Author: Lillian Eden

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.” 

Probe expands understanding of oral cavity homeostasis

Approach opens the door to a greater understanding of protein-microbe interactions

Lillian Eden | Department of Biology
June 7, 2023

Your mouth is a crucial interface between the outside world and the inside of your body. Everything you breathe, chew or drink interacts with your oral cavity—the proteins and the microbes, including microbes that can harm us. When things go awry, the result can range from the mild, like bad breath, to the serious, like tooth and gum decay to more dire effects in the gut and other parts of the body. 

Even though the oral microbiome plays a critical role as a front-line defense for human health and disease, we still know very little about the intricacies of host-microbe interactions in the complex physiological environment of the mouth; a better understanding of those interactions is key to developing treatments for human disease. 

In a recent study published in PNAS, a collaborative effort revealed that one of the most abundant proteins found in our saliva binds to the surface of select microbes found in the mouth. The findings shed light on how salivary proteins and mucus play a role in maintaining the oral cavity microbiome. 

The collaboration involved members of the Imperiali lab in the Department of Biology and the Kiessling lab in the Department of Chemistry at MIT, as well as the Ruhl group at the University at Buffalo School of Dental Medicine, and the Grimes group at the University of Delaware. 

The paper is focused on an abundant oral cavity protein called zymogen granule protein 16 homolog B (ZG16B). Finding ZG16B’s interaction partners and gaining insight into its function were the overarching goals of the project. To accomplish this, Ghosh and colleagues engineered ZG16B to add reporter tags such as fluorophores. They called these modified proteins “microbial glycan analysis probes (mGAPs)” because they allowed them to identify ZG16B binding partners using complementary methods. They applied the probes to samples of healthy oral microbiomes to identify target microbes and binding partners. 

The results excited them. 

“ZG16B didn’t just bind to random bacteria. It was very focused on certain species including a commensal bacteria called Streptococcus vestibularis,” says first author Soumi Ghosh, a postdoctoral associate in the Imperiali lab. 

Commensal bacteria are found in a normal healthy microbiome and do not cause disease. 

Using the mGAPs, the team showed that ZG16B binds to cell wall polysaccharides of the bacteria, which indicates that ZG16B is a lectin, a carbohydrate-binding protein. In general, lectins are responsible for cell-cell interactions, signaling pathways, and some innate immune responses against pathogens. “This is the first time that it has been proven experimentally that ZG16B acts as a lectin because it binds to the carbohydrates on the cell surface or cell wall of the bacteria,” Ghosh highlights.

ZG16B was also shown to recruit Mucin 7 (MUC7), a salivary glycoprotein in the oral cavity, and, together the results suggest that ZG16B may help maintain a healthy balance in the oral microbiome by forming a complex with MUC7 and certain bacteria. The results indicate that ZG16B regulates the bacteria’s abundance by preventing overgrowth through agglutination when the bacteria exceed a certain level of growth. 

blue dots with red and green smudges
ZG16B recruits salivary mucin MUC7 onto Streptococcus vestibularis and enhance microbial aggregation. In this super-resolution image, both ZG16B (shown in red) and salivary mucin MUC7-enriched samples (shown in green), localize to the surface of S. vestibularis (shown in blue), leading to the formation of a ternary complex between the lectin, the mucin, and the microbes. Enhanced microbial clustering occurs during the recruitment of MUC7 on S. vestibularis by ZG16B, potentially to regulate the bacterial load on the oral cavity surfaces.
The scale bar shown here represents a 3-micron (µm) length.

“ZG16B, therefore, seems to function as a missing link in the system; it binds to different types of glycans—the microbial glycans and the mucin glycans—and ultimately, maintains a healthy balance in our oral cavity,” Ghosh says. 

Further work with this probe and samples of oral microbiome from healthy and diseased subjects could also reveal the lectin’s importance for oral health and disease. 

Current attention is focused on developing and applying additional mGAPs based on other human lectins, such as those found in serum, liver, and intestine to reveal their binding specificities and their roles in host-microbe interactions. 

“The research carried out in this collaboration exemplifies the kind of synergy that made me excited to move to MIT 5 years ago,” says senior co-author Laura Kiessling. “I’ve been able to work with outstanding scientists who share my interest in the chemistry and the biology of carbohydrates.” 

The senior authors of the paper—Barbara Imperiali and Kiessling — came up with the term for the probes they’re creating: “mGAPS to fill in the gaps” in our understanding of the role of lectins in the human microbiome, according to Ghosh. 

