Abstract

TRPP2 (polycystin-2) is a member of the TRP family of nonselective cation channels that is mutated in human autosomal polycystic kidney disease. TRPP2 has been implicated in Ca²⁺-dependent mechanosensitive pathways in a variety of biological functions including cell proliferation, sperm fertilization, mating behavior, and asymmetric gene expression. Although its function as a Ca²⁺-permeable cation channel is well established, its precise role and subcellular localization in the plasma membrane, endoplasmic reticulum (ER), and cilium have remained controversial. The present review summarizes the most pertinent recent evidence regarding the structural and functional properties of TRPP2 channels, focusing on the regulation and physiology of mammalian TRPP2.

INTRODUCING THE TRPP PROTEIN SUBFAMILY

The transient receptor potential (TRP) channel superfamily currently includes 56 related six-transmembrane (TM) domain channels classified in seven subfamilies designated TRPC (“canonical”), TRPV (“vanilloid”), TRPM (“melastatin”), TRPN (“NOMPC,” from no mechanoreceptor potential-C), TRPA (“ankyrin-like with transmembrane domain-1”), TRPML (“mucolipin”), and TRPP (“polycystin”) [1,2]. TRPC, TRPV, and TRPM are related to canonical TRP proteins while TRPN, TRPA, and TRPP are more divergent. TRP channels are linked to a variety of sensory stimuli including phototransduction, thermosensation, and mechanosensation and to multiple integrative cellular functions including Ca²⁺ and Mg²⁺ homeostasis and cell cycle.

The TRPP subfamily was named after its founding member polycystin kidney disease-2 (TRPP2 encoded by the PKD2 gene), a gene product mutated in an inherited human disorder known as autosomal dominant polycystic kidney disease (ADPKD). TRPP2 is related to the TRP family of ion channels by virtue of its topological features/structural homology in the sixth TM region and the presence of an ion channel motif.

The polycystin proteins are found in the entire animal kingdom. In humans, the polycystin group contains eight members, which are widely expressed and can be divided structurally into two prototypical subgroups: the PKD1-like proteins and the PKD2 (TRPP2)-like proteins, both having a modest degree of sequence similarity in their C-termini [3–6]. The TRPP subfamily contains three homologous proteins, PKD2, PKD2L1, and PKD2L2, which are currently referred to as TRPP2, TRPP3, and TRPP5 [7]. The mammalian orthologues are highly conserved over the entire length (80–90 percent identity) but show sequence divergence in their cytosolic N-terminal regions. All TRPP2-like proteins are predicted to possess a putative coiled-coil domain at their C-termini, but only TRPP2 and TRPP3 have a Ca²⁺-binding EF-hand motif. The TRPP2 C-terminus also has a PACS-interacting binding motif, which plays a key role in regulating TRPP2 trafficking between organelles and the cell surface (reference 9 and see below).

TRPP3 was the first PKD2-like protein to be identified as a functional TRP-like cation channel [8]. TRPP2-related channels have large single-channel conductance (80–160 pS) and permeate a number of mono- and divalent cations, including Na⁺, K⁺, Ba²⁺, and Ca²⁺ [reviewed in reference 10]. Like many TRP channels, TRPP2 activity is blocked by La³⁺ and reduced by the diuretic amiloride [10]. TRPP3 is usually found to be more permeable to Ca²⁺ than to other monovalent cations such as Na⁺ and K⁺ (P_Ca/P_Na = 4), whereas TRPP2 has a P_Ca/P_Na selectivity ranging from 1 to 3. TRPP2-like cation channels therefore represent relevant routes for calcium entry or release.

