Inside the human airway, a certain cell type reigns supreme: multiciliated cells, decorated with dozens of hair-like cilia all beating in tandem. These cells are responsible for clearing out foreign bacteria and viruses.
“They break the normal architecture since almost all cells in your body have zero or one cilia each,” said Jeremy Reiter, a developmental geneticist at the University of California, San Francisco. “And whenever an individual cell type does something cool or unexpected, it’s an interesting subject for figuring out how that happens.”
Scientists have long been curious about how cells become multiciliated. Other researchers had previously discovered that several genes that were strongly linked to the canonical cell cycle were reused during the differentiation process for multiciliated cells.1 Reiter and his team wanted to find out how exactly the cell cycle might be related to differentiation in these special cells. In work recently published in Nature, the team found that these multiciliated cells actually leveraged a previously unknown variant of the cell cycle that they named the “multiciliation cycle.”2 This research, scientists said, could be useful for better understanding processes like cancer in which the cell cycle also goes awry.
To better understand how the multiciliation cycle worked, the scientists first identified changes in gene expression as the cells differentiated from stem cells into multiciliated cells. “The classic mitotic kinases that regulate division normally were expressed really nicely and sequentially during differentiation of multiciliated cells,” said Semil Choksi, a coauthor and postdoctoral researcher in Reiter’s lab. This sequential expression followed the phases of the traditional cell cycle: G0/G1 (the cell grows larger), S (the cell replicates its DNA), and G2/M (the cell prepares for division and then splits).
While the multiciliation cycle followed the transcriptional phases of the traditional cell cycle, DNA replication did not occur and the cells did not end up dividing. Instead, they grew a bunch of cilia. To figure out what might be driving these differences, the scientists sifted through the genes that were differentially expressed between this variant cell cycle and the traditional cell cycle.
A super-resolution micrograph depicts two airway cells stained for centrioles (magenta) and cilia (cyan). The primary ciliated cell (left) is a precursor cell, in which the multiciliation cycle is initiated. The differentiated cell (right) has hundreds of centrioles and cilia as a result of the multiciliation cycle.
Semil Choksi
They found that one factor, called E2F7, was expressed at higher levels in the multiciliation cycle. When the team knocked out E2F7 in mice, they found that the multiciliated cells in these mice had significantly increased markers of DNA synthesis, indicating that the multiciliation cycle in these mutated cells had shifted to be more like the canonical cell cycle.
Interestingly, these E2F7 knockout mice also had hydrocephalus, an abnormal buildup of fluid in the brain. This was due to the dysfunction of multiciliated cells in the brain, which were unable to adequately clear out fluid. When the scientists examined all the multiciliated cells in more detail, they found that the cells had fewer cilia. And instead of the neat phases of sequential gene expression found in normal multiciliated cell differentiation, the cell cycle-related genes expressed by these mutated cells melded into each other, without a distinct transcriptional change going from the S phase into the G2/M phase.
These findings demonstrate the importance of E2F7 as one of the key factors driving the multiciliation cycle. Reiter analogizes these various versions of the cell cycle to clocks. “I think that the multiciliated cell cycle gives us a different type of clock, one where the gears—or the fundamental mechanism underlying the progression—is very similar to the canonical cell cycle,” he said. “But the output is different.”
“This is a really beautiful piece of work,” said Cayla Jewett, a multiciliated cell researcher at Johns Hopkins University. She is curious about whether, beyond E2F7, the other genes that differ between this variant and the canonical cell cycle are functionally relevant. Better understanding how multiciliated cells form, she said, could provide insight into diseases like hydrocephalus that might be caused by dysfunction of these cells.
Jewett isn’t the only one interested in the nuances of multiciliated cells. So is Jacques Serizay, a biologist at the Pasteur Institute. Serizay and his colleagues recently posted a preprint that also outlined this variant cell cycle in multiciliated cells.3 While they found a similar cyclic gene expression pattern that drove the differentiation of these multiciliated cells, they focused on another group of differentially expressed genes called cyclins. When some of the missing cyclins were re-expressed, they partially restored cell cycle processes. “The idea of leveraging the cell cycle machinery for differentiation is quite striking,” Serizay added.
For Choksi, Reiter, and the rest of their team, the unearthing of this variant cell cycle gives rise to the possibility that there might be other such variants that are yet to be discovered—a probable multiverse of cell cycles. “Maybe lots and lots of cells throughout the body use a variant cell cycle to drive differentiation,” said Choksi. Finding these variants, he added, could provide important insights into how cells all over the body grow and develop.