Research Area: Stem Cell and Developmental Biology
Education
Graduate: PhD, 2016, Stanford University
Undergraduate: BA, 2009, Human Evolutionary Biology, Harvard University
Research Summary
We use the tiny, transparent jellyfish, Clytia hemisphaerica, to ask questions at the interface of nervous system evolution, development, regeneration, and function. Our foundation is in systems neuroscience, where we use genetic and optical techniques to examine how behavior arises from the activity of networks of neurons. Building from this work, we investigate how the Clytia nervous system is so robust, both to the constant integration of newborn neurons and following large-scale injury. Lastly, we use Clytia’s evolutionary position to study principles of nervous system evolution and make inferences about the ultimate origins of nervous systems.
Awards
Pathway to Independence Award (K99/R00), National Institute of Neurological Disorders and Stroke, 2020
Life Sciences Research Foundation Fellow, 2017
Eva Frederick | Whitehead Institute
July 20, 2022
Multicellular organisms evolved over millennia into a dazzling array of differently adapted creatures. With each generation, tiny worms, lavishly plumed birds, and even humans must create themselves anew from a single cell. To do so, they require a plan.
“How that multifunctional body plan is created is one of the deepest questions in developmental biology,” said Zak Swartz, until recently a postdoctoral researcher in the lab of Whitehead Institute Member Iain Cheeseman. “How do you take a single cell and pattern into a body that has different functions and features along it?”
Whitehead Institute researchers are tackling this question through a variety of different lenses. Researchers in Iain Cheeseman’s lab, including Swartz, have delved into the mysterious forces that underlie the polarity of an organism’s first cell. For the lab led by Pulin Li, research comes in at a later stage of development, when multiple cells combine to form a tissue and must communicate with each other to become an organized whole. Work on regeneration in Peter Reddien’s lab shows how some creatures can access their body blueprint throughout their lives to repair nearly any injury, and Yukiko Yamashita’s group studies how organisms pass on their body blueprints to their offspring through germ cells. Jonathan Weissman and his lab have created a “map” which researchers can use to find the function of a given gene, allowing them access to an organism’s most fundamental plans. Read on to learn about these scientists’ work, and more.
Laying out the plan
All multicellular organisms begin with a single cell, the fertilized egg. This cell has an essential role in setting out the body plan for the rest of an organism. It all starts with establishing polarity — in other words, figuring out which side of the cell is the top, and which is the bottom. This polarity establishes an axis of symmetry for the growing organism, and sets the stage for other developmental processes to come.
Once the initial polarity is established, the oocyte can divide, creating a bilaterally symmetric sea star larvae. The burgeoning cluster of cells must then undergo other processes to define the several axes of symmetry that adult sea stars are known for.
Talking through it
If an organism’s developmental blueprints are to be followed as development progresses, cells must be able to effectively communicate with each other. That cell to cell communication is the area of expertise of Whitehead Institute Member Pulin Li.
During her postdoctoral fellowship at the California Institute of Technology, Li studied tissue patterning — the mechanisms by which an organism’s newly forming tissues are laid out. Specifically, she investigated a developmental mechanism called morphogen gradient formation.
These gradients, composed of chemicals present in developing embryos, function as spatial coordinate systems and help determine how various cell types will be arranged in the organism — for example which groups of cells will form the liver, or the bones, or the brain, and where they will be within the body.
During cell division, chromosomes are replicated into two copies — one for each daughter cell. These copies, called sister chromatids, are usually considered identical. In fact, it’s the two pairs of sister chromatids that make up the symmetrical X shape usually shown when visualizing chromosomes.
A 2013 paper from the lab of Whitehead Institute Member Yukiko Yamashita showed that in the case of asymmetric cell division — such as when a stem cell is dividing into two different kinds of daughter cells (i.e. a stem cell and a differentiating daughter) — sister chromatids of sex chromosomes actually may carry distinct information, and the dividing cell “chooses” which of the daughters receive a specific copy.
What that “choice” means, and how it’s executed, has been a mystery — until now. A new paper from Yamashita, who is also a professor of biology at the Massachusetts Institute of Technology and an investigator with the Howard Hughes Medical Institute, published in Science Advances on July 27, illuminates the mechanisms that underlie nonrandom sister chromatid segregation, and suggests that the whole process may serve as a way to maintain the amount of ribosomal DNA (or rDNA) that is passed on to subsequent generations. “Tying together these two processes — rDNA copy number maintenance and nonrandom chromatid segregation — is an unexpected and exciting advance in our understanding of how germ cells are able to maintain their immortality,” said Yamashita.
George Watase, a postdoctoral scholar in the Yamashita Lab, led the study. Watase began his research intent on discovering the genetic underpinnings of nonrandom segregation of X and Y chromosomes in the fruit fly Drosophila melanogaster. As he surveyed the genome for genes that were essential to nonrandom segregation, it became apparent that ribosomal DNA was key for the process.
When rDNA was left intact, the sister chromatid with more rDNA was preferentially chosen by the daughter stem cell instead of the differentiating daughter cell. When rDNA was removed from X and Y chromosomes, however, Watase found that the sister chromatids segregated randomly to the daughter cells.
Ribosomal DNA, or rDNA, is composed of a long stretch of repeats of certain base pairs. The rDNA provides the instructions and material to make ribosomes, which are essential for cells to create proteins. “Most genes exist only as a single copy, but in the case of rDNA we have hundreds of copies in our genome,” Watase said. “The reason for this is that we need a massive amount of ribosomes to synthesize proteins to maintain our cells’ viability.”
