Author: Lillian Eden

3 Questions: Daniel Lew on what we can learn from yeast about cell movement, communication, and shape

New Professor of Biology Daniel Lew uses budding yeast to address fundamental questions in cell biology

Lillian Eden | Department of Biology
August 3, 2023

Sipping a beer on a warm summer evening, one might not consider that humans and yeast have been inextricably linked for thousands of years; winemaking, baking, and brewing all depend on budding yeast. Outside of baking and fermentation, researchers also use Saccharomyces cerevisiae, classified as a fungus, to study fundamental questions of cell biology.

Budding yeast gets its name from the way it multiplies. A daughter cell forms first as a swelling, protruding growth on the mother cell. The daughter cell projects further and further from the mother cell until it detaches as an independent yeast cell.

How do cells decide on a front and back? How do cells decode concentration gradients of chemical signals to orient in useful directions, or sense and navigate around physical obstacles? New Department of Biology faculty member Daniel “Danny” Lew uses the model yeast S. cerevisiae, and a non-model yeast with an unusual pattern of cell division, to explore these questions.

Q: Why is it useful to study yeast, and how do you approach the questions you hope to answer?

A: Humans and yeast are descended from a common ancestor, and some molecular mechanisms developed by that ancestor have been around for so long that yeast and mammals often use the same mechanisms. Many cells develop a front and migrate or grow in a particular direction, like the axons in our nervous system, using similar molecular mechanisms to those of yeast cells orienting growth towards the bud.

When I started my lab, I was working on cell cycle control, but I’ve always been interested in morphogenesis and the cell biology of how cells change shape and decide to do different things with different parts of themselves. Those mechanisms turn out to be conserved between yeast and humans.

But some things are very different about fungal and animal cells. One of the differences is the cell wall and what fungal cells have to do to deal with the fact that they have a cell wall.

Fungi are inflated by turgor pressure, which pushes their membranes against the rigid cell wall. This means they’ll die if there is any hole in the cell wall, which would be expected to happen often as cells remodel the wall in order to grow. We’re interested in understanding how fungi sense when any weak spots appear in the wall and repair them before those weak spots become dangerous.

Yeast cells, like most fungi, also mate by fusing with a partner. To succeed, they must do the most dangerous thing in the fungal lifecycle: get rid of the cell wall at the point of contact to allow fusion. That means they must be precise about where and when they remove the wall. We’re fascinated to understand how they know it is safe to remove the wall there, and nowhere else.

We take an interdisciplinary approach. We’ve used genetics, biochemistry, cell biology, and computational biology to try and solve problems in the past. There’s a natural progression: observation and genetic approaches tend to be the first line of attack when you know nothing about how something works. As you learn more, you need biochemical approaches and, eventually, computational approaches to understand exactly what mechanism you’re looking at.

I’m also passionate about mentoring, and I love working with trainees and getting them fascinated by the same problems that fascinate me. I’m looking to work with curious trainees who love addressing fundamental problems.

Q: How does yeast decide to orient a certain way—towards a mating partner, for example?

A: We are still working on questions of how cells analyze the surrounding environment to pick a direction. Yeast cells have receptors that sense pheromones that a mating partner releases. What is amazing about that is that these cells are incredibly small, and pheromones are released by several potential partners in the neighborhood. That means yeast cells must interpret a very confusing landscape of pheromone concentrations. It’s not apparent how they manage to orient accurately toward a single partner.

That got me interested in related questions. Suppose the cell is oriented toward something that isn’t a mating partner. The cell seems to recognize that there’s an obstacle in the way, and it can change direction to go around that obstacle. This is how fungi get so good at growing into things that look very solid, like wood, and some fungi can even penetrate Kevlar vests.

If they recognize an obstacle, they have to change directions and go around it. If they recognize a mating partner, they have to stick with that direction and allow the cell wall to get degraded. How do they know they’ve hit an obstacle? How do they know a mating partner is different from an obstacle? These are the questions we’d like to understand.

Q: For the last couple of years, you’ve also been studying a budding yeast that forms multiple buds when it reproduces instead of just one. How did you come across it, and what questions are you hoping to explore?

A: I spent several years trying to figure out why most yeasts make one bud and only one bud, which I think is related to the question of why migrating cells make one and only one front. We had what we thought was a persuasive answer to that, so seeing a yeast completely disobey that and make as many buds as it felt like was a shock, which got me intrigued.

We started working on it because my colleague,Amy Gladfelter, had sampled the waters around Woods Hole, Massachusetts. When she saw this specimen under a microscope, she immediately called me and said, “You have to look at this.”

A question we’re very intrigued by is if the cell makes five, seven, or 12 buds simultaneously, how do they divide the mother cell’s material and growth capacity five, seven, or 12 ways? It looks like all of the buds grow at the same rate and reach about the same size. One of our short-term goals is to check whether all the buds really get to exactly the same size or whether they are born unequal.