PKD1, PKD1L13, and PKDREJ (receptor for egg jelly) are large multidomain proteins and make up the PKD1-like subgroup. Because PKD1-like proteins show very limited sequence similarity with TRP channels, they are not considered members of the TRP superfamily. Each PKD1-like protein contains a large extra-cellular terminal domain, 11 predicted TM segments, and a short intracellular carboxyl terminus. They all possess the combination of REJ (receptor for egg jelly), GPS (G-protein-coupled receptor proteolytic site), and PLAT/LH2 (lipoxygenase homology/polycystin, lipoxygenase, α-toxin) domains that uniquely define them as PKD1 family members. The extracellular region of PKD1 spans more than ~3,000 amino acids and contains a number of adhesive domains that implicate PKD1 in cell–cell and cell–matrix interactions. PKD1 is cleaved at its predicted GPS [11], a feature common to members of the family-B (latrophilin) G-protein-coupled receptors and which may be important for receptor activation or localization. PKD1, PKD1L1, and PKD1L2 encompass a G-protein-interacting site that may be used to regulate at least four different classes of heterotrimeric G-protein activity and multiple downstream effectors, including among others phospholipase C, protein kinase C, adenylyl cyclase, protein kinase A, Janus kinase 2, and nuclear factor of activated T cells (NFAT) [reviewed in reference 4] (Figure 14.1). The intracellular carboxyl terminus of PKD1 also harbors a coiled-coil domain that is involved in physical interaction with TRPP2 [12] and possibly TRPP3a [13].

FIGURE 14.1

Signaling and regulation of TRPP2 channels. This illustration depicts the different models for the localization-dependent functions of TRPP2. TRPP2 mediates Ca²⁺ influx at the plasma membrane (PM) and the ciliary membrane (cilium), where it functions (more...)

PKD1 and PKD1L1, but not PKDREJ, PKD1L2, or PKD1L3, have this coiled-coil domain, suggesting that the PKD1-like family could be subdivided into two groups according to their structural domains. Likewise, though PKD1 and PKD1L1 lack a structurally defined surface channel pore domain, recent data suggest that PKD1L2, PKD1L3, and PKDREJ contain strong ion channel signature motifs [14], suggesting their potential functions as pore-forming channel subunits.

MUTATIONS IN TRPP2 AND POLYCYSTIN-1 CAUSE ADPKD

The founding members of the TRPP family were discovered as genes mutated in ADPKD. ADPKD is a common nephropathy affecting 4 to 6 million people worldwide and is a leading cause of end-stage kidney failure. The disease is typically characterized by defects in the polarized phenotype and function of epithelial kidney cells, leading to abnormal renal tubular cell growth and formation of numerous fluid-filled cysts. Eventually these cysts overwhelm the kidney and destroy the parenchyma. This disease is as yet incurable and is often associated with a number of systemic manifestations including hypertension, intracranial aneurysms, and cardiovalvular abnormalities such as mitral valve prolapse.

In the mid-1990s, mutated genes responsible for ADPKD were identified by positional cloning [15]. ADPKD has been shown to result from loss-of-function mutations either of polycystin-1 or of TRPP2, with PKD1 mutations being the most prevalent causes. The dominance of PKD1 and TRPP2 mutations appears to require both a germ-line mutation of PKD1 or TRPP2 and a subsequent somatic mutation of the wild-type allele. This would explain the relatively late development of ADPKD and the focal nature of epithelial cells giving rise to cysts. However, recent studies indicate that various karyotypic changes, not just loss of heterozygosity at the normal PKD allele, are associated with cystogenesis [16], a situation that illustrates the complexity of cyst formation and raises the question as to whether the two-hit mechanism is the only means to generate a cyst.

Consistent with the broad expression of both genes during early organogenesis, mouse models for ADPKD derived from targeted disruption of either PKD genes die in utero or perinatally with cardiac septal defects and severe cystic manifestations in nephrons and pancreatic ducts.

LOCALIZATION AND TRAFFICKING OF TRPP2

TRPP2 is expressed in a variety of tissues including epithelial cells, vascular smooth muscle, cardiac myocytes, adrenal glands, and ovaries [17]. A long-lasting matter of debate has been the subcellular localization of TRPP2. In recombinant cell-based systems and in most native cells, TRPP2 is found to be concentrated in intracellular compartments and most notably in the ER, as judged by immunofluorescence imaging, cofractionation with ER markers, and sensitivity to endoglycosidase H digestion [18]. TRPP2 encompasses an ER retention signal within its C-terminal domain [18], which seems to prevent trafficking to the cell surface when expressed on its own. Deletion mutants for this ER retention signal translocate to the cell surface and can be detected by immunological and electrophysiological means [19]. Opposing these findings are reports that electrogenic TRPP2 activity is present at the plasma membrane of several cell types following treatment with chemical chaperones/proteasome inhibitors [20,21] or upon overexpression [22]. TRPP2 has also been localized to basolateral plasma membranes, lamelopodia, primary cilia, and mitotic spindles [12,18,23–26].

