Regulation of Ciliogenesis

Primary cilia are tiny membrane-surrounded protrusions on almost every vertebrate cell. They possess a microtubule-based core with a ninefold symmetry, which is called the axoneme. The cilium arises from the basal body, which is the older of both centrioles. In general, cilia can be subdivided into motile and sensory subtypes, which differ mainly in their structure and function. Sensory cilia, which are also called primary cilia, form invariably as a single cilium per cell. They lack structural features as the central pair of microtubules and dynein arms. In contrast to motile cilia, which produce fluid flow by beating, primary cilia have important roles in detecting chemical or mechanical stimuli. They act as an antenna like unit, which relays and coordinates signaling pathways that are critical in embryonic and postnatal development as well as in tissue homeostasis in adulthood (sonic hedgehog pathway, PDGF receptor; signaling and Wnt signaling). Their physiological significance is further highlighted by a variety of human diseases, which are linked to ciliary defects, like polycystic kidney disease, retinal degeneration, developmental defects, obesity, diabetes and many more.

Ciliogenesis as well as cilia disassembly are processes, which are tightly regulated with the cell cycle. As the centrosome takes part in establishing the spindle poles during mitosis as well as giving rise to cilia, both events need to be temporally separated. Cilia formation occurs during interphase after progression into the G1 phase of the cell cycle, whereas ciliary shortening takes place at mitotic entry or when quiescent cells are stimulated to re-enter the cell cycle.

Ciliogenesis can be subdivided into four distinct stages. First, a Golgi-derived vesicle attaches to the distal end of the mother centriole, while the centrosome is still located deep in the cell close to the Golgi apparatus and the nucleus. At this point, the axoneme begins already to emerge. The attached vesicle becomes invaginated as the centriole extends. In the next step, more vesicles fuse with the membrane covering the elongating axonemal shaft. Third, the membrane-surrounded axoneme migrates towards the apical plasma membrane. When the axoneme reaches the cell surface the ciliary membrane fuses with the plasma membrane. After docking of the mother centriole at the cell surface, the nascent axoneme is elongated by the intraflagellar transport machinery (IFT) to form the mature primary cilium.

Although recent research has begun to shed light on cilia and several proteins involved in ciliogenesis were identified, this process is just beginning to be understood. To identify novel genes involved in ciliogenesis, we are performing a genome-wide RNA interference screen using the human cell line hTERT-RPE1 as a model for primary cilia formation. In addition, we are focusing on centrosomal proteins, which have a known role during ciliogenesis. We mainly use a proteomic approach to find new interaction partners in order to shed light on their still unkown function during this highly complex process.

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