And we’re interested in more than just growth rate. What about organelles? Do you give each bud the same number of mitochondria, nuclei, peroxisomes, and vacuoles? That question will inevitably lead to follow-up questions. If each bud has the same number of mitochondria, how does the cell measure mitochondrial inheritance to do that? If they don’t have the same amount, then buds are each born with a different complement and ratio of organelles. What happens to buds if they have very different numbers of organelles?

As far as we can tell, every bud gets at least one nucleus. How the cell ensures that each bud gets a nucleus is a question we’d also very much like to understand.

We have molecular candidates because we know a lot about how model yeasts deliver nuclei, organelles, and growth materials from the mother to the single bud. We can mutate candidate genes and see if similar molecular pathways are involved in the multi-budding yeast and, if so, how they are working.

It turns out that this unconventional yeast has yet to be studied from the point of view of basic cell biology. The other thing that intrigues me is that it’s a poly-extremophile. This yeast can survive under many rather harsh conditions: it’s been isolated in Antarctica, from jet engines, from all kinds of plants, and of course from the ocean as well. An advantage of working with something so ubiquitous is we already know it’s not toxic to us under almost any circumstances. We come into contact with it all the time. If we learn enough about its cell biology to begin to manipulate it, then there are many potential applications, from human health to agriculture.

Whitehead Institute researchers receive an HHMI Gilliam Fellowship
Merrill Meadow | Whitehead Institute
July 31, 2023
Freeman Hrabowski encourages students to ‘Hold fast to dreams’ and take time for laughter

On MIT campus, Hrabowski led a lively and inspiring Q&A with students.

Lillian Eden | Department of Biology
July 20, 2023

A group of more than 50 — predominantly MSRP-Bio students and alums and current students from the Meyerhoff Scholars program and the University of Maryland, Baltimore County — recently had the pleasure of sitting down for an informal chat at MIT with distinguished educator, author, and mathematician Freeman Hrabowski. 

Hrabowski is widely credited for transforming UMBC into a world-renowned, innovative institution while serving as its president from 1992-2022. The educator also ushered in a generation of Black students to earn PhDs in science and engineering, co-founding the Meyerhoff Scholars Program at UMBC. Founded in 1988, the program has become a national model for increasing diversity in STEM.  Hrabowski was also a member of the President’s Advisory Commission on Educational Excellence for African Americans during the Obama administration. 

A crowd of more than 50 in a lecture hall with Freeman Hrabrowski standing in front of them.
Freeman Hrabowski led a call-and-response recitation of a mantra until everyone could participate flawlessly: “Your thoughts, they become your words. Your words, they become your actions. Your actions, they become your habits. Your habits, they become your character. Your character becomes your destiny.” Photo credit: Mandana Sassanfar.

Hrabowski began by quoting poet William Carlos Williams “It is difficult to get the news from poems yet men die miserable every day for lack of what is found there,” and leading a call-and-response recitation of the poem Dreams by Langston Hughes as well as a mantra encouraging students to use their words, actions, and habits to shape their character and their destiny. Afterward, the students asked Hrabowski about his life and experiences. 

“The audience of high-achieving students asked terrific, insightful questions reflecting their contemplation of their own paths,” says Biology Department head Amy Keating. “When students spoke up, Hrabowski engaged with them, and their ideas and perspectives were welcomed and respected. By the end of his time with them, almost everyone had their hand up and wanted to contribute to the lively discussion.”

Tobias Coombs, a Meyerhoff Scholars program alumnus and current graduate student in the Spranger Lab, says the event was an example of “classic Freeman Hrabowski”: Hrabowski injected the crowd with excitement and energy. Coombs also remarked that Hrabowski, named by Time as one of the world’s most influential people in 2012, acknowledged to the group that he’s shy, something Hrabowski is still pushing himself to overcome.

“He makes a point of being this down-to-earth person that you feel you can talk to about real issues and have real conversations with,” Coombs says. “He genuinely wants to motivate you to think science and math are cool.”

Before taking questions from the students in attendance, Hrabowski posed one to them: what do you think it takes to be successful in research in STEM? Among the responses were passion, curiosity, and a supportive community. After each response, Hrabowski encouraged a round of applause for each student brave enough to stand and give an answer because “Everybody needs support.”

“The way that you think about yourselves, the language that you use, the way that you interact with each other, and the values that you hold, will be so important. You become like the things that you love,” Hrabowski says.

For his lifetime of accomplishments increasing diversity in STEM, the Howard Hughes Medical Institute recently announced a new program named after Hrabowski. The HHMI Freeman Hrabowski Scholars were selected for their potential to become leaders in their research fields and to foster diverse and inclusive lab environments. The inaugural class of 31 scholars includes MIT Biology faculty members Seychelle Vos, the Robert A. Swanson Career Development Professor of Life Sciences, and Hernandez Moura Silva, an assistant professor and Ragon Institute core member, as well as MIT Biology and Cheeseman lab alumna Kara McKinley, PhD ’16.

