Bigger equals faster

Cilia that sense orests 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 trans-duction 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 F Bigger equals faster ize matters, but how biological processes are scaled to match the size of their target—for organisms, organs, cells, and organelles—is largely a mystery. Now, Rzadzinska et al. (page 887) find that the turnover rates of neighboring actin structures depend on their respective sizes, thus renewing the entire superstructure synchronously. The structures are stereocilia—the mechanosensitive actin pillars sticking out of sensory hair cells of the inner ear. Adjacent parallel rows of stereocilia of graded height form S Longer stereocilia have more myosin XVa (green). 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 morpho-genesis. 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. ᭿ CMG-1 (red) identifies primary cilia on endothelial cells. a staircase-like pattern. Long regarded as the epitome of a stable cytoskeleton …