Category: Whitehead Institute

A genome-wide screen in live hosts reveals new secrets of parasite infection

Researchers in the Lourido Lab performed the first genome-wide screen of Toxoplasma gondii in live hosts, revealing genes that are important for infection but previously undetected in cell culture experiments. 

Greta Friar | Whitehead Institute
July 8, 2024

Apicomplexan parasites are a common cause of disease, infecting hundreds of millions of people each year. They are responsible for spreading malaria; cryptosporidiosis – a severe childhood diarrheal disease; and toxoplasmosis – a disease that endangers immune compromised people and fetuses, and is the reason why pregnant women are told to avoid changing cat litter. Apicomplexan parasites are very good at infecting humans and many other animals, and persisting inside of them. The more that researchers can learn about how apicomplexans infect hosts, the better they will be able to develop effective treatments against the parasites.

To this end, researchers in Whitehead Institute Member Sebastian Lourido’s lab, led by graduate student Christopher Giuliano, have now completed a genome-wide screen of the apicomplexan parasite Toxoplasma gondii (T. gondii), which causes toxoplasmosis, during its infection of mice. This screen shows how important each gene is for the parasite’s ability to infect a host, providing clues to genes’ functions. In the journal Nature Microbiology on July 8, the researchers share their approach for tracing lineages of parasites in a live host, and some specific findings of interest—including a possible anti-parasitic drug target.

From dish to animal

Researchers in Lourido’s lab previously developed a screen to test the function of every T. gondii gene in cells in a dish in 2016. They used CRISPR gene editing technology to make mutant parasites in which each lineage had one gene inactivated. The researchers could then assess the importance of each gene to a parasite’s fitness, or ability to thrive, based on how well the mutants missing that gene did. If a mutant died off, this implied that its inactivated gene is essential for the parasite’s survival.

This screen taught the researchers a lot about T. gondii’s biology but faced a common limitation: the parasites were studied in a dish rather than a live host. Cell culture provides an easier way to study parasites, but the conditions are not the same as what parasites face in an animal host. A host’s body is a more complex and dynamic environment, so it may require parasites to rely on genes that they don’t need in the artificial setting of cell culture.

To overcome this limitation, researchers in Lourido’s lab figured out how to repeat the T. gondii genome-wide screen, which their colleagues in the lab had previously done in cell culture, in live mice. This was a massive undertaking, which required solving various technical challenges and running a large number of parallel experiments. T. gondii has around eight thousand genes, so the researchers performed pooled experiments, with each mouse getting infected by many different mutants—but not so many as to overwhelm the mouse. This meant that the researchers needed a way to more closely monitor the trajectories of mutants in the mouse. They needed to track the lineages of parasites that carried the same mutation over time, as this would allow them to see how different replicate lineages of a particular mutant performed.

“This is an outstanding resource,” says Lourido. “The results of the screen reveal such a broader spectrum of ways in which the parasites are interacting with hosts, and enrich our perception of the parasites’ abilities and vulnerabilities.”
The researchers added barcodes to the CRISPR tools that inactivated a gene of interest in the parasite. When they harvested the parasites’ descendants, the barcode would identify the lineage, distinguishing replicate parasites that had been mutated in the same way. This allowed the researchers to use a population-based analytical approach to rule out false results and decrease experimental noise. Then they could draw conclusions about how well each lineage did. Lineage tracing allowed them to map how different populations of parasites traveled throughout the host’s body, and whether some populations grew better in one organ versus another.

The researchers found 237 genes that contribute to the parasite’s fitness more in a live host than in cell culture. Many of these were not previously known to be important for the parasite’s fitness. The genes identified in the current screen are active in different parts of the parasite, and affect diverse aspects of its interactions with a host. The researchers also found instances in which parasite fitness in a live host increased when a gene was inactivated; these genes may be, for example, related to signals that the host immune system uses to detect the parasites. Next, the researchers followed up on several of the fitness-improving genes that stuck out as of particular interest.

