Background-When challenged with extracellular fluid shear stress, vascular endothelial cells are known to release nitric oxide, an important vasodilator. Here, we show that the ability of cultured endothelial cells to sense a low range of fluid shear depends on apical membrane organelles, called cilia, and that cilia are compartments required for proper localization and function of the mechanosensitive polycystin-1 molecule. Methods and Results-Cells with the Pkd1null/null or Tg737 orpk/orpk mutation encoded for polycystin-1 or polaris, respectively, are unable to transmit extracellular shear stress into intracellular calcium signaling and biochemical nitric oxide synthesis. Cytosolic calcium and nitric oxide recordings further show that fluid shear sensing is a cilia-specific mechanism because other mechanical or pharmacological stimulation does not abolish calcium and nitric oxide signaling in polycystin-1 and polaris mutant endothelial cells. Polycystin-1 localized in the basal body of Tg737

Mutations in polycystin 2 (PC2), a Ca2؉ -permeable cation channel, cause autosomal dominant polycystic kidney disease. Whether PC2 functions in the endoplasmic reticulum (ER) or in the plasma membrane has been controversial. Here we generated and characterized a polyclonal antibody against PC2, determined the subcellular localization of both endogenous and transfected PC2 by immunohistochemistry and biotinylation of cell surface proteins, and assessed PC2 channel properties with electrophysiology. Endogenous PC2 was found in the plasma membrane and the primary cilium of mouse inner medullar collecting duct (IMCD) cells and Madin-Darby canine kidney (MDCK) cells, whereas heterologously expressed PC2 showed a predominant ER localization. Patch-clamping of IMCD cells expressing endogenous or heterologous PC2 confirmed the presence of the channel on the plasma membrane. Treatment with chaperone-like factors facilitated the translocation of the PC2 channel to the plasma membrane from intracellular pools. The unitary conductances, channel kinetics, and other characteristics of both endogenously and heterologously expressed PC2 were similar to those described in our previous study in Xenopus laevis oocytes. These results show that PC2 functions as a plasma membrane channel in renal epithelia and suggest that PC2 contributes to Ca 2؉ entry and transport of other cations in defined nephron segments in vivo.Polycystins represent an expanding family of membrane proteins composed of two subfamilies, polycystin 1-like and polycystin 2-like molecules. PC1-like molecules consist of polycystin 1 (PC1) (1, 16), polycystin-REJ (15), polycystin-1L1 (40), and polycystin-1L2 (unpublished data), which likely function as unorthodox G protein-coupled receptors (5, 24). PC2-like molecules are ion channels and encompass PC2 (20), polycystin-L (4, 23, 38), and polycystin-2L2 (11). While the disease associations of the other polycystins are unknown, mutations in PC1 and PC2 cause autosomal dominant polycystic kidney disease, the leading genetic cause of renal failure. Formation of a large number of fluid-filled cysts in the kidney is the main characteristic of the disease.PC1, encoded by PKD1, is predicted to be a Ϸ460-kDa integral membrane glycoprotein with a very large extracellular amino terminus, 11 transmembrane domains, and a small intracellular carboxyl terminus (1, 16). PC1 is found in the plasma membrane or in the cell-cell junction of cultured cells and in tissues (8,17,27). PC1 has recently been shown to function as a G protein-coupled receptor (5), although its ligand(s) has not been identified. PC2, the product of PKD2, is predicted to encode an integral membrane protein of Ϸ110 kDa with an EF-hand domain at its carboxyl terminus (20). Sequence homology to other ion channels suggests a pore-forming capacity of PC2. Since mutations in PC1 and PC2 result in similar phenotypes and PC2 is able to interact with PC1 through its coiled-coil domain in vitro (28,34), it has been speculated that PC1 and PC2 form a functional complex. Ho...