Mammalian cilia are ubiquitous appendages found on the apical surface of cells. Primary and motile cilia are distinct in both morphology and function. Most cells have a solitary primary cilium (9+0), which lacks the central microtubule doublet characteristic of motile cilia (9+2). The immotile primary cilia house unique signaling components and sequester several important transcription factors. In contrast, motile cilia commonly extend into the lumen of respiratory airways, fallopian tubes, and brain ventricles to move their contents and/or produce gradients. In this review, we focus on the composition of putative ion channels found in both types of cilia and in the periciliary membrane and discuss their proposed functions. Our discussion does not cover specialized cilia in photoreceptor or olfactory cells, which express many more ion channels.

A single cilium is about the size of a bacterium protruding from the surface of a cell (Fig. 1). Only 200–300 nm in diameter, cilia grow from periplasmic membrane centrioles and extend 1–15 µm into the extracellular space. Their hallmark structure is the axoneme, a rigid structure of microtubule doublets: in both primary and motile cilia, nine microtubular doublets form an internal cylindrical backbone that extends most of the length of the cilium. In motile cilia, two central doublets are also present, much like in the tail of spermatozoa (Fig. 1, A and B) or the cilia of unicellular organisms. In contrast, primary cilia lack fast motility and do not contain central doublets (Fig. 1 C). Nodal cilia are an exception to this rule, as they are motile but lack central doublets.

All eukaryotic cells have subcellular compartments, but the ciliary compartment differs from other organelles by its lack of a continuous membrane isolating it from the cytoplasm. However, this is not conceptually distinct from the nucleus, in which large nuclear pores allow ions and many proteins to exchange readily between compartments. In rough analogy to nuclear pores, cilia have a highly structured transition zone at the base of the cilia linking it, the plasma membrane, and the cytoplasmic centriole in the cytoplasm.

The fact that the primary cilium protrudes from the cell has evoked much speculation about it sensing flow force, the proximity of other cells, or chemical gradients. However, it is also noteworthy that the cilium is a semi-isolated cellular compartment with restricted protein trafficking, diffusion, and membrane lipid composition. Many cilia, such as those on adult neurons, are in tight intercellular spaces and do not experience high levels of flow. The volume of a cilium (e.g., ∼0.5 fl) is 10,000-fold smaller than that of the cell and has a >1.5-fold larger surface/volume ratio. Any or all of these properties may be significant to the regulation of ciliary function.

In this review, we will discuss the various ion channels proposed to function in mammalian cilia and the experiments leading to such conclusions, as well as the potentially unique environment the cilia provides for ion channel function. The types of channels differ between primary and motile cilia. Primary cilia channels so far appear to be predominantly members of the polycystin family (e.g., PKD2, PKD2-L1), but investigators have reported other channels in or near cilia, including other transient receptor potential (TRP) channels such as TRPM4 and TRPV4, as well as the calcium-activated chloride channel ANO1. Ependymal motile cilia appear at present to be largely autonomous motors without significant regulation by the few voltage-gated calcium (CaV) channels present in their membranes. Voltage-gated potassium channels, TRP channels, and epithelial sodium channels (ENaCs) are also reported on motile cilia. Sperm flagella, although similar to motile cilia, have cationic channels of sperm (CatSper) channels that are unique to sperm flagella and crucial to their motion and force generation. In addition, numerous other channels and transporters function or are proposed to function in sperm flagella, including P2X2, Slo3, and sNHE in mice and KSper and Hv1 in humans (Miller et al., 2015).

Ciliary structure and function

Scientists once considered primary cilia in mammalian cells to be vestigial with no clear function. The discovery that the absence or mutation of proteins that reside in cilia or are required for their assembly causes genetic disorders has dramatically altered this view (Novarino et al., 2011). To date, there are more than 50 known diseases, called ciliopathies, affecting primary or motile cilia. Because some of these proteins are present in both cilia and cytoplasm, or change in localization during cell division, this definition has some ambiguity.

Microtubule-based cilia and flagella were present in the ancestor of animals, the Urmetazoa, and their closest living relatives, the Choanoflagellates, more than 600 million years ago (Richter and King, 2013). Choanoflagellates are typically single-celled organisms that can form multicellular colonies and provide a template for understanding the development of multicellularity in animals. In these organisms as well as in the choanocyte cells of sponges, which are highly similar, the flagella are used to drive bacterial prey toward feeding collars for ingestion. Molecular phylogeny shows that the evolution of multicellular forms happened during a relatively short period during which signaling receptors and ligands (Wnt, Frizzled, Notch, Hedgehog [Hh], tyrosine kinases, and G protein–coupled receptors [GPCRs]) as well as adhesion (lectins and cadherins) and extracellular matrix receptors (integrins) greatly expanded (Richter and King, 2013). Because many of these transmembrane proteins and signaling pathways are found in cilia in modern-day animals, it is conceivable that this arrangement increased Urmetazoan fitness by acting as a chemosensory apparatus or trapping bacteria prey for ingestion. The fact that mammalian primary cilia retain many of these molecules and pathways might thus indicate that they preserve some essential functions required for formation of multicellular organisms.

Cilia play crucial roles even at the earliest stages of life. During embryogenesis, vertebrates establish a left–right axis that determines the development of organs such as the heart (Basu and Brueckner, 2008). Central to this early change from bilateral symmetry is a transient structure called the embryonic node, which contains several primary cilia. Although primary cilia are typically considered nonmotile, nodal cilia are known to be motile and beat in a stereotypic motion (Okada et al., 2005). This induces a leftward (viewed dorsally) fluid flow that is critical to the establishment of left–right asymmetry through poorly understood mechanisms (Nonaka et al., 1998; Buceta et al., 2005; Lee and Anderson, 2008). That the flow itself is essential has been proven by experiments in which reversal of the fluid flow leads to reversals of organ patterning (Nonaka et al., 2002). Popular hypotheses are that flow either leads to asymmetric distribution of secreted signaling components or that mechanosensors sense the flow and open a Ca2+-permeant pathway (Tanaka et al., 2005; Yoshiba et al., 2012). In either case, this eventually leads to asymmetric gene expression.

The Hh pathway is the clearest illustration of the intimate coupling between cilia and cell signaling. Crucially, vertebrate Hh signaling requires a primary cilium (Huangfu et al., 2003; Huangfu and Anderson, 2005; May et al., 2005; Goetz et al., 2009). In the absence of a ligand, the receptor (and potential transporter), Patched (Ptch), is concentrated in the cilium (Rohatgi et al., 2007). Upon activation with an Hh ligand, Ptch exits the cilia to be replaced by the GPCR-like receptor, Smoothened (Smo; Corbit et al., 2005). Smo activity then leads to release of Gli transcription factors from the cilium that can be processed and translocated to the nucleus to activate target genes (Haycraft et al., 2005). Ciliary membranes house other GPCRs such as SSTR3 and GRP161, as well as growth factor receptors (Hilgendorf et al., 2016). The ion channels present in the primary cilium that may also effect signal transduction are just beginning to be elucidated.

Cilia present an interesting context for ion channel function. If we assume a 7-µm-long cylindrical cilium with a diameter of 300 nm (Delling et al., 2013), these dimensions would yield ∼6.8-µm2 surface area and a 0.5-fl volume. Compared with the cell body, this constitutes a larger surface area to volume ratio, conceivably exposing a larger proportion of ion channels to the same spatially limited extracellular stimulus (e.g., secreted ligand). As is true for other subcellular organelles, the same density of ion channels or transporters could alter the state of the small cilioplasmic volume much more rapidly than in the cell. Indeed, in primary cilium, resting concentrations of calcium ([Ca2+]) are approximately sevenfold greater than in the cytoplasm (∼700 vs. ∼100 nM; Delling et al., 2013). Ca2+ changes in the cytoplasm propagate into the cilia, but the inverse is not true (Delling et al., 2013): the large cytoplasmic volume buffers and dissipates the number of free calcium ions diffusing from the small cilioplasmic volume.