Genes that make the difference in a live host

One gene that stuck out was GTP cyclohydrolase I (GCH), which codes for an enzyme involved in the production of the essential nutrient folate. Apicomplexans rely on folate, and so the researchers wanted to understand GCH’s role in securing it for the parasite. Cell culture media contains high levels of folate, and in this nutrient-rich environment, GCH is not essential. However, in a live mouse, the parasite must both scavenge folate and synthesize it using the metabolic pathway containing GCH. Lourido and Giuliano uncovered new details of how that pathway works.

Although previously GCH’s role was not fully understood, the importance of folate for apicomplexans is a well-known vulnerability that has been used to design anti-parasitic therapies. The anti-folate drug pyrimethamine was commonly used to treat malaria, but many parasites have developed resistance to it.

Some drug-resistant apicomplexans have increased the number of GCH gene copies that they have, suggesting that they may be using GCH-mediated folate synthesis to overcome pyrimethamine. The researchers found that combining a GCH inhibitor with pyrimethamine increased the efficacy of the drug against the parasites. The GCH inhibitor was also effective on its own. Unfortunately, the currently available GCH inhibitor targets mammalian as well as parasitic folate pathways, and so is not safe for use in animals. Giuliano and colleagues are working on developing a GCH inhibitor that is parasite-specific as a possible therapy.

“There was an entire half of the folate metabolism pathway that previously looked like it wasn’t important for parasites, simply because people add so much folate to cell culture media,” Giuliano says. “This is a good example of what can be missed in cell culture experiments, and what’s particularly exciting is that the finding has led us to a new drug candidate.”

Another gene of interest was RASP1. The researchers determined that RASP1 is not involved in initial infection attempts, but is needed if the parasites fail and need to mount a second attempt. They found that RASP1 is needed to reload an organelle of the parasites called a rhoptry that the parasites use to breach and reprogram host cells. Without RASP1, the parasites could only deploy one set of rhoptries, and so could only attempt one invasion.

Identifying the function of RASP1 in infection also demonstrated the importance of studying how parasites interact with different cell types. In cell culture, researchers typically culture parasites in fibroblasts, a connective tissue cell. The researchers found that parasites could invade fibroblasts with or without RASP1, suggesting that this cell type is easy for them to invade. However, when the parasites tried to invade macrophages, an immune cell, those without RASP1 often failed, suggesting that macrophages present the parasites with more of a challenge, requiring multiple attempts. The screen uncovered other probable cell-type specific pathways, which would not have been found using only model cell types in a dish.

The screen also highlighted a previously unnamed gene that the researchers are calling GRA72. Previous studies suggested that this gene plays a role in the vacuole or protective envelope that the parasite forms around itself. The Lourido lab researchers confirmed this, and discovered additional details of how the absence of GRA72 disrupts the parasite vacuole.

A rich resource for the future

Lourido, Giuliano, and colleagues hope that their findings will provide new insights into parasite biology and, especially in the case of GCH, lead to new therapies. They intend to continue pulling from the treasure trove of results—their screen identified many other genes of interest that require follow-up—to learn more about apicomplexan parasites and their interactions with mammalian hosts. Lourido says that other researchers in his lab have already used the results of the screen to guide them towards relevant genes and pathways in their own projects.

“This is an outstanding resource,” says Lourido, who is also an associate professor of biology at MIT. “The results of the screen reveal such a broader spectrum of ways in which the parasites are interacting with hosts, and enrich our perception of the parasites’ abilities and vulnerabilities.”

“Vaults” within germ cells offer more than safekeeping

Ribonucleoprotein (RNP) granules are believed to preserve maternal mRNA within eggs and developing embryos. The Lehman Lab reveals that a specific type of RNP granule also plays an active role in translating the mRNA that is crucial for specifying germ cells.

Shafaq Zia | Whitehead Institute
July 2, 2024

Maternal messenger RNAs (mRNAs), located within the cytoplasm of an immature egg, are crucial for jump starting development. Following fertilization, these mRNAs are passed onto the zygote, the first newly formed cell. Having been read from the maternal DNA genetic code, they serve as the sole templates for protein production essential for early development until the zygote’s own genes become active and take over.