As organisms age, most of their cells naturally lose some of those rDNA repeats, including germline stem cells. However, germline cells are sometimes called “immortal” — while all other cells in the body are made anew with each generation and die when an organism dies, germline cells such as sperm and eggs must carry DNA between generations. THerefore, the stem cells that produce sperm and eggs thus cannot keep losing rDNA repeats, and must bypass the mortality of other cells, by maintaining a high number of rDNA repeats over time.
By isolating proteins that bind to rDNA, Watase discovered one specific gene, the protein product of which bound to rDNA and somehow assigned the sister chromatid with more rDNA repeats to the daughter cell that was destined to remain a germline stem cell.
This particular gene had not been described before, and Watase and Yamashita were now tasked with naming it. Fruit fly genes are named after what happens to the animal when the gene is removed. When this new gene was knocked down, the germ cells of subsequent generations gradually lost the immortality that separates germline stem cells from their differentiated counterparts.
Watase wasn’t sure how to convey the intricacies of this outcome. In the end, it was Watase’s wife who came up with the perfect name: Indra. In Hindu scriptures, Indra, the lord of all deities, was given a garland of fragrant flowers by a sage called Durvasa. Indra placed the garland on the trunk of his elephant, but the animal was irritated by the smell of the flowers and threw the garland down, trampling it underfoot. When Durvasa saw this, he became enraged and cursed Indra, taking away his immortality.
The name also opened up a world of possibilities for naming future genes that are important in nonrandom sister chromatid segregation. “People sometimes pull from Roman or Greek myths when naming genes, but not as many people use names from Hindu myths,” he said. “And since this is new biology, if we identify additional related genes in the future, we can use names from Hindu myths again.”
Watase and Yamashita’s study opens new avenues for future research. For example, the paper focused primarily on male fruit flies and the production of sperm via asymmetric division. Indra is expressed in the female germline as well, and when the gene is knocked down in females, the resulting phenotype is much more severe. “There must be some mechanism in female germ cells to avoid rDNA copy number reduction,” said Watase. “We just don’t know what that mechanism is.”
In the future, Watase and Yamashita also hope to elucidate how exactly Indra is interacting with cell division machinery to influence which chromatid ends up in the stem cell and which in the differentiating cell, and beyond this mechanism, how the stem cell “selects” the longer chromatid.
“Many biologists study germ cells, but few specifically study how they maintain their immortality,” said Yamashita. “This study is a step towards understanding this fascinating property of germ cells. It’s a really fascinating area and we really have to keep digging deeper into this phenomenon.”
In a new study published July 7, 2022 in eLife, Adam Martin’s lab at the MIT Department of Biology identified the mechanical forces and molecular cues that help the spindles in cells located in the portion of the embryo destined to become the fly’s head to assume the same orientation.
Raleigh McElvery
July 7, 2022
Raleigh McElvery
During development, virtually all multicellular creatures must build themselves up from a ball of cells to a multilayered, fully-functioning organism. In the case of a fruit fly embryo, in order go from blob to organism, the cells must coordinate their divisions along the same axis to drive tissue growth in a specific direction — eventually going on to form the three germ layers known as the endoderm, ectoderm, and mesoderm.
Scientists have long studied how cells orient themselves to divide in a specific direction. It’s clear that this orientation is determined by a bundle of tiny fibers inside the cell called the spindle, which helps segregate the chromosomes so they can be distributed between the two daughter cells as the parent cell splits. When the spindles in neighboring cells are parallel with one another, then the cells will divide along the same axis.
In a new study published in eLife on July 7, 2022, Adam Martin’s lab at the MIT Department of Biology identified the mechanical forces and molecular cues that help the spindles in cells located in the portion of the embryo destined to become the fly’s head to assume the same orientation.
According to Martin, an associate professor of biology and the study’s senior author, his lab was among a handful of labs to take interest in this coordinated spindle orientation in the early fruit fly embryo — and identified the molecular, mechanical, and molecular cues that orient the spindle in a living organism.
“Cell divisions in the fly embryo was thought to be regulated independently of cell shape changes and morphogenetic movements,” Martin says. “However, we found that forces associated with cell invagination actually oriented cell divisions through a novel mechanism that we describe.” First author Jaclyn Camuglia, he explains, spearheaded the project from start to finish.
Prior to Camuglia’s work, researchers knew that spindle orientation was controlled by a complex of multiple proteins, including one protein in fruit flies called Pins (known as LGN in vertebrates). The entire protein complex, including Pins, is coupled to motor proteins that help to rotate the spindle. However, it was still unclear what mechanical forces were orienting Pins to dictate spindle rotation.
“It’s a very striking phenomenon when you look into the microscope at a fly embryo and see all the spindles orienting together,” Camuglia says. “We wanted to know: How are these spindles oriented and why is that directionality important?”
The fly is an ideal organism for probing cell division, because it has a very consistent and predictable cell cycle. During development, cells divide a set number of times, pause briefly, and then start up again in very discrete pockets across the embryo. The researchers wanted to know how the cells in a subset of those pockets in the fly’s head were coordinating their divisions.
