Cystic kidney diseases: all roads lead to the cilium.

Cystic kidney disorders are one of the leading causes of end-stage renal disease. Numerous experimental animal models have been used to understand the disease pathogenesis. Recent advancements in this field have provided a surprising finding: that many of the proteins associated with cystic kidney disease localize to a nearly forgotten organelle, the primary cilium.

A novel dynein light intermediate chain colocalizes with the retrograde motor for intraflagellar transport at sites of axoneme assembly in chlamydomonas and Mammalian cells.

The assembly of cilia and flagella depends on bidirectional intraflagellar transport (IFT). Anterograde IFT is driven by kinesin II, whereas retrograde IFT requires cytoplasmic dynein 1b (cDHC1b). Little is known about how cDHC1b interacts with its cargoes or how it is regulated. Recent work identified a novel dynein light intermediate chain (D2LIC) that colocalized with the mammalian cDHC1b homolog DHC2 in the centrosomal region of cultured cells. To see whether the LIC might play a role in IFT, we characterized the gene encoding the Chlamydomonas homolog of D2LIC and found its expression is up-regulated in response to deflagellation. We show that the LIC subunit copurifies with cDHC1b during flagellar isolation, dynein extraction, sucrose density centrifugation, and immunoprecipitation. Immunocytochemistry reveals that the LIC colocalizes with cDHC1b in the basal body region and along the length of flagella in wild-type cells. Localization of the complex is altered in a collection of retrograde IFT and length control mutants, which suggests that the affected gene products directly or indirectly regulate cDHC1b activity. The mammalian DHC2 and D2LIC also colocalize in the apical cytoplasm and axonemes of ciliated epithelia in the lung, brain, and efferent duct. These studies, together with the identification of an LIC mutation, xbx-1(ok279), which disrupts retrograde IFT in Caenorhabditis elegans, indicate that the novel LIC is a component of the cDHC1b/DHC2 retrograde IFT motor in a variety of organisms.

Identification of CHE-13, a novel intraflagellar transport protein required for cilia formation.

Cilia are present on cells of many eukaryotic organisms and recent data in the mouse suggest that ciliary defects can cause severe developmental abnormalities and disease. Studies across eukaryotic systems indicate that cilia are constructed and maintained through a highly conserved process termed intraflagellar transport (IFT), for which many of the proteins involved have yet to be identified. IFT describes the movement of large protein particles consisting of an A and a B complex along the cilia axoneme in anterograde and retrograde directions. Herein we describe a novel C. elegans gene, F59C6.7/9, that is required for cilia assembly and whose function is disrupted in che-13 ciliogenic mutants. As previously shown for all IFT complex B genes identified to date, expression of che-13 (F59C6.7/9) is regulated by the RFX-type transcription factor DAF-19, suggesting a conserved transcriptional pathway in ciliogenesis. Fluorescent-tagged CHE-13 protein concentrates at the base of cilia and moves along the axoneme as expected for an IFT protein. Furthermore, loss of che-13 differentially affects the localization of two known IFT complex B proteins, OSM-5 and OSM-6, implying that CHE-13 functions as part of this complex. Overall, our data confirm that CHE-13 is an IFT protein and further that the IFT particle assembles in an ordered process through specific protein-protein interactions.