To drive home the small size of a cilium, consider that 100 nM [Ca2+] corresponds to ∼30 free Ca2+ ions in the 0.5-fl volume of a cilium. This is an approximation, as the details of geometry, buffers, and transporters in the cilium will affect this estimate. Because there is no membrane to delay the Ca2+ diffusion (Fawcett, 1954), it appears that large ciliary [Ca2+] influx into the limited volume of the cilia is a sufficient mechanism to maintain a [Ca2+] gradient. Thus, the cilium is a distinct compartment from the cytoplasm. An analogy is that of a freshwater outlet into the ocean; the ocean’s waves and concentrations of saline affect the cove more profoundly than the cove’s waters can affect the ocean. Different ionic concentrations in the cilia would have consequences for cellular signaling (especially in the case of [Ca2+]) as well as for the electrochemical gradients that determine ion flux through ciliary ion channels. This is not the case for all cilia however; motile cilia have similarly small dimensions, but ciliary [Ca2+] is only slightly elevated over cytoplasmic [Ca2+] (Doerner et al., 2015). The difference may lie in the ion channels present in these different cilia.

The small dimensions of the cilium also introduce a considerable electrical resistance between cilioplasm and cytoplasm, thus creating another condition of nonuniformity. In another such case, the sharp geometric transition from the neuronal soma to the axon initial segment (AIS) is critical to the function of ion channels in the AIS membrane, which initiate action potentials (Baranauskas et al., 2013). In that situation, isolation of Na+ channels from the capacitive load of the cell body allows the channels to effectively change the membrane potential of the axon. Cilia are fivefold narrower than axons and thus could provide even higher resistance (assuming similar internal resistance). Currently, we do not know whether this is relevant to the physiology of ion channels in ciliary membranes. In the section on primary cilia, we discuss an interesting case in a unique population of neurons.

Cilia have unique protein and lipid compositions (Verhey and Yang, 2016). As with any other protein class, the local ionic environment, binding proteins, and lipids all affect ion channels. Proteins are shuttled in and out of cilia by the intraflagellar transport (IFT) system (Mourão et al., 2016; Taschner and Lorentzen, 2016) but may also migrate into the cilia membrane from the plasma membrane (Milenkovic et al., 2009) or exchange via simple diffusion in solution. Ciliary ion channels would most likely be subject to sorting, trafficking, and endocytosis (Pedersen et al., 2016) similar to other ciliary transmembrane proteins. The motors, adaptors, and fellow travelers in this system may regulate ciliary ion channels spatially and temporally.

Finally, the lipid content of cilia is also distinct from that of the plasma membrane. PI(4,5)P2, is a known modulator of many ion channel and transporter functions (Hille et al., 2015), but phosphatidylinositol 4-phosphate (PI4P) predominates over PI(4,5)P2 in cilia (as it does in Golgi). One hypothesis is that cilia could also contain significant PI(4,5)P2 at times (by diffusion from plasma membrane), which is switched to PI(4)P by Tubby-like protein (TULP3)–mediated trafficking of InPP5E (phosphoinositide 5-phosphatase) into cilia. Thus, this switch in the PIP state of the cilia may favor certain GPCRs or channels within cilia (Chávez et al., 2015; Garcia-Gonzalo et al., 2015). Lipid remodeling is a crucial part of sperm physiology, and cholesterol efflux is a marker of sperm capacitation (Cohen et al., 2016; Stival et al., 2016). Except for sperm flagella, the relative abundance of other lipids in the cilium’s membrane, such as cholesterol, is not well known.

Primary cilia ion channels

In several primary cilia measured to date, basal levels of [Ca2+] are higher (Delling et al., 2013; Yuan et al., 2015), and membrane potential is ∼30 mV depolarized relative to the cell body (Delling et al., 2013). Because most cells contain a nonmotile primary cilium, this may represent an important signaling environment within the cell. An interesting but unproven hypothesis is that ciliary calcium concentrations affect calcium-dependent adenylyl cyclases to regulate the known potent PKA-dependent inhibition of the Hh pathway (Tuson et al., 2011). Current experiments are only beginning to address links between ciliary calcium and cAMP levels (Moore et al., 2016). Other signaling pathways within primary cilia are present, and thus, many other possibilities exist.

Several proteins (e.g., TRPM4, TRPV4, TRPC1, PKD1, and PKD2; Tsiokas et al., 1999; Pazour et al., 2002; Yoder et al., 2002; Gradilone et al., 2007; Köttgen et al., 2008; Flannery et al., 2015; Kleene and Kleene, 2016) have been proposed as ciliary ion channels. With the exception of polycystin-1 (PKD1), these proteins are members of the TRP family of ion channels (Ramsey et al., 2006). PKD1s are a family of putative 11-transmembrane–spanning domain proteins of unknown function (The International Polycystic Kidney Disease Consortium, 1995; Hughes et al., 1995; Yuasa et al., 2002). PKD1 has been proposed to be a GPCR (Parnell et al., 1998, 2002), an integral component of a channel complex via its last six transmembrane domains (Ponting et al., 1999; Hanaoka et al., 2000), and a novel receptor (Kim et al., 2016). The PKD1 family member PKD1-L1 (L1 for Like-1) differs from PKD1 primarily in its N-terminal domain. Within this 1,748–amino acid N terminus are multiple extracellular recognition domains and an autocleavage domain known as the GAIN domain (Yuasa et al., 2002; Araç et al., 2012).

In contrast to the mysterious function of PKD1s, PKD2 family members have primary sequence and putative structure similar to TRP channels. PKD2-L1 was identified by homology to PKD2; mice with PKD2-L1 deletions have kidney and eye deficits (Nomura et al., 1998). PKD2-L1 has also been implicated in the detection of sour taste (Horio et al., 2011). Direct patch clamp of primary cilia from retina pigmented epithelium and embryonic fibroblasts (Fig. 2 A) showed that genetically ablating PKD1-L1 or PKD2-L1 reduces the constitutively active ciliary current (DeCaen et al., 2013). However, given the huge variety of cells that have cilia, other channels are likely to be present. Indeed, PKD2 has recently been identified as a subunit of the ciliary ion channel in the inner medullary kidney collecting duct (IMCD3) cell line (Kleene and Kleene, 2016), with a conductance similar to PKD1-L1/2-L1 heteromers but with distinct cation selectivity. The ciliary PKD2-dependent conductance prefers potassium over calcium and sodium ions, with relative permeabilities PK/PCa/PNa of 1:0.6:0.1 (Kleene and Kleene, 2016). PKD2’s preference for potassium over sodium is supported by the recent report of the PKD2 single partial cryo-electron microscopy structure (Shen et al., 2016). Here, grafting the pore helices of PKD2 onto the PKD2-L1 channel suggests that PKD2 is more permeant to K+ and Na+ than Ca2+, with PK/PCa/PNa of 2:0.5:1. In contrast, PKD1-L1/2-L1 channels were more selective for calcium with PK/PCa/PNa of 1:6:1 (DeCaen et al., 2013). These important developments confirm that PKD2 is a ciliary ion channel and challenges the view that PKD2 is a calcium-selective channel. Future work will determine which channels are present in the cilia of native epithelial cells within the lumen of the kidney-collecting duct. Interestingly, although heterologous expression of PKD2-L1, but not PKD1-L1, is capable of supporting currents in the plasma membrane, the full complement of ciliary ion channel features depends on their coexpression in cilia (DeCaen et al., 2013). PKD2-L1 coimmunoprecipitates PKD1-L1 and mice lacking PKD2-L1 lack the constitutively active current (Fig. 2 A; DeCaen et al., 2013). The exact composition/stoichiometry of a putative PKD1-L1/PKD2-L1 ciliary ion channel is unknown.

