Research Area: Genetics

Research Threads: One lab’s detective work reveals secrets of immortal cells

Most cells in our body live and die. But the germline, the cells that produce eggs and sperm, must carry on forever. How the germline successfully produces the next generation is a long-studied question. Through a string of discoveries made over years, the Yamashita lab is teasing apart how the germline remains immortal.

Madeleine Turner | Whitehead Institute
October 7, 2025

Most cells in our body live and die. But the germline, the cells that produce eggs and sperm, must carry on forever. How the germline successfully produces the next generation is a long-studied question. Research Threads examines how answering one question uncovers more questions to be solved. In our first installment of Research Threads, we follow the research of Whitehead Institute Member Yukiko Yamashita. Through a string of discoveries made over years, the Yamashita lab is teasing apart how the germline remains immortal.

“The germline is the only cell type responsible for transmitting the genome from generation to generation,” Whitehead Institute Member Yukiko Yamashita says. “We’ve done that for 1.5 billion years.”

The germline is the population of cells in an organism that give rise to gametes, either egg or sperm cells. These gametes contain genetic information, encoded in DNA, needed to produce the next generation. The act of transmitting this information — the instructions that a new individual needs to develop and function — is like a relay race that never ends. While a skin or gut cell may be prone to genetic errors and is generally replaceable, germline stem cells (GSCs) must maintain their genomes with precision. Otherwise, any mistakes or imbalances would be inherited by offspring and accumulated over generations, potentially driving a species to extinction. In other words, to keep passing the baton in this relay, the germline must be faithfully preserved.

Although germline preservation is paramount to the existence and survival of a species, some fundamental parts of its biology have remained a mystery. Yamashita, who is also a professor of biology at the Massachusetts Institute of Technology and a Howard Hughes Medical Institute Investigator, has focused her research on unraveling the secrets of the germline. To study these cells’ immortality, her lab utilizes the model organism Drosophila melanogaster, or the fruit fly. Along the way, research in the Yamashita lab has highlighted how the process of scientific discovery can be circuitous, often pulling scientists in surprising directions, revealing new questions and avenues to explore.

For decades, scientists had observed an aspect of germline behavior that was hard to explain. Most cells in the body divide to produce two identical copies, or daughter cells. GSCs in male fruit flies, however, divide “asymmetrically,” meaning they yield two daughter cells that are not identical. Instead, one daughter cell becomes a new GSC, while the other goes on to become a gamete, in this case a sperm cell. It might be easy to assume that asymmetric cell division is about producing gametes while maintaining a pool of stem cells. But an additional detail complicates this theory: when a daughter cell is on the path to becoming sperm, it can loop back around to become another stem cell, instead of continuing differentiation to become a sperm cell.

“If it can do that, why divide asymmetrically in the first place?” Yamashita says.

To shed light on why GSCs divide asymmetrically, researchers wanted to know how genetic information was actually divvied up between daughter cells. “After I started my own lab, there was this question hanging in the field,” Yamashita says. It made sense to find some difference in inheritance, DNA-based or otherwise: something to distinguish between the daughter fated to become a gamete, and the other that would remain in the GSC pool.

Preparing for division, a cell duplicates its DNA. Chromosomes happen to be shaped like the letter “X” as a result of this duplication: the left and right sides of the “X” are called matching sister chromatids, each a copy of the other. Later in cell division, these two sister chromatids part ways, winding up in separate daughter cells.

In 2013, Yamashita and her former graduate student, Swathi Yadlapalli, made a strange but important discovery. In fruit flies, for the X and Y chromosomes (the sex chromosomes), sister chromatids were not being sorted randomly. Instead, one was more likely to go to the daughter cell that would become a gamete; the other to the daughter on the GSC track. There had to be a reason for this preference, but no one had an explanation.

Initial experiments did not reveal obvious differences between these sister chromatid pairs. “Everyone would say, ‘oh, there’s probably some sort of epigenetic information in there,” Yamashita says, referring to heritable changes not carried in DNA. With few promising leads, the lab decided to take a systematic approach. George Watase, then a postdoc in the lab, began the painstaking work of removing different a parts of flies’ X chromosomes, seeing if the absence of any particular element would disrupt the pattern of preferential segregation.

“We thought it was going to be satellite DNA,” Yamashita says, referring to large swathes of DNA in the genome that are highly repetitive but don’t code for any genes. (While this initial guess was wrong, it kickstarted a separate project in the lab — one which led to discovering the unexpected role that satellite DNA plays when one species forks into two.)

Eventually the team narrowed in on the true culprit: ribosomal DNA (rDNA). The primary role of rDNA is to produce the components that make up ribosomes. Ribosomes, in turn, perform the crucial task of synthesizing proteins.

“We didn’t like this finding in the beginning. I always say that ribosomal DNA is ‘too important to be interesting.’ You don’t get excited about something you really need, like toilet paper,” Yamashita says. “In the case of ribosomal DNA, bacteria needs it, humans need it, everybody needs it.”

But what did rDNA have to do with asymmetric cell division in the germline?

“Around that time, we started reading lots of papers,” Yamashita says. “Then we discovered a phenomenon called rDNA magnification. These were studies from the 1960s and ’80s — most of the people in my lab were not even born yet.”

These studies from decades ago described mutant flies that lacked a sufficient amount of rDNA and, as a result, had a “bobbed” phenotype, or appearance. Since these flies possessed fewer ribosomes to produce proteins, the bristles on their back were shorter; the protective cuticle covering their bodies weakened. But when they reproduced, many of their offspring possessed a normal amount of rDNA. These observations pointed towards a mechanism that allowed flies to replenish their supply of rDNA.

At the time rDNA magnification was first observed, the phenomenon was regarded as an oddity, something that only happened in mutant flies and did not have physiological relevance. But Yamashita realized there was a connection between rDNA magnification and asymmetric division in the germline.

To make enough protein, a cell must produce ample ribosomes. To do that, the fruit fly genome contains hundreds of copies of rDNA in a row. But the DNA replication process can struggle to handle so many rDNA copies strung together, and can sometimes experience a hiccup, resulting in the loss of rDNA copies with each new division. It’s an outcome that might be okay on occasion, but would wreak havoc over many generations.

“All of a sudden, [rDNA magnification] was not about a mutant chromosome,” Yamashita says. “We were like, holy moly. This is about germline immortality.”

Soon many different pieces became part of a cohesive story: asymmetric cell division is not about producing a balance of gametes and stem cells; it’s about maintaining rDNA in the germline. Sister chromatids are almost identical — but one contains more copies of rDNA than the other, and that copy is fated to stay in the GSC pool. Through this asymmetry, the germline is replenished of rDNA; it can pass the baton to the next generation.

“For quite some time, for so many observations that everyone knew in the field, we felt we had an explanation. But from that ‘aha!’ moment, we took multiple years to test everything,” Yamashita says.

