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Unusual Labmates: Meet tardigrades, the crafters of nature’s ultimate survival kit

Whitehead Institute Member Siniša Hrvatin is studying tardigrades to decode the mechanisms enabling their survival in extreme environmental conditions. Learn about the biology of these microscopic “water bears” and what makes them a particularly fascinating model organism.

Shafaq Zia | Whitehead Institute
July 23, 2024

Tardigrades, also affectionately known as “water bears” or “moss piglets”, are remarkable microscopic organisms that have captured the imagination of scientists and nature enthusiasts alike.

With adults measuring anywhere from 0.2 to 1.2 millimeters in length — as big as a grain of salt — tardigrades possess the astounding ability to survive harsh environmental conditions. These resilient creatures have been found in habitats ranging from the depths of oceans and hot radioactive springs to the frigid expanses of Antarctica. It is their unparalleled adaptability that makes them invaluable as a model organism for researchers like Whitehead Institute Member Siniša Hrvatin, who’s studying physiological adaptation in animals with a focus on states that can slow down tissue damage, disease progression, and even aging.

Follow along to learn what’s behind tardigrades’ nearly indestructible nature, how researchers at Whitehead Institute — and beyond — are studying them, and what insights this work can offer into long-term organ preservation, space exploration, and more.

Big discovery of a tiny creature

In 1773, German naturalist Johann August Ephraim Goeze was analyzing moss samples under a microscope when he stumbled upon an unusual creature. Captivated by its peculiar appearance, he continued his observations and documented the discovery of Kleiner Wasserbär, translating to “little water bear”, in his publication. This work also featured the first-ever drawing of a tardigrade.

Since then, researchers’ understanding of this remarkable organism has evolved alongside advancements in imaging technology. Today, tardigrades are recognized as bilaterally symmetrical invertebrates with two eyes and eight chubby legs adorned with hook-like claws. Often described as a mix between nematodes and insects, these extremophiles are able to withstand freezing, intense radiation, vacuum of outer space, desiccation, chemical treatments, and possibly more.

And the best part? Despite their otherworldly appearance and surprising capabilities, tardigrades share plenty of similarities with larger, more complex organisms, including possessing a primordial brain, muscles, and even a digestive system.

The biology of an extremophile

Researchers trace the evolutionary origins of tardigrades back to panarthropods, a group that includes now-extinct worm-like organisms called lobopodians. To date, over a thousand species of tardigrades have been identified, with terrestrial species inhabiting environments like moss, leaf litter, and lichen, grassland, and deserts while aquatic ones are found in both fresh and saltwater.

Little is known about tardigrades’ diet but researchers are particularly drawn to herbivorous ones that like to munch on single-celled algae and thrive in water. There’s good reason for it: algae are inexpensive to grow in the lab with just light and basic nutrients. But it’s not just their diet that makes tardigrades an attractive model organism — they also have a short generation time (11 to 14 days), with eggs hatching within a four-day span. In fact, some species are able to reproduce without sexual reproduction through a process called parthenogenesis, during which the female egg undergoes cell division without fertilization by a male gamete.

Although genomic resources for studying tardigrades are limited to only a few species, researchers from Keio University and University of Edinburgh have successfully sequenced the genome of a moss-residing tardigrade commonly used in research called Hypsibius exemplaris. Its genome is less than half the size of a Drosophila melanogaster genome, consisting of 105 million base pairs that serve as the building blocks of DNA.

In spite of their small genome — and only a few thousand cells in the body — tardigrades have a well-defined miniaturized body plan, consisting of a head and four segments, that holds valuable insights for researchers looking to decode their adaptation prowess.

Inside tardigrade research at Whitehead Institute

In 2022, as Hrvatin was setting up his lab at Whitehead Institute, a question lingered in his mind. “I was trying to find animals that can survive being frozen for long periods of time and then continue living,” he says. “But there are not that many that fit the bill.”

Then, an undergraduate student at Massachusetts Institute of Technology (MIT) expressed her enthusiasm for astrobiology — the study of life across the universe — and highlighted tardigrades as a favorite among space researchers. Hrvatin was intrigued.