Camuglia found that, in healthy embryos, Pins was always recruited to the end of the cell along the anterior-posterior axis (from the fly’s head to rear). By cutting the embryo with a laser, treating it with chemical inhibitors, disrupting the connections between cells, and depleting certain transcription factor proteins, she was able to characterize the mechanical forces that directed Pins to the anterior-posterior axis and thus rotated the spindle in the proper direction. As it turns out, these forces stem from other large-scale tissue movements that occur at the same time to force the embryo to fold in on itself and eventually form the fly’s muscles.
The researchers don’t yet know the role that these coordinated divisions along the anterior-posterior axis play in the developing fruit fly head — or what might happen during development if the divisions were to go awry. But Martin and Camuglia suspect this process facilitates tissue elongation, and helps compensate for any cells that are lost from the tissue during the division process as the embryo folds in on itself.
“This study is unique because it connects mechanical forces to the specific molecular cues like Pins that coordinate cell division,” Camuglia explains. “The factors that shape a developing embryo are critical to understand, because all organisms — from fruit flies to humans — experience these driving forces.” This intersection between physics and biology, she says, is what makes the study so exciting.”
Video: Groups of cells divide in a coordinated and oriented manner in the fruit fly embryo.
Top Image: Enrichment of cortical cues at one end of the cell coordinates the orientation of the mitotic spindle. Credit: Jaclyn Camuglia
Citation: “Morphogenetic forces planar polarize LGN/Pins in the embryonic head during Drosophila gastrulation” eLife, online 07/07/2022, DOI: https://doi.org/10.7554/eLife.78779
Jaclyn Camuglia, Soline Chanet, and Adam C Martin
Eva Frederick | Whitehead Institute
June 16, 2022
In research published June 17 in the journal Science Advances, researchers in the lab of Whitehead Institute Founding Member Rudolf Jaenisch present a way to create fat cells that can be modified to display different levels of insulin sensitivity.
The cells accurately model healthy insulin metabolism, as well as insulin resistance, one of the key hallmarks of type 2 diabetes. “This system, I think, will be really useful for studying the mechanisms of this disease,” said Jaenisch, who is also a professor of biology at the Massachusetts Institute of Technology (MIT).
“It’s really exciting,” said Max Friesen, a postdoctoral researcher in Jaenisch’s lab and a first author of the study. “This is the first time that you can actually use a human stem cell-derived [fat cell] to show a real insulin response.”
Body fat — also known as adipose tissue — is essential for regulating your body’s metabolism and plays an important role in the storage and release of energy. When fat cells called adipocytes encounter the hormone insulin, they suck up sugar from the blood and store it for future use.
But over many years, factors such as genetics, stress, certain diets, or polluted air or water can cause this process to go awry, leading to type 2 diabetes. In this disease, adipocytes, as well as cells in the muscles and liver, become resistant to insulin and therefore unable to regulate the levels of sugar in the blood.
Tools to model diabetes in the lab generally rely on mice or on cells in a petri dish or test tube. Both these systems have their own problems. Mice, although they are comparable with humans in some respects, have a completely different metabolism and do not experience human diabetes comorbidities like heart attacks. And cell culture has, in the past, failed to replicate key markers of diabetes in a way that is comparable to human tissues.
That’s why Friesen and Andrew Khalil, another postdoc in Jaenisch’s lab, set out to create a new model. The researchers started with human pluripotent stem cells. These cells are the shapeshifters of the body — given the right conditions, they can assume the specific characteristics of almost any human cell type. The Jaenisch Lab has used them in the past to replicate liver cells, brain cells, and even cancerous tumors.
They decided to try to optimize an existing method for differentiating pluripotent cells into fat cells. The protocol created cells that looked like adipocytes, but these cells did not recreate the conditions of healthy insulin signaling or insulin resistance seen in the human body in type 2 diabetes. When healthy adipocytes encounter insulin in the human body, they respond by taking up glucose out of the bloodstream. These lab-made fat cells weren’t doing that, unless the researchers cranked up insulin levels to a thousand times higher than levels ever seen in humans. “Taking up glucose [in response to normal levels on insulin] is really the main function of an adipocyte, so if the model fails to do that, anything downstream in terms of disease research is not going to work either,” Friesen said.
Friesen and Khalil wondered if the lab-grown adipocytes’ low sensitivity to insulin could be a product of the conditions in which they grew. “We thought that maybe this happens because we’re feeding them an artificial culture medium, with all kinds of extra supplements that might be inhibiting their metabolic response,” Friesen said.
Friesen and Khalil decided to use a method called the Design of Experiments approach, which allows researchers to tease out the contributions of different factors to a specific outcome. Informed by this approach, they created nearly 30 different media compositions, each with slightly different levels of key ingredients such as glucose, insulin, the growth factor IGF-1, and albumin, a protein found in blood serum.
The medium that worked best had concentrations of insulin and glucose that were similar to the levels in the human body. When grown in this new medium, the cells responded to much lower concentrations of insulin, just like cells in the body. “So this is our healthy adipocyte,” Friesen said. “Next we wanted to see if we could make a disease model out of this — to make it an insulin-resistant adipocyte like you would see in the progression to type 2 diabetes.”
To desensitize the cells, they flooded the media with insulin for a short period of time. This caused the cells to become less sensitive to the hormone, and respond similarly to diabetic or pre-diabetic fat cells in a living person.