Group of 10 people standing in a line posing for a photo.
Hrabowski (5th from the right) was very excited to see UMBC alums as well as HHMI Freman Hrabowski Scholars Hernandez Moura Silva (3rd from the right) and Seychelle Vos (to the left of Hrabowski) at MIT. Photo credit: Mandana Sassanfar.

Vos and Moura Silva were among the faculty attending the event, and both say Hrabowski was an inspiring guest to have on campus.

“Dr. Hrabowski’s smile, energy, and words are a true force of nature,” Hernandez says. “His words of wisdom showed us that we can all make the impossible possible by bringing a positive attitude to build a strong, supportive, and diverse community. It was such an honor to have him here.”

Biology department undergraduate officer Adam Martin says he noticed the pride in Hrabowski’s eyes when Hrabowski discussed what his trainees and faculty in his programs have accomplished. Biology department graduate officer Mary Gehring said his visit made her remember why she wanted to be a professor: “to help others follow their passions to their full potential.” 

Hrabowski reflected on many topics, including the recent Supreme Court ruling on affirmative action. He pointed out that this was not the first time the Supreme Court had ruled on a racially conscious initiative, namely the 1995 decision that a UMBC scholarship program was unconstitutional. To continue the Meyerhoff Scholars Program, which was affected by the Supreme Court decision at the time, Hrabowski worked with Maryland’s Attorney General, found language and methods to encourage broad participation of diverse individuals, and focused on what the program was trying to achieve.

“My message to everyone was ‘where there’s a will, there’s a way.’ If the institution wants to continue to build diversity and broader participation, we can do it,” he says. “What we’re

working to achieve in the Meyerhoff program and in the Freeman Hrabowski Scholars program is to have everybody included.”

Hrabowski also offered advice on more everyday challenges: good students, himself included, can focus too much, forgetting to make time for other important aspects of their lives. He has learned to make time for Tai Chi, acupuncture, and getting his steps in; he encouraged the students similarly to take time for themselves outside work or school.

Six people standing in the Picower Institute for Learning and Memory, laughing.
Freeman Hrabowski (middle) chatting with MIT Biology faculty at MIT. Photo credit: Lillian Eden.

“When you can have fun and laugh, you’re a much better person. You can be a better thinker if you take care of yourself overall,” he says. “It’s the healthy person who can be most effective.”

As for being intimidated or nervous to talk to a superior, Hrabowski had the room roaring with laughter at his advice: “Just remember they go to the bathroom, too.”  

Keating noted that Hrabowski engaged with the audience with energy, compassion, and humor. 

She also observed, “No one can hide in Dr. Hrabowski’s classroom.”

“He put students front and center in his presentation, and his emphasis on the joys and importance of learning, knowledge, and achievement inspired us all to go back to the lab and classroom and be our best selves,” Keating says. “He acknowledged that paths in STEM demand much of us, and he encouraged students to have the discipline needed to stay the course while also taking care of themselves.”

It takes three to tango: transcription factors bind DNA, protein, and RNA
Greta Friar | Whitehead Institute
July 7, 2023

Transcription factors could be the Swiss Army knives of gene regulation; they are versatile proteins containing multiple specialized regions. On one end they have a region that can bind to DNA. On the other end they have a region that can bind to proteins. Transcription factors help to regulate gene expression—turning genes on or off and dialing up or down their level of activity—often in partnership with the proteins that they bind. They anchor themselves and their partner proteins to DNA at binding sites in genetic regulatory sequences, bringing together the components that are needed to make gene expression happen.

Transcription factors are a well-known family of proteins, but new research from Whitehead Institute Member Richard Young and colleagues shows that the picture we have had of them is incomplete. In a paper published in Molecular Cell on July 3, Young and postdocs Ozgur Oksuz and Jonathan Henninger reveal that along with DNA and protein, many transcription factors can also bind RNA. The researchers found that RNA binding keeps transcription factors near their DNA binding sites for longer, helping to fine tune gene expression. This rethinking of how transcription factors work may lead to a better understanding of gene regulation, and may provide new targets for RNA-based therapeutics.

“It’s as if, after carrying around a Swiss Army knife all your life for its blade and scissors, you suddenly realize that the odd, small piece in the back of the knife is a screwdriver,” Young says. “It’s been staring you in the face this whole time, and now that you finally see it, it becomes clear how many more uses there are for the knife than you had realized.”

How transcription factors’ RNA binding went overlooked

A few papers, including one from Young’s lab, had previously identified individual transcription factors as being able to bind RNA, but researchers thought that this was a quirk of the specific transcription factors. Instead, Young, Oksuz, Henninger and collaborators have shown that RNA-binding is in fact a common feature present in at least half of transcription factors.

“We show that RNA binding by transcription factors is a general phenomenon,” Oksuz says. “Individual examples in the past were thought to be exceptions to the rule. Other studies dismissed signs of RNA binding in transcription factors as an artifact—an accident of the experiment rather than a real finding. The clues have been there all along, but I think earlier work was so focused on the DNA and protein interactions that they didn’t consider RNA.”