Extracellular uridine and adenosine phosphates (Fig. 2 B), as well as the calmodulin blocker calmidazolium, potentiate the PKD2-L1 channel in recordings from cilia (DeCaen et al., 2013). Whole ciliary currents are more calcium selective and outwardly rectify, reminiscent of other TRP currents (Fig. 2 C). Estimates place channel density in the cilium at 30 channels/µm2, which is comparable with other specialized ion channel signaling domains in intracellular organelles such as lysosomes and mitochondria (DeCaen et al., 2013). As discussed previously, [Ca2+] permeability may explain how the cilia maintains [Ca2+] higher than in the cytoplasm (Delling et al., 2013). Intriguingly a mechanism for negative feedback may exist in the PKD2-L1 protein itself. Patch clamp experiments of heterologously expressed PKD2-L1 showed that at high internal [Ca2+] levels, the channel inactivates (DeCaen et al., 2016). Thus, this channel may help to ensure a desired range of ciliary [Ca2+] by allowing Ca2+ into the cilium when levels are low and by inactivating to prevent further Ca2+ entry when levels are high.

The fact that nucleoside phosphates and calmodulin antagonists potentiate the ciliary current (DeCaen et al., 2013) may be useful in dissecting out the function of the PKD2-L1 complex. The primary question is the functional significance of PKD2-L1 activation and subsequent alteration of primary cilia [Ca2+], [Na+], and the ciliary membrane potential. Although the most common hypothesis is that cilia are mechanical sensors that lead to Ca2+ influx (Praetorius and Spring, 2001; Nauli et al., 2003; Kindt et al., 2012; Lee et al., 2015; Grimes et al., 2016), no such changes in ciliary Ca2+ are detectable in primary cilia of various tissues over a wide range of mechanical force (Delling et al., 2016). In contrast, stereocilia (actually actin-based microvilli despite the name) exhibit a threefold increase in local Ca2+ upon physical deflection (Fig. 3; Delling et al., 2016). Unlike the higher density of Ca2+ permeant channels in primary cilia (DeCaen et al., 2013), the truly mechanosensitive stereocilia contain only a handful of mechanosensitive channels per stereocilium (Denk et al., 1995; Beurg et al., 2009). PKD2-L1 openings were not very sensitive to changes in membrane pressure (DeCaen et al., 2013), and thus, this channel is unlikely to be a physiologically relevant mechanosensor.

A recent study of cerebrospinal fluid–contacting neurons (CSF-cNs) contains a possibly important consequence of PKD2-L1’s large conductance. These neurons line the central canal of the nervous system with primary cilia on their cell bodies (Orts-Del’Immagine et al., 2014). Mathematical estimates indicate that a single PKD2-L1 opening in the primary cilium may be sufficient to trigger an action potential (Orts-Del’Immagine et al., 2016). Because PKD2-L1 is present throughout CSF-cNs, whether the cilium is the primary site for PKD2-L1 function in these neurons remains an open question.

Based on immunocytochemistry and colocalization analyses with PKD2, TRPV4 (Köttgen et al., 2008) and TRPC1 (Bai et al., 2008) have been proposed to be ciliary ion channels. Taken as further evidence are reports that both TRPV4 (Köttgen et al., 2008; Stewart et al., 2010) and TRPC1 (Bai et al., 2008; Kobori et al., 2009; Zhang et al., 2009) form heteromeric channel complexes with PKD2 that give rise to unique ion channel properties. But as we discuss, it is not clear that PKD2 channel current has been correctly identified. Most important, there are no direct ciliary measurements of TRPV4 and TRPC1 activity. One study in renal collecting duct cells indicates that TRPV4’s function may be more important in cells that do not have cilia (Zhang et al., 2013).

Whole cilia recordings have proposed that TRPM4, a Ca2+-activated, voltage-dependent channel, is active in cilia as well as in the apical membrane (Flannery et al., 2015). However, these whole cilia recordings encompassed the entire, intact cilia with the pipette forming a seal at the ciliary base, similar to methods used to measure from olfactory cilia (Kleene and Gesteland, 1991). Because these recordings were not confined to the cilia tip, it is possible that TRPM4 is located at the regions of the cilia near or in the plasma membrane. Further work should settle this issue.

Thus far, we have focused on channels present in the ciliary membrane. Interestingly, the ANO1 (TMEM16A) calcium-activated chloride channel is found on the apical membrane of cells before the formation of cilia. Loss of ANO1 function interferes with ciliogenesis (Ruppersburg and Hartzell, 2014), suggesting that this channel may regulate vesicle fusion at the site of future cilia formation.

Polycystic kidney disease proteins

Autosomal-dominant polycystic kidney disease (ADPKD) is caused by mutations in the Pkd1 (∼85%) or Pkd2 (∼15%) genes (Wu and Somlo, 2000). Morbidity is associated with primarily renal, but also liver and pancreatic cysts, and vascular abnormalities such as intracranial aneurysms and aortic artery defects. ADPKD is a common monogenetic disorder for which there are no effective therapies at present (Harris and Rossetti, 2010). Prior evidence suggests that ADPKD is associated with malfunction of cilia (i.e., ciliopathies) presumably caused by dysfunctional or mislocalized PKD1 and PKD2 proteins in the cilia membrane of IMCD epithelial cells (Qian et al., 1997; Grimm et al., 2003). Several studies show that PKD1 and PKD2 require each other for proper localization and function (Bertuccio et al., 2009; Chapin et al., 2010; Cai et al., 2014b; Kim et al., 2014; Gainullin et al., 2015; Trudel et al., 2016). A popular hypothesis is that PKD1 and PKD2 form a heteromeric ion channel that conducts Ca2+ through the ciliary or plasma membrane (Hanaoka et al., 2000). Observations of mechano- or chemically induced Ca2+ transients in kidney-derived Madin–Darby canine kidney (MDCK) and IMCD cells support this hypothesis (Praetorius and Spring, 2001; Nauli et al., 2003). However, these measurements do not resolve the origin of the Ca2+ transients, which are likely propagated from the cytoplasm (Delling et al., 2016). Indeed, cilia removal from MDCK cells did not abolish flow-induced Ca2+ transients, which instead were dependent on autocrine purinergic receptor activation (Hovater et al., 2008; Egorova et al., 2012; Rodat-Despoix et al., 2013). Interestingly, two groups have reported activation of cationic currents by purinergic activators when recording from the primary cilia of retinal pigmented epithelium (RPE) and IMCD cells (Kleene and Kleene, 2012; DeCaen et al., 2013). Based on single channel measurements in the primary cilia, channel modulation by ATP and ADP was shown to be indirect, possibly downstream of the activation of an unidentified GPCR.

Heterologous PKD2 ionic currents measured from the plasma membrane are controversial. Results vary with respect to PKD2 membrane localization or function in the absence of PKD1 (Hanaoka et al., 2000; Bertuccio et al., 2009) and whether a ligand is required for activity (Le et al., 2004; Ma et al., 2005; Kim et al., 2016). In some cases, PKD1 is reported to be a putative channel independent of PKD2 (Babich et al., 2004). In another study, heterologous expression of PKD1 and/or PKD2 did not enhance plasma membrane currents (DeCaen et al., 2013). Multiple studies suggested that heterologous PKD2 (or PKD1 + PKD2) produces a small ohmic current, but it is worrisome that they were not clearly distinct from endogenous channels found in host cells (Hanaoka et al., 2000; Feng et al., 2008), or indeed from leak current.

A recent study expressing a mutant form of PKD2 (F604P) in Xenopus laevis oocytes suggests that the WT PKD2 is not constitutively active, is cationic nonselective, and is blocked by Ca2+ (Arif Pavel et al., 2016). Furthermore, the recently identified PKD2 current recorded from IMCD cilia is most selective for potassium (Kleene and Kleene, 2016). These results contrast with reports that heterologous PKD2 (PKD1 + PKD2) is selective for calcium. The proposed location of F604 is in the fifth transmembrane helix of PKD2, and thus, the activating F604P mutation may create a bend in the activation gate, opening the pore and activating the channel (Arif Pavel et al., 2016). Given the lack of function of WT PKD2, these data suggest a significant conformational change must occur for the channel to open as supported by recent structural studies (Shen et al., 2016). In particular, the large extracellular “PKD domain” may function as a “lid” on the channel (Shen et al., 2016). It is possible that ligand binding or heteromerization with another channel subunit might surmount this energy barrier.