The researchers could then study how the cells responded to the change — such as what genes the insulin resistant cells expressed that healthy cells did not — in order to tease out the underlying genetics of insulin resistance. “We saw small changes in a lot of genes that are metabolism regulated, so that seems to be pointing to a deficiency of the metabolism or mitochondria of the insulin-resistant cells,” Friesen said. “That’s one thing we want to pursue in the future — figure out what is wrong with their metabolism, and then hopefully how to fix it.”
Now that they have created this new model for studying insulin resistance in fat cells, the researchers hope to develop similar procedures for other cells affected in diabetes. “It seems that with some modifications, we can apply this method to other tissues as well,” Friesen said. “In the future, this will hopefully lead to a unified system for all stem cell-derived tissues, including liver, skeletal muscle, and other cell types, to get a really robust insulin response.”
In a whirlwind team project, undergraduates Aniket Dehadrai SB ’22 and Brindha Rathinasabapathi SB ’24 of the Boyer lab pioneered a new method to study how hearts are built.
Celina Zhao
May 23, 2022
Can’t miss a beat
The lab was bustling with activity, with everyone working together on a team project comprised of many moving parts. Once one person finished a step of the experiment, it was whisked off to the next person. There was no time to lose.
During MIT’s Independent Activities Period (IAP) in January of 2022, several members of the Boyer Lab were hard at work — among them, Aniket Dehadrai, a junior studying Course 5-7 (Chemistry and Biology), and Brindha Rathinasabapathi, a sophomore studying Course 7 (Biology). Fueled with coffee every morning from the lab’s handy Keurig, the team was on a time crunch.
Working alongside Dehadrai and Rathinasabapathi were research scientist Vera Koledova, lab manager Kirsten Schneider, and fellow undergraduate researcher Caroline Zhang. They had a hard deadline at the end of the month to finish the project: studying how the absence of a certain protein affects the growth of cardiomyocytes, the cells responsible for pumping blood around the heart.
The Boyer lab — headed by Professor Laurie Boyer, the “Queen of Hearts” — specializes in heart cells. The lab is particularly interested in one intriguing question: Is it possible to heal the heart? Injuries like heart attacks often cause permanent damage that can eventually lead to heart failure. Scientists have found that at birth, injured heart cells are able to repair or replace themselves after such an event. However, that ability shuts off just a few days post-birth. Afterwards, heart cells, once damaged, are unfixable.
But what if adult cardiomyocytes could regain the ability to repair themselves, and thus repair trauma in heart tissue? The Boyer lab is intrigued by this possibility. But in order to answer that question, they must start from ground zero: learning how cardiomyocytes themselves develop.
The operation
Dehadrai, Rathinasabapathi, and the rest of the team were studying one part of that puzzle — the role histones play in cardiomyocyte growth. Histones are proteins that act as spools for DNA to wind around. DNA is extremely long, so histones help fit all this genetic information into the tiny space of a nucleus.
There are many types of histones (called “variants”), each of which has a unique effect on how DNA is wrapped. The tighter the DNA is packed, the more difficult it is for proteins to access the DNA — all of which affects how genes are expressed. As a result, each variant has a unique effect on how certain genes are regulated.
For the IAP project, the Boyer lab’s team focused on one histone variant called H2AZ.1. Prior studies have shown that H2AZ.1 is essential in most organisms, particularly when it comes to gene expression in stem cells. Stem cells are cells that essentially begin as blank slates, with the ability to form the many different cell types in the body. But through a differentiation process, they develop specific identities: skin, brain, or heart, to name a few.
By the end of the four weeks, the team planned to create and streamline a completely new process to “knock out,” or entirely remove, H2AZ.1 by degrading it during cardiomyocyte differentiation — the process where stem cells become specialized heart cells. Building this procedure to remove H2AZ.1 could later help identify what role H2AZ.1 plays in cardiomyocyte differentiation, a key step in both heart development and regeneration.
The histone variant H2A.Z.1 (red) is located in the nucleus (blue) of cardiac muscle cells. Actin, a component of the sarcomere, is shown in green. The striated structure of the muscle cells gives them strength to beat throughout our entire lives. Credit: Boyer lab
To begin creating the knockout procedure, the team started by culturing stem cells from a cell line specifically developed by the Boyer lab to study the H2AZ.1 histone. The goal was to see if removing H2AZ.1 would have a visible effect on how stem cells eventually become mature cardiomyocytes.
The amount of careful planning and execution to do in just one month — simply running through one full differentiation cycle took 15 days at a time — meant working together as a team was critical. “There was one late night with all five people in the lab, doing this giant experiment as well as we could without mixing up the different variables in play,” Rathinasabapathi says. “It was really critical for us to look over each other’s shoulders and double check each other.”
In all, the team tested out 10 different variations of a method to optimize the experimental procedure. Despite the time crunch, they succeeded in pioneering a procedure to efficiently remove H2AZ.1 during cardiac differentiation. It turns out that H2AZ.1 does, in fact, have a functional impact on heart cells.
Without H2AZ.1, the beating rate of mature cardiomyocytes was notably different, changing from rhythmic to arrhythmic. The research team also found varying levels of maturity in the cells, suggesting that the progression through the differentiation process was also changed.
All of this suggests that H2AZ.1 has a significant influence in gene regulation, which they plan to continue studying in greater detail in the future.
“We’re breaking new ground,” Dehadrai says. “And importantly, it’s a great framework for future work in this field.”
With the procedure the team developed, the lab is now able to ask and answer more questions. For one, they can zoom in on certain parts of cardiomyocyte differentiation to see when H2AZ.1 has the greatest impact on gene expression. They can also use this procedure as a model to study how other histone variants affect heart cell growth. Ultimately, they can begin piecing together how histones, their effect on gene regulation, and cardiomyocyte development unite to build the heart.
