Unexpected Stories and Spindle Mistakes: Discovering that Wild-type Cells are Full of Surprises

Natalie Nannas

Natalie Nannas is an Associate Professor of Biology at Hamilton College in Clinton, NY. She teaches courses in genetics, molecular biology, and bioethics. Dr. Nannas graduated from Grinnell College with bachelor’s degrees in biological chemistry and French. She received her Master’s and PhD from Harvard University in molecular biology and genetics. Dr. Nannas conducted her postdoctoral research at the University of Georgia where she won a National Science Foundation Plant Genome Postdoctoral Fellowship. At Hamilton College, Dr. Nannas enjoys teaching and sharing her passion for microscopy with her undergraduate research students. When not glued to a microscope, she loves spending time with her husband and two daughters. The narrative below by Natalie Nannas captures the human stories behind the science from a 2022 paper titled “Frequent spindle errors require structural rearrangement to complete meiosis in Zea mays” which was published by her group in 2022 in the International Journal of Molecular Sciences.

Science never works out the way we plan. As scientists, we ask questions, hypothesize and outline our goals … then reality of science occurs. The reality of science is often full of failed controls, endless troubleshooting, and sometimes strange findings that lead us in new and unpredictable directions. Our publications give the impression that we planned these scientific journeys from the beginning and do not tell the human side of the process with all of its twists and turns, dead-ends and U-turns. I want to tell you the real story behind my first publication as a faculty member with my own lab. It did not go as planned due to the COVID-19 pandemic. My lab was shut down in the middle of our investigation, and my students and I were unable to generate new data. In the beginning, it seemed like we were stranded with only control data and no story to tell, but the time away from the lab allowed us to spend more time looking carefully at wild-type cells. What seemed like a dead-end suddenly became its own story when we found something unexpected hiding within microscopy movies. Our wild-type cells were making mistakes, attempting fixes and changing directions, just like we do as scientists.

My scientific journey began with flickering green lights and a microscope (you can read more about it here). As an undergraduate, I was mesmerized by the beauty of watching living cells shuffle fluorescently labeled proteins throughout their cytoplasm. I followed this passion for microscopy into my doctoral dissertation research at Harvard University where I investigated how yeast cells build the machinery needed to pull their chromosomes apart. This machinery is a dynamic collection of long protein tubes called microtubules and other organizing proteins that help move and shuffle microtubules. I loved watching the delicate dance of chromosomes interacting with microtubules of the spindle, and I wanted to continue studying this process in my postdoctoral studies.

During postdoctoral studies at the University of Georgia, I won a fellowship from the National Science Foundation to develop a new technique in microscopy. No one had ever watched plants building their spindles in meiosis, the specialized cell division that produces egg and sperm. Other scientists had performed beautiful microscopy studies observing how mitotic spindles function inside of plant cells, but due to the technical challenges, no one had ever observed live plant cells building spindles in meiosis. I was thrilled to take on this challenge by using version of maize that had fluorescently labeled tubulin, the protein that makes up microtubules of the spindle. With this line of maize, spindles would glow fluorescent green, allowing me to image if only I could extract the meiotic cells.

We were so busy collecting data and prepping for our mutant studies that we never really took time to analyze the wild-type cells.

After almost a year spent dissecting maize plants, I finally managed to develop a method to isolate these tiny cells and keep them alive in a growth media long enough to image them. This new method of live imaging was going to serve as the foundation of my new lab at Hamilton College, a primarily undergraduate institution. With my students, I planned to investigate the pathways governed spindle assembly. Most animal mitotic cells have a structure called a centrosome that dictates how spindles are formed; however, female animal meiotic cells lack these structures and must use other pathways to direct spindle assembly. Plants also lack centrosomes, and I wanted to inhibit these known animal pathways in our plant live imaging system.

As I set up my lab, my students and I collected live movies of wild-type maize cells building their spindles. I told my students and myself that these movies were not the main event, they were just the control cells so we would have a baseline comparison for our experimental conditions. We were so busy collecting data and prepping for our mutant studies that we never really took the time to analyze the wild-type cells. At the surface level, they built spindles and segregated chromosomes in a generally expected amount of time, so we focused on preparing for our upcoming experiments…. then March 2020 occurred.

The pandemic forced us to slow down and look more carefully at our wild-type data, and I am grateful for the detour.

My students headed home for spring break with a warning that there may be a delay in coming back to campus due to the spread of COVID-19. None of us were prepared for the shutdown that followed. Like many colleges and universities, our campus was closed for the remainder of the spring 2020 semester and the summer of 2020. My students and I began meeting on Zoom, trying to make a new plan for our research. The only data we had to work with were the microscopy of wild-type maize cells, so we decided to spend time digging more deeply into these movies. Originally, we had only measured the total time it took to build a spindle as it would be a baseline for comparison to our mutants. We had not looked carefully at any of the intermediate time points in the assembly process. When my students looked more closely at our movies, they discovered that wild-type cells built an incorrectly shaped spindle over 60% of the time!

We found that maize meiotic cells often built spindles with three poles instead of two, and they had to actively rearrange their spindle structure to correct this mistake. We also found that in these cells, there was a delay in meiosis as cells refused to progress until this correction had been made. This is an exciting discovery as it showed that plants are error-prone in their spindle assembly, much like human female meiotic cells. Our findings also suggested that meiotic cells were monitoring their spindle shape when determining if they should move forward in meiosis. Previous work has shown that cells monitor the attachment of chromosomes to the spindle to make this decision, but our work adds a new dimension, showing that they also monitor spindle shape. As we continued to analyze our videos, we also learned that cells corrected their spindle morphology in a predictable way. They always collapsed the two poles that were closest together, creating a single pole and resulting in a correct bipolar spindle.

My students and I had begun our scientific journey planning to breeze over wild-type cells, moving on to what we envisioned would be a more exciting story of spindle mutants. The pandemic forced us to slow down and look more carefully at our wild-type data, and I am grateful for the detour. I rediscovered my love of closely watching flickering green fluorescent lights, the dance of microtubules sliding into place or making missteps and shuffling into new arrangements. Watching life attempt a complicated process, make mistakes, and try again, is a lesson that never grows old. It reminds me that our scientific journeys are just the same, they start in one direction but are fluid and constantly changing, and hopefully, they end with a functional spindle!

Read the Published Paper

Weiss, J.D., McVey, S.L., Stinebaugh, S.E., Sullivan, C.F., Dawe, R.K., and N.J. Nannas. 2022. Frequent spindle errors require structural rearrangement to complete meiosis in Zea mays. International Journal of Molecular Sciences, 23 (8):4293–4312.

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Natalie Nannas

Hamilton College

Natalie Nannas investigates how genetic information, packaged into chromosomes, is properly segregated when cells divide in meiosis, the specialized division that creates egg and sperm. Her research focuses on how the spindle machinery is assembled, how chromosomes are properly attached via kinetochores, and how this process is safeguarded as errors can lead to miscarriage, infertility, and genetics disorders. She has published articles in Genetics, PLoS Genetics, Molecular Biology of the Cell, Journal of Cell Science, Plant Cell, Proceedings of the National Academy of Science, Frontiers in Plant Science and Current Protocols in Plant Biology. She is the recipient of multiple grants from the National Science Foundation. Nannas earned a master's and doctorate from Harvard University in molecular biology and genetics. After her graduate work, she was a National Science Foundation Postdoctoral Fellow in Plant Genomics at the University of Georgia, studying mechanisms of meiotic chromosome segregation.