Up until this point, his research had centered upon two states of dormancy, or reduced metabolic activity, in animals: hibernation and a shorter, less intense torpor. But tardigrades possessed a survival mechanism unlike any other. When faced with harsh conditions like dehydration, they would expel water, retract their head and legs, and curl up in a small, dry ball, entering a state of suspended animation called crytobiosis or tun formation.

For decades, researchers hypothesized that the tun state might be responsible for tardigrades’ unparalleled ability to withstand a myriad of environmental assaults, including extremely low temperature. However, recent work has revealed that these animals utilize a separate and unique adaptation, distinct from the tun state, to survive being frozen for extended periods. In fact, preliminary evidence from a preprint by a team of scientists at UC Berkeley and UC San Francisco illustrates unique patterns of how tardigrades survive freezing while hydrated in water.

This phenomenon is markedly different from hibernation and its cousin torpor. “Unlike animals lowering their body temperature, we’re talking about putting tardigrades at minus 180 degrees Celsius, and then thawing them,” says Hrvatin. In fact, cryobiosis is so intense that tardigrades’ metabolic activity drops to undetectable levels, rendering them virtually, but not quite, dead. The organisms can then remain in this state from months to years, only to revive as healthy when conditions become favorable once again.

Frozen in time

In 2014, a group of Japanese researchers at Tokyo’s National Institute for Polar Research undertook an intriguing experiment. They began by thawing moss samples collected from East Antarctica in November 1983. Then, they carefully teased apart each sample using tweezers to retrieve tardigrades that might be nestled within. Among the tardigrades the researchers found, two stood out: Sleeping Beauty 1 and Sleeping Beauty 2 who were believed to be undergoing cold induced-dormancy. Turns out, the researchers were right — within the first day of being placed in the Petri dish with water, the tardigrades began exhibiting slow movements despite having been frozen for over 30 years.

The Swiss army knife in tardigrades’ toolbox

Yet, the remarkable resilience of tardigrades continues to baffle scientists. Recently, they’ve uncovered what could be another potential weapon in the creatures’ arsenal: intrinsically disordered proteins or IDPs. Picture them as putty — a group of proteins that do not have a well-defined three-dimensional structure and can interact with other molecules to produce a range of different outcomes. Some researchers have linked these tardigrade-specific IDPs to the animals extraordinary resilience: under extreme heat, these proteins remain stable. And when desiccated, they form protective glasses that shield cells and vital enzymes from dehydration.

If confirmed, the implications of this work would extend beyond tardigrades’ survival, potentially revolutionizing dry vaccine storage and the development of drought-resistant crops.

MIT affiliates named 2024 HHMI Investigators

Four faculty members and four others with MIT ties are recognized for pushing the boundaries of science and for creating highly inclusive and collaborative research environments.

School of Science
July 23, 2024

The Howard Hughes Medical Institute (HHMI) today announced its 2024 investigators, four of whom hail from the School of Science at MIT: Steven Flavell, Mary Gehring, Mehrad Jazayeri, and Gene-Wei Li.

Four others with MIT ties were also honored: Jonathan Abraham, graduate of the Harvard/MIT MD-PhD Program; Dmitriy Aronov PhD ’10; Vijay Sankaran, graduate of the Harvard/MIT MD-PhD Program; and Steven McCarroll, institute member of the Broad Institute of MIT and Harvard.

Every three years, HHMI selects roughly two dozen new investigators who have significantly impacted their chosen disciplines to receive a substantial and completely discretionary grant. This funding can be reviewed and renewed indefinitely. The award, which totals roughly $11 million per investigator over the next seven years, enables scientists to continue working at their current institution, paying their full salary while providing financial support for researchers to be flexible enough to go wherever their scientific inquiries take them.

Of the almost 1,000 applicants this year, 26 investigators were selected for their ability to push the boundaries of science and for their efforts to create highly inclusive and collaborative research environments.