The reason that researchers had not recognized transcription factors’ RNA binding region as such is because it is not a typical RNA binding domain. Typical RNA binding domains form stable structures that researchers can detect or predict with current technologies. Transcription factors do not contain such structures, and so standard searches for RNA binding domains had not identified them in transcription factors.

“We show that RNA binding by transcription factors is a general phenomenon,” Oksuz says.

Young, Oksuz and Henninger got their biggest clue that researchers might be overlooking something from the human immunodeficiency virus (HIV), which produces a transcription factor-like protein called Tat. Tat increases the transcription of HIV’s RNA genome by binding to the virus’ RNA and then recruiting cellular machinery to it. However, Tat does not contain a structured RNA binding site; instead, it binds RNA from a region called an arginine-rich motif (ARM) that is unstructured but has a high affinity for RNA. When the ARM binds to HIV RNA, the two molecules form a more stable structure together.

The researchers wondered if Tat might be more similar to human transcription factors than anyone had realized. They went through the list of transcription factors, and instead of looking for structured RNA binding domains, they looked for ARMs. They found them in abundance; the majority of human transcription factors contain an ARM-like region between their DNA and protein binding regions, and these sequences were conserved across animal species. Further testing confirmed that many transcription factors do in fact use their ARMs to bind RNA.

RNA binding fine tunes gene expression

Next, the researchers tested to see if RNA binding affected the transcription factors’ function. When transcription factors had their ARMs mutated so they couldn’t bind RNA, those transcription factors were less effective in finding their target sites, remaining at those sites and regulating genes. The mutations did not prevent transcription factors from functioning altogether, suggesting that RNA binding contributes to fine-tuning of gene regulation.

Further experiments confirmed the importance of RNA binding to transcription factor function. The researchers mutated the ARM of a transcription factor important to embryonic development, and found that this led to developmental defects in zebrafish. Additionally, they looked through a list of genetic mutations known to contribute to cancer and heritable diseases, and found that a number of these occur in the RNA binding regions of transcription factors. All of these findings point to RNA binding playing an important role in transcription factors’ regulation of gene expression.

They may also provide therapeutic opportunities. The transcription factors studied by the researchers were found to bind RNA molecules that are produced in the regulatory regions of the genome where the transcription factors bind DNA. This set of transcription factors includes factors that can increase or decrease gene expression. “With evidence that RNAs can tune gene expression through their interaction with positive and negative transcription factors,” says Henninger, “we can envision using existing RNA-based technologies to target RNA molecules, potentially increasing or decreasing expression of specific genes in disease settings.”

Notes

Ozgur Oksuz, Jonathan E. Henninger, Robert Warneford-Thomson, Ming M. Zheng, Hailey Erb, Adrienne Vancura, Kalon J. Overholt, Susana Wilson Hawken, Salman F. Banani, Richard Lauman, Lauren N. Reich, Anne L. Robertson, Nancy M. Hannett, Tong I. Lee, Leonard I. Zon, Roberto Bonasio, Richard A. Young. “Transcription factors interact with RNA to regulate genes.” Molecular Cell, July 3, 2023. https://doi.org/10.1016/j.molcel.2023.06.012.

Department of Biology opens its doors for Community College outreach

15 from Bunker Hill Community College visited campus as part of an outreach initiative to build stronger ties with local institutions that serve diverse, nontraditional learners.

Lillian Eden | Department of Biology
July 6, 2023

Although many undergraduates may be home for the summer, the halls and labs of MIT are still teeming with activity. On a sunny Thursday in June, 15 students from Bunker Hill Community College (BHCC) got to peek behind the curtain of research at MIT. 

The Community College Partnership builds ties with two local community colleges that serve diverse, nontraditional students. The program was first conceived in 2020 as part of the biology department’s participation in #ShutDownSTEM, a day to consider equity and inclusion for marginalized communities and to educate and take action against injustice. 

The visit is part of a larger effort to encourage students to pursue research opportunities and careers in research at and beyond MIT; other initiatives include virtual career panels and workshops for students at BHCC and Roxbury Community College. In addition, two community college students perform research as part of MRSP-Bio each summer, thanks to funding from the Packard Foundation acquired by Ankur Jain, Assistant Professor of Biology and Core Member of the Whitehead Institute. 

BHCC students toured the MIT Cryo-EM facility with director Sarah Sterling. Photo Credit: Mandana Sassanfar

Sarah Sterling, Director of the Cryo-EM facility, loves giving tours because students ask such great questions, and the BHCC students were no exception: they were curious and inquisitive at every stop of their tour. Sterling explained that she chose her position, in part, because she enjoys “facilitating science”—helping researchers use cutting-edge equipment to find answers to their questions. 

The students also visited labs in Building 68, Whitehead Institute, and the Picower Institute for Learning and Memory. 