“If we want to develop therapeutics against bacterial infection, we need a better understanding of host-microbe interactions,” Ghosh says. “The significance of our study is to prove that we can make very good probes for microbial glycans, find out their importance in the frontline defense of the immune system, and, ultimately, come up with a therapeutic approach to disease.” 

This research was supported by the National Institute of Health.

New peptide modulators of the pro-apoptotic protein BAK

Biophysical characteristics such as peptide binding affinity and kinetics do not determine cell death function

Lillian Eden | Department of Biology
May 9, 2023

Billions of times a day, every day of our lives, cells receive signals to initiate the process of cell death. This strategic cell death, also called apoptosis, is one of the tools multicellular organisms use to maintain tissues and regulate immune responses: damaged, old, or superfluous cells are given the green light to, as it were, turn out the lights for the last time.

Programmed cell death is both extremely powerful and extremely regulated: for example, the careful culling of cells between our digits during embryonic development reveals fingers and toes. When programmed cell death goes awry, however, it can have serious consequences. Cells left unchecked can divide unstoppably and aggressively, leading to cancer. Dysregulated apoptotic pathways have also been implicated in neurodegenerative diseases like Alzheimer’s, where unrestrained cell death may play a part in the severity of the disease.

MIT Professor H. Robert Horvitz ‘68 shared a Nobel prize in 2002 for his foundational research on the genetics of programmed cell death and organ development in the nematode, a microscopic roundworm. Horvitz discovered that ced-9, a key gene in programmed cell death in nematodes, was similar in structure and function to the human gene bcl-2.

Targeting members of the BCL-2 protein family has already shown promise in the fight against cancer. For example, approved by the FDA in 2016, the oral drug Venetoclax is a BCL-2 inhibitor used to treat certain types of leukemia.

In a study published online Jan. 26 in Structure, Fiona Aguilar PhD ‘22 (Keating lab) and collaborators focused on a member of the BCL-2 protein family called BAK. When it is active, BAK promotes mitochondrial outer membrane disruption, leading to cell death, and is therefore referred to as a pro-apoptotic protein. But precisely how BAK becomes activated – or inhibited – is unknown.

“A greater understanding of BAK activation is interesting both from a fundamental biochemical and biophysical perspective as well as from the more translational one of BAK as a potential therapeutic target,” says lead author Fiona Aguilar.

BAK exists in two different forms: an inactive monomer and an active oligomer. A few activators of BAK (BIM, truncated BID, and PUMA) have already been identified and these proteins bind directly to BAK, leading to the model that binding of activators trigger changes in protein shape that allow BAK to transition from the inactive to active forms. To further explore this idea, Aguilar identified and characterized a number of other peptides that bind to and regulate BAK. To identify new peptide binders, the team used cell-surface display screening and computational protein design methods, including techniques developed by Keating lab alum Gevorg Grigoryan– dTERMen and TERMify – that use protein structural data to generate new protein sequences likely to bind a protein of interest.

In total, Aguilar et al. discovered 10 diverse new peptide binders of BAK that regulate its function.

Interestingly, some of the BAK-binding peptides inhibited activation rather than promoting it. Aguilar et al. found that inhibitors and activators of BAK shared many characteristics including structure as well as binding affinity and kinetics – the strength and rate that binders associate with and dissociate from BAK.

Newly identified activators had sequences both dissimilar from one another and from the previously known BAK activators BIM, truncated BID, and PUMA. The similarity of the sequence was not necessarily a good indicator of activation or inhibition. For example, an inhibitor and an activator differed by just two amino acids.

Aguilar and colleagues solved the crystal structures of two inhibitor-BAK complexes and one activator-BAK complex and found that the activator interacted with BAK with similar geometry as the two inhibitors. Also, the two inhibitors have only about 40% sequence identity, but bind very similarly to BAK.

Amy Keating, the senior author on the study, says “Fiona was tireless in identifying new peptides, testing their interactions with BAK, determining their functions, and solving structures to look for differences between activators and inhibitors. We were surprised that peptides with such different behaviors shared such common interaction properties.”

Although the puzzle is not yet solved, Aguilar believes the “transition state” between inactive and active forms of BAK is key.

“We think of activators as peptides that preferentially bind to the BAK transition state, whereas inhibitors are those that preferentially bind to the monomeric state,” Aguilar says. “Overall, we should be thinking more about the transition state, what steps are necessary to reach the transition state, and how to target the transition state.”

