Ciliary localization of the transient receptor potential polycystin 2 channel (TRPP2/PKD-2) is evolutionarily conserved, but how TRPP2 is targeted to cilia is not known. In this study, we characterize the motility and localization of PKD-2, a TRPP2 homolog, in C. elegans sensory neurons. We demonstrate that GFP-tagged PKD-2 moves bidirectionally in the dendritic compartment. Furthermore, we show a requirement for different molecules in regulating the ciliary localization of PKD-2. PKD-2 is directed to moving dendritic particles by the UNC-101/adaptor protein 1 (AP-1) complex. When expressed in non-native neurons, PKD-2 remains in cell bodies and is not observed in dendrites or cilia, indicating that cell-type specific factors are required for directing PKD-2 to the dendrite. PKD-2 stabilization in cilia and cell bodies requires LOV-1, a functional partner and a TRPP1 homolog. In lov-1 mutants, PKD-2 is greatly reduced in cilia and forms abnormal aggregates in neuronal cell bodies. Intraflagellar transport (IFT) is not essential for PKD-2 dendritic motility or access to the cilium, but may regulate PKD-2 ciliary abundance. We propose that both general and cell-type-specific factors govern TRPP2/PKD-2 subcellular distribution by forming at least two steps involving somatodendritic and ciliary sorting decisions.

Cilia are endowed with membrane receptors, channels, and signaling components whose localization and function must be tightly controlled. In primary cilia of mammalian kidney epithelia and sensory cilia of Caenorhabditis elegans neurons, polycystin-1 (PC1) and transient receptor polycystin-2 channel (TRPP2 or PC2), function together as a mechanosensory receptor-channel complex. Despite the importance of the polycystins in sensory transduction, the mechanisms that regulate polycystin activity and localization, or ciliary membrane receptors in general, remain poorly understood. We demonstrate that signal transduction adaptor molecule STAM-1A interacts with C. elegans LOV-1 (PC1), and that STAM functions with hepatocyte growth factor-regulated tyrosine kinase substrate (Hrs) on early endosomes to direct the LOV-1-PKD-2 complex for lysosomal degradation. In a stam-1 mutant, both LOV-1 and PKD-2 improperly accumulate at the ciliary base. Conversely, overexpression of STAM or Hrs promotes the removal of PKD-2 from cilia, culminating in sensory behavioral defects. These data reveal that the STAM-Hrs complex, which down-regulates ligand-activated growth factor receptors from the cell surface of yeast and mammalian cells, also regulates the localization and signaling of a ciliary PC1 receptor-TRPP2 complex. INTRODUCTIONCilia are specialized organelles that function in motility (motile or nodal cilia) or sensation (sensory or primary cilia). Several human genetic diseases are linked to defects in cilia formation or function Badano et al., 2006). Ciliary assembly via intraflagellar transport (IFT) and sensory transduction capabilities are evolutionarily conserved . These sensory devices, recently referred to as "antennae" or "nanomachines," transduce a plethora of sensory stimuli and must be fine-tuned both temporally and spatially to execute their cellular functions (Marshall and Nonaka, 2006;Scholey and Anderson, 2006;Singla and Reiter, 2006). Significant advances have been made in understanding cilia biogenesis and the genetic basis of human ciliary disease. In contrast, little is known regarding how cilia perceive, integrate, and transduce multiple extracellular stimuli into precise developmental and physiological responses.Sensory cilia are best known for their roles in photoreception and olfaction, which require G protein-coupled receptors (GPCRs) on the ciliary membrane (Buck and Axel, 1991;Marszalek et al., 2000). Cilia also act in mechanosensory and osmotic capacities and require ciliary localization of transient receptor potential (TRP) ion channels (Tobin et al., 2002;Kim et al., 2003;Nauli et al., 2003). Recently, vertebrate cilia have been shown to mediate not only environmental inputs, but also the Hedgehog (Hh) developmental cue that triggers translocation of the Smoothened (Smo) GPCR into the cilium (May et al., 2005;Huangfu and Anderson, 2006). Vertebrate cilia also express the somatostatin receptor sst3, serotonin 5-HT 6 receptor, platelet-derived growth factor receptor ␣ (PDGFR ␣) and epidermal growth fa...