Many maternal mRNAs are stored in ribonucleoprotein (RNP) granules, which are a type of membrane-less compartments, or condensates, within eggs and developing embryos. These granules are believed to preserve the mRNA in a “paused” state until the encoded proteins are needed for specific developmental processes upon fertilization of the egg cell. Then, certain developmental signals kick in to instruct the RNP granules to release the stored mRNA so the instructions can be translated into a functional protein.

One type of RNP granules called germ granules is found in embryo germplasm, a cytoplasmic region that gives rise to germ cells, which become the eggs or sperms of adult flies. Whitehead Institute Director Ruth Lehmann studies how germ cells form and transmit their genetic information across generations. Her lab is particularly interested in understanding how germ granules in embryos localize and regulate maternal mRNAs.

Now, Lehmann, along with graduate student Ruoyu Chen and colleagues, has uncovered that the role of germ granules in fruit flies (Drosophila melanogaster) extends beyond safeguarding maternal mRNAs. Their findings, published in the journal Nature Cell Biology on July 4, demonstrate that germ granules also play an active role in translating, or making into protein, a specific maternal mRNA, called nanos, crucial for specifying germ cells and the abdomen of the organism.

“Traditionally, scientists have thought of RNP granules as a dead zone for translation,” says Chen. “But through high-resolution imaging, we’ve challenged this notion and shown that the surface of these granules is actually a platform for translation of nanos mRNA.”

RNP granules act as vaults

Within a developing embryo, various fate-determining proteins dictate whether a cell will become a muscle, nerve, or skin cell in a fully-formed body. Nanos, a gene with conserved function in Drosophila and humans, guides the production of Nanos protein which instructs cells to develop into germline. Mutations in the nanos gene cause sterility in animals.

During early embryonic development, Nanos protein also helps establish the body plan of the fruit fly embryo — it specifies the posterior end or abdominal region, and guides the ordered development of tissues along the length of the body, from head to tail. In embryos with impaired Nanos function, the consequences are fatal.

“When Nanos protein isn’t functioning properly, the fruit fly embryos are really short,” says Chen. “This is because the embryo has no abdomen, which is basically half of its body. Nanos also has a second function that is conserved from flies to humans. This function is very local and instructs the cells with lots of Nanos to become germ cells. ”

Given Nanos’ vital role, embryos must safeguard instructions for its production until the embryo reaches a specific stage of development, when it is time to define the posterior region. Previous work has indicated that germ granules in the germplasm and germ cells can act like vaults, shielding the nanos mRNA from degradation or premature translation.

However, while the mRNA instructions for building the protein are distributed throughout the embryo, Nanos protein is found only in regions where germ granules reside. The mRNA does not get translated elsewhere in the embryo because of a regulatory protein called Smaug, named after the golden dragon depicted in J. R. R. Tolkien’s 1937 novel The Hobbit. Smaug binds to a non-protein coding segment of the mRNA known as the 3’ untranslated region (3’ UTR), extending beyond the protein-coding sequence, effectively suppressing the translation process.

For Lehmann, Chen, and their colleagues, this hinted at an intriguing relationship between nanos mRNA and germ granules. Are the granules essential for translating nanos mRNA into a functional protein? And if they are, is their role primarily to serve as a safekeeping place to evade repression by Smaug or do they actively facilitate the translation of nanos mRNA too?

To answer these questions, the researchers combined high-resolution imaging with a technique called the SunTag system to directly visualize the translation of nanos mRNA within Drosophila germ granules at the single-molecule level.

Unlike green fluorescent protein tagging, where a single fluorescent molecule is used, the SunTag system allows scientists to recruit multiple GFP copies for an amplified signal. First, a small protein tag, known as the SunTag, is fused with the protein-producing region of the nanos mRNA. As the mRNA instructions undergo translation, GFP molecules stick to the newly synthesized SunTag-Nanos protein, resulting in a bright fluorescent signal. Overlying this translation signal with fluorescent probes specifically labeling the mRNA then allows researchers to precisely visualize and track when and where the translation process is taking place.