In subsequent years, the Yamashita lab pinned down additional details about how rDNA is diverted back to the germline. First, in 2022, the team identified a specific protein, which they named Indra, which binds to rDNA. The presence of Indra helps assign the sister chromatid containing more rDNA copies to the GSC daughter cell.

Their next discovery was another plot twist. If one sister chromatid contained more rDNA than the other, and those rDNA copies weren’t appearing out of thin air, it meant that one chromatid needed to be stealing rDNA from its sister. The lab discovered a genetic element that facilitated this transfer: a retrotransposon.

Retrotransposons are usually considered “genetic parasites,” copying and pasting themselves into the genome. In an attempt to reinsert itself, this particular retrotransposon, called R2, slices open sections containing rDNA on both chromatids. As the cell repairs these breaks, it may inadvertently stitch copies of rDNA from one chromosome to the other, creating an unequal number of copies between the two.

“Not many people thought retrotransposons could be beneficial to the host. They’re seen as parasites,” Yamashita says. “But it turns out that they are essential for germline immortality.”

Sometimes, one research project is a spin off of a spin off. That was true for Xuefeng Meng, a graduate student in the lab who was working on satellite DNA, the genetic element that turned out to be unrelated to asymmetric cell division, but was interesting in its own right.

While studying satellite DNA, Meng noticed that a particular stock of flies had a problem producing normal sperm, that their cells’ nuclei were abnormally packaged. The problem had to do with a gene called Stellate on the flies’ X chromosome. While most flies have few copies of Stellate, these flies had a higher number of copies.

Stellate was already known in the field as a meiotic driver, or “selfish-gene”: a genetic element that has evolved ways to preferentially transmit itself to the next generation. Some meiotic drivers, including Stellate, are on the sex chromosomes and, when not suppressed, cause an excess of either male or female progeny. In this case, Stellate produces a protein, Ste, which is found to concentrate in Y-carrying cells during meiosis, the specialized type of cell division that produces gametes (meiosis follows the earlier round of asymmetric cell division described above). High concentrations of Ste impede the proper packaging of nuclei in cells, leading to their eventual death. When Stellate is widely expressed, fewer male flies emerge in the next generation.

But here lies an inherent tension: if a selfish gene is too good at propagating itself, and produces only males or females, its host species would go extinct — and so would the gene. Meng and Yamashita were interested in this paradox. Through this work, they identified a novel mechanism that keeps Stellate in check. To balance selfish propagation with the host species’ survival, Stellate has a built-in system for pumping the brakes. After Ste concentrates in Y-carrying cells during the first meiotic division, the protein is unevenly distributed a second time. This second step spares a portion of Y-carrying cells that go on to create males.

How the germline is able to counter disruptive forces, thereby renewing itself, continues to be a ripe research area. Researchers still don’t know all the secrets to how a line of cells can achieve perpetuity — but the Yamashita lab continues to investigate the question.

“What I really like about people in my lab is that they don’t jump to the easiest conclusion,” Yamashita says. “If you start embracing surprise, then good things happen. Because you are surprised, you start testing your finding in multiple ways. Then you can build confidence about the result.”

Notes

Xuefeng Meng and Yukiko Yamashita (2025). “Intrinsically weak sex chromosome drive through sequential asymmetric meiosis.” Science Advances. https://doi.org/10.1126/sciadv.adv7089

Jonathan O. Nelson, Tomohiro Kumon, Yukiko M. Yamashita. (2023) “rDNA magnification is a unique feature of germline stem cells.” PNAS. https://doi.org/10.1073/pnas.2314440120

Jonathan O. Nelson, Alyssa Slicko, Yukiko M. Yamashita. (2023) “The retrotransposon R2 maintains Drosophila ribosomal DNA repeats.” PNAS. https://doi.org/10.1073/pnas.2221613120

George J. Watase, Jonathan O. Nelson, Yukiko M. Yamashita. (2022) “Nonrandom sister chromatid segregation mediates rDNA copy number maintenance in Drosophila.” Science Advances. https://www.science.org/doi/10.1126/sciadv.abo4443

Madhav Jagannathan and Yukiko Yamashita. (2021) “Defective satellite DNA clustering into chromocenters underlies hybrid incompatibility in Drosophila.” Molecular Biology and Evolution. https://doi.org/10.1093/molbev/msab221

Swathi Yadlapalli and Yukiko Yamashita (2013) “Chromosome-specific nonrandom sister chromatid segregation during stem-cell division.” Nature. 10.1038/nature12106

Neural activity helps circuit connections mature into optimal signal transmitters

By carefully tracking the formation and maturation of synaptic active zones in fruit flies, MIT scientists have discovered how neural activity helps circuit connections become tuned to the right size and degree of signal transmission capability over a period of days.

David Orenstein | The Picower Institute for Learning and Memory
October 14, 2025

Nervous system functions, from motion to perception to cognition, depend on the active zones of neural circuit connections, or “synapses,” sending out the right amount of their chemical signals at the right times. By tracking how synaptic active zones form and mature in fruit flies, researchers at The Picower Institute for Learning and Memory at MIT have revealed a fundamental model for how neural activity during development builds properly working connections.

Understanding how that happens is important, not only for advancing fundamental knowledge about how nervous systems develop, but also because many disorders such as epilepsy, autism, or intellectual disability can arise from aberrations of synaptic transmission, said senior author Troy Littleton, Menicon Professor in The Picower Institute and MIT’s Department of Biology. The new findings, funded in part by a 2021 grant from the National Institutes of Health, provide insights into how active zones develop the ability to send neurotransmitters across synapses to their circuit targets. It’s not instant or predestined, the study shows. It can take days to fully mature and that is regulated by neural activity.

If scientists can fully understand the process, Littleton said, then they can develop molecular strategies to intervene to tweak synaptic transmission when it’s happening too much or too little in disease.

“We’d like to have the levers to push to make synapses stronger or weaker, that’s for sure,” Littleton said. “And so knowing the full range of levers we can tug on to potentially change output would be exciting.”

Littleton Lab research scientist Yuliya Akbergenova led the study published Oct. 14 in the Journal of Neuroscience.

How newborn synapses grow up 

In the study, the researchers examined neurons that send the neurotransmitter glutamate across synapses to control muscles in the fly larvae. To study how the active zones in the animals matured, the scientists needed to keep track of their age. That hasn’t been possible before, but Akbergenova overcame the barrier by cleverly engineering the fluorescent protein mMaple, which changes its glow from green to red when zapped with 15 seconds of ultraviolet light, into a component of the glutamate receptors on the receiving side of the synapse. Then, whenever she wanted, she could shine light and all the synapses already formed before that time would glow red and any new once that formed subsequently would glow green.