“When scientists create environments in which others can thrive, we all benefit,” says HHMI president Erin O’Shea. “These newest HHMI Investigators are extraordinary, not only because of their outstanding research endeavors but also because they mentor and empower the next generation of scientists to work alongside them at the cutting edge.”

Steven Flavell

Steven Flavell, associate professor of brain and cognitive sciences and investigator in the Picower Institute for Learning and Memory, seeks to uncover the neural mechanisms that generate the internal states of the brain, for example, different motivational and arousal states. Working in the model organism, the C. elegans worm, the lab has used genetic, systems, and computational approaches to relate neural activity across the brain to precise features of the animal’s behavior. In addition, they have mapped out the anatomical and functional organization of the serotonin system, mapping out how it modulates the internal state of C. elegans. As a newly named HHMI Investigator, Flavell will pursue research that he hopes will build a foundational understanding of how internal states arise and influence behavior in nervous systems in general. The work will employ brain-wide neural recordings, computational modeling, expansive research on neuromodulatory system organization, and studies of how the synaptic wiring of the nervous system constrains an animal’s ability to generate different internal states.

“I think that it should be possible to define the basis of internal states in C. elegans in concrete terms,” Flavell says. “If we can build a thread of understanding from the molecular architecture of neuromodulatory systems, to changes in brain-wide activity, to state-dependent changes in behavior, then I think we’ll be in a much better place as a field to think about the basis of brain states in more complex animals.”

Mary Gehring

Mary Gehring, professor of biology and core member and David Baltimore Chair in Biomedical Research at the Whitehead Institute for Biomedical Research, studies how plant epigenetics modulates plant growth and development, with a long-term goal of uncovering the essential genetic and epigenetic elements of plant seed biology. Ultimately, the Gehring Lab’s work provides the scientific foundations for engineering alternative modes of seed development and improving plant resiliency at a time when worldwide agriculture is in a uniquely precarious position due to climate changes.

The Gehring Lab uses genetic, genomic, computational, synthetic, and evolutionary approaches to explore heritable traits by investigating repetitive sequences, DNA methylation, and chromatin structure. The lab primarily uses the model plant A. thaliana, a member of the mustard family and the first plant to have its genome sequenced.

“I’m pleased that HHMI has been expanding its support for plant biology, and gratified that our lab will benefit from its generous support,” Gehring says. “The appointment gives us the freedom to step back, take a fresh look at the scientific opportunities before us, and pursue the ones that most interest us. And that’s a very exciting prospect.”

Mehrad Jazayeri

Mehrdad Jazayeri, a professor of brain and cognitive sciences and an investigator at the McGovern Institute for Brain Research, studies how physiological processes in the brain give rise to the abilities of the mind. Work in the Jazayeri Lab brings together ideas from cognitive science, neuroscience, and machine learning with experimental data in humans, animals, and computer models to develop a computational understanding of how the brain creates internal representations, or models, of the external world.

Before coming to MIT in 2013, Jazayeri received his BS in electrical engineering, majoring in telecommunications, from Sharif University of Technology in Tehran, Iran. He completed his MS in physiology at the University of Toronto and his PhD in neuroscience at New York University.

With his appointment to HHMI, Jazayeri plans to explore how the brain enables rapid learning and flexible behavior — central aspects of intelligence that have been difficult to study using traditional neuroscience approaches.

“This is a recognition of my lab’s past accomplishments and the promise of the exciting research we want to embark on,” he says. “I am looking forward to engaging with this wonderful community and making new friends and colleagues while we elevate our science to the next level.”

Gene-Wei Li,

Gene-Wei Li, associate professor of biology, has been working on quantifying the amount of proteins cells produce and how protein synthesis is orchestrated within the cell since opening his lab at MIT in 2015.

Li, whose background is in physics, credits the lab’s findings to the skills and communication among his research team, allowing them to explore the unexpected questions that arise in the lab.