“The better we can understand how heart cell development works, the better we can understand heart development, injury, and response — all of which have a lot of different implications in the medical field,” Rathinasabapathi says.
Following their hearts
The two credit the cohesiveness of the team as a big part of their success. “Brindha is really responsible, helpful, and willing to put in the hours,” Dehadrai says . “You can’t take stuff like that for granted.”
“Ani is just as dependable, and I’ve learned a lot from him as a senior with a lot of experience in the lab,” Rathinasabapathi says.
Another strength of the team was their ability to draw upon many different academic areas: a hallmark of the Boyer lab, which is known for its interdisciplinary approach to heart research. Members come from all sorts of backgrounds: biology, chemistry, biological engineering, mechanical engineering, and more. Research in the lab also spans a wide expanse, from uncovering the secrets of heart regeneration to building better microscopy techniques to study the heart. In fact, that was one of the reasons why Dehadrai initially chose to join the lab. “Here, there’s people who pretty much know how to do everything,” he says.
Although the IAP project has concluded, both Dehadrai and Rathinasabapathi are committed to continuing their passion for medical research. Dehadrai, who is graduating in the spring, is planning to take a gap year to work on clinical research projects before applying to medical school.
Rathinasabapathi, on the other hand, still has two years at MIT. She plans to stay in the Boyer Lab and is eager to take more advanced courses in the Department of Biology. “I’m impatient — I wish I already had the solid foundation to attack the research at different angles and come up with more cool new things,” she says. “There’s just so much more that I want to know.”
Greta Friar | Whitehead Institute
May 18, 2022
When animals experience a large injury, such as the loss of a limb, the body immediately begins a wound healing response that includes sealing the wound site and repairing local damage. In many animals, including humans, when the local wound site is taken care of, this response ends. However, in some animals, the initial wound response soon transitions into another stage of healing: regeneration, regrowing the parts that were lost.
Whitehead Institute Member Peter Reddien, also a professor of biology at MIT and a Howard Hughes Medical Investigator (HHMI), has long studied a flatworm known as the planarian (Schmidtea mediterranea), capable of regrowing any part of its body, to understand the mechanisms underlying regeneration. New research from staff scientist M. Lucila Scimone, graduate students Jennifer Cloutier and Chloe Maybrun, and Reddien identifies a previously undescribed gene, equinox, as playing a key role in initiating the transition from the initial wound healing stage into the regeneration stage in planarians. The work, published in Nature Communications on May 18, also reveals an important role for the wound epidermis, the skin that grows to cover a wound site, in initiating regeneration. Discovering what enables animals like planarians to regrow lost body parts can inform the field of regenerative medicine, which seeks to understand the limits of wound healing in humans and to improve our capacity for recovery and regeneration.
“The more we understand about the genes and mechanisms that play key roles in regeneration in animals that are capable of it, the better we may understand why humans lack that ability and, perhaps, the feasibility of future approaches to improve human wound healing,” says co-first author Scimone.
The case of the mystery gene
When the researchers began this project, they had no idea that it would lead them to identify a new gene that was crucial for regeneration. They originally set out to learn more about bmp4, a gene they had previously studied. BMP signaling, which includes bmp4, is involved in dorsal-ventral patterning, or the formation of the body around an axis between its top (dorsal) and bottom (ventral) sides. Previously, Reddien had found that bmp4 was necessary for regeneration after injuries to an animal’s side. Using new technologies that had not been around when they first studied the gene, the researchers now found that planarians without bmp4 failed to regenerate after large injuries anywhere on the body. This suggested a much more fundamental role for bmp4 in regeneration than the researchers expected, given that its main function relates to only one body axis. The researchers hypothesized that along with its role in dorsal-ventral patterning, bmp4 might help to activate an unknown gene that played some important, as yet unidentified role in regeneration. Bmp4 would therefore be necessary for regeneration because of its connection to this mystery gene.
The researchers started looking at genes regulated by bmp4 and found a promising candidate. They learned that bmp4 was needed to activate their mystery gene during the initial wound healing response, and that the mystery gene was crucial for wound healing to progress into regeneration after large injuries. When the gene was not activated, the steps that usually follow the initial wound healing response to prepare the body for regeneration would not occur. The wound would heal but the missing parts would never regrow, much like what would happen in a human. The researchers named the mystery gene equinox in honor of its appearance during a key transition period to move the body towards renewal.
“We know of a few genes that, when they are inactivated, the hallmarks of regeneration do not occur,” says co-first author Cloutier. “When equinox is not activated, we see an even more powerful inhibition of regeneration at an early phase. It appears to be required early on to allow for the other steps to proceed.”
Skin gets a starring role
The researchers found that equinox is expressed, or active in, wound epidermis, a skin tissue that is integral to regeneration after large injuries in a number of animals and yet had not been known to play a role in the signaling that initiates regeneration in planarians. After an injury, the wound epidermis covers and protects the wound site. As animals begin regeneration, the wound epidermis facilitates the formation of an outgrowth of cells called a blastema, in which the body produces the cell types it needs to replace the parts lost in the injury. Correspondingly, the researchers found that equinox is needed for regeneration in any injury that requires a blastema—essentially any large external injury where the replacement tissues grow out from the body.