Recent data are clarifying the confusing picture of TRPP2 localization. First, Köttgen et al. [9] have reported that TRPP2 trafficking between the ER, Golgi, and plasma membrane compartments may be directed by the phosphofurin acidic cluster proteins PACS-1 and PACS-2, two sorting proteins that bind to an acidic cluster in the C-ter domain of TRPP2. Binding of these adaptor proteins to TRPP2 is promoted by protein kinase casein kinase-2-dependent phosphorylation of Ser812. TRPP2 accumulates at the plasma membrane only when both PACS-1 and PACS-2 molecules are absent or upon inhibition of protein kinase casein kinase-2 activity. Mutation of Ser812 to alanine or destruction of the acidic cluster (TRPP2 D815–817A) abrogates the interaction between TRPP2 and PACS proteins and increases whole-cell TRPP2 currents. Thus, mechanisms that regulate the interaction of PACS proteins with TRPP2 are likely to play key roles in routing TRPP2 between the ER and the plasma membrane. TRPP3 and TRPP5 lack the PACS-binding acidic cluster in their C-tails, suggesting that their trafficking is regulated differently from that of TRPP2. This may explain why TRPP3 is targeted to the cell surface when overexpressed in oocytes, while TRPP2 is retained in the ER. Second, a recently discovered protein of 14 kDa dubbed PIGEA-14 (polycystin-2 interactor, Golgi- and ER-associated protein) has been shown to interact with the C-ter of TRPP2. Coexpression of both proteins in HeLa cells and in LLC-PK₁ induces a redistribution of TRPP2 as well as PIGEA-14 from the ER to an unorthodox trans-Golgi compartment [27], indicating that intracellular trafficking of TRPP2 is regulated both at the levels of the ER and the trans-Golgi network. Trafficking of TRPP2 is therefore rapidly becoming a key issue to understanding TRPP2 function and dysfunction [28].

TRPP2: A CA²⁺-REGULATED CATION CHANNEL LOCATED IN THE ENDOPLASMIC RETICULUM

TRPP2 may act as a Ca²⁺-release channel in ER membranes, which amplifies Ca²⁺ transients initiated by InsP₃-generating plasma membrane receptors (Figure 14.1) [29]. This led to the suggestion that TRPP2 represents a new type of intracellular receptor that, along with IP₃Rs and RyRs, may be involved in mediating Ca²⁺-induced Ca²⁺ release. TRPP2 appears to be directly activated by Ca²⁺ and displays a bell-shaped dependence on cytoplasmic Ca²⁺ [29, see 10 for review]. Although it is yet unclear whether the Ca²⁺-binding EF hand of TRPP2 is involved in Ca²⁺-dependent modulation of TRPP2, it is noteworthy that pathogenic TRPP2 mutants with premature termination of the peptide chain in their C-ter lost their ability to sense Ca²⁺. However, TRPP3 C-terminal artificial truncation mutants lacking the EF hand exhibit basal and Ca²⁺-activated channel activities, suggesting that, at least for TRPP3, the EF hand and other parts of the carboxyl tail are not key determinants of the Ca²⁺-dependent activation [30].

Phosphorylation of Ser812 by a putative casein kinase 2 results in a significant increase in the sensitivity of the TRPP2 channel to calcium stimulation [31]. The S812A substitution, which results in loss of phosphorylation of TRPP2, shifts the Ca²⁺ dependence such that TRPP2 S812A has a maximum open probability at tenfold higher Ca²⁺ concentrations (~3 μM [Ca²⁺]) than normal TRPP2. Thus, the maximum open probability of a wild-type TRPP2 channel occurs at a Ca²⁺ concentration at which the nonphosphorylated form remains closed. Intracellular TRPP2 is therefore likely to have enhanced Ca²⁺ sensitivity, because the protein kinase casein kinase-2 is opportunely associated with the ER and most TRPP2 is found to be phosphorylated in vivo [31]. As stated above, phosphorylated TRPP2 fails to escape the ER. Only dephosphorylation of S812 would promote TRPP2 translocation to the plasma membrane. Thus, it can be hypothesized that the dephosphorylated form of TRPP2, possibly the form found in the plasma or the ciliary membranes, is unlikely to be activated by a modest increase in bulk Ca²⁺.