“Using this system, we’ve discovered that when nanos mRNA is translated, it protrudes slightly from the surface of the granules like snakes peeking out of a box,” says Chen. “But they can’t fully emerge; a part of their sequence, specifically their “back” end, the 3’ UTR, remains tucked inside the granules. When the RNA is not translated, like during oogenesis, the tip coils back and is hidden inside the granule.”

With their high-resolution SunTag imaging technique, Lehmann, Chen and their colleagues have directly added to the work of other researchers with similar observations: mRNAs in the process of translation are in an extended configuration, while the 5’UTR curls back to the 3’UTR when the mRNAs are repressed.

Flipping on nanos translation

Then, the researchers went on to take a closer look at how these granules help initiate translation, while Smaug is able to inhibit the same nanos mRNA molecules from being translated in other areas of the embryo. They hypothesized that the untranslated region (UTR) of nanos mRNA, which remains concealed within the granules, might be playing a pivotal role in the translation process by localizing the mRNA instructions within germ cell granules. This localization, they speculated, protects the mRNA from Smaug’s inhibitory actions and facilitates Nanos protein production, so the posterior region can develop properly.

However, counter-intuitive to a simple protection model, they found that rather than being depleted, Smaug is enriched within germ granules, indicating that additional mechanisms within the RNP granule must counteract Smaug’s inhibitory effects. To explore this, the researchers turned to another regulatory protein called Oskar, which is known to interact with Smaug.

Discovered by Lehmann in a 1986 study, and named after a character in the German novel The Tin Drum, the oskar gene in Drosophila is known to help with the development of the posterior region. Later research has revealed that, during the development of oocytes, Oskar acts as a scaffold protein by initiating the formation of germ granules in germ cells and directing mRNA molecules, including nanos, towards the granules.

To gain a deeper understanding of Oskar’s full role in translational regulation in germ granules and its interaction with Smaug, the researchers engineered a modified version of Oskar protein. This altered Oskar protein retained its ability to initiate the formation of germ granules and localize nanos mRNA within them. However, Smaug no longer localized to the germ granules assembled by this altered Oskar.

The researchers then studied whether the mutant protein had any effect on nanos mRNA translation. In the germ cells with this mutant version of Oskar, the researchers saw a significant reduction in the translation of nanos mRNA. These findings, combined, suggested that Oskar regulates nanos translation in fruit fly embryos by recruiting Smaug to the granules and then counteracting its repression of translation.

“Condensates composed of RNAs and proteins are found in the cytoplasm of pretty much every cell and are thought to mediate mRNA storage or transport,” says Lehmann, who is also a professor of biology at the Massachusetts Institute of Technology. “But our results provide new insights into condensate biology by suggesting that condensates can be also used to specifically translate stored mRNAs.”

Indeed, in the oocyte, the germ granules are silent and only become activated when the egg is fertilized.

“This suggests that there might also be other ‘on and off switches’ governing translation within condensates during early development,” adds Lehmann. “How this is achieved and whether we could engineer this to happen at will in these and other granules is a question for the future.”

A day in the life — graduate student and genomics researcher Neha Bokil

Neha Bokil is studying mechanisms that regulate expression of genes located on the X and Y chromosomes in order to better understand sex-biased conditions that predominantly affect one sex.

Shafaq Zia | Whitehead Institute
June 25, 2024

Graduate student Neha Bokil moves around the Page lab with urgency. Today, she’s running an experiment using white blood cells from patients with varying numbers of X and Y chromosomes.

The lab of Whitehead Institute Member David Page investigates the role of the X and Y chromosomes beyond determining sex. While most females have two X chromosomes (XX) and most males have one X and one Y chromosome (XY), there are individuals whose sex chromosome constitution varies from this, having instead, for example, XXY, XXX, or XXXXY. With the goal of understanding why certain conditions are more prevalent in one sex versus than the other, Bokil is using this experiment to explore if and how cellular processes, such as gene regulation, vary among individuals with these atypical combinations of sex chromosomes.

Partially hidden in the cell culture hood, Bokil finally locates what she’s been searching for: a pipette for dispensing 99 microliters of the cell suspension she’s meticulously prepared this afternoon, a type of culture where cells float in nutrient-rich liquid, free to function and grow.

