Multiciliated Cell Differentiation Regulated by an Alternative Cell Cycle

A fertilized egg develops into a baby through cell division, while a malignant cell can become a tumor. Due to its importance, nature tightly regulates this process via the cell cycle, a mechanism scientists once believed they fully understood.

 Researchers at UC San Francisco have shown that cells can produce cilia, which are projections that resemble hair, by using the cell cycle.

The cell cycle has been studied intensively for decades, and here we have found it working in a new way, this old dog – the cell cycle – is capable of more tricks than we realized.”

Jeremy Reiter, PhD, Study Senior Author and Professor, Department of Biophysics and Biochemistry, University of California San Francisco

Breaking a Rule that Prevents Cancer

Human health depends on multi ciliated cells. The cilia in the lungs oscillate to prevent the buildup of fluids like mucus. They also assist the fallopian tubes in carrying eggs into the uterus inside the reproductive system and clear waste from the brain's cerebrospinal fluid. Serious diseases result from their dysfunction.

Reiter and his colleagues employed a method known as single-cell RNA sequencing to determine which genes activated and inactivated in individual multiciliated cells inside the lungs, gaining insight into their development.

In an attempt to get a peek at the genetic instructions required for cilia growth, they caught them at various stages of maturity. What they discovered resembled the cell cycle.

Previous studies had discovered that centrioles, which attach the two sets of chromosomes during cell division, and a small group of cell cycle proteins known as cyclins were active during the formation of cilia.

Even though the lung cells weren't proliferating, Reiter’s team discovered that a large number of cell cycle genes—far beyond simply the cyclins—were expressed at high levels in the cells.

In developing multiciliated cells, we saw the same sequential expression of cell cycle regulators, like cyclins and CDKs, that we would expect to see in stem cells.”

Semil Choksi, PhD, Study First Author and Researcher, University of California San Francisco

This was obviously not the normal cell cycle. First of all, compared to the four centrioles produced during cell division, this alternative cell cycle, or “multiciliation cycle,” as the scientists called it, was creating an abnormally large number of centrioles.

Choksi added, “If you have something go wrong in the cell cycle, and you make too many centrioles, it can lead to cancer, somehow, this strict cancer-preventing rule, no more than four centrioles per cell, is very specifically broken in multiciliated cells to make hundreds of centrioles.”

The Cellular Orchestra Plays Something New

Choksi and Reiter investigated how the multiciliation cycle in lung cells differs from the classic cell cycle in dividing stem cells, gene by gene. A specific gene, E2F7, stood out. Its expression was moderate in stem cells but significantly higher in mature, multiciliated cells.

Indeed, when E2F7 was entirely inhibited or knocked out in an animal model, multiciliated cells could not grow properly, causing issues in the brain.

Reiter stated, “We thought one of the knobs that evolution might have turned was by upregulating E2F7, to change the canonical cell cycle into the multiciliation cycle.”

The researchers subsequently discovered that multiciliated cells lacking E2F7 started producing additional DNA, a sign of cell division. Hundreds of centrioles, which were expected to be used to form cilia at the cell surface, got stuck in the cell body.

If the cell cycle were a specific score performed by a molecular orchestra, then E2F7 would be the new conductor, leading the same instruments to play a new tune, the multiciliation cycle.

Reiter concluded, “Evolution clearly has adapted the cell cycle to carry out a variety of cellular projects well beyond cell division, it will be exciting to see what else it is capable of.”