Motile cilia

The most well understood role of ion channels in vertebrate cilia is that of the CatSper Ca2+-selective channels in sperm flagella (CatSpers 1–4 and additional subunits, β, γ, δ; Ren et al., 2001; Kirichok et al., 2006; Liu et al., 2007; Qi et al., 2007; Chung et al., 2011). A change in sperm environment (pH, voltage, progesterone; Ren et al., 2001; Kirichok et al., 2006; Lishko et al., 2011; Strünker et al., 2011) triggers the CatSper channels to allow Ca2+ influx into sperm. Calcium sensors linked to phosphorylation cascades alter the shape and frequency of flagellar motion, a process called hyperactivated motility. This process is essential for mammalian sperm to traverse the oviduct and fertilize the oocyte. Mouse sperm express CatSpers, Slo3, and P2X2 channels, whereas human sperm express CatSpers, Slo3, and Hv1 channels. Although motile cilia and sperm flagella have many structural similarities, they have diverged significantly in their regulation, perhaps because of the significant evolutionary pressure on sperm (Cai et al., 2014a). To date, none of the sperm channels appear to be present in mammalian motile or primary cilia. A thorough discussion of CatSpers and the other ion channels in mammalian sperm has been recently published (Miller et al., 2015).

Although sperm are the only flagellated mammalian cells, motile cilia are present in ependymal cells lining the brain ventricles and epithelial cells lining respiratory airways and fallopian tubes. Whereas sperm use their modified cilium to propel themselves through fluid, somatic cells use their cilia to propel fluid through luminal spaces (Enuka et al., 2012; Faubel et al., 2016). As is the case in primary cilia, members of the TRP family of ion channels (TRPV4, PKD1, and PKD2) have been reported to be present in motile cilia (Bloodgood, 2010).

Immunohistochemistry suggests that TRPV4 localizes to the cilia of tracheal cells and epithelia of reproductive organs (Teilmann et al., 2005; Lorenzo et al., 2008). Activation of TRPV4 with stimuli such as agonists, warm temperature, or increased fluid viscosity led to increased cytosolic Ca2+ and ciliary beat frequency (Andrade et al., 2005; Lorenzo et al., 2008). At present, there are no recordings of TRPV4 currents from these cilia. PKD1 and PKD2 have also been localized by immunohistochemistry to epithelial cells of reproductive organs, but their function there has yet to be clarified (Teilmann et al., 2005). Given the frequency of inaccurate identification of proteins by antibodies alone, we place more credence in definitive functional measurements of ion channels under voltage clamp to establish localization.

Whole-cilia patch clamping of brain ependymal cells revealed that they contain a large potassium conductance and CaV channels, although the precise identities are not established (Doerner et al., 2015). Unlike sperm flagella and TRPV4 activation in tracheal cells, depolarization-induced increases in ciliary [Ca2+] did not lead to any significant changes in ciliary beating (Doerner et al., 2015). Moreover, CaV channels do not seem to be enriched in the ciliary membrane compared with the rest of the cell membrane (Doerner et al., 2015). We suspect that the membranes of ependymal motile cilia are more similar to the cell’s plasma membrane, unlike primary cilia. Motile ependymal cilia have similar resting [Ca2+] levels and membrane potentials as cytoplasm. Currently, it is unknown whether similar observations will hold true for motile cilia in the respiratory tracts and fallopian tubes.

Immunohistochemical experiments have identified abundant and uniform expression of ENaCs on the motile cilia of the fallopian tubes and bronchi (Enuka et al., 2012). However, functional measurements have not yet confirmed these findings.

Future directions

Ciliary ion channels are topics of active investigation. Although several channels localize to cilia via antibody labeling, the direct measure of function is patch clamp of these channels in their native environments (Fig. 4). Correlation of phenotypes of knockout animals with cilia-specific function will further corroborate results of functional and structural studies. The large body of research on ciliopathies will greatly aid this endeavor. Much more work is required to understand how ion channels regulate these subcellular structures’ functions. Thus far, different types of cilia (primary vs. motile) seem to possess different ion channels. Another area of future research will be to determine the true diversity of ion channels, or lack thereof, in cilia of different organs during development and in adults.

Acknowledgments

The authors declare no competing financial interests.

Lesley C. Anson served as editor.