Bokil carefully extracts this volume and transfers it to a flat plate — also called a 96-well plate — with tiny holes for growing small cell samples. Now, it’s a waiting game until she can find out how these cells are growing, and whether their proliferation rate depends on the number of sex chromosomes in a cell.

Bokil dives into the intricacies of human genetics every day, hoping her work will eventually help reshape how sex differences are understood in medicine and improve treatment outcomes. The dynamic research Bokil is conducting at Whitehead Institute is her calling, but she has other passions as well. Here’s what a typical day in her life as a graduate student looks like, both in and outside the lab.

An inherited love of numbers

When she isn’t rushing out the door, Bokil loves brewing and savoring the perfect cup of morning chai, a traditional South Asian loose-leaf tea with milk. Every family has their own recipe, and Bokil makes hers with ginger, a touch of cardamom, and some sugar.

“Chai is comforting at any time, but I’ve noticed my mood vastly improves when I’m able to have a cup in the morning,” she says.

On her walk to the Whitehead Institute, she often listens to Bollywood songs. But these predilections — chai and Indian cinema — are more than just rituals for her. They symbolize tradition and cherished connections with family and friends.

In fact, family bonds have greatly influenced Bokil’s career path. As a child, she loved mathematics. It wasn’t a trait passed on genetically, but one that flourished through moments of connection with her grandmother, a math teacher in India. During summer visits to Bokil’s family in the U.S., she’d enthusiastically impart her passion for numbers onto her granddaughter. By the time Bokil went to high school and later college, she had become fluent in the language of logic and patterns.

“My time with her made me realize just how beautiful and fun math is, and I could see its practical applications in everyday life, all around me,” Bokil says.

For her PhD, she sought to combine her undergraduate training in mathematics and molecular biology to tackle a real-world problem. With genetics at the crossroads of these disciplines, and the Page Lab leading the way in transforming scientific understanding of X and Y chromosomes beyond reproduction, Bokil knew she had to get involved.

This morning, as she sits at her desk, poring over a research paper before an afternoon lab meeting, she ponders how insights from the study could enhance her manuscript writing process. Bokil’s graduate project uses a collection of cell lines derived from patients with atypical numbers of X and Y chromosomes to investigate mechanisms that regulate — or dial up and down the expression of — genes located on one of the X chromosomes in females called the “inactive” X chromosome.

Although the X and Y sex chromosomes in mammals began as a pair with similar structures, over time, the Y chromosome underwent degeneration, leading to the loss of numerous active genes. In contrast, the X chromosome preserved its original genes and even gained new ones. To maintain balance in gene expression across the two sexes — XX and XY — an evolutionary mechanism called X chromosome inactivation emerged.

This process is known to randomly silence one X chromosome in each XX pair, ensuring that both sexes have an equal dosage of genes from the X chromosome. However, in recent years, the Page lab has discovered that there are powerful distinctions within females’ pair of X chromosomes, and the so-called “inactive” X chromosome is far from passive. Instead, it plays a crucial role in regulating gene expression on the active X chromosome.

“That’s not all,” adds Bokil. “There are still genes expressed from that “inactive” X chromosome. Cracking how these genes are regulated could answer longstanding questions about sex differences in health.”

Bokil is unraveling this genetic mystery with the help of chemical tags called histone marks. These tags cling to a family of proteins that function like spools, allowing long strands of DNA to coil around them — like thread around a bobbin — so genetic information remains neatly packaged within the cell’s nucleus.

This complex of DNA, RNA, and proteins is called chromatin, the genetic material that eventually forms chromosomes. Chromatin also lays the groundwork for gene regulation by keeping some genes tightly wound around the histones, rendering them inaccessible, and unwinding others for active use.

Certain histone marks are associated with open chromatin structure and active gene expression, while others indicate closed chromatin structure and gene silencing. By examining the specific histone marks on proteins near genes on the “inactive” X chromosome, Bokil aims to decipher if and how these genes are turned on and off.

