Many environmental signals are detected by specialized sensory neurons, which have cilia extending from the cell surface as long appendices and exposed to the environment. Cilia consist of a microtubular axonemal core surrounded by a membrane and are anchored in the cell by the basal body. The end of the basal body and the beginning of the axoneme are called the transition zone. Since cilia do not have the capacity to synthesize proteins, all components, both structural and signaling molecules, need to be transported into and out of the cilia. This is probably achieved by a process called intraflagellar transport (IFT) and is driven by three motors in Caenorhabditis elegans. Two motors, kinesin-II and OSM-3 kinesin, are used for anterograde transport from the base of the cilium to the distal tip. Both these kinesin motor complexes are involved in the transport in the first 4 µm of the cilium, called the middle segment, whereas only OSM-3 kinesin is required for the transport in the last 2,5 µm of the cilium until the distal tip, called the distal segment. Retrograde transport from the distal tip back to the basal body is dependent on the dynein motor complex. IFT is not only responsible for the transport of structural components but recently three signalling molecules have been described to be transported in the cilia i.e. OSM-9, a transient receptor potential vanilloid channel involved in sensory signal transduction, a PKD associated protein called qilin and Smoothened, a plasma membrane protein involved in hedgehog signaling.

G. Jansen (Gert) , F.G. Grosveld (Frank)
Center for Biomedical Genetics, Grosveld, Prof. Dr. F.G., Koninklijke Nederlandse Akademie van Wetenschappen (KNAW), Nederlandse Vereniging voor Wetenschappelijk Onderzoek (NWO)
Erasmus University Rotterdam
Erasmus MC: University Medical Center Rotterdam