Whitehead Institute Member and assistant professor of biology at MIT Sebastian Lourido. You can learn more about Whitehead science on our website at wi.mit.edu.  And listen to other AudioHelicase episodes on SoundCloud and iTunes. For Whitehead Institute, I’m Lisa Girard. Thanks for listening.