She’s particularly interested in a group of genes that have counterparts on the Y chromosome. These genes, known as homologous X-Y gene pairs, are typically dosage-sensitive and play a crucial role in regulating essential processes throughout the body like the transcription of DNA into RNA and the translation of RNA into proteins.

Celebrating small triumphs

Graduate school can feel like a marathon — progress is slow but every small step counts towards a breakthrough. For Bokil, stumbling upon a captivating scientific puzzle has been a stroke of luck she deeply appreciates. In fact, the mystery of how genes are controlled on the “inactive” X chromosome has not only shaped her scientific pursuits but also her artwork — on one quiet evening at home, she found herself inspired to capture an experiment, called CUT&RUN, in her painting.

During the early days of her PhD, Bokil spent hundreds of hours using this technique to identify the precise locations of histone protein and DNA interactions. Right as she was prepared to expand these experiments across multiple cell lines, the COVID-19 hit, throwing her plans — and progress — off course.

During these challenging times, Bokil found solace in her cultural roots and the warmth of community. She began teaching virtual BollyX classes — a dance similar to Zumba, but on Bollywood tunes — every Tuesday evening as a means to stay connected, a commitment she’s upheld ever since throughout her time in graduate school.

Beyond nurturing a sense of togetherness through dance, Bokil is committed to mentoring in science and celebrating improbable victories along a tedious research journey.

“I had a former lab mate who used to do what she called a data dance every time she had a graph she felt happy with,” Bokil recalls. “I think that should catch on a little bit more because it’s always a really good feeling to see how these experiments that have taken up so much of your time and effort are leading somewhere.”

Whitehead Institute Member Siniša Hrvatin named a 2024 McKnight Scholar

The McKnight Endowment Fund for Neuroscience has selected Whitehead Institute Member Siniša Hrvatin as one of ten early career scientists to receive a 2024 McKnight Scholar Award, supporting his research on mechanisms underlying certain animals’ capacity to enter states of torpor and hibernation.

Merrill Meadow | Whitehead Institute
June 20, 2024
Rudolf Jaenisch receives the ISTT Prize for contributions to transgenic technologies

The International Society for Transgenic Technologies recognized Whitehead Institute Founding Member Rudolf Jaenisch for his exceptional contribution to the field of animal transgenesis over the past five decades.

Merrill Meadow | Whitehead Institute
June 11, 2024
Whitehead Institute Director Ruth Lehmann elected as a Fellow of the Royal Society

Whitehead Institute Director and President Ruth Lehmann has been named a Foreign Member of the Royal Society. The election recognizes her “pioneering studies of the mechanisms underlying the embryonic development and reproduction of the fruit fly Drosophila.” It honors her work establishing the role of messenger RNA localization in specifying the antero-posterior body axis and germ line development and additionally notes her discoveries that revealed the role of lipid-based signaling pathways in the migration of germ cells to the developing gonads.

Lisa Girard | Whitehead Institute
May 22, 2024
Q&A: Pulin Li on recreating development in the lab

In the whirlwind of activity that occurs simultaneously in a developing embryo, it can be difficult for scientists to pinpoint critical moments of a particular trait. In this Q&A, Pulin Li discusses how her lab ventures beyond mere observation to actually engineer developmental events in a petri dish, and why this approach is vital for understanding health and disease more broadly.

Shafaq Zia | Whitehead Institute
May 1, 2024
New findings activate a better understanding of Rett syndrome’s causes

Rett syndrome is caused by mutations to the gene MECP2, which is highly expressed in the brain and appears to play important roles in maintaining healthy neurons. Researchers led by Rudolf Jaenisch have used cutting-edge techniques to create an epigenome map of MECP2, which may help guide future research on the disease.

Greta Friar | Whitehead Institute
April 25, 2024
The Whitehead Innovation Initiative is established to advance the use of artificial intelligence in biomedical research

The Whitehead Innovation Initiative launched in April 2024 and, under the expert guidance of President and Director Ruth Lehmannn, will pioneer the melding of AI and biology. The initiative was made possible by a $10 million gift from Michael and Victoria Chambers.

Merrill Meadow | Whitehead Institute
April 8, 2024