In line with a role of ER-localized TRPP2 in regulating intracellular Ca²⁺, TRPP2^+/− vascular smooth muscle cells show a lower level of TRPP2 and have altered intracellular Ca²⁺ homeostasis [32]. Furthermore, TRPP2 has been recently shown to interact functionally and physically with IP₃R in oocyte expression systems (Figure 14.1) [33]. The physiological relevance of these results remains to be clarified.

COASSEMBLY OF PKD1 AND TRPP2 RECONSTITUTES A CELL SURFACE CA²⁺-PERMEABLE CATION CHANNEL WITH MULTIPLE FUNCTIONS

The differential cellular and subcellular pattern of expression of PKD1 and TRPP2 in some systems strongly argues that both proteins have independent functions [25]. Notwithstanding, a central issue surrounding TRPP2 is how much TRPP2 channel activity depends on the presence of PKD1. In a lipid bilayer system, recombinant TRPP2 can reconstitute cation channel activity in the absence of PKD1 [34]. Hanaoka et al. [35] provided a functional substratum for the well-established demonstration that PKD1 and TRPP2 interact physically via their C-termini and form heteromeric complexes in vivo [12]. Coexpression of PKD1 and TRPP2 in CHO cells as well as in sympathetic neurons promotes the translocation of TRPP2 to the plasma membrane and generates a nonselective cation channel with perm-selectivity and sensitivity to di-and trivalent cations very similar to those of homomeric TRPP2 (Figure 14.1) [35,36]. The channel activity is not observed when C-terminal interaction between PKD1 and TRPP2 is not allowed, implying that coassembly of PKD1 and TRPP2 is required for the targeting/retention of TRPP2 to the plasma membrane.

The PKD1/TRPP2 complex has been reconstituted at the cell surface of sympathetic neurons [36]. In this model, TRPP2 is likely to act as the ion-translocating component of the polycystin complex because the pharmacological and permeation properties of the PKD1/TRPP2 channel complex resemble those of recombinant homomeric TRPP2 and because homomeric PKD1 cannot form an ion channel by itself [37]. Within the complex, PKD1 and TRPP2 have reciprocal “stabilizing” effects on each other’s function [34,36]. Indeed, the association of PKD1 appears to repress the constitutive activity of TRPP2 [36], which would be detrimental to the cell if uncontrolled. Conversely, TRPP2 binding to PKD1 represses PKD1’s ability to constitutively activate G proteins possibly by steric/competitive interaction among the different PKD1-binding partners [36,37]. These data favor the view that besides its ion channel function, TRPP2 also regulates the downstream effects of PKD1 on its target effectors and genes. Therefore, the balance between TRPP2 and PKD1 expression, which is manifestly disrupted in ADPKD, may play a critical role in normal PKD1–TRPP2 signaling. TRPP2 repression of G-protein activation by PKD1 has been confirmed in the case of PKD1-mediated NFAT (nuclear factor of activated T-cells) activation [38]; more recently, TRPP2 has been shown to impair the nuclear translocation of the PKD1 C-terminus by binding to, and sequestering of, the latter (Figure 14.1) [39].

TRPP2, in conjunction with PKD1, has been shown to regulate messages to the nucleus by preventing the pro-proliferative helix-loop-helix protein Id2 from entering the nucleus (Figure 14.1) [65]. Id2 is known to associate with E proteins and blocks their ability to turn on growth-suppressive genes. Id2 is normally prevented from translocating to the nucleus through its association with the serine-phosphorylated C-terminal domain of TRPP2, which is promoted by PKD1. These data predict that loss-of-function mutations in either TRPP2 or PKD1, or disruption of their functional interaction, cause Id2 to enter the nucleus and turn off growth-suppressive genes, making a case for the involvement of such a mechanism in the pathogenesis of ADPKD.