This study also added two sequences in the human proteome – BNIP5 and PXT1 – to the repertoire of known BAK binders. Not much is known about these sequences, Aguilar says, but the fact that they activate BAK could indicate that they may play a role in apoptotic pathways that have not yet been determined.

“The finding is something that people in the field are pretty excited about,” Aguilar says.

Ultimately, work remains to establish what characteristics of the binders determine their function, and how binding to BAK triggers the conformational changes that activate or inhibit this complex protein.

“It’s still unclear what it is about these sequences that trigger the allosteric network leading to BAK activation, but at least for now we can rule out the hypothesis that binding mode, affinity, and kinetics fully determine how this occurs,” Aguilar says.

Aguilar suggests that it will be interesting also to explore how these peptides interact with BAX, another pro-apoptotic protein in the BCL-2 family that is both structurally and functionally similar to BAK.

Fiona Aguilar is lead author and Amy Keating is senior author; Bob Grant and graduate students Sebastian Swanson, Dia Ghose, and Bonnie Su contributed. Collaborators Stacey Yu and Kristopher Sarosiek, from the Harvard T.H. Chan School of Public Health, helped with cell-based experiments. The research was funded by a National Institute of General Medical Sciences award, the MIT School of Science Fellowship in Cancer Research award, the John W. Jarve (1978) Seed Fund for Science Innovation (MIT) award, an award from the National Cancer Institute, a National Institute of Diabetes and Digestive and Kidney Diseases award, and Alex’s Lemonade Stand Foundation for Childhood Cancers award.

Remembering Stephen Goldman: An institution at MIT

Faculty and staff across MIT recall Goldman's unending commitment to his work for more than three decades.

Lillian Eden | Department of Biology
May 1, 2023

On Sept. 30, 2022, Stephen “Steve” Goldman, 59, passed away after a courageous battle with ALS. Goldman worked for MIT for more than 30 years, first with IS&T, then for the CSBi research program, and then in the biology department.

“Steve was an Institution,” says Stuart Levine, Director of the BioMicro center and Goldman’s supervisor for more than a decade. “He did a little bit of everything, and that’s really hard to find these days.”

Levine says he was the type of person who had his “whole being” wrapped up in the job. Steve Goldman was one of the first hires for the fledgling BioMicro Center, according to former supervisor Peter Sorger, whose current role is Otto Krayer Professor of Systems Pharmacology, Department of Systems Biology at Harvard Medical School. Steve Goldman, he said, was essential for setting up the Biology Department’s first server-based computing system. 

“He brought great enthusiasm and skill to the role and I also appreciated his sangfroid and sense of humor. This was essential because we were inventing the Center’s infrastructure and mission on the fly and were often in the dark–and also down in the steam tunnels. Steve was a real pioneer,” Sorger says. 

Goldman worked for MIT for more than 30 years and was known for his workspace filled with piles of memory sticks, CDs, cables, and devices in various states of repair.

Before MIT, Goldman lived in New York and worked on Wall Street. He met his wife of 32 years, Brenda Goldman (née Mahar) on a boat in the middle of the Caribbean.  

“He came up to me in a white tuxedo and asked me to have dinner,” Brenda Goldman recalls.

They clicked immediately. 

Around the time of their wedding two years later, Brenda Goldman had found a job in Cambridge and they were both eager for Steve Goldman to find work in Massachusetts, far from the high stress environment of Wall Street.  

“I found an ad at MIT and I said ‘this sounds very much like you,’” Brenda Goldman says. “He interviewed two, three times because MIT is very slow about getting people in the door. But most of the people that get in the door end up sticking around.”

Steve Goldman was no exception: he found out he’d gotten a job at MIT the day before the wedding and the rest, as they say, is history.

Whether it was a weekend or a holiday, if Steve Goldman got an alert that something was wrong, he would always try to follow up, fix the problem, or go in to offer hands-on help, according to Levine. 

Brenda Goldman even accompanied him a few times, noticing that her husband always found a friendly face. 

“There was always somebody around who waved or said hello. We couldn’t get out of the building without seeing someone, no matter which building it was,” she says.

Former department head Alan Grossman recalls many casual conversations about sports, especially baseball and softball. 

“He always greeted me with a warm smile and ‘hello professor,’” Grossman says. “He truly loved working in our department and we miss him.” 

Goldman’s second love, according to Brenda Goldman, was refereeing sports. Steve Goldman would often get to work early so he could wrap up in time to referee games. 

