The researchers believe that this practice of emptying the trash helps cells give their offspring a “clean slate” — without the trash accumulated by the mother cell.
“Our hypothesis is that cells may be throwing away stuff that’s accumulating, toxic components or just stuff that isn’t functioning properly that you don’t want to be there,” said Teemu Miettinen, a research scientist at MIT and lead author of the new study. It allows new cells to be born with more functional content.”
Scott Manalis, the David H. Koch Professor of Engineering in the Departments of Bioengineering and Mechanical Engineering and a member of the Koch Institute for Integrative Cancer Research, is the senior author of the research paper, published May 10 in eLife. Kevin Ly and Alice Lam, undergraduates in MIT’s Department of Bioengineering, are also authors of the paper.
Measuring the dry mass of a cell — the weight of its contents excluding water — is usually done using a microscopy technique called quantitative phase microscopy. While this technique can measure cell growth, it cannot reveal information about the molecular content of dry mass, and it is difficult to use for cells grown in suspension.
Manalis’ lab had previously developed a technique to measure the buoyant mass of cells, which is how much they float in a liquid such as water. The method measures buoyant mass by flowing cells through a channel embedded in a vibrating cantilever, which can be repeated to track how the mass of a particular cell changes over many hours or days.
The researchers hope to adapt the technique so that it can be used to calculate the dry mass of cells and the density of dry mass. In fact, about 10 years ago, they discovered that if they measured cells in normal water and then in heavy water (containing deuterium instead of normal hydrogen), they could calculate the dry mass of the cells. These two measurements can be used to calculate the dry mass of the cells.
However, heavy water is toxic to cells, so they could only make one measurement per cell. Last year, Miettinen began investigating whether he could design a system that could measure cells repeatedly and minimize heavy water exposure.
In the system he came up with, as cells flowed through microfluidic channels, they were exposed to heavy water very briefly. Because cells can completely exchange their water content in just one second, the researchers can measure the mass of the cells when they are filled with heavy water and compare it to the mass in normal water, then calculate the dry mass.
“The idea is that if we minimize the exposure of cells to heavy water, we can design this system so that we can repeat this measurement over long periods of time without harming the cells,” Miettinen said. “This allows us, for the first time, to not only be able to Tracking the dry mass of cells — which is what others have done using microscopy, but also tracking the density of dry mass, gives us an understanding of the biomolecular makeup of cells.”
The researchers say their dry mass measurements are qualitatively consistent with previous work using quantitative phase microscopy. And in addition to providing the density of dry matter, the MIT team’s method enables higher temporal resolution, which has proven to be very useful for revealing dynamics during mitosis (cell division).
throw away the trash
In cells undergoing mitosis, the researchers used their new technique to study what happened to cell mass and composition during the process. In a 2019 paper, Miettinen and Manalis found that buoyant mass increases slightly as mitosis begins. However, other studies using quantitative phase microscopy have shown that cells may retain or lose dry mass early in cell division.
In the new study, the MIT team measured three types of cancer cells. Because these cells divide more frequently than healthy cells, they are easier to study. To the researchers’ surprise, they found that the dry mass of the cells actually decreased as the cells entered the cell division cycle. This mass was later restored before the split was complete.
Further experiments showed that when cells entered mitosis they increased the activity of a process called lysosome extravasation. Lysosomes are organelles that break down or recycle cellular waste, while exocytosis is the process they use to throw away any molecules that are no longer needed.
The researchers also found that as cells lost dry matter, the density of dry matter increased, leading them to believe that cells were losing low-density molecules such as lipids or lipoproteins. They hypothesized that cells use this process to remove toxic molecules before dividing.
The researchers speculate that their findings may help explain why neurons that don’t divide are more likely to accumulate toxic proteins such as tau or amyloid beta, which have been implicated in the development of Alzheimer’s disease.
In addition, the findings may also be related to cancer. Cancer cells can use extracellular extravasation to expel some chemotherapy drugs, helping them become resistant to the drugs. In theory, blocking the production of exosomes before cells divide could help make cancer cells more susceptible to such drugs.
“In some diseases, we might want to increase exocrine secretion, like in neurodegenerative diseases, but there are also diseases like cancer, where we might want to turn it down. In the future, if we can better understand what’s behind this Molecular mechanisms and finding a way to trigger it outside of mitosis or prevent it during mitosis, then we could really use a new switcher in the treatment of disease,” Miettinen said.