With the ability to track each active zone’s birthday, the authors could then document how active zones developed their ability to increase output over the course of days after birth. The researchers actually watched as synapses were built over many hours by tagging each of eight kinds of proteins that make up an active zone. At first, the active zones couldn’t transmit anything. Then, as some essential early proteins accumulated, they could send out glutamate spontaneously, but not if evoked by electrical stimulation of their host neuron (simulating how that neuron might be signaled naturally in a circuit). Only after several more proteins arrived did active zones possess the mature structure for calcium ions to trigger the fusion of glutamate vesicles to the cell membrane for evoked release across the synapse.

Activity matters

Of course, construction does not go on forever. At some point, the fly larva stops building one synapse and then builds new ones further down the line as the neuronal axon expands to keep up with growing muscles. The researchers wondered whether neural activity had a role in driving that process of finishing up one active zone and moving on to build the next.

To find out, they employed two different interventions to block active zones from being able to release glutamate, thereby preventing synaptic activity. Notably, one of the methods they chose was blocking the action of a protein called Synaptotagmin 1. That’s important because mutations that disrupt the protein in humans are associated with severe intellectual disability and autism. Moreover, the researchers tailored the activity-blocking interventions to just one neuron in each larva because blocking activity in all their neurons would have proved lethal.

In neurons where the researchers blocked activity, they observed two consequences: the neurons stopped building new active zones and instead kept making existing active zones larger and larger. It was as if the neuron could tell the active zone wasn’t releasing glutamate and tried to make it work by giving it more protein material to work with. That effort came at the expense of starting construction on new active zones.

“I think that what it’s trying to do is compensate for the loss of activity,” Littleton said.

Testing indicated that the enlarged active zones the neurons built in hopes of restarting activity were functional (or would have been if the researchers weren’t artificially blocking them). This suggested that the way the neuron sensed that glutamate wasn’t being released was therefore likely to be a feedback signal from the muscle side of the synapse. To test that, the scientists knocked out a glutamate receptor component in the muscle and when they did, they found that the neurons no longer made their active zones larger.

Littleton said the lab is already looking into the new questions the discoveries raise. In particular, what are the molecular pathways that initiate synapse formation in the first place, and what are the signals that tell an active zone it has finished growing? Finding those answers will bring researchers closer to understanding how to intervene when synaptic active zones aren’t developing properly.

In addition to Littleton and Akbergenova, the paper’s other authors are Jessica Matthias and Sofya Makeyeva.

In addition to the National Institutes of Health, The Freedom Together Foundation provided funding for the study.

Little picture, large revelations

A summer intensive using microscopy to study a unique type of yeast was a dream come true for BSG-MSRP-Bio student Adryanne Gonzalez.

Lillian Eden | Department of Biology
September 11, 2025

For Adryanne Gonzalez, studying yeast using microscopy at MIT this summer has been a dream come true. 

“Whatever world we’re living in, there’s an even smaller one,” Gonzalez says. “Knowing and understanding the smaller one can help us learn about the bigger stuff, and I think that’s so fascinating.” 

Gonzalez was part of the Bernard S. and Sophie G. Gould MIT Summer Research Program in Biology, working in the Lew Lab this summer. The program offers talented undergraduates from institutions with limited research opportunities at their home institutions the chance to spend 10 weeks at MIT, where they gain experience, hone skills, and create the types of connections with potential collaborators and future colleagues that are critical for success in academia. 

Gonzalez was so excited about the opportunity that she didn’t apply for any other summer programs.  

“I really wanted to work on becoming more independent in the lab, and this program was research-intensive, and you get to lead your own project,” she says. “It was this or nothing.”

two people standing at a bench in front of a computer
Adryanne Gonzalez, right, with her mentor, Lew Lab graduate student Clara Fikry, left. Gonzalez spent the summer studying Aureobasidium pullulans, a type of yeast that produces large, root-like networks. Photo credit: Mandana Sassanfar/MIT Department of Biology

The fun of science & the rigors of mentoring

The Lew Lab works with two different specimens: a model baker’s yeast that multiplies by producing a round growth called a bud that eventually separates into a separate, daughter cell; and Aureobasidium pullulans, which is unusual because it can create multiple buds at the same time, and can also spread in large networks of branching, rootlike growths called hyphae. A. pullulans is an emerging model system, meaning that researchers are still defining what normal growth and behavior is for the fungus, like how it senses and responds to obstacles, and how resources and molecular machinery are allocated to its branching structures.  

“I’m really interested in all the diversity of biology that we don’t get to study if we’re only focused on the model species,” says Clara Fikry, a graduate student in the Lew Lab and Gonzalez’s mentor for the summer. 

On the mentoring side, Fikry learned how to balance providing a rigorous workload while not overwhelming her mentee with information. 

“Science should be fun,” Fikry says. “The goal of this isn’t to produce as much data as possible; it’s to learn what the process of science is like.”

Although her day-to-day work was with Fikry, Gonzalez also received guidance from Daniel Lew himself. For example, his advice was invaluable for honing a draft of her research statement for potential graduate school applications, which she’d previously written as part of a class assignment.

“It was an assignment where I needed to hit a page count, and he pointed out that I kind of wrote the same thing three times in the first paragraph,” she shares with a laugh. He helped her understand that “when you’re writing something professionally, you want your writing to be concise and understandable to a broad spectrum of readers.” 

Life in the cohort

The BSG-MSRP-Bio program gives undergraduate students a taste of what the day-to-day life of graduate school might feel like, from balancing one’s workload and reading research papers to learning new techniques and troubleshooting when experiments don’t go as planned. Gonzalez recalls that the application process felt very “adult” and “professional” because she was responsible for reaching out to the faculty member of the lab she was interested in on her own behalf, rather than going through a program intermediary. 

Gonzalez is one of just three students from Massachusetts participating in the program this year—the program draws students from across the globe to study at MIT. 

Every student also arrives with different levels of experience, from Gonzalez, who can only work in a lab during the school year about once a week, to Calo Lab student Adriana Camacho-Badillo, who is in her third consecutive summer in the program, and continuing work on a project she began last year.

“We’re all different levels of novice, and we’re coming together, and we’re all really excited about research,” Gonzalez says.

Gonzalez is a Gould Fellow, supported at MIT through the generous donations of Mike Gould and Sara Moss. The program funding was initiated in 2015 to honor the memory of Gould’s parents, Bernard S. and Sophie G. Gould. Gould and Moss take the time to come to campus and meet the students they’re supporting every year. 

“You don’t often get to meet the person that’s helping you,” Gonzalez said. “They were so warm and welcoming, and at the end, when they were giving everyone a nice, firm handshake, Mike Gould said, ‘Make sure you keep going. Don’t give up,’ which was so sweet.” 