Stephen “Steve” Goldman, far left, loved refereeing sports in his spare time.

“He had something for almost every season of the year except winter,” Brenda Goldman says. “He liked it for the exercise, but he also liked it because it got him off his office chair and interacting with people.” 

Steve Goldman was organized—but not a neat person. His workspace was always filled with stuff—piles of memory sticks, CDs, cables, and devices open and in various stages of repair.

But “If you told him something broke, he knew what pile of things to pull the magic out of to make it work,” Brenda Goldman says. 

Levine says Steve Goldman’s death came as a bit of a shock: Goldman had been answering emails just days before his death.

“He always, always loved working for MIT,” Brenda Goldman says. “He loved computers and the work gave his life purpose.”

Following his death, the Biology Department made a contribution in Steve Goldman’s memory to the ALS Association of Massachusetts. He leaves behind his wife, children Kevin and Jason Goldman, in-laws, and many nieces and nephews.

3 Questions: Brady Weissbourd on a new model of nervous system form, function, and evolution

Developing a new neuroscience model is no small feat. New faculty member Brady Weissbourd has risen to the challenge in order to study nervous system evolution, development, regeneration, and function.

Lillian Eden | Department of Biology
April 26, 2023

How does animal behavior emerge from networks of connected neurons?  How are these incredible nervous systems and behaviors actually generated by evolution? Are there principles shared by all nervous systems or is evolution constantly innovating? What did the first nervous system look like that gave rise to the incredible diversity of life that we see around us?

Combining the study of animal behavior with studies of nervous system form, function, and evolution, Brady Weissbourd, a new faculty member in the Department of Biology and investigator in The Picower Institute for Learning and Memory, uses the tiny, transparent jellyfish Clytia hemisphaerica, a new neuroscience model.

Q: In your work, you developed a new model organism for neuroscience research, the transparent jellyfish Clytia hemisphaerica. How do these jellyfish answer questions about neuroscience, the nervous system, and evolution in ways that other models cannot?

A: First, I believe in the importance of more broadly understanding the natural world and diversifying the organisms that we deeply study. One reason is to find experimentally tractable organisms to identify generalizable biological principles – for example, we understand the basis of how neurons “fire” from studies of the squid giant axon. Another reason is that transformative breakthroughs have come from identifying evolutionary innovations that already exist in nature – for example, green fluorescent protein (GFP, from jellyfish) or CRISPR (from bacteria). In both ways, this jellyfish is a valuable complement to existing models.

I have always been interested in the intersection of two types of problems: how nervous systems generate our behaviors; and how these incredible systems were actually created by evolution.

On the systems neuroscience side, ever since working on the serotonin system during my PhD I have been fascinated by the problem of how animals control all of their behaviors simultaneously in a flexible and context-dependent manner, and how behavioral choices depend not just on incoming stimuli but on how those stimuli interact with constantly changing states of the nervous system and body. These are extremely complex and difficult problems, with the particular challenge of interactions across scales, from chemical signaling and dynamic cell biology to neural networks and behavior.

To address these questions, I wanted to move into a model organism with exceptional experimental tractability.

There have been exciting breakthroughs in imaging techniques for neuroscience, including these incredible ways in which we can actually watch and manipulate neuronal activity in a living animal. So, the first thing I wanted was a small and transparent organism that would allow for this kind of optical approach. These jellyfish are a few millimeters in diameter and perfectly transparent, with interesting behaviors but relatively compact nervous systems. They have thousands of neurons where we have billions, which also puts them at a nice intermediate complexity compared to other transparent models that are widely used – for example, C. elegans have 302 neurons and larval zebrafish have something like 100,000 in the brain alone. These features will allow us to look at the activity of the whole nervous system in behaving animals to try to understand how that activity gives rise to behaviors and how that activity itself arises from networks of neurons.

On the evolution side of our work, we are interested in the origins of nervous systems, what the first nervous systems looked like, and broadly what the options are for how nervous systems are organized and functioning: to what extent there are principles versus interesting and potentially useful innovations, and if there are principles, whether those are optimal or somehow constrained by evolution. Our last common ancestor with jellyfish and their relatives (the cnidarians) was something similar to the first nervous system, so by comparing what we find in cnidarians with work in other models we can make inferences about the origins and early evolution of nervous systems. As we further explore these highly divergent animals, we are also finding exciting evolutionary innovations: specifically, they have incredible capabilities for regenerating their nervous systems. In the future, it will be exciting to better understand how these neural networks are organized to allow for such robustness.