 Reddien Lab postdoc Thomas Cooke described exploring mechanisms of regeneration in planarians, and Professor Laurie Boyer described the core questions underlying her research on heart development. 

“The process of forming tissues and organs works sufficiently well that we’re all here, and we’re all relatively healthy,” Boyer says. “To me, that is remarkable.” 

What isn’t well understood, she explains, is how faulty regulation can lead to disease and congenital malformations, and research using model systems can provide answers. For example, creating a model system in a dish can lead to a better understanding of the formation of circuits and molecular players. That, in turn, can lead to therapies or early diagnosis. The lab also works on tools to visualize what is occurring inside cells because “seeing is believing.”

“As scientists, we are not only trying to plan the best experiments possible, but we are also trying to develop new tools that push the boundaries of what we can discover,” she says. “Keeping an eye on the big picture is important because you’re never studying a problem in isolation. You’re studying a biological mechanism that has implications for many different things.” 

It was “really eye-opening” to see what’s happening in some of the labs, according to BHCC student Robinson Le. Le is a dancer turned Biology major but had only ever come to campus for breakdancing practice—a skill they showed off to cheering BHCC students during lunch. 

BHCC students with Department of Biology Faculty Laurie Boyer. Photo Credit: Mandana Sassanfar

Badara Mbengue, another BHCC student, was excited to learn “what everyday life is like in the department.” 

“It makes me very happy to see how much this has grown and continues to grow,” says Sheena Vasquez, PhD ‘23, who helped spearhead the initiative. 

BHCC alums at MIT also showed students around the labs they are working in and shared their experiences at MIT, including as MSRP-Bio students, Quantitative Methods Workshop students, and as an undergraduate transfer student. 

Libby Dunphy, a professor at BHCC, helped arrange the visit. She says the trip was an excellent opportunity for her students, who don’t get much exposure to real research.

“Seeing actual researchers, seeing that they’re real people, and that they’re nice, can help students imagine themselves in this place,” Dunphy says. “The Bunker Hill motto is ‘imagine the possibilities.’ And it’s cheesy, but we’re imagining the possibilities here.” 

Boyer also offered advice for pursuing research at this stage in the students’ careers. 

“The opportunities are unlimited, and so many people would be happy to support you—but sometimes, you have to ask,” Boyer advises. “Stay ambitious. You should be so proud of yourselves for embarking on this journey.” 

Without a key extracellular protein, neuronal axons break and synaptic connections fall apart
David Orenstein | The Picower Institute for Learning and Memory
June 23, 2023

Perhaps the most obvious feature of a neuron is the long branch called an axon that ventures far from the cell body to connect with other neurons or muscles. If that long, thin projection ever seems like it could be vulnerable, a new MIT study shows that its structural integrity may indeed require the support of a surrounding protein called perlecan. Without that protein in Drosophila fruit flies, researchers at The Picower Institute for Learning and Memory found axonal segments can break apart during development and the connections, or synapses, that they form end up dying away.

Perlecan helps make the extracellular matrix, the proteins and other molecules that surround cells, stable and flexible so that cells can develop and function in an environment that is supportive without being rigid.

“What we found was that the extracellular matrix around nerves was being altered and essentially causing the nerves to break completely. Broken nerves eventually led to the synapses retracting,” says study senior author Troy Littleton, the Menicon Professor in MIT’s departments of Biology and Brain and Cognitive Sciences.

Humans need at least some perlecan to survive after birth. Mutations that reduce, but don’t eliminate, perlecan can cause Schwartz-Jampel syndrome, in which patients experience neuromuscular problems and skeletal abnormalities. The new study may help explain how neurons are affected in the condition, Littleton says, and also deepen scientists’ understanding of how the extracellular matrix supports axon and neural circuit development.

Ellen Guss PhD ’23, who recently defended her doctoral thesis on the work, led the research published June 8 in eLife.

At first she and Littleton didn’t expect the study to yield a new discovery about the durability of developing axons. Instead, they were investigating a hypothesis that perlecan might help organize some of the protein components in synapses that fly nerves develop to connect with muscles. But when they knocked out the gene called “trol” that encodes perlecan in flies, they saw that the neurons appeared to “retract” many synapses at a late stage of larval development. Proteins on the muscle side of the synaptic connection remained, but the neuron side of the connection withered away. That suggested that perlecan had a bigger role than they first thought.

Indeed, the authors found that the perlecan wasn’t particularly enriched around synapses. Where it was pronounced was in a structure called the neural lamella, which surrounds axon bundles and acts a bit like the rubbery cladding around a TV cable to keep the structure intact. That suggested that a lack of perlecan might not be a problem at the synapse, but instead causes trouble along axons due to its absence in the extracellular matrix surrounding nerve bundles.

Littleton’s lab had developed a technique for daily imaging of fly neural development called serial intravital imaging. They applied it to watch what happened to the fly axons and synapses over a four-day span. They observed that while fly axons and synapses developed normally at first, not only synapses but also whole segments of axons faded away.