Gonzalez is also supported by Cedar Tree, a Boston-based family foundation that primarily funds local environmental initiatives. In the interest of building a pipeline for future scientists with potential interest in the environmental sciences and beyond, Cedar Tree recently established a grant program for local high school and undergraduate students pursuing STEM research and training opportunities. 

Gonzalez discusses her summer research with attendees of the poster session that serves as the culmination of the 10-week summer research intensive for talented non-MIT undergraduate students from around the world. Photo credit: Lillian Eden/MIT Department of Biology.

Preparing for the future

The BSG-MSRP-Bio program culminates with a lively poster session where students present their summer projects to the MIT community—the first time some students are presenting their data to the public in that format.

Although the program is aimed at students who foresee a career in academia, the majority of students who participate are uncertain about the specific field, organism, or process they’ll eventually want to study during a PhD program. For Gonzalez, the program has helped her feel more prepared for the potential rigors of academic research.

“I think the hardest thing about this program is convincing yourself to apply,” she says. “Don’t let that hinder you from exploring opportunities that may seem out of reach.” 

Can a pill help you live longer? The science behind NAD and longevity

MIT professor, Dr. Leonard Guarente, conducts research into sirtuin genes and the power of a molecule called NAD.

WCVB
July 22, 2025

It might sound too good to be true: a pill that could help you live a longer, healthier life. But Leonard Guarente, a longtime MIT biologist, believes the idea holds promise.

Guarente, the Novartis Professor of Biology at MIT, has spent more than 40 years studying the science of aging. He started small, working with yeast cells.

“We decided to look for genes that could make yeast live longer,” he said. That’s when a gene called SIR2 caught his attention. Boosting SIR2 activity helped yeast cells live longer—and when the same effect was observed in roundworms, Guarente turned his attention to humans.

Humans, it turns out, have seven genes similar to SIR2. Collectively, these are called sirtuins, a group of proteins essential to cell health. According to Guarente, sirtuins help power cells, repair damage, and regulate which genes are turned on or off.

Guarente says sirtuins need NAD (nicotinamide adenine dinucleotide) to stay active, but NAD levels naturally decline as we get older.

“If we could restore NAD levels in an older person back to youthful levels, we thought that would do a lot of good,” he explained.

That idea became the foundation for Elysium Health, a company Guarente co-founded. Some critics question the ethics of a scientist selling supplements based on his own research, but Guarente stands by the rigor of his approach. “We ended up with eight Nobel Prize winners on the board,” he noted.

Of course, whether restoring NAD levels leads to longer life is still uncertain. “A person who is very healthy might not notice much initially because where is there to go?” Guarente explained. “But what about in 30 years? There’s no way to answer that question right now.”

A selfish gene unlike any other

Certain genes are “selfish," cheating the rules of inheritance to increase their chances of being transmitted. Researchers in the Yamashita Lab have uncovered a unique "self-limiting" mechanism keeping the selfish gene Stellate in check

Shafaq Zia | Whitehead Institute
May 7, 2025

When a species reproduces, typically, each parent passes on one of their two versions, or alleles, of a given gene to their offspring. But not all alleles play fair in their quest to be passed onto future generations.

Certain alleles, called meiotic drivers, are “selfish”—they cheat the rules of inheritance to increase their chances of being transmitted, often at the expense of the organism’s fitness.

The lab of Whitehead Institute Member Yukiko Yamashita investigates how genetic information is transmitted across generations through the germline—cells that give rise to egg and sperm. Now, Yamashita and first author Xuefeng Meng, a graduate student in the Yamashita Lab, have discovered a meiotic driver that operates differently from previously known drivers.

The researchers’ findings, published online in Science Advances on May 7, reveal that the Stellate (Ste) gene—which has multiple copies located close to one another—on the X chromosome in Drosophila melanogaster, a fruit fly species, is a meiotic driver that biases the transmission of the X chromosome. However, it also has a unique “self-limiting” mechanism that helps preserve the organism’s ability to have male offspring.

“This mechanism is an inherent remedy to the gene’s selfish drive,” says Yamashita, who is also a professor of biology at Massachusetts Institute of Technology and an investigator of the Howard Hughes Medical Institute. “Without it, the gene could severely skew the sex ratio in a population and drive the species to extinction—a paradox that has been recognized for a long time.”

Fatal success

Meiosis is a key process underlying sexual reproduction. This is when cells from the germline undergo two rounds of specialized cell division—meiosis I and meiosis II—to form gametes (egg and sperm cells). In males, this typically results in an equal number of X-bearing and Y-bearing sperm, which ensures an equal chance of having a male or female offspring.

Meiotic drivers located on sex chromosomes can skew this sex ratio by selectively destroying gametes that do not carry the driver allele. Among them is the meiotic driver Ste.

In male germline cells of fruit flies, Ste is kept in check by small RNA molecules, called piRNAs, produced by Suppressor of Stellate (Su(Ste)) located on the Y chromosome. These RNA molecules recruit special proteins to silence Ste RNA. This prevents the production of Ste protein that would otherwise disrupt the development of Y-bearing sperm, which helps maintain the organism’s ability to have male offspring.

“But the suppressing mechanism isn’t foolproof,” Meng explains. “When the meiotic driver and its suppressor are located on different chromosomes, they can get separated during reproduction, leaving the driver unchecked in the next generation.”

A skewed sex ratio toward females offers a short-term advantage: having more females than males could increase a population’s reproductive potential. But in the long run, the meiotic driver risks fatal success—driving the species toward extinction through depletion of males.

Interestingly, prior research suggests that un-silencing Ste only modestly skews a population’s sex ratio, even in the absence of the suppressor, unlike other meiotic drivers that almost exclusively produce females in the progeny. Could another mechanism be at play, keeping Ste’s selfish drive in check?

Practicing self-restraint

To explore this intriguing possibility, researchers in the Yamashita Lab began by examining the process of sperm development. Under moderate Ste expression, pre-meiotic germ cell development and meiosis proceeded normally but defects in sperm development began to emerge soon after. Specifically, a subset of spermatids—immature sperm cells produced after meiosis—failed to incorporate essential DNA-packaging proteins called protamines, which are required to preserve the integrity of genetic information in sperm.

To confirm if the spermatids impacted were predominantly those that carried the Y chromosome, the researchers used an imaging technique called immunofluorescence staining, which uses antibodies to attach fluorescent molecules to a protein of interest, making it glow. They combined this with a technique called FISH (fluorescence in-situ hybridization), which tags the X and Y chromosomes with fluorescent markers, allowing researchers to distinguish between cells that will become X-bearing or Y-bearing following meiosis.

Indeed, the team found that while Ste protein is present in all spermatocytes before meiosis I, it unevenly divides between the two daughter cells—a phenomenon called asymmetric segregation—during meiosis I and gets concentrated in Y-bearing spermatids, eventually inducing DNA-packaging defects in these spermatids.