ACTIVATION OF THE PKD1–TRPP2 COMPLEX

That PKD1 and TRPP2 are interacting partners within a heteromultimeric polycystin complex has been intuited from the observation that mutations in PKD1 or TRPP2 produce virtually identical clinical presentations, irrespective of the causative gene [15]. Reconstituted PKD1–TRPP2 complexes can be activated by applying antibodies directed against the extracellular REJ domain of PKD1 [36]. This activates bidirectional signaling events, concordantly enhancing TRPP2 activity and stimulating heterotrimeric G-protein pathways. Thus, PKD1 and TRPP2 form functionally associated subunits of a receptor-ion channel signaling complex in which PKD1 acts as a “receptor/regulator” that controls TRPP2 activity and G proteins (Figure 14.1) [3,34,36]. The activation of TRPP2 and G proteins appears to proceed through a structural rearrangement of the polycystin complex that requires both proteins to have intact C-termini. This mechanism may mimic a yet-to-be-determined (mechanical? ligand?) extracellular signal that activates the polycystin complex.

This proposed mechanism may be paradigmatic for the function of other polycystin orthologues in a variety of tissues [3]. For example, PKD1 signaling has fascinating functional parallels with the acrosome reaction (AR) in sea urchin spermatozoa, a prerequisite for sperm-egg fusion [40]. AR requires the activation of suREJ1 or suREJ3, two PKD1 orthologues harboring REJ modules; each binds components of the egg jelly [41]. Importantly, antibodies directed against the REJ domain of suREJs induce the AR by opening Ca²⁺-permeant channels [42]. Recently, suREJ3 has been shown to physically bind to the sea urchin sperm orthologue of TRPP2 in the acrosome plasma membrane [43], raising the possibility that suTRPP2 may be involved in the Ca²⁺-regulated AR. Collectively, these findings add further weight to the primary importance of the REJ domain in activating the polycystin complex.

Other early evidence in support of the idea that PKD1 and TRPP2 form heteromultimeric complexes came from studies of C. elegans orthologues of ADPKD genes, lov-1 (location of vulva) and pkd-2 [44]. Lov-1, however, has an unrelated extracellular domain to PKD1 in that it lacks the REJ domain and harbors instead several mucin domains [3]. Mutation analysis shows identical male sensory behavioral defects in single or double lov-1 and pkd-2 mutants, indicating that both proteins act together in a single sensory pathway necessary for normal mating behavior [45]. Lov-1 and pkd-2 concentrate in cilia and cell bodies of male-specific sensory neurons, consistent with their functions as associated subunits of a receptor-ion channel complex involved in mechanosensitive signaling.

TRPP2: A BONA FIDE MECHANOSENSITIVE CHANNEL?

Recent studies have provided evidence that PKD1 and TRPP2 colocalize in primary cilia of renal epithelial cells, where they may function in transducing sensory information, such as shear fluid stress [23]. The primary cilium of renal epithelial cells is a solitary nonmotile structure of a few micrometers that arises from the basal body or centriole and projects into the lumen of the tubule. Its central role in cyst formation has been suggested from the primary observation that defects in proteins necessary for the assembly or function of primary cilia such as cystin, polaris, inversin, and kinesin-II cause polycystic kidney diseases. The cilium is proposed to serve as a flow sensor because it can reversibly bend in response to fluid flow rates comparable to those observed in renal tubules and because it was shown to be essential for Madin-Darby canine kidney (MDCK) cells’ ability to sense flow, because deciliated cells are irresponsive to changes in flow rate [46,47]. Fluid shear-force bending of the cilium causes Ca²⁺ influx through mechanically sensitive channels. Although it is not known whether these mechanosensitive channels reside at the base of cilia or throughout the cilium membrane, one could predict that they localize in the intervening bilayer regions that increase in tension during cilium bending.

