“Chlamydomonas reinhardtii as a model to study cilia-related disease”

Abstract: Cilia and eukaryotic flagella are slender cell projections with motile and sensory functions. They lack ribosomes and all ciliary building blocks need to be imported posttranslationally from the cell body. This task involves intraflagellar transport (IFT), a motor-based protein shuttle. Using single particle imaging in the unicellular alga Chlamydomonas reinhardtii, we determined key aspects of ciliary protein transport such as cargoes and unloading sites. The transport frequencies of structural proteins such as tubulin are upregulated when a cilium is too short revealing that cells sense the length of their cilia and adjust the cargo load of IFT, accordingly. In mammals, cilia malfunction leads to a plethora of diseases, named ciliopathies. Many of the disease-related proteins are conserved in protists. Biochemical analyses of isolated Chlamydomonas cilia and live imaging revealed that the BBSome, an octameric protein complex, is an adapter mediating the export of certain signaling proteins from cilia by IFT. This led to the concept that Bardet-Biedl syndrome (BBS, characterized by obesity, kidney anomalies, polydactyly and blindness) results from the abnormal accumulation of (signaling) proteins in cilia. One of the most common inherited single-gene, life-threatening disorders is autosomal dominate polycystic kidney disease (ADPKD), which affects ~1:1,000 adults. ADPKD results from mutations in the TRP cation channel PKD2, but the role of PKD2 in cilia remains unclear. In Chlamydomonas, PKD2 anchors the mastigonemes, large extracellular glycoprotein polymers, to the ciliary membrane. The PKD2-mastigoneme complexes are arranged in two rows along the axoneme positioning them perpendicular to the plane of the ciliary beating. Association with extracellular components, the cytoskeleton or both is characteristic for many mechanically gated channels in eukaryotes. We proposed that pull on these polymers during bending of the axoneme could generate a force to open the PKD2 channel.