These findings clarified Ste’s role as a meiotic driver but the researchers still wondered why expression of Ste only led to a moderate sex ratio distortion. The answer soon became clear when they observed Ste undergo another round of asymmetric segregation during meiosis II. This meant that even if a secondary spermatocyte inherited Ste protein after meiosis I, only half of the spermatids produced in this round of cell division ended up retaining the protein. Hence, only half of the Y-bearing spermatids were going to be killed off.

“This self-limiting mechanism is the ultimate solution to the driver-suppressor separation problem,” says Yamashita. “But the idea is so unconventional that had it been proposed as just a theory, without the evidence we have now, it would’ve been completely dismissed.”

These findings have solved some questions and raised others: Unlike female meiosis, which is known to be asymmetrical, male meiosis has traditionally been considered symmetrical. Does the unequal segregation of Ste suggest there’s an unknown asymmetry in male meiosis? Do meiotic drivers like Ste trigger this asymmetry, or do they simply exploit it to limit their selfish drive?

Answering them is the next big step for Yamashita and her colleagues. “This could fundamentally change our understanding of male meiosis,” she says. “The best moments in science are when textbook knowledge is challenged and it turns out to have been tunnel vision.”

Biologists identify targets for new pancreatic cancer treatments

Research from MIT and Dana-Farber Cancer Institute yielded hundreds of “cryptic” peptides that are found only on pancreatic tumor cells and could be targeted by vaccines or engineered T cells.

Anne Trafton | MIT News
May 7, 2025

Researchers from MIT and Dana-Farber Cancer Institute have discovered that a class of peptides expressed in pancreatic cancer cells could be a promising target for T-cell therapies and other approaches that attack pancreatic tumors.

Known as cryptic peptides, these molecules are produced from sequences in the genome that were not thought to encode proteins. Such peptides can also be found in some healthy cells, but in this study, the researchers identified about 500 that appear to be found only in pancreatic tumors.

The researchers also showed they could generate T cells targeting those peptides. Those T cells were able to attack pancreatic tumor organoids derived from patient cells, and they significantly slowed down tumor growth in a study of mice.

“Pancreas cancer is one of the most challenging cancers to treat. This study identifies an unexpected vulnerability in pancreas cancer cells that we may be able to exploit therapeutically,” says Tyler Jacks, the David H. Koch Professor of Biology at MIT and a member of the Koch Institute for Integrative Cancer Research.

Jacks and William Freed-Pastor, a physician-scientist in the Hale Family Center for Pancreatic Cancer Research at Dana-Farber Cancer Institute and an assistant professor at Harvard Medical School, are the senior authors of the study, which appears today in Science. Zackery Ely PhD ’22 and Zachary Kulstad, a former research technician at Dana-Farber Cancer Institute and the Koch Institute, are the lead authors of the paper.

Cryptic peptides

Pancreatic cancer has one of the lowest survival rates of any cancer — about 10 percent of patients survive for five years after their diagnosis.

Most pancreatic cancer patients receive a combination of surgery, radiation treatment, and chemotherapy. Immunotherapy treatments such as checkpoint blockade inhibitors, which are designed to help stimulate the body’s own T cells to attack tumor cells, are usually not effective against pancreatic tumors. However, therapies that deploy T cells engineered to attack tumors have shown promise in clinical trials.

These therapies involve programming the T-cell receptor (TCR) of T cells to recognize a specific peptide, or antigen, found on tumor cells. There are many efforts underway to identify the most effective targets, and researchers have found some promising antigens that consist of mutated proteins that often show up when pancreatic cancer genomes are sequenced.

In the new study, the MIT and Dana-Farber team wanted to extend that search into tissue samples from patients with pancreatic cancer, using immunopeptidomics — a strategy that involves extracting the peptides presented on a cell surface and then identifying the peptides using mass spectrometry.

Using tumor samples from about a dozen patients, the researchers created organoids — three-dimensional growths that partially replicate the structure of the pancreas. The immunopeptidomics analysis, which was led by Jennifer Abelin and Steven Carr at the Broad Institute, found that the majority of novel antigens found in the tumor organoids were cryptic antigens. Cryptic peptides have been seen in other types of tumors, but this is the first time they have been found in pancreatic tumors.

Each tumor expressed an average of about 250 cryptic peptides, and in total, the researchers identified about 1,700 cryptic peptides.

“Once we started getting the data back, it just became clear that this was by far the most abundant novel class of antigens, and so that’s what we wound up focusing on,” Ely says.

The researchers then performed an analysis of healthy tissues to see if any of these cryptic peptides were found in normal cells. They found that about two-thirds of them were also found in at least one type of healthy tissue, leaving about 500 that appeared to be restricted to pancreatic cancer cells.

“Those are the ones that we think could be very good targets for future immunotherapies,” Freed-Pastor says.

Programmed T cells

To test whether these antigens might hold potential as targets for T-cell-based treatments, the researchers exposed about 30 of the cancer-specific antigens to immature T cells and found that 12 of them could generate large populations of T cells targeting those antigens.

The researchers then engineered a new population of T cells to express those T-cell receptors. These engineered T cells were able to destroy organoids grown from patient-derived pancreatic tumor cells. Additionally, when the researchers implanted the organoids into mice and then treated them with the engineered T cells, tumor growth was significantly slowed.

This is the first time that anyone has demonstrated the use of T cells targeting cryptic peptides to kill pancreatic tumor cells. Even though the tumors were not completely eradicated, the results are promising, and it is possible that the T-cells’ killing power could be strengthened in future work, the researchers say.

Freed-Pastor’s lab is also beginning to work on a vaccine targeting some of the cryptic antigens, which could help stimulate patients’ T cells to attack tumors expressing those antigens. Such a vaccine could include a collection of the antigens identified in this study, including those frequently found in multiple patients.

This study could also help researchers in designing other types of therapy, such as T cell engagers — antibodies that bind an antigen on one side and T cells on the other, which allows them to redirect any T cell to kill tumor cells.

Any potential vaccine or T cell therapy is likely a few years away from being tested in patients, the researchers say.

The research was funded in part by the Hale Family Center for Pancreatic Cancer Research, the Lustgarten Foundation, Stand Up To Cancer, the Pancreatic Cancer Action Network, the Burroughs Wellcome Fund, a Conquer Cancer Young Investigator Award, the National Institutes of Health, and the National Cancer Institute.

Manipulating time with torpor

New research from the Hrvatin Lab recently published in Nature Aging indicates that inducing a hibernation-like state in mice slows down epigenetic changes that accompany aging.

Shafaq Zia | Whitehead Institute
March 7, 2025

Surviving extreme conditions in nature is no easy feat. Many species of mammals rely on special adaptations called daily torpor and hibernation to endure periods of scarcity. These states of dormancy are marked by a significant drop in body temperature, low metabolic activity, and reduced food intake—all of which help the animal conserve energy until conditions become favorable again.

