In fall 2016, ASCB awarded ASCB-Gibco Emerging Leader Prizes to three cell biology researchers. The prizes honor not-yet-tenured independent investigators with outstanding scientific accomplishments and strong publication track records. The prizes were underwritten by Gibco, a brand of Thermo Fisher Scientific.
Peter Walter, 2016 ASCB President, invited each of the prize winners and each of the seven additional finalists to contribute an essay to the ASCB Newsletter. The writers were encouraged to provide a personal statement that articulates who they are, their science, and how they got into it; to describe their major scientific accomplishments; and to discuss their “dream results” and where they see themselves in the next five years.
This issue of the Newsletter features the essays of finalists Don Fox, Stephanie Gupton, Amy Ralston, and Pere Rocha-Cusachs.
Although most cells in our bodies contain two chromosome sets, both normal and diseased cells can acquire extra chromosome sets. This phenomenon is known as polyploidy. For the past 10 years, I’ve studied the biology of polyploid cells in the contexts of organ development, organ injury repair, and disease. My lab’s fundamental question is: How does polyploidy alter cell and tissue biology?
I didn’t set out to study polyploidy. Rather, in my postdoc with Allan Spradling, I decided to explore the biology of a somewhat forgotten organ, the adult fruit fly hindgut (the large intestine equivalent). My idea was that if I applied the powerful cell biological and genetic tools available in flies to an essentially unstudied tissue I was bound to uncover new and interesting biology. I found the hindgut is a great model to study how polyploidy is employed in tissue biology, and how it alters fundamental cellular processes such as mitosis.
I wasn’t completely in the dark when beginning this work—two inspirational papers from the 1930s offered clues. The first paper showed that the hindgut regenerates following the developmental “injury” of metamorphosis. We then asked whether the same regeneration takes place following injury to the mature adult gut. Instead, we found that a region of the injured adult hindgut called the pylorus repairs itself without making a single new cell. Rather, the remaining pyloric cells increase in both ploidy and size to perfectly match the pre-injury number of genomes and tissue mass. Using the hindgut as a model, a major ongoing focus of our work is to understand why some injured tissues, including our own liver and heart, increase ploidy instead of restoring cell number after injury.
The second inspirational paper suggested that part of the hindgut forms through somatic ploidy reductional divisions—from 64N to 2N! Using modern techniques, we instead found that polyploid divisions do occur, but these cells maintain polyploidy. This was interesting in and of itself, as developmentally programmed polyploidy usually leads to terminal differentiation. Most surprisingly, we found these divisions (which build salt-absorbing structures called rectal papillae) were incredibly inaccurate—not the way one would think an organ would be built!
Since then, we showed that papillar cells tolerate frequent chromosome number imbalances, or aneuploidy. We’re very interested in exploiting this extreme aneuploidy tolerance to identify mechanisms that maintain cells with chromosome imbalances. Much like papillar cells, over one-third of human cancers derive from inaccurate polyploid divisions. We’re excited about the possibility that our system might identify targetable signatures of polyploid cancers.
It’s amazing how little we know about why diploid organisms acquire polyploidy in specific tissues. In thinking about an ambitious five-year plan, I hope to uncover functional states that tissues can achieve only through polyploidy. One clue along this path to discovery is the numerous ways that polyploidy initiates additional genomic changes such as aneuploidy. While aneuploidy unquestionably creates fitness disadvantages in diploid cells, the extra chromosome sets in polyploid cells may lead to a more dynamic and well-tolerated range of gene expressions. A dream experiment to identify such novel gene expression states requires simultaneous interrogation of the genome and transcriptome at the single cell level, coupled with genetic manipulation of ploidy. Such experiments are increasingly possible to do. If I’ve learned one thing about polyploid cells, it’s that they follow their own surprising cellular rulebook. I look forward to more surprises!