Cilia serve as sensory devices in a diversity of organisms and their defects contribute to many human diseases. In primary cilia of kidney cells, the transient receptor potential polycystin (TRPP) channels polycystin-1 (PC-1) and polycystin-2 (PC-2) act as a mechanosensitive channel, with defects resulting in autosomal dominant polycystic kidney disease. In sensory cilia of Caenorhabditis elegans male-specific neurons, the TRPPs LOV-1 and PKD-2 are required for mating behavior. The mechanisms regulating TRPP ciliary localization and function are largely unknown. We identified the regulatory subunit of the serine-threonine casein kinase II (CK2) as a binding partner of LOV-1 and human PC-1. CK2 and the calcineurin phosphatase TAX-6 modulate male mating behavior and PKD-2 ciliary localization. The phospho-defective mutant PKD-2(S534A) localizes to cilia, whereas a phospho-mimetic PKD-2(S534D) mutant is largely absent from cilia. Calcineurin is required for PKD-2 ciliary localization, but is not essential for ciliary gene expression, ciliogenesis, or localization of cilium structural components. This unanticipated function of calcineurin may be important for regulating ciliary protein localization. A dynamic phosphorylation-dephosphorylation cycle may represent a mechanism for modulating TRPP activity, cellular sensation, and ciliary protein localization.

Caenorhabditis elegans is a powerful model to study the molecular basis of autosomal dominant polycystic kidney disease (ADPKD). ADPKD is caused by mutations in the polycystic kidney disease (PKD)1 or PKD2 gene, encoding polycystin (PC)-1 or PC-2, respectively. The C. elegans polycystins LOV-1 and PKD-2 are required for male mating behaviors and are localized to sensory cilia. The function of the evolutionarily conserved polycystin/lipoxygenase/␣-toxin (PLAT) domain found in all PC-1 family members remains an enigma. Here, we report that ATP-2, the ␤ subunit of the ATP synthase, physically associates with the LOV-1 PLAT domain and that this interaction is evolutionarily conserved. In addition to the expected mitochondria localization, ATP-2 and other ATP synthase components colocalize with LOV-1 and PKD-2 in cilia. Disrupting the function of the ATP synthase or overexpression of atp-2 results in a male mating behavior defect. We further show that atp-2, lov-1, and pkd-2 act in the same molecular pathway. We propose that the ciliary localized ATP synthase may play a previously unsuspected role in polycystin signaling. INTRODUCTIONAutosomal dominant polycystic kidney disease (ADPKD) is one of the most common monogenic diseases, affecting one per 400-1000 individuals (Igarashi and Somlo, 2002). This syndrome is characterized by progressive development of fluid-filled, epithelial cysts in the kidney, liver, and pancreas and accounts for ϳ10% of all cases of endstage renal disease. Mutation in either the polycystic kidney disease (PKD)1 or PKD2 gene accounts for 95% of all ADPKD cases. PKD1 encodes polycystin (PC)-1, a 4302-amino acid protein with a large extracellular domain, a G protein-coupled receptor proteolytic site (GPS), 11 predicted transmembrane (TM) domains, and an intracellular C terminus. The polycystin/lipoxygenase/␣-toxin (PLAT) domain is located in the first cytoplasmic loop between TM1 and TM2 and has been postulated to be involved in membrane-protein or protein-protein interactions (Bateman and Sandford, 1999). This domain is conserved in all PC-1 family members and also found in a variety of membrane-or lipid-associated proteins. Polycystin-2 (PC-2, encoded by PKD2) shares homology with the transient receptor protein (TRP) channels and acts as a nonselective cation channel. Defects in PC-1 or PC-2 signaling may result in epithelial dedifferentiation and cyst formation. Several signaling pathways regulated by PC-1 and PC-2 have been identified using in vitro approaches, including G protein signaling, JAK/STAT cell cycle regulation, and mechanotransduction (Boletta and Germino, 2003), but the physiological relevance to normal and disease states remains unclear.The nematode C. elegans is a simple but powerful animal model for studying basic molecular mechanisms underlying human ADPKD. The C. elegans LOV-1 and PKD-2 proteins are homologues of human PC-1 and PC-2 (Barr and Sternberg, 1999;Barr et al., 2001). lov-1 (location of vulva) and pkd-2 act nonredundantly in the same genetic pathway and are requi...