The lab of Whitehead Institute Member Siniša Hrvatin studies daily torpor, which lasts several hours, and its longer counterpart, hibernation, in order to understand their effects on tissue damage, disease progression, and aging. In their latest study, published in Nature Aging on March 7, first author Lorna Jayne, Hrvatin, and colleagues show that inducing a prolonged torpor-like state in mice slows down epigenetic changes that accompany aging.

“Aging is a complex phenomenon that we’re just starting to unravel,” says Hrvatin, who is also an assistant professor of biology at Massachusetts Institute of Technology. “Although the full relationship between torpor and aging remains unclear, our findings point to decreased body temperature as the central driver of this anti-aging effect.”

Tampering with the biological clock

Aging is a universal process, but scientists have long struggled to find a reliable metric for measuring it. Traditional clocks fall short because biological age doesn’t always align with chronology—cells and tissues in different organisms age at varying rates.

To solve this dilemma, scientists have turned to studying molecular processes that are common to aging across many species. This, in the past decade, has led to the development of epigenetic clocks, new computational tools that can estimate an organism’s age by analyzing the accumulation of epigenetic marks in cells over time.

Think of epigenetic marks as tiny chemical tags that cling either to the DNA itself or to the proteins, called histones, around which the DNA is wrapped. Histones act like spools, allowing long strands of DNA to coil around them, much like thread around a bobbin. When epigenetic tags are added to histones, they can compact the DNA, preventing genetic information from being read, or loosen it, making the information more accessible. When epigenetic tags attach directly to DNA, they can alter how the proteins that “read” a gene bind to the DNA.

While it’s unclear if epigenetic marks are a cause or consequence of aging, this much is evident: these marks change over an organism’s lifespan, altering how genes are turned on or off, without modifying the underlying DNA sequence. These changes have enabled researchers to track the biological age of individual cells and tissues using dedicated epigenetic clocks.

In nature, states of stasis like hibernation and daily torpor help animals survive by conserving energy and avoiding predators. But now, emerging research in marmots and bats hints that hibernation may also slow down epigenetic aging, prompting researchers to explore whether there’s a deeper connection between prolonged bouts of torpor and longevity.

However, investigating this link has been challenging, as the mechanisms that trigger, regulate, and sustain torpor remain largely unknown. In 2020, Hrvatin and colleagues made a breakthrough by identifying neurons in a specific region of the mouse hypothalamus, known as the avMLPA, which act as core regulators of torpor.

“This is when we realized that we could leverage this system to induce torpor and explore mechanistically how the state of torpor might have beneficial effects on aging,” says Jayne. “You can imagine how difficult it is to study this in natural hibernators because of accessibility and the lack of tools to manipulate them in sophisticated ways.”

The age-old mystery

The researchers began by injecting adeno-associated virus in mice, a gene delivery vehicle that enables scientists to introduce new genetic material into target cells. They employed this technology to instruct neurons in the mice’s avMLPA region to produce a special receptor called Gq-DREADD, which does not respond to the brain’s natural signals but can be chemically activated by a drug. When the researchers administered this drug to the mice, it bound to the Gq-DREADD receptors, activating the torpor-regulating neurons and triggering a drop in the animals’ body temperature.

However, to investigate the effects of torpor on longevity, the researchers needed to maintain these mice in a torpor-like state for days to weeks. To achieve this, the mice were continuously administered the drug through drinking water.

The mice were kept in a torpor-like state with periodic bouts of arousal for a total of nine months. The researchers measured the blood epigenetic age of these mice at the 3-, 6-, and 9-month marks using the mammalian blood epigenetic clock. By the 9-month mark, the torpor-like state had reduced blood epigenetic aging in these mice by approximately 37%, making them biologically three months younger than their control counterparts.

To further assess the effects of torpor on aging,  the group evaluated these mice using the mouse clinical frailty index, which includes measurements like tail stiffening, gait, and spinal deformity that are commonly associated with aging. As expected, mice in the torpor-like state had a lower frailty index compared to the controls.

With the anti-aging effects of the torpor-like state established, the researchers sought to understand how each of the key factors underlying torpor—decreased body temperature, low metabolic activity, and reduced food intake—contributed to longevity.

To isolate the effects of reduced metabolic rate, the researchers induced a torpor-like state in mice, while maintaining the animal’s normal body temperature. After three months, the blood epigenetic age of these mice was similar to that of the control group, suggesting that low metabolic rate alone does not slow down epigenetic aging.

Next, Hrvatin and colleagues isolated the impact of low caloric intake on blood epigenetic aging by restricting the food intake of mice in the torpor-like state, while maintaining their normal body temperature. After three months, these mice were a similar blood epigenetic age as the control group.

When both low metabolic rate and reduced food intake were combined, the mice still exhibited higher blood epigenetic aging after three months compared to mice in the torpor state with low body temperature. These findings, combined, led the researchers to conclude that neither low metabolic rate nor reduced caloric intake alone are sufficient to slow down blood epigenetic aging. Instead, a drop in body temperature is necessary for the anti-aging effects of torpor.

Although the exact mechanisms linking low body temperature and epigenetic aging are unclear, the team hypothesizes that it may involve the cell cycle, which regulates how cells grow and divide: lower body temperatures can potentially slow down cellular processes, including DNA replication and mitosis. This, over time, may impact cell turnover and aging. With further research, the Hrvatin Lab aims to explore this link in greater depth and shed light on the lingering mystery.

Taking the pulse of sex differences in the heart

Work led by Talukdar and Page Lab postdoc Lukáš Chmátal shows that there are differences in how healthy male and female heart cells—specifically, cardiomyocytes, the muscle cells responsible for making the heart beat—generate energy.

Greta Friar | Whitehead Institute
February 18, 2025

Heart disease is the number one killer of men and women, but it often presents differently depending on sex. There are sex differences in the incidence, outcomes, and age of onset of different types of heart problems. Some of these differences can be explained by social factors—for example, women experience less-well recognized symptoms when having heart attacks, and so may take longer to be diagnosed and treated—but others are likely influenced by underlying differences in biology. Whitehead Institute Member David Page and colleagues have now identified some of these underlying biological differences in healthy male and female hearts, which may contribute to the observed differences in disease.

“My sense is that clinicians tend to think that sex differences in heart disease are due to differences in behavior,” says Harvard-MIT MD-PhD student Maya Talukdar, a graduate student in Page’s lab. “Behavioral factors do contribute, but even when you control for them, you still see sex differences. This implies that there are more basic physiological differences driving them.”

