Normal cilia beat smoothly (top), but hydin-lacking cilia (bottom) are stiff.
Beating cilia circulate cerebrospinal fluid through the brain. If they falter, fluid can accumulate, causing the brain-damaging condition hydrocephalus. Mice carrying mutations in the gene for the protein hydin develop hydrocephalus, but it wasn't known why. Last year, the researchers discovered a clue by finding that the absence of hydin caused flagella in the protist Chlamydomonas to freeze up. Now the team has observed a similar effect in the cilia of mice.
Cilia from mice with Hydin mutations beat abnormally. Instead of showing a smooth back-and-forth movement, the cilia merely vibrated. They also beat more slowly and often stopped. Though the precise function of the hydrin protein is unclear, it is known to be part of the cilia's axoneme. Indeed, the Hydin mice also had a subtle structural defect in their cilia—a knob on one of the central microtubules was absent.
The authors believe that this defect might cause a lack of coordination among the dynein motors that move cilia. Normally, motors on one side of the shaft kick into gear to produce the forward stroke. Then they flip off and motors on the opposite side turn on, producing the return stoke. But the researchers suspect that this switch doesn't occur in hydin-lacking cilia, causing the shaft to relax prematurely. The results predict that humans with defects in hydin will develop hydrocephalus and primary ciliary dyskinesia, an inherited disorder caused by defective airway cilia. 
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