They also saw that the farther an axon segment was from the fly’s brain, the more likely it was to break apart, suggesting that the axon segments became more vulnerable the further out they extended. Looking segment by segment, they found that where axons were breaking down, synapse loss would soon follow, suggesting that axon breakage was the cause of the synapse retraction.

“The breakages were happening in a segment-wide manner,” Littleton says. “In some segments the nerves would break and in some they wouldn’t. Whenever there was a breakage event, you would see all the neuromuscular junctions (synapses) across all the muscles in that segment retract.”

When they compared the structure of the lamella in mutant versus healthy flies, they found that the lamella was thinner and defective in the mutants. Moreover, where the lamella was weakened, axons were prone to break and the microtubule structures that run the length of the axon would become misdirected, protruding outward and becoming tangled up in dramatic bundles at sites of severed axons.

In one other key finding, the team showed that perlecan’s critical role depended on its secretion from many cells, not just neurons. Blocking the protein in just one cell type or another did not cause the problems that total knockdown did, and enhancing secretion from just neurons was not enough to overcome its deficiency from other sources.

Altogether, the evidence pointed to a scenario where lack of perlecan secretion caused the neural lamella to be thin and defective, with the extracellular matrix becoming too rigid. The further from the brain nerve bundles extended, the more likely movement stresses would cause the axons to break where the lamella had broken down. The microtubule structure within the axons then became disorganized. That ultimately led to synapses downstream of those breakages dying away because the disruption of the microtubules means the cells could no longer support the synapses.

“When you don’t have that flexibility, although the extracellular matrix is still there, it becomes very rigid and tight and that basically leads to this breakage as the animal moves and pulls on those nerves over time,” Littleton says. “It argues that the extracellular matrix is functional early on and can support development, but doesn’t have the right properties to sustain some key functions over time as the animal begins to move and navigate around. The loss of flexibility becomes really critical.”

In addition to Littleton and Guss, the paper’s other authors are Yulia Akbergenova and Karen Cunningham.

Support for the study came from the National Institutes of Health. The Littleton Lab is also supported by The Picower Institute for Learning and Memory and The JPB Foundation.

Focus on function helps identify the changes that made us human

It can be difficult to tell which of the many small genetic differences between us and chimps have been significant to our evolution. New research from Jonathan Weissman and colleagues narrowed in on the key differences in how humans and chimps rely on certain genes, including how humans became able to grow comparatively large brains.

Greta Friar | Whitehead Institute
June 22, 2023

Humans split away from our closest animal relatives, chimpanzees, and formed our own branch on the evolutionary tree about seven million years ago. In the time since—brief, from an evolutionary perspective—our ancestors evolved the traits that make us human, including a much bigger brain than chimpanzees and bodies that are better suited to walking on two feet. These physical differences are underpinned by subtle changes at the level of our DNA. However, it can be hard to tell which of the many small genetic differences between us and chimps have been significant to our evolution.

New research from Whitehead Institute Member Jonathan Weissman; University of California, San Francisco Assistant Professor Alex Pollen; Weissman lab postdoc Richard She; Pollen lab graduate student Tyler Fair; and colleagues uses cutting edge tools developed in the Weissman lab to narrow in on the key differences in how humans and chimps rely on certain genes. Their findings, published in the journal Cell on June 20, may provide unique clues into how humans and chimps have evolved, including how humans became able to grow comparatively large brains.

Studying function rather than genetic code

Only a handful of genes are fundamentally different between humans and chimps; the rest of the two species’ genes are typically nearly identical. Differences between the species often come down to when and how cells use those nearly identical genes. However, only some of the many differences in gene use between the two species underlie big changes in physical traits. The researchers developed an approach to narrow in on these impactful differences.

Their approach, using stem cells derived from human and chimp skin samples, relies on a tool called CRISPR interference (CRISPRi) that Weissman’s lab developed. CRISPRi uses a modified version of the CRISPR/Cas9 gene editing system to effectively turn off individual genes. The researchers used CRISPRi to turn off each gene one at a time in a group of human stem cells and a group of chimp stem cells. Then they looked to see whether or not the cells multiplied at their normal rate. If the cells stopped multiplying as quickly or stopped altogether, then the gene that had been turned off was considered essential: a gene that the cells need to be active–producing a protein product–in order to thrive. The researchers looked for instances in which a gene was essential in one species but not the other as a way of exploring if and how there were fundamental differences in the basic ways that human and chimp cells function.

By looking for differences in how cells function with particular genes disabled, rather than looking at differences in the DNA sequence or expression of genes, the approach ignores differences that do not appear to impact cells. If a difference in gene use between species has a large, measurable effect at the level of the cell, this likely reflects a meaningful difference between the species at a larger physical scale, and so the genes identified in this way are likely to be relevant to the distinguishing features that have emerged over human and chimp evolution.