References

References
Andrade
,
Y.N.
,
J.
Fernandes
,
E.
Vázquez
,
J.M.
Fernández-Fernández
,
M.
Arniges
,
T.M.
Sánchez
,
M.
Villalón
, and
M.A.
Valverde
.
2005
.
TRPV4 channel is involved in the coupling of fluid viscosity changes to epithelial ciliary activity
.
J. Cell Biol.
168
:
869
874
.
Araç
,
D.
,
A.A.
Boucard
,
M.F.
Bolliger
,
J.
Nguyen
,
S.M.
Soltis
,
T.C.
Südhof
, and
A.T.
Brunger
.
2012
.
A novel evolutionarily conserved domain of cell-adhesion GPCRs mediates autoproteolysis
.
EMBO J.
31
:
1364
1378
.
Arif Pavel
,
M.
,
C.
Lv
,
C.
Ng
,
L.
Yang
,
P.
Kashyap
,
C.
Lam
,
V.
Valentino
,
H.Y.
Fung
,
T.
Campbell
,
S.G.
Møller
, et al
2016
.
Function and regulation of TRPP2 ion channel revealed by a gain-of-function mutant
.
Proc. Natl. Acad. Sci. USA.
113
:
E2363
E2372
.
Babich
,
V.
,
W.Z.
Zeng
,
B.I.
Yeh
,
O.
Ibraghimov-Beskrovnaya
,
Y.
Cai
,
S.
Somlo
, and
C.L.
Huang
.
2004
.
The N-terminal extracellular domain is required for polycystin-1-dependent channel activity
.
J. Biol. Chem.
279
:
25582
25589
.
Bai
,
C.X.
,
A.
Giamarchi
,
L.
Rodat-Despoix
,
F.
Padilla
,
T.
Downs
,
L.
Tsiokas
, and
P.
Delmas
.
2008
.
Formation of a new receptor-operated channel by heteromeric assembly of TRPP2 and TRPC1 subunits
.
EMBO Rep.
9
:
472
479
.
Baranauskas
,
G.
,
Y.
David
, and
I.A.
Fleidervish
.
2013
.
Spatial mismatch between the Na+ flux and spike initiation in axon initial segment
.
Proc. Natl. Acad. Sci. USA.
110
:
4051
4056
.
Basu
,
B.
, and
M.
Brueckner
.
2008
.
Chapter six cilia: Multifunctional organelles at the center of vertebrate left-right asymmetry
.
Curr. Top. Dev. Biol.
85
:
151
174
.
Bertuccio
,
C.A.
,
H.C.
Chapin
,
Y.
Cai
,
K.
Mistry
,
V.
Chauvet
,
S.
Somlo
, and
M.J.
Caplan
.
2009
.
Polycystin-1 C-terminal cleavage is modulated by polycystin-2 expression
.
J. Biol. Chem.
284
:
21011
21026
.
Beurg
,
M.
,
R.
Fettiplace
,
J.H.
Nam
, and
A.J.
Ricci
.
2009
.
Localization of inner hair cell mechanotransducer channels using high-speed calcium imaging
.
Nat. Neurosci.
12
:
553
558
.
Bloodgood
,
R.A.
2010
.
Sensory reception is an attribute of both primary cilia and motile cilia
.
J. Cell Sci.
123
:
505
509
.
Buceta
,
J.
,
M.
Ibañes
,
D.
Rasskin-Gutman
,
Y.
Okada
,
N.
Hirokawa
, and
J.C.
Izpisúa-Belmonte
.
2005
.
Nodal cilia dynamics and the specification of the left/right axis in early vertebrate embryo development
.
Biophys. J.
89
:
2199
2209
.
Cai
,
X.
,
X.
Wang
, and
D.E.
Clapham
.
2014
a
.
Early evolution of the eukaryotic Ca2+ signaling machinery: conservation of the CatSper channel complex
.
Mol. Biol. Evol.
31
:
2735
2740
.
Cai
,
Y.
,
S.V.
Fedeles
,
K.
Dong
,
G.
Anyatonwu
,
T.
Onoe
,
M.
Mitobe
,
J.D.
Gao
,
D.
Okuhara
,
X.
Tian
,
A.R.
Gallagher
, et al
2014
b
.
Altered trafficking and stability of polycystins underlie polycystic kidney disease
.
J. Clin. Invest.
124
:
5129
5144
.
Chapin
,
H.C.
,
V.
Rajendran
, and
M.J.
Caplan
.
2010
.
Polycystin-1 surface localization is stimulated by polycystin-2 and cleavage at the G protein-coupled receptor proteolytic site
.
Mol. Biol. Cell.
21
:
4338
4348
.
Chávez
,
M.
,
S.
Ena
,
J.
Van Sande
,
A.
de Kerchove d’Exaerde
,
S.
Schurmans
, and
S.N.
Schiffmann
.
2015
.
Modulation of ciliary phosphoinositide content regulates trafficking and Sonic Hedgehog signaling output
.
Dev. Cell.
34
:
338
350
.
Chung
,
J.J.
,
B.
Navarro
,
G.
Krapivinsky
,
L.
Krapivinsky
, and
D.E.
Clapham
.
2011
.
A novel gene required for male fertility and functional CATSPER channel formation in spermatozoa
.
Nat. Commun.
2
:
153
.
Chung
,
J.J.
,
S.H.
Shim
,
R.A.
Everley
,
S.P.
Gygi
,
X.
Zhuang
, and
D.E.
Clapham
.
2014
.
Structurally distinct Ca2+ signaling domains of sperm flagella orchestrate tyrosine phosphorylation and motility
.
Cell.
157
:
808
822
.
Cohen
,
R.
,
C.
Mukai
, and
A.J.
Travis
.
2016
.
Lipid regulation of acrosome exocytosis
.
Adv. Anat. Embryol. Cell Biol.
220
:
107
127
.
Corbit
,
K.C.
,
P.
Aanstad
,
V.
Singla
,
A.R.
Norman
,
D.Y.
Stainier
, and
J.F.
Reiter
.
2005
.
Vertebrate Smoothened functions at the primary cilium
.
Nature.
437
:
1018
1021
.
DeCaen
,
P.G.
,
M.
Delling
,
T.N.
Vien
, and
D.E.
Clapham
.
2013
.
Direct recording and molecular identification of the calcium channel of primary cilia
.
Nature.
504
:
315
318
.
DeCaen
,
P.G.
,
X.
Liu
,
S.
Abiria
, and
D.E.
Clapham
.
2016
.
Atypical calcium regulation of the PKD2-L1 polycystin ion channel
.
eLife.
5
:
e13413
.
Delling
,
M.
,
P.G.
DeCaen
,
J.F.
Doerner
,
S.
Febvay
, and
D.E.
Clapham
.
2013
.
Primary cilia are specialized calcium signalling organelles
.
Nature.
504
:
311
314
.
Delling
,
M.
,
A.A.
Indzhykulian
,
X.
Liu
,
Y.
Li
,
T.
Xie
,
D.P.
Corey
, and
D.E.
Clapham
.
2016
.
Primary cilia are not calcium-responsive mechanosensors
.
Nature.
531
:
656
660
.
Denk
,
W.
,
J.R.
Holt
,
G.M.
Shepherd
, and
D.P.
Corey
.
1995
.
Calcium imaging of single stereocilia in hair cells: localization of transduction channels at both ends of tip links
.
Neuron.
15
:
1311
1321
.
Doerner
,
J.F.
,
M.
Delling
, and
D.E.
Clapham
.
2015
.
Ion channels and calcium signaling in motile cilia
.
eLife.
4
:
e11066
.
Egorova
,
A.D.
,
K.
van der Heiden
,
R.E.
Poelmann
, and
B.P.
Hierck
.
2012
.
Primary cilia as biomechanical sensors in regulating endothelial function
.
Differentiation.
83
:
S56
S61
.
Enuka
,
Y.
,
I.
Hanukoglu
,
O.
Edelheit
,
H.
Vaknine
, and
A.
Hanukoglu
.
2012
.
Epithelial sodium channels (ENaC) are uniformly distributed on motile cilia in the oviduct and the respiratory airways
.
Histochem. Cell Biol.
137
:
339
353
.
Faubel
,
R.
,
C.
Westendorf
,
E.
Bodenschatz
, and
G.
Eichele
.
2016
.
Cilia-based flow network in the brain ventricles
.
Science.
353
:
176
178
.
Fawcett
,
D.W.
1954
.
The study of epithelial cilia and sperm flagella with the electron microscope
.
Laryngoscope.
64
:
557
567
.
Feng
,
S.
,
G.M.
Okenka
,
C.X.
Bai
,
A.J.
Streets
,
L.J.
Newby
,
B.T.
DeChant
,
L.
Tsiokas
,
T.
Obara
, and
A.C.
Ong
.
2008
.
Identification and functional characterization of an N-terminal oligomerization domain for polycystin-2