I became a cell biologist after being enamored of the stunning beauty, the complex organization, and the emergent properties of the cell revealed by live-cell microscopy. My overarching research goal is to understand how the components of the cell organize to achieve complex cellular behaviors. This fascination began in a freshman lecture on photosynthesis. After seeing images of the chloroplast and the mechanisms by which it harnessed energy from the sun, I practically ran to the laboratory of Nina Strömgren Allen, one of the co-developers of Allen video-enhanced differential interference contrast microscopy, in the Botany Department at North Carolina State University. I worked in Nina’s lab throughout my undergraduate career, using live-cell microscopy to observe the subcellular dynamics of the cytoplasm and the endoplasmic reticulum in plant cells, and I attended my first of many ASCB meetings.
During my PhD work in Clare Waterman’s laboratory at the Scripps Research Institute, I continued training in high-resolution, quantitative live-cell microscopy techniques, including fluorescent speckle microscopy, to define the organization and coordination of focal adhesions and the microtubule and actin cytoskeletons during epithelial cell migration. After completing my thesis, I did my postdoctoral training in Frank Gertler’s laboratory at MIT, studying the morphogenesis of developing neurons, and how the extracellular environment modulates both the cytoskeleton and the membrane delivery critical to neuronal morphogenesis.
Newly born neurons are symmetrical, sphere-shaped cells that morph into a highly elongated, polarized shape with long axonal and dendritic extensions. These extensions innervate synaptic partners to establish neural networks. If correct connections are not formed, the neural circuits and the behaviors they encode may be compromised, resulting in neurological disorders. The overall research goal of my laboratory is to define cellular and molecular mechanisms mediating the development and maintenance of a functional nervous system, and the consequences of their going astray. We are interested particularly in molecules that spatially and temporally coordinate cytoskeletal reorganization at, and vesicle delivery to, the expanding plasma membrane of developing neurons.
In the six years I have been a primary investigator, we have focused on the role of the brain-enriched E3 ubiquitin ligases TRIM9 and TRIM67 in controlling cytoskeletal dynamics and vesicle trafficking during axon guidance and branching in embryonic cortical neurons. We have uncovered mechanisms by which these ligases alter the stability of growth cone filopodia and membrane delivery via exocytosis, which are critical for axon guidance and axon branching. The functions of TRIM9 and TRIM67 we have uncovered are dependent upon their ligase activity, yet surprisingly so far, independent of protein degradation, thus revealing novel roles for this post-translational modification in cytoskeletal and trafficking regulation and morphogenesis. Together with the development of microfluidic-based approaches, we are parsing new mechanisms of axon guidance. By adding neuroanatomical and behavioral studies, our work extends the molecular mechanisms regulating neuronal morphogenesis and function to disruptions in specific axon fiber tracts in vivo and to compromised animal behaviors.
As scientists, our collective goal is to continue the search for truth and make that knowledge available for all. My research program will continue to explore the fundamental mechanisms by which neurons develop and function. I hope this will improve our understanding of how the brain works and what goes wrong in certain disease states, which will be critical to developing therapeutics.
My goal is to discover how genes regulate stem cell behaviors in the embryo. My approach is to learn lessons from the mouse embryo and then apply these lessons to devise new stem cell therapies and improve pregnancy outcomes for humans. My research is funded by the National Institute of General Medical Sciences, and my lab is located at Michigan State University, where I am the James K. Billman, Jr., M.D., Endowed Professor of Biochemistry and Molecular Biology. One of my favorite aspects of being a professor and a principal investigator is having the opportunity to show others how creative science can be. This feeling undoubtedly stems from the way my own entry into science occurred.
When I was 13, I toured a cell biology lab at the local university as part of middle school career day. During the visit, I was offered my first job—as a glassware washer—which I promptly accepted. The job was meditative, but the real excitement came one day with the arrival of a new shipment of urchins that were spawning. Graduate students in the lab undertook an impromptu fertilization experiment, and I was thrilled beyond belief to watch as two-cell embryos magically appeared. The next day, the embryos had hatched and were swimming around. I was baffled by how a collection of cells could produce so much complexity overnight. Later, as an undergraduate at Oberlin College, the professor who had the greatest influence on me, Yolanda Cruz, set me on my course as a budding developmental biologist.