Page, who is also an HHMI Investigator and a professor of biology at the Massachusetts Institute of Technology, and members of his lab study the underlying biology of sex differences in health and disease, and recently they have turned their attention to the heart. In a paper published on February 17 in the women’s health edition of the journal Circulation, work led by Talukdar and Page lab postdoc Lukáš Chmátal shows that there are differences in how healthy male and female heart cells—specifically, cardiomyocytes, the muscle cells responsible for making the heart beat—generate energy.

“The heart is a hard-working pump, and heart failure often involves an energy crisis in which the heart can’t summon enough energy to pump blood fast enough to meet the body’s needs,” says Page. “What is intriguing about our current findings and their relationship to heart disease is that we’ve discovered sex differences in the generation of energy in cardiomyocytes, and this likely sets up males and females differently for an encounter with heart failure.”

Page and colleagues began their work by looking for sex differences in healthy hearts because they hypothesize that these impact sex differences in heart disease. Differences in baseline biology in the healthy state often affect outcomes when challenged by disease; for example, people with one copy of the sickle cell trait are more resistant to malaria, certain versions of the HLA gene are linked to slower progression of HIV, and variants of certain genes may protect against developing dementia.

Identifying baseline traits in the heart and figuring out how they interact with heart disease could not only reveal more about heart disease, but could also lead to new therapeutic strategies. If one group has a trait that naturally protects them against heart disease, then researchers can potentially develop medical therapies that induce or recreate that protective feature in others. In such a manner, Page and colleagues hope that their work to identify baseline sex differences could ultimately contribute to advances in prevention and treatment of heart disease.

The new work takes the first step by identifying relevant baseline sex differences. The researchers combined their expertise in sex differences with heart expertise provided by co-authors Christine Seidman, a Harvard Medical School professor and director of the Cardiovascular Genetics Center at Brigham and Women’s Hospital; Harvard Medical School Professor Jonathan Seidman; and Zoltan Arany, a professor and director of the Cardiovascular Metabolism Program at the University of Pennsylvania.

Along with providing heart expertise, the Seidmans and Arany provided data collected from healthy hearts. Gaining access to healthy heart tissue is difficult, and so the researchers felt fortunate to be able to perform new analyses on existing datasets that had not previously been looked at in the context of sex differences. The researchers also used data from the publicly available Genotype-Tissue Expression Project. Collectively, the datasets provided information on bulk and single cell gene expression, as well as metabolomics, of heart tissue—and in particular, of cardiomyocytes.

The researchers searched these datasets for differences between male and female hearts, and found evidence that female cardiomyocytes have higher activity of the primary pathway for energy generation than male cardiomyocytes. Fatty acid oxidation (FAO) is the pathway that produces most of the energy that powers the heart, in the form of the energy molecule ATP. The researchers found that many genes involved in FAO have higher expression levels in female cardiomyocytes. Metabolomic data reinforced these findings by showing that female hearts had greater flux of free fatty acids, the molecules used in FAO, and that female hearts used more free fatty acids than did males in the generation of ATP.

Altogether, these findings show that there are fundamental differences in how female and male hearts generate energy to pump blood. Further experiments are needed to explore whether these differences contribute to the sex differences seen in heart disease. The researchers suspect that an association is likely, because energy production is essential to heart function and failure.

In the meantime, Page and his lab members continue to investigate the biology underlying sex differences in tissues and organs throughout the body.

“We have a lot to learn about the molecular origins of sex differences in health and disease,” Chmátal says. “What’s exciting to me is that the knowledge that comes from these basic science discoveries could lead to treatments that benefit men and women, as well as to policy changes that take sex differences into account when determining how doctors are trained and patients are diagnosed and treated.”

A planarian’s guide to growing a new head

Researchers at the Whitehead Institute have described a pathyway by which planarians, freshwater flatworms with spectacular regenerative capabilities, can restore large portions of their nervous system, even regenerating a new head with a fully functional brain.

Shafaq Zia | Whitehead Institute
February 6, 2025

Cut off any part of this worm’s body and it will regrow. This is the spectacular yet mysterious regenerative ability of freshwater flatworms known as planarians. The lab of Whitehead Institute Member Peter Reddien investigates the principles underlying this remarkable feat. In their latest study, published in PLOS Genetics on February 6, first author staff scientist M. Lucila Scimone, Reddien, and colleagues describe how planarians restore large portions of their nervous system—even regenerating a new head with a fully functional brain—by manipulating a signaling pathway.

This pathway, called the Delta-Notch signaling pathway, enables neurons to guide the differentiation of a class of progenitors—immature cells that will differentiate into specialized types—into glia, the non-neuronal cells that support and protect neurons. The mechanism ensures that the spatial pattern and relative numbers of neurons and glia at a given location are precisely restored following injury.

“This process allows planarians to regenerate neural circuits more efficiently because glial cells form only where needed, rather than being produced broadly within the body and later eliminated,” said Reddien, who is also a professor of biology at Massachusetts Institute of Technology and an Investigator with the Howard Hughes Medical Institute.

Coordinating regeneration

Multiple cell types work together to form a functional human brain. These include neurons and a more abundant group of cells called glial cells—astrocytes, microglia, and oligodendrocytes. Although glial cells are not the fundamental units of the nervous system, they perform critical functions in maintaining the connections between neurons, called synapses, clearing away dead cells and other debris, and regulating neurotransmitter levels, effectively holding the nervous system together like glue. A few years ago, Reddien and colleagues discovered cells in planarians that looked like glial cells and performed similar neuro-supportive functions. This led to the first characterization of glial cells in planarians in 2016.

Unlike in mammals where the same set of neural progenitors give rise to both neurons and glia, glial cells in planarians originate from a separate, specialized group of progenitors. These progenitors, called phagocytic progenitors, can not only give rise to glial cells but also pigment cells that determine the worm’s coloration, as well as other, lesser understood cell types.

Why neurons and glia in planarians originate from distinct progenitors—and what factors ultimately determine the differentiation of phagocytic progenitors into glia—are questions that still puzzled Reddien and team members. Then, a study showing that planarian neurons regenerate before glia formation led the researchers to wonder whether a signaling mechanism between neurons and phagocytic progenitors guides the specification of glia in planarians.

The first step to unravel this mystery was to look at the Notch signaling pathway, which is known to play a crucial role in the development of neurons and glia in other organisms, and determine its role in planarian glia regeneration. To do this, the researchers used RNA interference (RNAi)—a technique that decreases or completely silences the expression of genes—to turn off key genes involved in the Notch pathway and amputated the planarian’s head. It turned out Notch signaling is essential for glia regeneration and maintenance in planarians—no glial cells were found in the animal following RNAi, while the differentiation of other types of phagocytic cells was unaffected.

Of the different Notch signaling pathway components the researchers tested, turning of the genes notch-1, delta-2, and suppressor of hairless produced this phenotype. Interestingly, the signaling molecules Delta-2 was found on the surface of neurons, whereas Notch-1 was expressed in phagocytic progenitors.