.
J. Biol. Chem.
283
:
28471
28479
.
Flannery
,
R.J.
,
N.K.
Kleene
, and
S.J.
Kleene
.
2015
.
A TRPM4-dependent current in murine renal primary cilia
.
Am. J. Physiol. Renal Physiol.
309
:
F697
F707
.
Gainullin
,
V.G.
,
K.
Hopp
,
C.J.
Ward
,
C.J.
Hommerding
, and
P.C.
Harris
.
2015
.
Polycystin-1 maturation requires polycystin-2 in a dose-dependent manner
.
J. Clin. Invest.
125
:
607
620
.
Garcia-Gonzalo
,
F.R.
,
S.C.
Phua
,
E.C.
Roberson
,
G.
Garcia
III
,
M.
Abedin
,
S.
Schurmans
,
T.
Inoue
, and
J.F.
Reiter
.
2015
.
Phosphoinositides regulate ciliary protein trafficking to modulate Hedgehog signaling
.
Dev. Cell.
34
:
400
409
.
Goetz
,
S.C.
,
P.J.R.
Ocbina
, and
K.V.
Anderson
.
2009
.
The primary cilium as a Hedgehog signal transduction machine
.
Methods Cell Biol.
94
:
199
222
.
Gradilone
,
S.A.
,
A.I.
Masyuk
,
P.L.
Splinter
,
J.M.
Banales
,
B.Q.
Huang
,
P.S.
Tietz
,
T.V.
Masyuk
, and
N.F.
Larusso
.
2007
.
Cholangiocyte cilia express TRPV4 and detect changes in luminal tonicity inducing bicarbonate secretion
.
Proc. Natl. Acad. Sci. USA.
104
:
19138
19143
.
Grimes
,
D.T.
,
J.L.
Keynton
,
M.T.
Buenavista
,
X.
Jin
,
S.H.
Patel
,
S.
Kyosuke
,
J.
Vibert
,
D.J.
Williams
,
H.
Hamada
,
R.
Hussain
, et al
2016
.
Genetic analysis reveals a hierarchy of interactions between polycystin-encoding genes and genes controlling cilia function during left-right determination
.
PLoS Genet.
12
:
e1006070
.
Grimm
,
D.H.
,
Y.
Cai
,
V.
Chauvet
,
V.
Rajendran
,
R.
Zeltner
,
L.
Geng
,
E.D.
Avner
,
W.
Sweeney
,
S.
Somlo
, and
M.J.
Caplan
.
2003
.
Polycystin-1 distribution is modulated by polycystin-2 expression in mammalian cells
.
J. Biol. Chem.
278
:
36786
36793
.
Hanaoka
,
K.
,
F.
Qian
,
A.
Boletta
,
A.K.
Bhunia
,
K.
Piontek
,
L.
Tsiokas
,
V.P.
Sukhatme
,
W.B.
Guggino
, and
G.G.
Germino
.
2000
.
Co-assembly of polycystin-1 and -2 produces unique cation-permeable currents
.
Nature.
408
:
990
994
.
Harris
,
P.C.
, and
S.
Rossetti
.
2010
.
Molecular diagnostics for autosomal dominant polycystic kidney disease
.
Nat. Rev. Nephrol.
6
:
197
206
.
Haycraft
,
C.J.
,
B.
Banizs
,
Y.
Aydin-Son
,
Q.
Zhang
,
E.J.
Michaud
, and
B.K.
Yoder
.
2005
.
Gli2 and Gli3 localize to cilia and require the intraflagellar transport protein polaris for processing and function
.
PLoS Genet.
1
:
e53
.
Hilgendorf
,
K.I.
,
C.T.
Johnson
, and
P.K.
Jackson
.
2016
.
The primary cilium as a cellular receiver: organizing ciliary GPCR signaling
.
Curr. Opin. Cell Biol.
39
:
84
92
.
Hille
,
B.
,
E.J.
Dickson
,
M.
Kruse
,
O.
Vivas
, and
B.C.
Suh
.
2015
.
Phosphoinositides regulate ion channels
.
Biochim. Biophys. Acta.
1851
:
844
856
.
Horio
,
N.
,
R.
Yoshida
,
K.
Yasumatsu
,
Y.
Yanagawa
,
Y.
Ishimaru
,
H.
Matsunami
, and
Y.
Ninomiya
.
2011
.
Sour taste responses in mice lacking PKD channels
.
PLoS One.
6
:
e20007
.
Hovater
,
M.B.
,
D.
Olteanu
,
E.L.
Hanson
,
N.L.
Cheng
,
B.
Siroky
,
A.
Fintha
,
P.
Komlosi
,
W.
Liu
,
L.M.
Satlin
,
P.D.
Bell
, et al
2008
.
Loss of apical monocilia on collecting duct principal cells impairs ATP secretion across the apical cell surface and ATP-dependent and flow-induced calcium signals
.
Purinergic Signal.
4
:
155
170
.
Huangfu
,
D.
, and
K.V.
Anderson
.
2005
.
Cilia and Hedgehog responsiveness in the mouse
.
Proc. Natl. Acad. Sci. USA.
102
:
11325
11330
.
Huangfu
,
D.
,
A.
Liu
,
A.S.
Rakeman
,
N.S.
Murcia
,
L.
Niswander
, and
K.V.
Anderson
.
2003
.
Hedgehog signalling in the mouse requires intraflagellar transport proteins
.
Nature.
426
:
83
87
.
Hughes
,
J.
,
C.J.
Ward
,
B.
Peral
,
R.
Aspinwall
,
K.
Clark
,
J.L.
San Millán
,
V.
Gamble
, and
P.C.
Harris
.
1995
.
The polycystic kidney disease 1 (PKD1) gene encodes a novel protein with multiple cell recognition domains
.
Nat. Genet.
10
:
151
160
.
Kim
,
H.
,
H.
Xu
,
Q.
Yao
,
W.
Li
,
Q.
Huang
,
P.
Outeda
,
V.
Cebotaru
,
M.
Chiaravalli
,
A.
Boletta
,
K.
Piontek
, et al
2014
.
Ciliary membrane proteins traffic through the Golgi via a Rabep1/GGA1/Arl3-dependent mechanism
.
Nat. Commun.
5
:
5482
.
Kim
,
S.
,
H.
Nie
,
V.
Nesin
,
U.
Tran
,
P.
Outeda
,
C.X.
Bai
,
J.
Keeling
,
D.
Maskey
,
T.
Watnick
,
O.
Wessely
, and
L.
Tsiokas
.
2016
.
The polycystin complex mediates Wnt/Ca2+ signalling
.
Nat. Cell Biol.
18
:
752
764
.
Kindt
,
K.S.
,
G.
Finch
, and
T.
Nicolson
.
2012
.
Kinocilia mediate mechanosensitivity in developing zebrafish hair cells
.
Dev. Cell.
23
:
329
341
.
Kirichok
,
Y.
,
B.
Navarro
, and
D.E.
Clapham
.
2006
.
Whole-cell patch-clamp measurements of spermatozoa reveal an alkaline-activated Ca2+ channel
.
Nature.
439
:
737
740
.
Kleene
,
N.K.
, and
S.J.
Kleene
.
2012
.
A method for measuring electrical signals in a primary cilium
.
Cilia.
1
:
17
.
Kleene
,
S.J.
, and
R.C.
Gesteland
.
1991
.
Transmembrane currents in frog olfactory cilia
.
J. Membr. Biol.
120
:
75
81
.
Kleene
,
S.J.
, and
N.K.
Kleene
.
2016
.
The native TRPP2-dependent channel of murine renal primary cilia
.
Am. J. Physiol. Renal Physiol.
10
:
2016
.
Kobori
,
T.
,
G.D.
Smith
,
R.
Sandford
, and
J.M.
Edwardson
.
2009
.
The transient receptor potential channels TRPP2 and TRPC1 form a heterotetramer with a 2:2 stoichiometry and an alternating subunit arrangement
.
J. Biol. Chem.
284
:
35507
35513
.
Köttgen
,
M.
,
B.
Buchholz
,
M.A.
Garcia-Gonzalez
,
F.
Kotsis
,
X.
Fu
,
M.
Doerken
,
C.
Boehlke
,
D.
Steffl
,
R.
Tauber
,
T.
Wegierski
, et al
2008
.
TRPP2 and TRPV4 form a polymodal sensory channel complex
.
J. Cell Biol.
182
:
437
447
.
Le
,
N.H.
,
P.
van der Bent
,
G.
Huls
,
M.
van de Wetering
,
M.
Loghman-Adham
,
A.C.
Ong
,
J.P.
Calvet
,
H.
Clevers
,
M.H.
Breuning
,
H.
van Dam
, and
D.J.
Peters
.
2004
.
Aberrant polycystin-1 expression results in modification of activator protein-1 activity, whereas Wnt signaling remains unaffected
.
J. Biol. Chem.
279
:
27472
27481
.
Lee
,
J.D.
, and
K.V.
Anderson
.
2008
.
Morphogenesis of the node and notochord: the cellular basis for the establishment and maintenance of left-right asymmetry in the mouse
.
Dev. Dyn.
237
:
3464
3476
.
