Spring cleaning cells take out the trash before they divide

Before cells divide by undergoing mitosis, they do a little bit of a clean-up and toss out molecules they no longer need, according to a new study.

Scientists have used a new method that measures the dry mass of cells – the weight of its contents not including water – to find that cells lose about 4% of their mass as they enter cell division.

They believe that this cellular spring clean helps cells give their offspring a “fresh start” by getting rid of the accumulated junk of the parent cell – like toxic by-products.

“Our hypothesis is that cells might be throwing out things that are building up, toxic components or just things that don’t function properly that you don’t want to have there,” says lead author Teemu Miettinen, a research scientist in the Centre for Precision Cancer Medicine at Massachusetts Institute of Technology in the US.

“It could allow the newborn cells to be born with more functional contents.”

Losing baggage before mitosis

The team looked at cells undergoing mitosis – the process through which a single cell divides into two identical daughter cells – to study what happens to cell mass and composition.

They measured three types of cancer cells (which are easier to study than healthy cells because they divide more frequently) and were surprised to find that the dry mass of cells actually decreases when they enter the cell division cycle, with cells then regaining the lost mass after division is complete.

The researchers found that this occurs because cells ramp up the activity of a process called lysosomal exocytosis as they enter mitosis. Lysosomes are intracellular organelles that enclose and breakdown or recycle cellular waste products, and exocytosis is the process they use to dump unneeded molecules outside of the cell. 

The researchers also found that the density of the dry mass of cells increased as the cells lost dry mass, leading them to believe that the cells are losing low-density molecules such as lipids or lipoproteins during this process.

“What we are seeing is that cells might be trying to throw out damaged components before dividing,” says Miettinen.

Implications for disease

The authors think that their research could help explain why neurons, which do not divide, are more likely to accumulate toxic proteins such as tau or amyloid beta, which are linked to the development of Alzheimer’s disease.

And because cancer cells can expel some chemotherapy drugs using exocytosis and become resistant to them, in theory, preventing this from occurring before cell division could make cancer cells more susceptible.

“There are diseases where we might want to upregulate exocytosis, for example in neurodegenerative diseases, but then there are diseases like cancer where maybe we want to dial it down,” Miettinen explains. “In the future, if we could better understand the molecular mechanism behind this and find a way to trigger it outside of mitosis or prevent it during mitosis, we could really have a new toggle to use when treating disease.”

The research has been published in the journal eLife.

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