“The problem with looking at expression changes or changes in DNA sequences is that there are many of them and their functional importance is unclear,” says Weissman, who is also a professor of biology at the Massachusetts Institute of Technology and an Investigator with the Howard Hughes Medical Institute. “This approach looks at changes in how genes interact to perform key biological processes, and what we see by doing that is that, even on the short timescale of human evolution, there has been fundamental rewiring of cells.”

After the CRISPRi experiments were completed, She compiled a list of the genes that appeared to be essential in one species but not the other. Then he looked for patterns. Many of the 75 genes identified by the experiments clustered together in the same pathways, meaning the clusters were involved in the same biological processes. This is what the researchers hoped to see. Individual small changes in gene use may not have much of an effect, but when those changes accumulate in the same biological pathway or process, collectively they can cause a substantive change in the species. When the researchers’ approach identified genes that cluster in the same processes, this suggested to them that their approach had worked and that the genes were likely involved in human and chimp evolution.

“Isolating the genetic changes that made us human has been compared to searching for needles in a haystack because there are millions of genetic differences, and most are likely to have negligible effects on traits,” Pollen says. “However, we know that there are lots of small effect mutations that in aggregate may account for many species differences. This new approach allows us to study these aggregate effects, enabling us to weigh the impact of the haystack on cellular functions.”

Researchers think bigger brains may rely on genes regulating how quickly cells divide

One cluster on the list stood out to the researchers: a group of genes essential to chimps, but not to humans, that help to control the cell cycle, which regulates when and how cells decide to divide. Cell cycle regulation has long been hypothesized to play a role in the evolution of humans’ large brains. The hypothesis goes like this: Neural progenitors are the cells that will become neurons and other brain cells. Before becoming mature brain cells, neural progenitors divide multiple times to make more of themselves. The more divisions that the neural progenitors undergo, the more cells the brain will ultimately contain—and so, the bigger it will be. Researchers think that something changed during human evolution to allow neural progenitors to spend less time in a non-dividing phase of the cell cycle and transition more quickly towards division. This simple difference would lead to additional divisions, each of which could essentially double the final number of brain cells.

Consistent with the popular hypothesis that human neural progenitors may undergo more divisions, resulting in a larger brain, the researchers found that several genes that help cells to transition more quickly through the cell cycle are essential in chimp neural progenitor cells but not in human cells. When chimp neural progenitor cells lose these genes, they linger in a non-dividing phase, but when human cells lose them, they keep cycling and dividing. These findings suggest that human neural progenitors may be better able to withstand stresses—such as the loss of cell cycle genes—that would limit the number of divisions the cells undergo, enabling humans to produce enough cells to build a larger brain.

“This hypothesis has been around for a long time, and I think our study is among the first to show that there is in fact a species difference in how the cell cycle is regulated in neural progenitors,” She says. “We had no idea going in which genes our approach would highlight, and it was really exciting when we saw that one of our strongest findings matched and expanded on this existing hypothesis.”

More subjects lead to more robust results

Research comparing chimps to humans often uses samples from only one or two individuals from each species, but this study used samples from six humans and six chimps. By making sure that the patterns they observed were consistent across multiple individuals of each species, the researchers could avoid mistaking the naturally occurring genetic variation between individuals as representative of the whole species. This allowed them to be confident that the differences they identified were truly differences between species.

The researchers also compared their findings for chimps and humans to orangutans, which split from the other species earlier in our shared evolutionary history. This allowed them to figure out where on the evolutionary tree a change in gene use most likely occurred. If a gene is essential in both chimps and orangutans, then it was likely essential in the shared ancestor of all three species; it’s more likely for a particular difference to have evolved once, in a common ancestor, than to have evolved independently multiple times. If the same gene is no longer essential in humans, then its role most likely shifted after humans split from chimps. Using this system, the researchers showed that the changes in cell cycle regulation occurred during human evolution, consistent with the proposal that they contributed to the expansion of the brain in humans.

The researchers hope that their work not only improves our understanding of human and chimp evolution, but also demonstrates the strength of the CRISPRi approach for studying human evolution and other areas of human biology. Researchers in the Weissman and Pollen labs are now using the approach to better understand human diseases—looking for the subtle differences in gene use that may underlie important traits such as whether someone is at risk of developing a disease, or how they will respond to a medication. The researchers anticipate that their approach will enable them to sort through many small genetic differences between people to narrow in on impactful ones underlying traits in health and disease, just as the approach enabled them to narrow in on the evolutionary changes that helped make us human.

MIT alum filling in the gaps in urology research

Now an assistant professor at UT Dallas, Nicole De Nisco draws on love of problem solving and interdisciplinary skills honed as an undergraduate and graduate student at MIT

Lillian Eden | Department of Biology
June 12, 2023

There were early signs that Nicole De Nisco, SB ‘07, PhD ‘13, might become a scientist. She ran out of science classes to take in high school and fondly remembers the teacher that encouraged her to pursue science instead of the humanities. But she ended up at MIT, in part, out of spite. 

“I applied because my guidance counselor told me I wouldn’t get in,” she said. The rest, as they say, is history for the first-generation college student from Los Angeles. 