A calcium signal through mechanically activated channels is then amplified by Ca²⁺ release from IP₃R stores in MDCK cells and spreads to neighboring cells through gap junctions [46]. These data conflict, however, with a study in murine embryonic kidney epithelial (MEK) cells, where ryanodine receptors instead of IP₃Rs have been implicated in Ca²⁺ amplification [23]. Thus, the cilium acts as the mechanosensor of changes in laminar fluid flow and transduces stimulus energy into change in membrane permeability. In this model, the PKD1–TRPP2 polycystin complex may be envisioned as a mechanotransducer, which is used to signal relevant intratubular information such as flow rates, directing attention to the regulation of Ca²⁺ influx as a crucial misstep that initiates cystogenesis. However, whether the PKD1–TRPP2 complex is mechanosensitive still awaits direct experimental evidence.

Recently, TRPP2 has been shown to play a central role in establishing the left-right (LR) asymmetry of visceral organs [48], which occurs during early embryonic development when the nodal gene, initially expressed throughout the node, becomes limited to the left margin of the node. The node is a triangular-shaped structure at the distal tip of ~E7 embryos, consisting of endodermally derived cells, each carrying a single cilium on their apical surface. The monocilia located at the center of the node express dynein, a microtubular motor protein, and are motile, producing rotational movement that creates a leftward fluid flow across the node. This leftward nodal flow is critical for the sideness of asymmetric gene expression because mice with immotile cilia develop laterality defects. In contrast, monocilia located in the periphery of the node are immobile (lacking dynein) and act as sensors of directional nodal flow by generating an asymmetric Ca²⁺ signal [48]. TRPP2 is expressed in both motile and immotile monocilia, yet a perinodal Ca²⁺ signal is absent in TRPP2^−/− mice embryos, suggesting that TRPP2 functions as a mechanotransducer in immotile monocilia and transduces leftward nodal flow into an increase in Ca²⁺ at the left border of the node. This function would be key for the establishment of a morphogenic gradient at the embryonic node and consistent with the observation that targeted disruption in TRPP2 causes situs inversus in addition to the hallmark cardiac and kidney defects [49]. The lack of laterality defects in PKD1 knockout embryos correlates with the absence of PKD1 in cilia [50], favoring the idea that TRPP2 may be involved in mechanosensation in the absence of PKD1. In the same vein, the orthologue of TRPP2 encoded by the amo gene (almost there) in Drosophila melanogaster is localized to the distal tip of the sperm flagella and, apparently in the absence of the PKD1 orthologue, plays a critical role for directional movement inside the female reproductive tract [51,52]. This suggests that TRPP2 in Drosophila sperm is part of a signaling pathway involved in detecting directional cues that are necessary for entry into the female storage organs, perhaps supporting a common role for Ca²⁺-dependent TRPP2 signals in both motile and immotile axonemal-based structures.

Despite the widespread utilization of mechanosensitive channels in a variety of physiological processes including the detection of touch, hearing function, blood pressure control, and osmotic pressure, little is known about the molecular structure and organization of vertebrate mechanotransducers. In this respect, the recent identification of TRP channels as core components of mechanoreceptors in C. elegans, Drosophila melanogaster, and vertebrates may offer clues to the conservative mechanoreceptive structural elements of mechanotransducers [53,54].

In the fruit fly, mechanoelectrical responses in bristle sensory neurons occur rapidly upon deflection of the bristle hair shaft and result from the opening—among others—of the NOMPC channel, a member of the TRPN subfamily. NOMPC has a particularly long intracellular amino-terminal tail harboring 29 ankyrin repeats, which are considered to anchor the channel to the cytoskeleton and may mediate the protein–protein interaction of a tethered mechanism that might be required for mechanical gating.

The nematode C. elegans senses nose touch by stimulating ciliated nociceptive sensory neurons, which detect, among others, mechanical and osmotic stimuli. The OSM-9 channel is thought to be part of the mechanosensitive channel because OSM-9 mutants are defective in osmotic avoidance and in sensitivity to nose touch. OSM-9 is a homologue to members of the TRPV channels, with three ankyrin-repeat domains at its amino-terminal intracellular domain. TRPV2 and TRPV4, its invertebrate TRP counterparts, have multiple ankyrin-repeat domains and are implicated in vertebrate mechanosensation in that they can sense membrane stretch [55] and hypo-osmotic stress [56], respectively. With regard to mechanical gating, it is noteworthy that TRPV4 requires the amino-terminal domain with the three ankyrin-repeats to sense physical challenges [57]. More recently, Corey et al. [58] have shown that TRPA1 (also called ANKTM1), which harbors 17 ankyrin domains, constitutes or is a component of the mechanosensitive transduction channel of vertebrate hair cells. Although the mechanism of activation by mechanical force is not yet established, an ankyrin repeat has been hypothesized to form a springlike gating structure, consistent with a “tethered channel” model [4].

