JEng Hughes, an endocrinologist at Washington University in St. Louis, was working late in the lab recently, imaging cilia in mouse pancreatic beta cells. These cilia, which are tiny hair-like organelles, were thought to be static sensors that help the pancreas manage blood sugar, but nonmotile cilia in general are poorly characterized in comparison to their mobile and mobile counterparts. Thus, Hughes’ objective was to observe and record the distribution of these “primarycilia within the organ’s well-defined clusters of cells, called islets. Then she saw one of them move.
“I didn’t believe it at first,” says Hughes. She had stayed up late working on her microscopy, she explains, so “I thought I was just tired. These things weren’t supposed to move.
I thought I was just tired. These things weren’t supposed to move.
—Jing Hughes, Washington University in St. Louis
Intrigued, Hughes and his colleagues imaged pancreatic cilia in many different conditions, observing the same movement over and over again, the team reports (September 23) in Nature. The study marks the first time scientists have suggested that pancreatic cilia – in fact any primary cilia – can move due to the force generated inside the structures. This active movement, says Hughes, also has an important role in regulating insulin secretion.
The findings shocked the scientific community, says Hughes The scientist. The team’s close examination of the organelles revealed that they were something of a hybrid between primary and motile cilia with molecular and structural aspects of both, upending the long-standing binary sorting of cilia. “We received criticism from reviewers. . . . Many of them challenged us to really defend our definition of this hybrid eyelash. [by asking] “How do you know it’s not an accident?” she adds.
Ron Orbach, a biologist from Yale University who was not involved in the study, also admits to being surprised. “You have apples, you have oranges. Those are two different things. But now we see that there is also something in between,” he says.
After his overnight observation, Hughes and his colleagues began their investigation by imaging primary cilia in human and mouse pancreatic islets. Each cilia is usually arranged in a specific arrangement called “9+0”, where nine fused pairs of microtubules form a hollow cylinder. However, again to the scientists’ surprise, the cilia of pancreatic cells deviated from this expected arrangement and had eight outer microtubule doublets and a central microtubule doublet or singlet.
The researchers also used immunofluorescence microscopy to visualize proteins on live beta cells. They observed that the cilia contain motor proteins that are responsible for active movement in so-called motile cilia, the type known for wiggling and only seen in the lungs, middle ear and airways. “It was another big surprise. We thought we might find one or two [motor proteins]. In fact, we found a whole bunch of them,” says Hughes.
When Hughes and his team eliminated these proteins through targeted genetic deletion, cilia movement of beta cells ceased. And when the team exposed the motor protein-deficient beta cells to a bolus of glucose to trigger insulin secretion, they observed that a key step in this response – the influx of calcium – was delayed. This told the researchers that motor proteins were necessary for cilia movement and that these organelles did not move passively in response to the flow of fluids around them. Instead, they were looking at a previously unknown type of cilia that could not only sense their environment, but also respond to it by modulating beta cell function.
Although, according to Orbach, there is more work to be done to discover how the structural arrangement of this new type of cilia regulates its movement, he says this study has opened many new doors.
Hughes is equally excited. His group is currently focusing on work that will further prove the functionality of cilia movement inside a live animal model. “I think like anything, it takes a lot of work to build momentum to prove or disprove dogma. . . . I hope a lot of colleagues will join forces and start looking into this issue,” she says.