For all animals, the removal of certain cells is a necessary part of embryonic development. Living cells are also naturally eliminated in mature tissues; for example, the lining of the intestine turns over every few days.
One way that organisms get rid of unnecessary cells is through a process called extrusion, which allows cells to be pulled from a layer of tissue without disrupting the layer of cells left behind. MIT biologists have now discovered that this process is triggered when cells are unable to replicate their DNA during cell division.
The researchers discovered this mechanism in the C. elegans worm, and they showed that the same process can be carried out by mammalian cells; they believe that extrusion can serve as a way for the body to eliminate cancerous or precancerous cells.
“Cell extrusion is a mechanism of cellular elimination used by organisms as diverse as sponges, insects and humans,” says H. Robert Horvitz, David H. Koch professor of biology at MIT, member of the McGovern Institute for Brain Research and the Koch Institute for Integrative Cancer Research, a Howard Hughes Medical Institute researcher and lead author of the study. “The discovery that extrusion is driven by DNA replication failure was unexpected and offers a new way of thinking and possibly intervening in certain diseases, in particular cancer.”
MIT post-doctoral fellow Vivek Dwivedi is the lead author of the article, which appears today in Nature. The other authors of the article are Carlos Pardo-Pastor, researcher at King’s College London, Rita Droste, research specialist at MIT, Ji Na Kong, graduate student of MIT Nolan Tucker, scientist at Novartis and former post-doctoral fellow at MIT Daniel Denning, and professor of biology at King’s College London. Jody Rosenblatt.
Stuck in the cell cycle
In the 1980s, Horvitz was one of the first scientists to analyze a type of programmed cell suicide called apoptosis, which organisms use to shed cells that are no longer needed. He made his discoveries using C. elegans, a tiny nematode that contains exactly 959 cells. The developmental lineage of each cell is known and embryonic development follows the same pattern each time. Throughout this developmental process, 1,090 cells are generated and 131 cells undergo programmed cell suicide by apoptosis.
Horvitz’s lab later showed that if the worms were genetically mutated so that they could not kill cells by apoptosis, a few of those 131 cells would instead be eliminated by cell extrusion, which appears to be able to serve safeguard mechanism for apoptosis. How this extrusion process is triggered, however, has remained a mystery.
To unravel this mystery, Dwivedi performed a large-scale screening of over 11,000 C. elegans genes. One by one, he and his colleagues reversed the expression of each gene in the worms that could not perform apoptosis. This screen allowed them to identify genes that are essential to activate cell extrusion during development.
To the researchers’ surprise, many genes that were needed for extrusion were involved in the cell division cycle. These genes were mainly active during the early stages of the cell cycle, which involves the initiation of the cell division cycle and the copying of the cell’s DNA.
Further experiments revealed that cells that are ultimately extruded initially enter the cell cycle and begin to replicate their DNA. However, they seem to get stuck in this phase, which leads them to be extruded.
Most of the cells that end up being extruded are unusually small and are produced from patchy cell division which results in one large daughter cell and a much smaller one. The researchers showed that if they interfered with the genes that control this process, so that the two daughter cells were closer to the same size, the cells that would normally have been extruded were able to successfully complete the cell cycle and n ‘have not been extruded.
Researchers have also shown that the failure of very small cells to complete the cell cycle stems from a shortage of proteins and DNA building blocks needed to copy DNA. Of the other key proteins, cells probably don’t have enough of an enzyme called LRR-1, which is essential for DNA replication. When DNA replication stops, the proteins responsible for detecting replication stress quickly stop cell division by inactivating a protein called CDK1. CDK1 also controls cell adhesion, so the researchers hypothesize that when CDK1 is deactivated, cells lose their adhesion and break off, leading to extrusion.
Horvitz’s lab then teamed up with researchers from King’s College London, led by Rosenblatt, to determine if the same mechanism could be used by mammalian cells. In mammals, cell extrusion plays an important role in replacing the lining of the intestines, lungs, and other organs.
Researchers used a chemical called hydroxyurea to induce DNA replication stress in canine kidney cells grown in cell culture. The processing quadrupled the extrusion rate, and the researchers found that the extruded cells reached the phase of the cell cycle where DNA is replicated before being extruded. They also showed that in mammalian cells, the well-known cancer suppressor p53 is involved in initiating the extrusion of cells undergoing replication stress.
This suggests that in addition to its other cancer-protective roles, p53 may help eliminate cancerous or precancerous cells by forcing them to extrude, Dwivedi says.
“Replication stress is one of the characteristics of precancerous or cancerous cells. And what this finding suggests is that extruding cells that are under replication stress is potentially a tumor suppressor mechanism, ”he says.
The fact that cell extrusion is seen in so many animals, from sponges to mammals, has led researchers to hypothesize that it may have evolved as a very early form of cell elimination which was later supplanted by programmed cell suicide involving apoptosis.
“This cellular elimination mechanism depends only on the cell cycle,” explains Dwivedi. “It does not require any specialized mechanism like that required for apoptosis to eliminate these cells, so what we have proposed is that it could be a primordial form of cellular elimination. This means that it may have been. one of the first ways the cell eliminates occurs because it depends on the same process an organism uses to generate many more cells. ”
Dwivedi, who received his doctorate from MIT, was a Khorana scholar before entering MIT for graduate studies.
This research was supported by the Howard Hughes Medical Institute and the National Institutes of Health.