Millions of hair-like structures called cilia move in unison on the inner surface of the lungs to sweep out mucus and prevent respiratory infections. But cilia cover many more animal epithelia - tissues that cover surfaces in the body - and their versatile functions include moving cerebrospinal fluid in the brain and coaxing an egg down a fallopian tube. To push fluid in one direction, cilia move in time, similar to eight rowers in a boat steered by a coxswain. While the mechanisms behind ciliar motility are reasonably well understood, only little is known about how movements are coordinated. When trying to find the coxswain in charge of ciliar movements, Chris Kintner and his team from the Salk Institute for Biological Studies and the University of California, San Diego recently proposed a positive feedback mechanism that ensures cilia row in the same direction.
Cilia are protrusions of a cell's plasma membrane and are supported by longitudinal filaments called microtubules that are arranged in nine pairs around a central pair. Motor molecules, called dyneins, between the microtubule pairs facilitate sliding of the filaments against each other in a process that is dependent on calcium. As the filaments slide, the cilium eventually bends as the ends of the microtubules are anchored inside the cell by a structure called the basal body, which has a foot at its bottom. This foot is always orientated in the direction of the ciliar stroke.
To analyze the coordination of ciliar movements, the team of Californian scientists needed an appropriate experimental system. They took skin samples from frog larvae and placed them in cell culture because each of the skin's epithelial cells are covered with hundreds of cilia. By scoring the orientation of countless basal feet using transmission electron microscopy the scientists observed that all of them roughly pointed to the back of the larvae during early development. A few hours later in development, however, the cilia refined their polarity and precisely aligned on the front-back axis. Interestingly, when the team removed the skin before the front-back axis had established and transferred it to the cell culture, the cilia failed to align. The latter finding suggested that the cilia of the prospective skin miss some signal that helps to `agree' on a common orientation.
To further analyze ciliar orientation, the team took advantage of known genetic defects that affect ciliar motility and cause primary ciliary dyskinesia (PCD), a rare congenital disease that impairs ciliary flow and causes persistent respiratory infections, infertility, ear inflammation or fluid accumulation in the brain. After the team blocked the expression of these genes in the frog embryo, the cilia didn't work properly, which is typical in PCD patients. Although the cilia's basal feet still pointed towards the back of the larvae, they could not generate a detectable flow on the skin's surface and the cilia also didn't reorient during development. The team suspected that the initial fluid flow causes the cilia to refine their alignment and decide on a common direction. To prove this hypothesis the scientists constructed a flow chamber, in which they traced the ciliar orientations in response to different external flow. Indeed, the cilia could actively sense and respond to externally applied fluid flow by changing their orientation.
By showing that the flow itself influences ciliar orientation, Kintner and his team have identified a type of coxswain determining the polarity of the strokes. The cilia use the flow to self-correct the polarity and motion in a positive feedback mechanism. Although it is unclear how the cilia sense and respond to the flow, the finding might yet deliver new insights into the molecular details of PCD.