The Cilium: Not Just a Receiver, but a Transmitter, Too
Most cells have a cilium, a small hair-like projection from the cell membrane. Some cilia are motile, like those in the respiratory tract that sweep away dust, but many are stationary. All cilia (sometimes called flagella, though different from the bacterial flagellum) use “inter-flagellar transport” (IFT), described in Michael Behe’s book The Edge of Evolution -- a system of “ore cars” that move materials up to the tip and remove waste. Non-motile cilia have been known to act as “antennae” for the cell, sensing the external environment and sending signals into the cell.
Now researchers have found that the cilium can transmit, too. A paper in Current Biology describes how cilia on Chlamydomonas send out packages, called ciliary ectosomes, that release enzymes that can alter the extracellular matrix. That adds a secretory function as to the cilium’s well-known sensory function. The paper, “The Cilium Secretes Bioactive Ectosomes,” states, “Our results suggest that cilia may be an underappreciated source of bioactive, extracellular membrane vesicles.” (Emphasis added.)
In the same issue of Current Biology, Prachee Avasthi and Wallace Marshall comment on this finding. In “Ciliary Secretion: Switching the Cellular Antenna to ‘Transmit,’” they remark on the growing repertoire of these organelles:
Cilia are microtubule-based protrusions of the plasma membrane that were first noticed for their role in generating fluid flow, such as the flow of mucus in the airway. In recent decades, it has become clear that cilia also have important sensory roles and act as antennae, sensing the cell's environment: for example, kidney cilia can transduce calcium signals mediated by mechanosensitive channels sensing fluid flow; photoreceptor cilia capture light and transduce visual signals to electrical signals via the G-protein-coupled receptor (GPCR) rhodopsin; and cilia from olfactory sensory neurons can detect and transduce odor stimuli also via specialized GPCRs. In addition, cilia can play roles in processing signals within cells; for example, developmental patterning of vertebrate limbs is regulated by ciliary transport of Hedgehog signaling components. Given the varied functions of ciliary signaling, defects in conserved ciliary structure often result in disorders with seemingly unrelated pleiotropic phenotypes. A new finding reported in a recent issue of Current Biology by Wood et al. reveals an interesting twist on the signaling roles of cilia, by showing that the motile flagella of the unicellular green alga Chlamydomonas can release biochemical signals into the extracellular environment via membrane budding of enzyme-containing ciliary ectosomes. If anyone still doubted the importance of the cilium in essential cellular functions, this new demonstration of the multitasking abilities of this nearly ubiquitous organelle should convince them otherwise.
Since many of the initial discoveries about cilia were made with these simple blue-green algae, but were later found to apply to cilia in vertebrates, “We thus have every reason to expect that the ability of cilia to function as secretory organelles is likely to hold true in mammalian cilia.” In fact, there appears to be such a function occurring in the retina of the eye, and another in the kidneys. It is also well known that defects in ciliogenesis often have wide-ranging, pleiotropic effects. The commentary concludes:
Ciliary secretion might also modulate internal ciliary signaling via sensitization, habituation or extrusion of unwanted or recycling material, as is the case for photoreceptor cilia. Considering all these possible functions, ciliopathies may turn out to involve alterations in signals transmitted from cilia, not just in signals they receive.
And that’s not all. Cilia are a virtual grand central station for proteins, according to Johns Hopkins School of Medicine. Cilia seemed too small and narrow to admit many proteins (after all, they represent 1/10,000th the volume of the cell). Now, however, it appears that 90 percent of mammalian proteins can fit inside, including ones ten times larger than previously thought. Most proteins probably “wander in” at some point, although only certain ones are retained within the cilium. The discovery means, though, that the functional roles of cilia are probably greater than realized: “Figuring out how cilia select their captives is a question for another study.”
If intelligent design is a productive scientific theory, we would expect systems judged irreducibly complex to continue to exhibit design in deeper and more profound ways, and to provoke additional questions for research. In the case of cilia, that’s exactly what is happening.