Lee
,
K.L.
,
M.D.
Guevarra
,
A.M.
Nguyen
,
M.C.
Chua
,
Y.
Wang
, and
C.R.
Jacobs
.
2015
.
The primary cilium functions as a mechanical and calcium signaling nexus
.
Cilia.
4
:
7
.
Lishko
,
P.V.
,
I.L.
Botchkina
, and
Y.
Kirichok
.
2011
.
Progesterone activates the principal Ca2+ channel of human sperm
.
Nature.
471
:
387
391
.
Liu
,
J.
,
J.
Xia
,
K.H.
Cho
,
D.E.
Clapham
, and
D.
Ren
.
2007
.
CatSperβ, a novel transmembrane protein in the CatSper channel complex
.
J. Biol. Chem.
282
:
18945
18952
.
Lorenzo
,
I.M.
,
W.
Liedtke
,
M.J.
Sanderson
, and
M.A.
Valverde
.
2008
.
TRPV4 channel participates in receptor-operated calcium entry and ciliary beat frequency regulation in mouse airway epithelial cells
.
Proc. Natl. Acad. Sci. USA.
105
:
12611
12616
.
Ma
,
R.
,
W.P.
Li
,
D.
Rundle
,
J.
Kong
,
H.I.
Akbarali
, and
L.
Tsiokas
.
2005
.
PKD2 functions as an epidermal growth factor-activated plasma membrane channel
.
Mol. Cell. Biol.
25
:
8285
8298
.
May
,
S.R.
,
A.M.
Ashique
,
M.
Karlen
,
B.
Wang
,
Y.
Shen
,
K.
Zarbalis
,
J.
Reiter
,
J.
Ericson
, and
A.S.
Peterson
.
2005
.
Loss of the retrograde motor for IFT disrupts localization of Smo to cilia and prevents the expression of both activator and repressor functions of Gli
.
Dev. Biol.
287
:
378
389
.
Milenkovic
,
L.
,
M.P.
Scott
, and
R.
Rohatgi
.
2009
.
Lateral transport of Smoothened from the plasma membrane to the membrane of the cilium
.
J. Cell Biol.
187
:
365
374
.
Miller
,
M.R.
,
S.A.
Mansell
,
S.A.
Meyers
, and
P.V.
Lishko
.
2015
.
Flagellar ion channels of sperm: similarities and differences between species
.
Cell Calcium.
58
:
105
113
.
Moon
,
H.
,
J.
Song
,
J.O.
Shin
,
H.
Lee
,
H.K.
Kim
,
J.T.
Eggenschwiller
,
J.
Bok
, and
H.W.
Ko
.
2014
.
Intestinal cell kinase, a protein associated with endocrine-cerebro-osteodysplasia syndrome, is a key regulator of cilia length and Hedgehog signaling
.
Proc. Natl. Acad. Sci. USA.
111
:
8541
8546
.
Moore
,
B.S.
,
A.N.
Stepanchick
,
P.H.
Tewson
,
C.M.
Hartle
,
J.
Zhang
,
A.M.
Quinn
,
T.E.
Hughes
, and
T.
Mirshahi
.
2016
.
Cilia have high cAMP levels that are inhibited by Sonic Hedgehog-regulated calcium dynamics
.
Proc. Natl. Acad. Sci. USA.
113
:
13069
13074
.
Mourão
,
A.
,
S.T.
Christensen
, and
E.
Lorentzen
.
2016
.
The intraflagellar transport machinery in ciliary signaling
.
Curr. Opin. Struct. Biol.
41
:
98
108
.
Nauli
,
S.M.
,
F.J.
Alenghat
,
Y.
Luo
,
E.
Williams
,
P.
Vassilev
,
X.
Li
,
A.E.
Elia
,
W.
Lu
,
E.M.
Brown
,
S.J.
Quinn
, et al
2003
.
Polycystins 1 and 2 mediate mechanosensation in the primary cilium of kidney cells
.
Nat. Genet.
33
:
129
137
.
Nomura
,
H.
,
A.E.
Turco
,
Y.
Pei
,
L.
Kalaydjieva
,
T.
Schiavello
,
S.
Weremowicz
,
W.
Ji
,
C.C.
Morton
,
M.
Meisler
,
S.T.
Reeders
, and
J.
Zhou
.
1998
.
Identification of PKDL, a novel polycystic kidney disease 2-like gene whose murine homologue is deleted in mice with kidney and retinal defects
.
J. Biol. Chem.
273
:
25967
25973
.
Nonaka
,
S.
,
Y.
Tanaka
,
Y.
Okada
,
S.
Takeda
,
A.
Harada
,
Y.
Kanai
,
M.
Kido
, and
N.
Hirokawa
.
1998
.
Randomization of left-right asymmetry due to loss of nodal cilia generating leftward flow of extraembryonic fluid in mice lacking KIF3B motor protein
.
Cell.
95
:
829
837
.
Nonaka
,
S.
,
H.
Shiratori
,
Y.
Saijoh
, and
H.
Hamada
.
2002
.
Determination of left-right patterning of the mouse embryo by artificial nodal flow
.
Nature.
418
:
96
99
.
Novarino
,
G.
,
N.
Akizu
, and
J.G.
Gleeson
.
2011
.
Modeling human disease in humans: the ciliopathies
.
Cell.
147
:
70
79
.
Okada
,
Y.
,
S.
Takeda
,
Y.
Tanaka
,
J.C.
Izpisúa Belmonte
, and
N.
Hirokawa
.
2005
.
Mechanism of nodal flow: a conserved symmetry breaking event in left-right axis determination
.
Cell.
121
:
633
644
.
Orts-Del’Immagine
,
A.
,
A.
Kastner
,
V.
Tillement
,
C.
Tardivel
,
J.
Trouslard
, and
N.
Wanaverbecq
.
2014
.
Morphology, distribution and phenotype of polycystin kidney disease 2-like 1-positive cerebrospinal fluid contacting neurons in the brainstem of adult mice
.
PLoS One.
9
:
e87748
.
Orts-Del’Immagine
,
A.
,
R.
Seddik
,
F.
Tell
,
C.
Airault
,
G.
Er-Raoui
,
M.
Najimi
,
J.
Trouslard
, and
N.
Wanaverbecq
.
2016
.
A single polycystic kidney disease 2-like 1 channel opening acts as a spike generator in cerebrospinal fluid-contacting neurons of adult mouse brainstem
.
Neuropharmacology.
101
:
549
565
.
Parnell
,
S.C.
,
B.S.
Magenheimer
,
R.L.
Maser
,
C.A.
Rankin
,
A.
Smine
,
T.
Okamoto
, and
J.P.
Calvet
.
1998
.
The polycystic kidney disease-1 protein, polycystin-1, binds and activates heterotrimeric G-proteins in vitro
.
Biochem. Biophys. Res. Commun.
251
:
625
631
.
Parnell
,
S.C.
,
B.S.
Magenheimer
,
R.L.
Maser
,
C.A.
Zien
,
A.M.
Frischauf
, and
J.P.
Calvet
.
2002
.
Polycystin-1 activation of c-Jun N-terminal kinase and AP-1 is mediated by heterotrimeric G proteins
.
J. Biol. Chem.
277
:
19566
19572
.
Pazour
,
G.J.
,
J.T.
San Agustin
,
J.A.
Follit
,
J.L.
Rosenbaum
, and
G.B.
Witman
.
2002
.
Polycystin-2 localizes to kidney cilia and the ciliary level is elevated in orpk mice with polycystic kidney disease
.
Curr. Biol.
12
:
R378
R380
.
Pedersen
,
L.B.
,
J.B.
Mogensen
, and
S.T.
Christensen
.
2016
.
Endocytic control of cellular signaling at the primary cilium
.
Trends Biochem. Sci.
41
:
784
797
.
Ponting
,
C.P.
,
K.
Hofmann
, and
P.
Bork
.
1999
.
A latrophilin/CL-1-like GPS domain in polycystin-1
.
Curr. Biol.
9
:
R585
R588
.
Praetorius
,
H.A.
, and
K.R.
Spring
.
2001
.
Bending the MDCK cell primary cilium increases intracellular calcium
.
J. Membr. Biol.
184
:
71
79
.
Qi
,
H.
,
M.M.
Moran
,
B.
Navarro
,
J.A.
Chong
,
G.
Krapivinsky
,
L.
Krapivinsky
,
Y.
Kirichok
,
I.S.
Ramsey
,
T.A.
Quill
, and
D.E.
Clapham
.
2007
.
All four CatSper ion channel proteins are required for male fertility and sperm cell hyperactivated motility
.
Proc. Natl. Acad. Sci. USA.
104
:
1219
1223
.
Qian
,
F.
,
F.J.
Germino
,
Y.
Cai
,
X.
Zhang
,
S.
Somlo
, and
G.G.
Germino
.
1997
.
PKD1 interacts with PKD2 through a probable coiled-coil domain
.
Nat. Genet.