With these findings in hand, the researchers hypothesized that interaction between Delta-2 on neurons and Notch-1 on phagocytic progenitors could be governing the final fate determination of glial cells in planarians.

To test the hypothesis, the researchers transplanted eyes either from planarians lacking the notch-1 gene or from planarians lacking the delta-2 gene into wild-type animals and assessed the formation of glial cells around the transplant site. They observed that glial cells still formed around the notch-1 deficient eyes, as notch-1 was still active in the glial progenitors of the host wild-type animal. However, no glial cells formed around the delta-2 deficient eyes, even with the Notch signaling pathway intact in phagocytic progenitors, confirming that delta-2 in the photoreceptor neurons is required for the differentiation of phagocytic progenitors into glia near the eye.

“This experiment really showed us that you have two faces of the same coin—one is the phagocytic progenitors expressing Notch-1, and one is the neurons expressing Delta-2—working together to guide the specification of glia in the organism,”said Scimone.

The researchers have named this phenomenon coordinated regeneration, as it allows neurons to influence the pattern and number of glia at specific locations without the need for a separate mechanism to adjust the relative numbers of neurons and glia.

The group is now interested in investigating whether the same phenomenon might also be involved in the regeneration of other tissue types.

A cell protector collaborates with a killer

New research from the Horvitz Lab reveals what it takes for a protein that is best known for protecting cells against death to take on the opposite role.

Jennifer Michalowski | McGovern Institute
November 1, 2024

From early development to old age, cell death is a part of life. Without enough of a critical type of cell death known as apoptosis, animals wind up with too many cells, which can set the stage for cancer or autoimmune disease. But careful control is essential, because when apoptosis eliminates the wrong cells, the effects can be just as dire, helping to drive many kinds of neurodegenerative disease.

By studying the microscopic roundworm Caenorhabditis elegans—which was honored with its fourth Nobel Prize last month—scientists at MIT’s McGovern Institute have begun to unravel a longstanding mystery about the factors that control apoptosis: how a protein capable of preventing programmed cell death can also promote it. Their study, led by McGovern Investigator Robert Horvitz and reported October 9, 2024, in the journal Science Advances, sheds light on the process of cell death in both health and disease.

“These findings, by graduate student Nolan Tucker and former graduate student, now MIT faculty colleague, Peter Reddien, have revealed that a protein interaction long thought to block apoptosis in C. elegans, likely instead has the opposite effect,” says Horvitz, who shared the 2002 Nobel Prize for discovering and characterizing the genes controlling cell death in C. elegans.

Mechanisms of cell death

Horvitz, Tucker, Reddien and colleagues have provided foundational insights in the field of apoptosis by using C. elegans to analyze the mechanisms that drive apoptosis as well as the mechanisms that determine how cells ensure apoptosis happens when and where it should. Unlike humans and other mammals, which depend on dozens of proteins to control apoptosis, these worms use just a few. And when things go awry, it’s easy to tell: When there’s not enough apoptosis, researchers can see that there are too many cells inside the worms’ translucent bodies. And when there’s too much, the worms lack certain biological functions or, in more extreme cases, can’t reproduce or die during embryonic development.

Work in the Horvitz lab defined the roles of many of the genes and proteins that control apoptosis in worms. These regulators proved to have counterparts in human cells, and for that reason studies of worms have helped reveal how human cells govern cell death and pointed toward potential targets for treating disease.

A protein’s dual role

Three of C. elegans’ primary regulators of apoptosis actively promote cell death, whereas just one, CED-9, reins in the apoptosis-promoting proteins to keep cells alive. As early as the 1990s, however, Horvitz and colleagues recognized that CED-9 was not exclusively a protector of cells. Their experiments indicated that the protector protein also plays a role in promoting cell death. But while researchers thought they knew how CED-9 protected against apoptosis, its pro-apoptotic role was more puzzling.

CED-9’s dual role means that mutations in the gene that encode it can impact apoptosis in multiple ways. Most ced-9 mutations interfere with the protein’s ability to protect against cell death and result in excess cell death. Conversely, mutations that abnormally activate ced-9 cause too little cell death, just like mutations that inactivate any of the three killer genes.

An atypical ced-9 mutation, identified by Reddien when he was a PhD student in Horvitz’s lab, hinted at how CED-9 promotes cell death. That mutation altered the part of the CED-9 protein that interacts with the protein CED-4, which is proapoptotic. Since the mutation specifically leads to a reduction in apoptosis, this suggested that CED-9 might need to interact with CED-4 to promote cell death.

The idea was particularly intriguing because researchers had long thought that CED-9’s interaction with CED-4 had exactly the opposite effect: In the canonical model, CED-9 anchors CED-4 to cells’ mitochondria, sequestering the CED-4 killer protein and preventing it from associating with and activating another key killer, the CED-3 protein —thereby preventing apoptosis.

To test the hypothesis that CED-9’s interactions with the killer CED-4 protein enhance apoptosis, the team needed more evidence. So graduate student Nolan Tucker used CRISPR gene editing tools to create more worms with mutations in CED-9, each one targeting a different spot in the CED-4-binding region. Then he examined the worms. “What I saw with this particular class of mutations was extra cells and viability,” he says—clear signs that the altered CED-9 was still protecting against cell death, but could no longer promote it. “Those observations strongly supported the hypothesis that the ability to bind CED-4 is needed for the pro-apoptotic function of CED-9,” Tucker explains. Their observations also suggested that, contrary to earlier thinking, CED-9 doesn’t need to bind with CED-4 to protect against apoptosis.

When he looked inside the cells of the mutant worms, Tucker found additional evidence that these mutations prevented CED-9’s ability to interact with CED-4. When both CED-9 and CED-4 are intact, CED-4 appears associated with cells’ mitochondria. But in the presence of these mutations, CED-4 was instead at the edge of the cell nucleus. CED-9’s ability to bind CED-4 to mitochondria appeared to be necessary to promote apoptosis, not to protect against it.

Looking ahead

While the team’s findings begin to explain a long-unanswered question about one of the primary regulators of apoptosis, they raise new ones, as well. “I think that this main pathway of apoptosis has been seen by a lot of people as more or less settled science. Our findings should change that view,” Tucker says.

The researchers see important parallels between their findings from this study of worms and what’s known about cell death pathways in mammals. The mammalian counterpart to CED-9 is a protein called BCL-2, mutations in which can lead to cancer.  BCL-2, like CED-9, can both promote and protect against apoptosis. As with CED-9, the pro-apoptotic function of BCL-2 has been mysterious. In mammals, too, mitochondria play a key role in activating apoptosis. The Horvitz lab’s discovery opens opportunities to better understand how apoptosis is regulated not only in worms but also in humans, and how dysregulation of apoptosis in humans can lead to such disorders as cancer, autoimmune disease and neurodegeneration.