First Person is a series of interviews with the first authors of a selection of papers published in Journal of Cell Science, helping researchers promote themselves alongside their papers. Julia Spear is first author on ‘ Single-cell intracellular pH dynamics regulate the cell cycle by timing the G1 exit and G2 transition’, published in JCS. Julia is a PhD student in the lab of Katharine White at the Harper Cancer Research Institute, University of Notre Dame, South Bend, IN, USA, investigating single-cell intracellular pH dynamics during the cell cycle and intracellular pH heterogeneity in diseases such as cancer.

Julia Spear

How would you explain the main findings of your paper in lay terms?

Intracellular pH (pHi) regulation is important for normal cell health and behaviors. Normal cells have a pHi of 7.0–7.2, but temporary changes in pHi (pHi 6.8–7.6) are required for behaviors such as cell movement, cell division and programmed cell death. However, we don't know how pHi varies across a population of cells or how pHi changes over time in single cells during division or migration, which can take hours to days to complete. We investigated how pHi changes as a cell grows, replicates its DNA, aligns and separates the duplicated DNA, and divides into two daughter cells: collectively called the cell cycle. Using time-lapse microscopy and a fluorescent biosensor for pHi, we found that pHi increases and decreases with distinct cell cycle phases at the single-cell level. Low pHi is required for cells to enter the cell cycle and begin the process of DNA replication, and high pHi is required for cells to successfully divide. When we disrupted these normal pHi changes, we could cause cells to delay or halt cell division. Not only does this research lay a framework for understanding pHi during the entire cell cycle, it also shows that pHi changes might be a cue for certain transitions and cell division. This is particularly important for understanding diseases with dysregulated pHi, such as cancer, where cells are rapidly dividing.

Were there any specific challenges associated with this project? If so, how did you overcome them?

In cells, pH is dynamic, as cells are constantly regulating their pH levels to account for a wide range of behaviors. We had to establish technically challenging microscopy data collection and analysis pipelines to accurately measure pHi in single cells. Another big challenge to my experimentation was the limited range of pHi manipulation techniques. Although our lab is currently developing tools to spatiotemporally raise and lower pHi in single cells, for this paper we used small molecules to manipulate pHi. Unfortunately, these drugs can have off-target effects, which might alter additional factors. To combat this issue, we used multiple manipulation techniques to change pHi and checked for off-target effects (like DNA damage) that are known to affect cell cycle progression for each manipulation technique.

When doing the research, did you have a particular result or ‘eureka’ moment that has stuck with you?

Using time-lapse microscopy, it was amazing to be able to measure pHi in a single cell during an entire cell cycle. When I aligned pHi to cell cycle progression in each cell, we could see trends in the pHi oscillations that correlated with cell cycle phase transitions. This was a really exciting moment, as we also found that dividing cells experienced a rapid acidification during mitosis that had not been shown before. Although we have not examined why this may be occurring, we are excited to use new pHi manipulation tools to see how this acidification is important for successful cell division.

“This was a really exciting moment, as we also found that dividing cells experienced a rapid acidification during mitosis that had not been shown before.”

Why did you choose Journal of Cell Science for your paper?

I have always had a deep appreciation for microscopy, and it was so amazing to learn that JCS started as a journal that ‘encompassed all branches of science related to the microscope’. I was very hopeful to be accepted into this journal and share my beloved images, eager that my work could reach a large audience that also loves analyzing microscopy.

Have you had any significant mentors who have helped you beyond supervision in the lab? How was their guidance special?

My PI, Katie White, has been a significant mentor in my time as a graduate student. This paper started as an idea to measure single-cell pHi in clonal cell lines and investigate their heterogeneity, but she had the idea to try out cell synchronization and study whether cell cycle differences play a role. From that experimental pivot, we were able to turn this paper into the masterpiece it is, and for that I will always be thankful for her guidance.

What motivated you to pursue a career in science, and what have been the most interesting moments on the path that led you to where you are now?

As an undergraduate at a small liberal arts school, I was not exposed to research until the semester before my senior year. But that summer research experience, where I learned about Drosophila genetics, dissections and confocal imaging, really initiated my love for bench science and microscopy. To be able to continue using microscopy in my everyday life is something I am so thankful for, and I hope to continue my passion for it!

What's next for you?

Write my thesis and defend my PhD! After that, if I can find a postdoc that fits my research interests and has a healthy lab environment, I would be open to pursuing academia. If not, industry with a biology focus would be nice as well.

Tell us something interesting about yourself that wouldn't be on your CV

My first job ever was at a soft-serve ice cream store, and if there was a competition for making the tallest ice cream cone, I'm confident I would be placed in the top three!

Julia Spear's contact details: Harper Cancer Research Institute, University of Notre Dame, South Bend, IN 46617, USA.


J. S.
K. A.
Single-cell intracellular pH dynamics regulate the cell cycle by timing the G1 exit and G2 transition
J. Cell Sci