For example, two of his graduate student researchers found that the coordination between transcription and translation fundamentally differs between the model organisms E. coli and B. subtilis. In B. subtilis, the ribosome lags far behind RNA polymerase, a process the lab termed “runaway transcription.” The discovery revealed that this kind of uncoupling between transcription and translation is widespread across many species of bacteria, a study that contradicted the long-standing dogma of molecular biology that the machinery of protein synthesis and RNA polymerase work side-by-side in all bacteria.

The support from HHMI enables Li and his team the flexibility to pursue the basic research that leads to discoveries at their discretion.

“Having this award allows us to be bold and to do things at a scale that wasn’t possible before,” Li says. “The discovery of runaway transcription is a great example. We didn’t have a traditional grant for that.”

Gene silencing tool has a need for speed

Small changes in the molecular machines that carry out RNA interference can lead to big differences in the efficacy of gene silencing. These new findings from the Bartel Lab have implications for the design of gene-silencing therapeutics.

Greta Friar | Whitehead Institute
July 17, 2024

RNA interference (RNAi) is a process that many organisms, including humans, use to decrease the activity of target RNAs in cells by triggering their degradation or slicing them in half. If the target is a messenger RNA, the intermediary between gene and protein, then RNAi can decrease or completely silence expression of the gene. Researchers figured out how to tailor RNAi to target different RNAs, and since then it has been used as a research tool to silence genes of interest. RNAi is also used in a growing number of therapeutics to silence genes that contribute to disease.

However, researchers still do not understand some of the biochemistry underlying RNAi. Slight differences in the design of the RNAi machinery can lead to big differences in how effective it is at decreasing gene expression. Through trial and error, researchers have worked out guidelines for making the most effective RNAi tools without understanding exactly why they work. However, Whitehead Institute Member David Bartel and graduate student in his lab Peter Wang have now dug deeper to figure out the mechanics of the main cellular machine involved in RNAi. The researchers’ findings, shared in Molecular Cell on July 17, not only provide explanations for some of the known rules for RNAi tool design, but also provide new insights that could improve future designs.

Slicing speed is highly variable

The cellular machine that carries out RNAi has two main parts. One is a guide RNA, a tiny RNA typically only 22 bases or nucleotides long. RNA, like DNA, is made of four possible bases, although RNA has the base uracil (U) instead of the DNA base thymine (T). RNA bases bind to each other in certain pairings—guanines (G) pair to cytosines (C) and adenines (A) pair to U’s—and the sequence of bases in the guide RNA corresponds to a complementary sequence within the target RNA. When the guide RNA comes across a target, the corresponding bases pair up, binding the RNAs. Then the other part of the RNAi machine, an Argonaute protein bound to the guide RNA, can slice the target RNA in half or trigger the cell to break it down more gradually.

In humans, AGO2 is the Argonaute protein that is best at slicing. Only a couple dozen RNA targets actually get sliced, but these few targets play essential roles in processes such as neuron signal control and accurate body shape formation. Slicing is also important for RNAi tools and therapeutics.

In order for AGO2 to slice its target, the target must be in the exact right position. As the guide and target RNAs bind together, they go through a series of motions to ultimately form a double helix. Only in that configuration can AGO2 slice the target.

Researchers had assumed that AGO2 slices through different target RNAs at roughly the same rate, because most research into this process used the same few guide RNAs. These guide RNAs happen to have similar features, and so similar slicing kinetics—but they turn out not to be representative of most guide RNAs.

Wang paired AGO2 with a larger variety of guide RNAs and measured the rate at which each AGO2-guide RNA complex sliced its targets. He found big differences. Whereas the commonly used guide RNAs might differ in their slicing rate by 2-fold, the larger pool of guide RNAs differed by as much as 250-fold. The slicing rates were often much slower than the researchers expected. Previously, researchers thought that all targets could be sliced relatively quickly, so the rate wasn’t considered as a limiting factor – other parts of the process were thought to determine the overall pace – but Wang found that slicing can sometimes be the slowest step.

“The important consideration is whether the slicing rate is faster or slower than other processes in the cell,” Wang says. “We found that for many guide RNAs, the slicing rate was the limiting factor. As such, it impacted the efficacy.”