16
:
179
183
.
Ramsey
,
I.S.
,
M.
Delling
, and
D.E.
Clapham
.
2006
.
An introduction to TRP channels
.
Annu. Rev. Physiol.
68
:
619
647
.
Ren
,
D.
,
B.
Navarro
,
G.
Perez
,
A.C.
Jackson
,
S.
Hsu
,
Q.
Shi
,
J.L.
Tilly
, and
D.E.
Clapham
.
2001
.
A sperm ion channel required for sperm motility and male fertility
.
Nature.
413
:
603
609
.
Richter
,
D.J.
, and
N.
King
.
2013
.
The genomic and cellular foundations of animal origins
.
Annu. Rev. Genet.
47
:
509
537
.
Rodat-Despoix
,
L.
,
J.
Hao
,
M.
Dandonneau
, and
P.
Delmas
.
2013
.
Shear stress-induced Ca2+ mobilization in MDCK cells is ATP dependent, no matter the primary cilium
.
Cell Calcium.
53
:
327
337
.
Rohatgi
,
R.
,
L.
Milenkovic
, and
M.P.
Scott
.
2007
.
Patched1 regulates hedgehog signaling at the primary cilium
.
Science.
317
:
372
376
.
Ruppersburg
,
C.C.
, and
H.C.
Hartzell
.
2014
.
The Ca2+-activated Cl− channel ANO1/TMEM16A regulates primary ciliogenesis
.
Mol. Biol. Cell.
25
:
1793
1807
.
Shen
,
P.S.
,
X.
Yang
,
P.G.
DeCaen
,
X.
Liu
,
D.
Bulkley
,
D.E.
Clapham
, and
E.
Cao
.
2016
.
The structure of the polycystic kidney disease channel PKD2 in lipid nanodiscs
.
Cell.
167
:
763
773.e11
.
Stewart
,
A.P.
,
G.D.
Smith
,
R.N.
Sandford
, and
J.M.
Edwardson
.
2010
.
Atomic force microscopy reveals the alternating subunit arrangement of the TRPP2-TRPV4 heterotetramer
.
Biophys. J.
99
:
790
797
.
Stival
,
C.
,
L.C.
Puga Molina
,
B.
Paudel
,
M.G.
Buffone
,
P.E.
Visconti
, and
D.
Krapf
.
2016
.
Sperm capacitation and acrosome reaction in mammalian sperm
.
Adv. Anat. Embryol. Cell Biol.
220
:
93
106
.
Strünker
,
T.
,
N.
Goodwin
,
C.
Brenker
,
N.D.
Kashikar
,
I.
Weyand
,
R.
Seifert
, and
U.B.
Kaupp
.
2011
.
The CatSper channel mediates progesterone-induced Ca2+ influx in human sperm
.
Nature.
471
:
382
386
.
Tanaka
,
Y.
,
Y.
Okada
, and
N.
Hirokawa
.
2005
.
FGF-induced vesicular release of Sonic hedgehog and retinoic acid in leftward nodal flow is critical for left-right determination
.
Nature.
435
:
172
177
.
Taschner
,
M.
, and
E.
Lorentzen
.
2016
.
The intraflagellar transport machinery
.
Cold Spring Harb. Perspect. Biol.
8
:
a028092
.
Teilmann
,
S.C.
,
A.G.
Byskov
,
P.A.
Pedersen
,
D.N.
Wheatley
,
G.J.
Pazour
, and
S.T.
Christensen
.
2005
.
Localization of transient receptor potential ion channels in primary and motile cilia of the female murine reproductive organs
.
Mol. Reprod. Dev.
71
:
444
452
.
The International Polycystic Kidney Disease Consortium
.
1995
.
Polycystic kidney disease: the complete structure of the PKD1 gene and its protein
.
Cell
.
81
:
289
298
.
Trudel
,
M.
,
Q.
Yao
, and
F.
Qian
.
2016
.
The Role of G-protein-coupled receptor proteolysis site cleavage of polycystin-1 in renal physiology and polycystic kidney disease
.
Cells.
5
:
E3
.
Tsiokas
,
L.
,
T.
Arnould
,
C.
Zhu
,
E.
Kim
,
G.
Walz
, and
V.P.
Sukhatme
.
1999
.
Specific association of the gene product of PKD2 with the TRPC1 channel
.
Proc. Natl. Acad. Sci. USA.
96
:
3934
3939
.
Tuson
,
M.
,
M.
He
, and
K.V.
Anderson
.
2011
.
Protein kinase A acts at the basal body of the primary cilium to prevent Gli2 activation and ventralization of the mouse neural tube
.
Development.
138
:
4921
4930
.
Verhey
,
K.J.
, and
W.
Yang
.
2016
.
Permeability barriers for generating a unique ciliary protein and lipid composition
.
Curr. Opin. Cell Biol.
41
:
109
116
.
Voronina
,
V.A.
,
K.
Takemaru
,
P.
Treuting
,
D.
Love
,
B.R.
Grubb
,
A.M.
Hajjar
,
A.
Adams
,
F.Q.
Li
, and
R.T.
Moon
.
2009
.
Inactivation of Chibby affects function of motile airway cilia
.
J. Cell Biol.
185
:
225
233
.
Wu
,
G.
, and
S.
Somlo
.
2000
.
Molecular genetics and mechanism of autosomal dominant polycystic kidney disease
.
Mol. Genet. Metab.
69
:
1
15
.
Yoder
,
B.K.
,
X.
Hou
, and
L.M.
Guay-Woodford
.
2002
.
The polycystic kidney disease proteins, polycystin-1, polycystin-2, polaris, and cystin, are co-localized in renal cilia
.
J. Am. Soc. Nephrol.
13
:
2508
2516
.
Yoshiba
,
S.
,
H.
Shiratori
,
I.Y.
Kuo
,
A.
Kawasumi
,
K.
Shinohara
,
S.
Nonaka
,
Y.
Asai
,
G.
Sasaki
,
J.A.
Belo
,
H.
Sasaki
, et al
2012
.
Cilia at the node of mouse embryos sense fluid flow for left-right determination via Pkd2
.
Science.
338
:
226
231
.
Yuan
,
S.
,
L.
Zhao
,
M.
Brueckner
, and
Z.
Sun
.
2015
.
Intraciliary calcium oscillations initiate vertebrate left-right asymmetry
.
Curr. Biol.
25
:
556
567
.
Yuasa
,
T.
,
B.
Venugopal
,
S.
Weremowicz
,
C.C.
Morton
,
L.
Guo
, and
J.
Zhou
.
2002
.
The sequence, expression, and chromosomal localization of a novel polycystic kidney disease 1-like gene, PKD1L1, in human
.
Genomics.
79
:
376
386
.
Zhang
,
P.
,
Y.
Luo
,
B.
Chasan
,
S.
González-Perrett
,
N.
Montalbetti
,
G.A.
Timpanaro
,
M.R.
Cantero
,
A.J.
Ramos
,
W.H.
Goldmann
,
J.
Zhou
, and
H.F.
Cantiello
.
2009
.
The multimeric structure of polycystin-2 (TRPP2): structural-functional correlates of homo- and hetero-multimers with TRPC1
.
Hum. Mol. Genet.
18
:
1238
1251
.
Zhang
,
Z.R.
,
W.F.
Chu
,
B.
Song
,
M.
Gooz
,
J.N.
Zhang
,
C.J.
Yu
,
S.
Jiang
,
A.
Baldys
,
P.
Gooz
,
S.
Steele
, et al
2013
.
TRPP2 and TRPV4 form an EGF-activated calcium permeable channel at the apical membrane of renal collecting duct cells
.
PLoS One.
8
:
e73424
.

    Abbreviations used:
     
  • ADPKD

    autosomal-dominant polycystic kidney disease

  •  
  • CatSper

    cationic channels of sperm

  •  
  • CaV

    voltage-gated calcium

  •  
  • GPCR

    G protein–coupled receptor

  •  
  • Hh

    Hedgehog

  •  
  • IMCD

    inner medullary kidney collecting duct

  •  
  • Smo

    Smoothened

  •  
  • TRP

    transient receptor potential

This article is distributed under the terms of an Attribution–Noncommercial–Share Alike–No Mirror Sites license for the first six months after the publication date (see http://www.rupress.org/terms/). After six months it is available under a Creative Commons License (Attribution–Noncommercial–Share Alike 4.0 International license, as described at https://creativecommons.org/licenses/by-nc-sa/4.0/).