Neither PKD1 nor TRPP2 display ankyrin-repeats that would allow tight interactions between the channel complex and the cytoskeleton. TRPP2, however, has been shown to connect indirectly with the actin cytoskeletal network, though it remains to be shown whether these actin-based elements play a role in cilium mechanotransduction, given that the cilium is primarily the domain of microtubules rather than actin filaments [3]. On this last issue, the extracellular domain of PKD1 has been shown to display a dynamic extensibility whereby its length might be regulated through unfolding/refolding its Ig-like domains [59]. Although these mechanical properties of PKD1 are important in the context of mechanosensation, they seem more appropriate to provide structural support in cell–cell or cell–matrix interactions at basolateral membranes than mechanosensation in the solitary cilium.

At this point, the available information does not entirely support the candidacy of a PKD1–TRPP2 complex as the core component of the primary cilium’s mechanosensitive apparatus. This would seem ample reason to consider alternative models in which mechanical force is transmitted indirectly to the protein complex or via an auxiliary subunit. On the one hand, this may imply that the TRPP2 channel acts as a nonmechanosensitive amplifier of a true mechanically gated channel, with cytosolic Ca²⁺ acting as a suitable activator of TRPP2. A critical issue to be established is the polymodal nature of TRPP2 regulation (i.e., whereby channel opening is not only dependent on mechanical stimuli but also modulated by PKD1, Ca²⁺, H⁺, and phosphorylation). Thus, the basic distinction between physical and chemical mechanisms of PKD1–TRPP2 mechanotransduction is not yet made. On the other hand, it is also conceivable that TRPP2 coassembles with other TRP channels to form a mechanosensitive channel as many invertebrate TRP-related channels do. A tantalizing link points to TRPV4, because it is expressed in renal epithelial cells, particularly in the distal nephron and collecting ducts, which are flow-sensitive segments [60]. In this regard, preliminary evidence by Walz’s group indicate that TRPV4 and TRPP2 functionally interact and colocalize in the primary cilium [61]. TRPC1 has also been recently found to be a component of the vertebrate mechanosensitive cation channel [62]. In contrast to TRPA1, TRPC1 is gated by tension developed in the lipid bilayer. Interestingly, TRPC1 is known to interact with TRPP2 in expression systems [63] and to form functional heterotetramers with TRPP2 [64], suggesting that TRPC1 as well may contribute to the mechanosensory TRPP2 apparatus (Figure 14.1).

CONCLUDING REMARKS

In the past five years, considerable progress has been achieved in evaluating the distribution, pathophysiology, and functional characteristics of TRPP proteins. Most notably, TRPP2 channels have been shown to traffic to different subcellular compartments and to display specific subcellular functions. In renal primary cilia, TRPP2 interacts with PKD1, a process that may be essential functionally for the regulation of mechanosensation and the cell cycle. In the ER, TRPP2 serves as an intracellular Ca²⁺ release channel. It would be important to identify ligands for PKD1 that affect the function of the PKD1–TRPP2 complex and to identify the factors interacting with TRPP2 that determine and regulate its compartment-specific functions. These studies will contribute not only to a better understanding of TRPP physiological functions but also to the development of new strategies for targeted therapeutic intervention.

Acknowledgments

ACKNOWLEDGMENTS

This work was supported by the Centre National de la Recherche Scientifique (CNRS), and by grants from the Agence Nationale de la Recherche (ANR Cardiovasculaire, obésité et diabète, N° ANR-05-PCOD-029-02), la Fondation Schlumberger pour l’Education et la Recherche, the French Ministère délégué à la Recherche (ACI Jeune Chercheurs, N° 5294), and a fellowship from the French Research Ministry and the University of Méditerranée Marseille.

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