The slower AGO2 is to slice targets, the more messenger RNAs will remain intact to be made into protein, meaning that the corresponding gene will continue being expressed. The researchers observed this in action: the guide RNAs with slower slicing rates decreased target gene expression by less than the faster ones.

Small changes lead to big differences in slicing rate

Next, the researchers explored what could be causing such big differences in slicing rate between guide RNAs. They mutated guide RNAs to swap out single bases along the guide RNA’s sequence—say, switching the 10th base in the sequence from a C to an A—and measured how this changed the slicing rate.

“The important consideration is whether the slicing rate is faster or slower than other processes in the cell,” Wang says. “We found that for many guide RNAs, the slicing rate was the limiting factor. As such, it impacted the efficacy.”

The researchers found that slicing rate increased when the base at position 7 was an A or a U. The bases A and U pair more weakly than C and G. The researchers found that having a weak A-U pair at that position, or a fully mismatched pair at position 6 or 7, may allow a kink to form in the double helix shape that actually makes the target easier to slice. Wang also found that slicing rate increases with certain substitutions at the 10th and the 17th base positions, although the researchers could not yet determine possible underlying mechanisms.

These observations correspond to existing recommendations for RNAi design, such as not using a G at position 7. The new work demonstrates that the reason these recommendations work is because they affect the slicing rate, and, in the case of position 7, the new work further identifies the specific mechanism at play.

Interplay between regions matters

People designing synthetic guide RNAs thought that the bases at the tail end, past the 16th position, were not very important. This is because in the case of the most commonly used guide RNAs, the target will be rapidly cleaved even if all of the tail end positions are mismatches that cannot pair.

However, Wang and Bartel found that the identity of the tail end bases are only irrelevant in a specific scenario that happens to be true of the most commonly used guide RNAs: when the bases in the center of the guide RNA (positions 9-12) are strong-pairing Cs and Gs. When the center pairings are weak, then the tail end bases need to be perfect matches to the target RNA. The researchers found that guide RNAs could have up to a 600-fold difference in tolerance for tail end mismatches based on the strength of their central pairings.

The reason for this difference has to do with the final set of motions that the two RNAs must perform in order to assume their final double helix shape. A perfectly paired tail end makes it easier for the RNAs to complete these motions. However, a strong enough center can pull the RNAs into the double helix even if the tail ends are not ideally suited for doing so.

The observation that weak central pairing requires perfect or near perfect tail end matches could provide a useful new guideline for designing synthetic RNAs. Any guide RNA runs the risk of sometimes binding other messenger RNAs that are similar enough to the intended target RNA. In the case of a therapy, this off-target binding can lead to negative side effects. Bartel and Wang suggest that researchers could design guide RNAs with weak centers, which would require more perfect pairing in the tail end, so that the guide RNA will be less likely to bind non-target RNAs; only the perfect pairing of the target’s RNA sequence would suffice.

Altogether, Wang and Bartel’s findings explain how small differences between guide RNAs can make such large differences in the efficacy of RNAi, providing a rationale for the long-standing RNAi design guidelines. Some of the findings even suggest new guidelines that could help with future synthetic guide RNA designs.

“Discovering the interplay between the center and tail end of the guide RNA was unexpected and satisfying,” says Bartel, who is also a professor at the Massachusetts Institute of Technology and a Howard Hughes Medical Investigator. “It explains why, even though the guidelines suggested that tail-end sequence doesn’t matter, the target RNAs that are sliced in our cells do have pairing to the tail end. This observation could prove useful to reduce off-target effects in RNAi therapeutics.”

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.”

Brady Weissbourd named Searle Scholar

With an eye on regenerative medicine, Weissbourd's lab will study how jellyfish manage to constantly integrate new neurons into their nervous system.