Q: What work is required to develop a new organism as a model, and why did you choose this particular species of jellyfish?

A: If you’re choosing a new animal model, it’s not just about whether it has the right features for the questions you want to ask, but also whether it technically lets you do the right experiments. The model we’re using was first developed by a research group in France, who spent many years doing the really hard work of figuring out how to culture the whole life cycle in the lab, injecting eggs, and developing other key resources. For me, the big question was whether we’d be able to use the genetic tools that I was describing earlier for looking at neural activity. Working closely with collaborators in France, our first step was figuring out how to insert things into the jellyfish genome. If we couldn’t figure that out, I was going to switch back to working with mice. It took us about two years of troubleshooting, but now we can routinely generate genetically modified jellyfish in the lab.

Switching to a new animal model is tough – I have a mouse neuroscience background and joined a postdoc lab that used mice and flies; I was the only person working with jellyfish but had no experience. For example, building an aquaculture system and figuring out how to keep jellyfish healthy is not trivial, particularly now that we’re trying to do genetics. One of my goals is now to optimize and simplify this whole process so that when other labs want to start working with jellyfish we have a simple aquaculture platform to get them started, even if they have no experience.

In addition to the fact that these things are tiny and transparent, the main reason that we chose this particular species is because it has an amazing life cycle that makes it an exciting laboratory animal.

They have separate sexes that spawn daily with the fertilized eggs developing into larvae that then metamorphose into polyps. We grow these polyps on microscope slides, where they form colonies that are thought to be immortal. These colonies are then constantly releasing jellyfish, which are all genetically identical “clones” that can be used for experiments. That means that once you create a genetically modified strain, like a transgenic line or a knockout, you can keep it forever as a polyp colony – and since the animals are so small, we can culture them in large numbers in the lab.

There’s still a huge amount of foundational work to do, like characterizing their behavioral repertoire and nervous system organization. It’s shocking how little we know about the basics of jellyfish biology – particularly considering that they kill more people per year than sharks and stingrays combined – and the more we look into it the more questions there are.

Q: What drew you to a faculty position at MIT?

A: I wanted to be in a department that does fundamental research, is enthusiastic about basic science, is open-minded, and is very diverse in what people work on and think about. My goal is also to be able to ultimately link mechanisms at the molecular and cellular level to organismal behavior, which is something that MIT Biology is particularly strong at doing. It’s been an exciting first few months! MIT Biology is such an amazing place to do science and it’s been wonderful how enthusiastic and supportive everyone in the department has been.

I was additionally drawn to MIT by the broader community and have already found it so easy to start collaborations with people in neuroscience, engineering, and math. I’m also thrilled to have recently become a member of The Picower Institute for Learning and Memory, which further enables these collaborations in a way that I believe will be transformational for the work in my lab.

It’s a new lab. It’s a new organism. There isn’t a huge, well-established field that is taking these approaches. There’s so much we don’t know, and so much that we have to establish from scratch. My goal is for my lab to have a sense of adventure and fun, and I’m really excited to be doing that here in MIT Biology.

3 Questions: Sara Prescott on the brain-body connection

New faculty member Sara Prescott investigates how sensory input from within the body control mammalian physiology and behavior.

Lillian Eden | Department of Biology
April 26, 2023

Many of our body’s most important functions occur without our conscious knowledge, such as digestion, heartbeat, and breathing. These vital functions depend on the signals generated by the “interoceptive nervous system,” which enables the brain to monitor our internal organs and trigger responses that sometimes save our lives. One second you are breathing normally as you eat your salad and the next, when a vinegar-soaked crouton enters your throat, you are coughing or swallowing to protect and clear your airway. We know our bodies are sensitive to cues like irritants, but we still have a lot to learn about how the interoceptive system works to meet our physiological needs, keep organs safe and healthy, and affect our behavior. We can also learn how chronic insults may lead to organ dysfunction and use what we learn to create therapeutic interventions.

Focusing on the airway, Sara Prescott, a new faculty member in the Department of Biology and Investigator in The Picower Institute for Learning and Memory, seeks to understand the ways our nervous systems detect and respond to stimuli in health and disease.

Q: You’re interested in interoceptive biology. What makes the nervous system of mice a good model for doing that?

A: Our flagship system is the mammalian airway. We use a mouse model and modern molecular neuroscience tools to manipulate various neural pathways and observe what the effects are on respiratory function and animal health.