Previously, the Reddien lab had found key genes required for regeneration expressed largely in muscle. Muscle in planarians maintains an active blueprint of the body, a network of positional genes that lets cells and tissues know where they are supposed to be. After an injury requiring regeneration, these positional genes rescale their body map near the wound site and guide new cells in building replacement tissues in the correct places. However, if equinox is not expressed, then the muscle tissue does not rescale its map. The body also fails to ramp up production of planarian stem cells or to begin differentiating stem cells into the cell types that were lost. Together, these findings flesh out the researchers’ understanding of the complete steps needed for regeneration to occur, revealing an early key role for wound epidermis, through its expression of equinox, in the signaling sequence that enables regrowth after an injury.
“There’s a cascade of events in which wound signaling activates, among other genes, equinox; equinox promotes wound-induced gene expression in muscle; and that promotes positional information resetting that can then lead to regeneration,” Reddien says. “What’s exciting about filling in this picture is that we’re identifying the key regulatory logic that can bring about regeneration.”
HMS grad Jennifer Cloutier has a habit of pushing limits
Christine Paul | Harvard Medical School News
May 17, 2022
When Jennifer Cloutier receives her MD from Harvard Medical School in May, it will be 12 years since she won a Canadian national waterskiing championship.
Although that feat alone is impressive, it’s even more extraordinary because the competition was designed for individuals with disabilities, and because of her lower-body paralysis, Cloutier, now 30, performed tricky slalom turns and acrobatics from a special seat bolted to her skis.
But then, pushing limits has been Cloutier’s signature style.
“In the 20-second period allowed for trick skiing, if you fall off the seat, your performance is over,” she said. “So, my goal was to always perform the hardest trick I could do without falling.”
Skiing triumphs were just the beginning of many of Cloutier’s achievements, demonstrating her refusal to be deterred by the spinal-cord injury she experienced at age 6 in a car accident, which also left her younger brother paralyzed.
Pushing limits
Cloutier was encouraged by her parents not to let her injury impede her future ambitions, and during the six months she was initially hospitalized after the accident, she gained firsthand appreciation of the marvels of rehabilitative medicine, which she says helped inspire her to become a doctor.
But childhood came first. At age 10, the Ottawa, Ontario, native also embraced alpine skiing, becoming a ski instructor during high school.
Then, turning to watersports, she competed internationally and became a volunteer administrator for SkiAbility Ottawa, a waterskiing organization for people with chronic illnesses and disabilities.
Winning medal after medal, Cloutier’s athletic successes and volunteer work with disabled people culminated in her being selected in 2011 to Canada’s Top 20 Under 20, a prestigious list published by Youth in Motion.
At the time, Cloutier was already at Harvard College, graduating with a bachelor’s in human developmental and regenerative biology in 2013, and serving as president of Women in Science at Harvard-Radcliffe from 2011 to 2013.
She says her early traumatic injury was pivotal in defining her research goal—to understand how tissues regenerate after they are damaged. HMS and the Massachusetts Institute of Technology (MIT) have given her a unique opportunity to pursue this goal.
Enrolled in the joint Harvard-MIT Program in Health Sciences and Technology (HST), which immerses students in rigorous interdisciplinary studies on both campuses, Cloutier will receive an MD in 2022 from HMS, complementing the PhD in biology she received from MIT in 2020.
Compressed into overlapping years, HST students on the MD track receive training to become physician-scientists. In addition to classroom studies on the HMS campus and clinical rotations at HMS-affiliated hospitals, they spend long hours in HMS or MIT laboratories, working with leading scientists on critical questions.
“As a physician-scientist, I am very interested in how organs and tissues re-form in adult organisms that are attempting to regenerate from injury,” Cloutier said.
She has studied the regenerative ability of a tiny planarian, or flatworm, named Schmidtea mediterranea.
For two centuries, this freshwater planarian has been a model organism for studying development and regeneration, because of its distinct anatomical features—eyes, gut, brain, central nervous system, and more—and its capacity to regenerate any missing body region, even the whole body, from minuscule body parts.
Working in the lab of Peter Reddien, professor and associate head of the MIT Department of Biology, Cloutier’s research has focused on planarian signaling pathways that recruit stem cells for regenerating tissues.
“Our research team is seeking to identify the genes and signals involved in initiating regeneration,” Cloutier said. “We are converging on a promising regulator that is expressed within hours of injury in the planarian wound epidermis. Such a discovery would offer key insights to the cellular signals that drive regeneration and could potentially lead someday to therapeutic strategies for better repair after injury,” she said.
Precursor cells
While at Harvard College, she worked as an undergraduate in the labs of Konrad Hochedlinger, HMS professor of genetics at Massachusetts General Hospital, and Guillermo Garcia-Cardeña, HMS associate professor of pathology at Brigham and Women’s Hospital, again focusing on questions of cell fate.
Before beginning her research at MIT, Cloutier gained experience as a doctoral student in the laboratory of HMS Dean George Q. Daley, well known for his research on hematopoietic stem cells, precursor cells in the blood and bone marrow that give rise to mature blood cells which are often used in therapies designed to replace or rebuild a patient’s hematopoietic system.
Then, in 2018, something besides regenerative biology caught her eye. Crossing the MIT campus one day, Cloutier noticed a group of mechanical engineering students designing their capstone project—a wheelchair with an adjustable seat that could be raised 12 inches.