In graduate school, I used fruit flies to understand how genes create patterns within tissues during embryogenesis. My work, with Seth S. Blair at the University of Wisconsin, Madison, demonstrated multiple activities for Chordin-like proteins that exist in flies and humans. For my postdoctoral studies with Janet Rossant in Toronto, I identified transcription factors and signaling pathways that establish the first cell types of the mouse embryo, including stem cell progenitors. Since I started my own laboratory in 2009, my trainees and I have discovered that pluripotency factors, such as Oct4 and Sox2, have additional roles beyond pluripotency, in embryos and in reprogramming. Our work opens opportunities to discover new mechanisms regulating reprogramming factors and new ways to use reprogramming to model human development. In 2016, I was honored to receive the Presidential Early Career Award for Scientists and Engineers from President Barack Obama.
Since mentoring is in my blood, I derive much satisfaction from my opportunities to mentor and interact with students. At Michigan State University, I teach introductory biology to 250 undergraduates each year, and I mentor students and postdocs in my lab. In addition, I am in my third year as an instructor of the lab course Mouse Development, Stem Cells, and Cancer at Cold Spring Harbor Laboratory in New York. In the next five years, my goal is to lead my trainees into underexplored territory to discover new cell biology in the context of human extraembryonic tissues, such as placenta or yolk sac. This goal is important because the extraembryonic tissues direct the formation of many fetal cell types in mice, but much less is known about this process in humans. My lab is now in the process of applying our collective creativity to develop new tools and approaches to confront this exciting opportunity.
Shortly after completing my BSc (as a physics major) at the University of Barcelona, and while finding myself at a complete loss as to what to do with my life, I went one day to visit a friend who was doing a PhD. After leaving his office and getting tired of waiting for the elevator, I took the stairs. Two floors down, I randomly saw an ad on a bulletin board: “Biophysics PhD students wanted.” I called, joined the lab of Daniel Navajas as a graduate student, and immediately got hooked.
Unlike my undergraduate experience, tightly compartmentalized in lectures, disciplines, and subdisciplines, graduate research was about solving problems and understanding phenomena, often willfully ignoring interdisciplinary boundaries. For this experience, nothing could be better than mechanobiology, which combines mechanical, molecular, and nanotechnology tools to understand how cells and tissues respond to and interact with their physical environment. As a graduate student, I started by using atomic force microscopy to measure cell mechanical properties and to understand their implications, for instance, in neutrophil circulation. As a postdoc and under the advice of Michael Sheetz (Columbia University), I shifted my attention to the molecular scale, studying how integrins and adaptor proteins sense and transmit forces.
I then opened my own laboratory at the Institute for Bioengineering of Catalonia in Barcelona. Here, I devoted the efforts of my lab to understanding how cells detect tissue rigidity, a fundamental parameter that drives embryonic development, wound healing, and tumor growth. We first found that the properties of different integrins under force can lead breast cells to adapt to tissue rigidities corresponding to either healthy or malignant tissue. Then we unveiled that the differential effect of force on integrins and the adaptor protein talin leads talin to unfold only above a threshold in tissue rigidity. This results in vinculin binding to talin and activation of the major oncogene YAP, providing a molecular mechanism of rigidity sensing. More recently, we have discovered in joint work with the lab of Xavier Trepat that force transmission across cell monolayers is sufficient to explain the phenomenon of durotaxis, i.e., directed cell migration toward stiffer regions of their environment.
This topic has been extremely rewarding to study, and has led to a fundamental realization: A deep, mechanistic understanding of cellular physical interactions requires the combination of experimental tools in biomechanics, cell and molecular biology, and single molecule studies, but also theoretical modeling. This combination is in my view essential to unravel cell responses, which are often strongly, and non-intuitively, nonlinear. It also requires the generosity of strong collaborators across disciplines, which I’ve been extremely fortunate to have with several outstanding laboratories. I now intend to explore the limits of this approach, with the aim of understanding not only early events of cell interactions with their environment, but also how this is integrated to drive gene transcription and downstream cell and tissue response in different systems. Where this will lead me I don’t know, and experience has taught me that the most exciting findings are rarely (if ever) predictable in advance. After all, the thrill of the unexpected is one of the most rewarding aspects of our work as scientists, and I still wake up every day eager to see what will be on the bulletin board when I walk down the stairs.