Neuroscience and mouse work have a reputation for being a little challenging and intense, but I think this is also where we can ask really important questions that are useful for our everyday lives — and the only place where we can fully recapitulate the complexity of nervous system signaling all the way down to our organs, back to our brain, and back to our organs.

It’s a very fun place to do science with lots of open questions.

One of the core discoveries from my postdoctoral work was focusing on the vagus nerve as a major body-to-brain conduit, as it innervates our lungs, heart and gastrointestinal tract. We found that there were about 40 different subtypes of sensory neurons within this small nerve, which is really a remarkable amount of diversity and reflects the massive sensory space within the body. About a dozen of those vagal neurons project to the airways.

We identified a rare neuron type specifically responsible for triggering protective responses like coughing when water or acid entered the airway. We also discovered a separate population of neurons that make us feel and act sick when we get a flu infection. The field now knows what four to five vagal populations of neurons are actually sensing in the airways, but the remaining populations are still a mystery to us; we don’t know what those populations of sensory neurons are detecting, what their anatomy is, and what reflex effects those neurons are evoking.

Looking ahead, there are many exciting directions for the interoceptive biology field. For example, there’s been a lot of focus on characterizing the circuits underlying acute motor reflexes, like rapid responses to visceral stimuli on the timescale of minutes to hours. But we don’t have a lot of information about what happens when these circuits are activated over long periods of time. For example, respiratory tract infections often last for weeks or longer. We know that the airways undergo changes in composition when they’re exposed to different types of infection or stress to better accommodate future threats. One of the hypotheses we’re testing is that chronically activating neural circuits may drive changes in organ composition. We have this idea, which we’re calling reflexive remodeling: neurons may be communicating with stem cells and progenitor cells in the periphery to drive adaptive remodeling responses.

We have the genetic, molecular and circuit scale tools to explore this pheno­­­menon in mice. In parallel, we’re also setting up some in vitro models of the mouse airway mucosa to expedite receptor screening and to explore basic mechanisms of neuron-epithelium crosstalk. We hope this will inform our understanding of how the airway surface senses and responds to different types of irritants or damage. 

Q: Why is understanding the peripheral nervous system important, and what parts of your background are you drawing on for your current research?

A: The lab focuses on really trying to explore the body-brain connection. 

People often think that our mind exists in a vacuum, but in reality, our nervous system is heavily integrated with the rest of the body, and those neural interfaces are important, both for taking information from our body or environment and turning it into an internal representation of the world, and, in reverse, being able to process that information and being able to enact changes throughout the body. That includes things like autonomic reflexes, basic functions of the body like breathing, blood-gas regulation, digestion, and heart rate.

I’ve integrated both my graduate training and postdoctoral training into thinking about biology across multiple scales.

Graduate school for me was quite focused on deep molecular mechanism questions, particularly gene regulation, so I feel like that has been very useful for me in my general approach to neuroscience because I take a very molecular angle to all of this.

It also showed me the power of in vitro models as reductionist tools to explore fundamental aspects of cell biology. During my postdoc, I focused on larger, emergent phenotypes. We were able to manipulate specific circuits and see very impressive behavioral responses in animals. You could stimulate about 100 neurons in a mouse and see that their breathing would just stop until you remove the stimulation, and then the breathing would return to normal.

Both of those experiences inform how we approach a problem in my research. We need to understand how these circuits work, not just their connectivity at the anatomical level but what is driving their changes in sensitivity over time, the receptor expression programs that affect how they sense and signal, how these circuits emerge during development, and their gene expression.

There are still s­o many foundational questions that haven’t been answered that there’s enough to do in the mouse for quite some time.

Q: This all sounds fascinating. Where does it lead?

A: Human health has been my north star for a long time and I’ve taken a long, wandering path to find particular areas where I can scratch whatever intellectual itch that I have.

I originally thought I would be a doctor and then realized that I felt like I could have a more lasting impact by discovering fundamental truths about how our bodies work. I think there are a number of chronic diseases in which autonomic imbalance is actually a huge clinical component of the disorder.

We have a lot of interest in some of these very common airway remodeling diseases, like chronic obstructive pulmonary disorder—COPD—asthma, and potentially lung cancer. We want to ask questions like how autonomic circuits are altered in disease contexts, and when neurons actually drive features of disease. 

Perhaps this research will help us come up with better molecular, cellular or tissue engineering approaches to improve the outcomes for a variety of autonomic diseases. 

It’s very easy for me to imagine how one day not too far from now we can turn these findings into something actionable for human health.