Recognizing the potential benefit of this device for physicians with disabilities when they are performing patient procedures and physical exams, she provided valuable feedback to the group of 20 students.
Feeling at home
On-campus mobility was essential to her studies, and Cloutier credits Harvard for striving to make its facilities accessible. One of her favorite aspects of Harvard life was living as an undergrad at Quincy House in Cambridge, where she later returned during her time as an HST student to serve as resident tutor and mentor.
“Harvard’s house system is incredible and unique,” she said. “It feels like home.”
Even after Cloutier graduates from HMS, she’s not planning on going far. Her residency assignment is at Mass General, an HMS affiliate, where she’ll train in internal medicine.
Again, her primary interest is tissue regeneration, particularly the liver, which has the greatest regenerative capacity of any organ in the body. But if the liver is injured beyond its ability to regenerate itself, a transplant is often called for.
“During my principal clinical experience at HMS, I realized how much liver disease affects a patient,” Cloutier said, which led to more questions. “With the current shortage of livers available for transplant, is there a way to prolong the liver’s regeneration capacity and extend the bridge to transplant?”
Harkening back to the accident that changed her life, Cloutier’s also concerned about people who cope daily with chronic illnesses and pain, like her father, whose leg injury from that car accident that injured her still causes him significant discomfort.
“As physicians, we can’t fix all problems,” she said. “But we can focus on what matters to our patients and try to improve their quality of life.”
With her future before her, Cloutier is reminded of the powerful life lesson she learned at an early age.
“One day you may wake up, and things might be quite different,” she said. “But you still can do really cool things.”
Image: Gretchen Ertl
Greta Friar | Whitehead Institute
April 20, 2022
Stem cells are the versatile building blocks from which every cell type in the body, from neurons, to skin cells, to blood cells, is ultimately descended. Researchers have also figured out how to turn stem cells into different cell types in the lab, which has been helpful for studying health and disease in their normal cellular contexts, and could be used to generate cells for medical transplants. Whitehead Institute Founding Member Rudolf Jaenisch not only uses these cells in his research, but has spent much of his career discovering and improving the methods for making accurate laboratory models out of stem cell-derived cells.
One challenge that Jaenisch’s lab is focusing on is how to eliminate the differences between cell types as they are found in the body and their stem cell-derived equivalents. In particular, they have found that stem-cell derived cells are often immature, more closely resembling the cells found in fetuses rather than in adults. These differences can make the cells less accurate research models and prevent them from being medically useful as functional transplant cells. Stem cells in the body receive complex cocktails of molecular signals as they transform into different cell types. The challenge for researchers lies in figuring out which of the many molecular signals in the body are relevant and then get the recipe exactly right in their recreations.
Postdoc Haiting Ma in the Jaenisch lab decided to tackle this problem for hepatocytes, the main type of cell in the liver. In work published in Cell Stem Cell on April 21, Jaenisch and Ma share their findings on why stem cell-derived liver cells resemble fetal liver cells, and what’s needed to make them mature—including an important role for a thyroid hormone.
The liver filters everything that enters the body through the digestive system. It helps to store and modify nutrients, safely break down toxins and waste, process medications, and more. There is still a lot to learn about how the liver functions, and what goes wrong in a number of liver-associated diseases, and accurate stem cell-derived models will help with that research. Liver cells are also needed to treat end-stage liver disease, and if researchers could mass produce stem cell-derived liver cells that can function safely in an adult liver, this could help to meet the demand for liver cell transfusions.
For this study, Jaenisch and Ma grew liver cells from stem cells in two setups: a typical 2D culture, in which the cells were grown in a dish, and a 3D spheroid, in which cells that started out in the normal culture were then allowed to grow into three-dimensional balls of cells. The spheroids can be designed to mimic some aspects of the cells’ natural environment in ways that a 2D culture cannot. In each case, the researchers exposed the cells to a carefully timed mixture of signals to prompt them to develop into liver cells. The researchers then analyzed cells from both the 2D and 3D cultures and compared them to primary liver cells, or cells from a body, using a variety of techniques to look for differences related to DNA and gene expression. They found that the cells cultivated in the 3D system were closer to cells from the adult body than those in the 2D system.
“The 3D culture not only contributes to maturation of the liver cells, but it can also be used to scale up production of the cells, which could be very useful for cell therapies in the future,” Ma says.
However, both sets of lab-derived cells lacked important features of adult liver cells. The analyses pointed to one important missing factor in particular: in the adult liver cells, a hormone receptor called Thyroid Hormone Receptor Beta (THRB) binds to a number of places in the DNA. THRB then senses the presence or absence of thyroid hormones, and regulates a variety of gene expression processes accordingly. However, the researchers found that while the stem cell-derived liver cells made the right amount of THRB, something was preventing it from binding where it should and performing its function.
Normally, THRB has a partner that helps it bind to DNA, the thyroid hormone T3. When the researchers added T3 to their 2D and 3D cultures, this led to more typical binding of THRB, which in turn made the cells—especially the cells from the 3D culture—more closely resemble adult liver cells in a number of ways. Improved THRB binding increased the expression of key liver genes, restored the activity of regulatory elements in the DNA that modify gene expression, and reduced the expression of a fetal liver gene. The researchers also gained insights into the molecules that THRB interacts with and the mechanisms by which it affects liver maturation, painting a more complete picture of its key roles in liver cells.