David Orenstein | The Picower Institute for Learning and Memory
July 8, 2024

Scientists who dream of a future in which regenerative medicine has advanced enough to enable repairs in human nervous systems currently have more questions than answers. As a recently named Searle Scholar, MIT Assistant Professor Brady Weissbourd will seek to learn some of the needed fundamentals by studying a master of neural regeneration: the jellyfish, Clytia hemisphaerica.

Weissbourd, a faculty member in the Department of Biology and The Picower Institute for Learning and Memory, has helped to pioneer use of the seafaring species in neuroscience research for many reasons. It is transparent for easy imaging, reproduces rapidly, and shares many basic nervous system properties with mammals despite diverging evolutionarily 600 million years ago (just after the development of the earliest nervous systems). Meanwhile, with about 10,000 neurons, the jellyfish fills a gap in the field in terms of that degree of complexity.

But what Weissbourd didn’t appreciate until he began experimenting with the jellyfish was that they are also incredibly good at refreshing and rebuilding their nervous systems with new cells. After becoming the first researcher to develop the ability to genetically manipulate the organism, he started teasing out how its highly distributed nervous system (there is no central brain), was organized to enable its many behaviors. When he ablated a subnetwork of cells to test whether it was indeed responsible for a particular feeding behavior, he found that within a week it had completely regrown. Moreover, he has observed that the jellyfish constantly produce and integrate new cells, even in the absence of major injury.

Looking for the logic

The finding raised a proverbial boatload of intriguing questions that his support of $100,000 a year for the next three years from the Searle Scholars Program will help him pursue.

“Where are these newborn neurons coming from in both the normal and regenerative contexts?” Weissbourd asked. “What rules guide them to the correct locations to rebuild these networks, both to integrate these newborn neurons into the network without messing it up and also to recreate it during regeneration? Are the rules the same or different between these contexts?”

Additionally, by using a combination of techniques such as imaging neural activity during behavior, sequencing gene expression cell by cell, and computational modeling, Weissbourd’s lab has discerned that within their web-like mesh of neurons, jellyfish harbor more than a dozen different functional subnetworks that enable its variety of different behaviors. Can all the subnetworks regenerate? If not, why do some forgo the remarkable ability? Among those that do regenerate, do they all do so the same way? If they employ different means, then learning what those are could provide multiple answers to the question of how new neurons can successfully integrate into existing neural networks.

Building on support provided by a Klingenstein-Simons Fellowship Weissbourd earned last year, he’ll be able to pursue experiments designed to understand the “logic” of how jellyfish manage neural regeneration.

“The ability to understand how nervous systems regenerate has significant implications for regenerative medicine,” Weissbourd said.

A complete 3D ‘wiring diagram’

As part of the new award, Weissbourd also plans to create a major new resource for jellyfish neurobiology to advance not only this project, but also the research of any other scientist who wants to study the organism. Working with collaborator Jeff Lichtman, a professor of molecular and cellular biology at Harvard University, Weissbourd will create a complete 3D reconstruction of a jellyfish’s nervous system at the subcellular resolution enabled by electron microscopy. The resource, which Weissbourd plans to provide openly online, will amount to a full “wiring diagram” of a jellyfish where every circuit connection can be mapped.

Being able to see how every neural circuit is constructed in a whole animal will enable Weissbourd to answer questions about how the circuits are built and therefore how new neurons integrate. Having a complete and detailed view of every circuit will improve the computational models his lab is building to predict how anatomy helps give rise to function and behavior. And given that new neurons are being born, migrating and integrating all the time, Weissbourd said, the imaging will also likely yield a snapshot of neural regeneration in action in its many stages.

Weissbourd said he was grateful for the honor of being named a Searle Scholar, which not only provides support for his lab’s work, but also welcomes him into a new community of young scientists.

“I’m honored and super excited,” Weissbourd said. “I’m excited to interact with the other scholars as well.”

 

“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.”

Sara Prescott named Pew Scholar in the Biomedical Sciences

Assistant Professor Sara Prescott and her lab plan to test whether and how neurons have a role in airway remodeling, which goes awry in many diseases.

David Orenstein | The Picower Institute for Learning and Memory
June 17, 2024
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