Forests of cilia wave fluids in desired directions. But recent excitement about primary cilia—which are unique in occurring at just one copy per cell—has focused on their possible signal transduction abilities. Now, on page 811, Iomini et al. have rediscovered primary cilia on endothelial cells and found that the growth of these cilia responds to outside signals.
Buried in papers from the 1980s, the existence of primary cilia in endothelial cells was all but forgotten. The team detected the primary cilia in cultured human umbilical vein endothelial cells (HUVECs) using antibodies to capillary morphogenesis gene-1 product (CMG-1), the human orthologue of Chlamydomonas IFT71. They exposed the cells to laminar shear stress and found that this terminates intraflagellar transport (IFT) and causes disassembly of the primary cilia. Such a response suggests that primary cilia may act as antennae to trigger rearrangement of the cytoskeleton in appropriate conditions.
Evidence for a signal transduction function for primary cilia has come from kidney epithelia, where mouse PKD-1 is necessary for the production of a calcium influx after shear stress. Failure of this response leads to inapropriate tissue morphogenesis. PKD-1 is a membrane protein mutated in polycystic kidney disease and found in many different kinds of primary cilia. Its detection by Iomini et al. in only a subset of endothelial cilia may reflect either low or heterogeneous expression.
The new study was performed on cultured cells, and past sightings of primary cilia on endothelial cells have not resolved whether the structures are usually found on the luminal side of the endothelial cell or toward the basal lamina connecting to other tissues. The answer to this question will suggest whether primary cilia might be acting as a mechanosensor of fluid flow, chemosensor, or both. ▪