Altogether, this work led to a better recipe for making adult liver cells from stem cells in the lab–using the 3D spheroid culture and adding T3. When cells developed with this approach were incorporated into the livers of mice, the cells integrated successfully and the liver maintained normal function long term.
The new and improved stem cell-derived liver cells are still not a perfect match for adult liver cells—the researchers have ideas about which missing characteristics they could tackle next—but the current cells’ ability to seamlessly integrate into the liver, as well as indicators from the analyses that they would be good models for liver-associated diseases, suggest that they will be useful in a variety of projects.
“As we improve the authenticity of our stem cell-derived cell types, we open up new opportunities for research,” Jaenisch says. “We can build more accurate models in which to study high-impact diseases, such as liver diseases, diabetes, and chronic viral infections, and using those models we can develop strategies for treatment and prevention.”
The MIT biologist’s research has shed light on the immortality of germline cells and the function of “junk DNA.”
Anne Trafton | MIT News Office
March 22, 2022
When cells divide, they usually generate two identical daughter cells. However, there are some important exceptions to this rule: When stem cells divide, they often produce one differentiated cell along with another stem cell, to maintain the pool of stem cells.
Yukiko Yamashita has spent much of her career exploring how these “asymmetrical” cell divisions occur. These processes are critically important not only for cells to develop into different types of tissue, but also for germline cells such as eggs and sperm to maintain their viability from generation to generation.
“We came from our parents’ germ cells, who used to be also single cells who came from the germ cells of their parents, who used to be single cells that came from their parents, and so on. That means our existence can be tracked through the history of multicellular life,” Yamashita says. “How germ cells manage to not go extinct, while our somatic cells cannot last that long, is a fascinating question.”
Yamashita, who began her faculty career at the University of Michigan, joined MIT and the Whitehead Institute in 2020, as the inaugural holder of the Susan Lindquist Chair for Women in Science and a professor in the Department of Biology. She was drawn to MIT, she says, by the eagerness to explore new ideas that she found among other scientists.
“When I visited MIT, I really enjoyed talking to people here,” she says. “They are very curious, and they are very open to unconventional ideas. I realized I would have a lot of fun if I came here.”
Exploring paradoxes
Before she even knew what a scientist was, Yamashita knew that she wanted to be one.
“My father was an admirer of Albert Einstein, so because of that, I grew up thinking that the pursuit of the truth is the best thing you could do with your life,” she recalls. “At the age of 2 or 3, I didn’t know there was such a thing as a professor, or such a thing as a scientist, but I thought doing science was probably the coolest thing I could do.”
Yamashita majored in biology at Kyoto University and then stayed to pursue her PhD, studying how cells make exact copies of themselves when they divide. As a postdoc at Stanford University, she became interested in the exceptions to that carefully orchestrated process, and began to study how cells undergo divisions that produce daughter cells that are not identical. This kind of asymmetric division is critical for multicellular organisms, which begin life as a single cell that eventually differentiates into many types of tissue.
Those studies led to a discovery that helped to overturn previous theories about the role of so-called junk DNA. These sequences, which make up most of the genome, were thought to be essentially useless because they don’t code for any proteins. To Yamashita, it seemed paradoxical that cells would carry so much DNA that wasn’t serving any purpose.
“I couldn’t really believe that huge amount of our DNA is junk, because every time a cell divides, it still has the burden of replicating that junk,” she says. “So, my lab started studying the function of that junk, and then we realized it is a really important part of the chromosome.”
In human cells, the genome is stored on 23 pairs of chromosomes. Keeping all of those chromosomes together is critical to cells’ ability to copy genes when they are needed. Over several years, Yamashita and her colleagues at the University of Michigan, and then at MIT, discovered that stretches of junk DNA act like bar codes, labeling each chromosome and helping them bind to proteins that bundle chromosomes together within the cell nucleus.
Without those barcodes, chromosomes scatter and start to leak out of the cell’s nucleus. Another intriguing observation regarding these stretches of junk DNA was that they have much greater variability between different species than protein-coding regions of DNA. By crossing two different species of fruit flies, Yamashita showed that in cells of the hybrid offspring flies, chromosomes leak out just as they would if they lost their barcodes, suggesting that the codes are specific to each species.
“We think that might be one of the big reasons why different species become incompatible, because they don’t have the right information to bundle all of their chromosomes together into one place,” Yamashita says.
Stem cell longevity
Yamashita’s interest in stem cells also led her to study how germline cells (the cells that give rise to eggs and sperm cells) maintain their viability so much longer than regular body cells across generations. In typical animal cells, one factor that contributes to age-related decline is loss of genetic sequences that encode genes that cells use continuously, such as genes for ribosomal RNAs.
A typical human cell may have hundreds of copies of these critical genes, but as cells age, they lose some of them. For germline cells, this can be detrimental because if the numbers get too low, the cells can no longer form viable daughter cells.
Yamashita and her colleagues found that germline cells overcome this by tearing sections of DNA out of one daughter cell during cell division and transferring them to the other daughter cell. That way, one daughter cell has the full complement of those genes restored, while the other cell is sacrificed.
That wasteful strategy would likely be too extravagant to work for all cells in the body, but for the small population of germline cells, the tradeoff is worthwhile, Yamashita says.
“If skin cells did that kind of thing, where every time you make one cell, you are essentially trashing the other one, you couldn’t afford it. You would be wasting too many resources,” she says. “Germ cells are not critical for viability of an organism. You have the luxury to put many resources into them but then let only half of the cells recover.”