Now, she’s an assistant professor of biological sciences at UT Dallas studying urinary tract infections (UTIs) and the urinary microbiome in postmenopausal women. 

De Nisco has already made some important advancements in the field: she developed a new technique for visualizing bacteria in the bladder and used it to demonstrate that bacteria form reservoirs in human bladder tissue, leading to chronic or recurrent UTIs. 

It was known that in mice, bacteria are able to create communities within the bladder tissue, forming reservoirs and staying there long term—but no one had shown that occurring in human tissue before. 

People in lab coats looking at something Nicole De Nisco is holding in her hand.
De Nisco says MIT prepared her well for the type of interdisciplinary work she does every day at UT Dallas, where all research buildings are fully integrated. She works closely with mathematicians, chemists, and engineers. Photo provided by The University of Texas at Dallas

De Nisco found that reservoirs of tissue-resident bacteria exist in human patients with recurring UTIs, a condition which may ultimately lead to women needing to have their bladder removed. De Nisco now mostly works with postmenopausal women who have been suffering from decades of recurring UTIs. 

There was a big gap in the field, De Nisco explained, so entering the field of urology was also an opportunity to make new discoveries and find new ways to treat those recurring infections.  

De Nisco said she’s in the minority, both as a woman studying urology and as someone studying diseases that affect female patients. Most researchers in the urology field are men, and most focus on the prostate. 

But things are changing. 

“I think there are a lot of women in the field who are now pushing back, and I actually collaborate with a lot of other female investigators in the field. We’re trying to support each other so that we can survive and, hopefully, actually advance the science—instead of it being in the same place it was 15 years ago,” De Nisco says.

De Nisco first fell in love with biomedical research as an undergrad doing a UROP in Catherine Drennan’s lab, back when Drennan was still located in the chemistry building. 

“Cathy herself was incredibly encouraging, and is probably the main reason I decided to pursue a career in science—or felt that I could,” De Nisco said. 

De Nisco became fascinated with the dialogue between a microbe and a host organism during an undergraduate course in microbial physiology with Graham Walker, which led to De Nisco’s decision to remain at MIT for her PhD work and to perform her doctoral research in rhizobia legume symbiosis in Walker’s lab. 

De Nisco said during her time at MIT, Drennan and Walker gave her a lot of encouragement and “room to do my own thing,” fostering a love of discovery and problem solving. It’s a mentoring style she’s using now with her own graduate students; she currently has eight working in her lab. 

“Every student is different: some just want a project and they want to know what they’re doing, and some want to explore,” she said. “I was the type that wanted to do my own thing and so they gave me the room and the patience to be able to explore and find something new that I was interested in and excited about.” 

As a low-income student sending financial help home, she also pursued teaching opportunities outside of her usual duties; Walker was very supportive of pursuing other teaching opportunities. De Nisco was a graduate student tutor for Next House watching over 40 undergrads, served as a teaching fellow with the Harvard Extension School, and worked with Eric Lander to help launch the course 7.00x Introduction to Biology – The Secret of Life for EdX, one of the most highly rated MOOCs of all time.  

She said MIT definitely prepared her for a life as a professor, teacher, and mentor; the most important thing about graduate school isn’t choosing “the most cutting-edge research project,” but making sure you have a good training experience and an advisor who can provide that. 

“You don’t need to start building your name in the field when you’re a grad student. The lab environment is much more important than the topic. It’s easy to get burned out or to be turned off to a career in academia altogether if you have the wrong advisor,” she said. “You need to learn how to be a scientist, and you have plenty of time later in your career to follow whatever path you want to follow.”

She knows this from experience: her current research is somewhat parallel but unrelated to her previous research experience. 

“I think my motivation for being a scientist is rooted in my desire to help people doing something I enjoy,” she said. “I was not doing this kind of research as a graduate student, and that doesn’t mean that I wasn’t able to end up at this point in my career where I’m doing research that is focused on improving the lives of women, specifically.”

She did her postdoctoral work at UT Southwestern Medical Center studying Vibrio parahaemolyticus, a human pathogen that causes gastroenteritis. The work was a marriage of her interests in biochemistry and host-microbiome interactions.

She said MIT prepared her well for the type of interdisciplinary work that she does every day: At UT Dallas, all the research buildings are fully integrated, with engineers, chemists, physicists, and biologists sharing lab spaces in the same building. Her closest collaborators are mathematicians, chemists, and engineers. 

Although she may not be fully literate, she has a common language with the people she works with thanks to MIT’s undergraduate course requirements in many different topics and MIT’s focus on interdisciplinary research, which is “how real advancement is made.” 

Ultimately, De Nisco said she is glad to this day that she attended MIT. 

“Getting that acceptance letter to attend MIT probably changed the trajectory of my life,” she said. “You never know, on paper, what someone is going to achieve eventually, and what kind of force they’re going to be. I’m always grateful to whoever was on the admissions committees that made the decision to accept me—twice.”