Polycystin-2 cation channel function in the human syncytiotrophoblast is regulated by microtubular structures

TRiPPing the sensors: The osmosensing pathway of Polycystin 2

Mutations to polycystin-2 (PC2), a non-selective cation permeant transient receptor potential channel, results in polycystic kidney disease (PKD). Despite the disease relevance of PC2, the physiological agonist that activates PC2 has remained elusive. As one of the earliest symptoms in PKD is a urine concentrating deficiency, we hypothesized that shifts in osmolarity experienced by the collecting duct cells would activate PC2 and loss of PC2 would prevent osmosensing. We found that mice with inducible PC2 knocked out (KO) in renal tubules had dilute urine. Hyperosmotic stimuli induced a rise in endoplasmic reticulum (ER)-mediated cytosolic calcium which was absent in PC2 KO mice and PC2 KO cells. A pathologic point mutation that prevents ion flux through PC2 inhibited the calcium rise, pointing to the centrality of PC2 in the osmotic response. To understand how an extracellular stimulus activated ER-localized PC2, we examined microtubule-ER dynamics, and found that the osmotically induced calcium increase was preceded by microtubule destabilization. This was due to a novel interaction between PC2 and the microtubule binding protein MAP4 that tethers the microtubules to the ER. Finally, disruption of the MAP4-PC2 interaction prevented incorporation of the water channel aquaporin 2 following a hyperosmotic challenge, in part explaining the dilute urine. Our results demonstrate that MAP4-dependent microtubule stabilization of ER-resident PC2 is required for PC2 to participate in the osmosensing pathway. Moreover, osmolarity represents a bona fide physiological stimulus for ER-localized PC2 and loss of PC2 in renal epithelial cells impairs osmosensing ability and urine concentrating capacity.

Placental ion channels: potential target of chemical exposure

The placenta is an important organ for the exchange of substances between the fetus and the mother, hormone secretion, and fetoplacental immunological defense. Placenta has an organ-specific distribution of ion channels and trophoblasts, and placental vessels express a large number of ion channels. Several placental housekeeping activities and pregnancy complications are at least partly controlled by ion channels, which are playing an important role in regulating hormone secretion, trophoblastic homeostasis, ion transport, and vasomotor activity. The function of several placental ion channels (Na, Ca, and Cl ion channels, cation channel, nicotinic acetylcholine receptors, and aquaporin-1) is known to be influenced by chemical exposure, i.e., their responses to different chemicals have been tested and confirmed in experimental models. Here, we review the possibility that placental ion channels are targets of toxicological concern in terms of placental function, fetal growth, and development.

Modelling human placental villous development: designing cultures that reflect anatomy

The use of in vitro tools to study trophoblast differentiation and function is essential to improve understanding of normal and abnormal placental development. The relative accessibility of human placentae enables the use of primary trophoblasts and placental explants in a range of in vitro systems. Recent advances in stem cell models, three-dimensional organoid cultures, and organ-on-a-chip systems have further shed light on the complex microenvironment and cell-cell crosstalk involved in placental development. However, understanding each model's strengths and limitations, and which in vivo aspects of human placentation in vitro data acquired does, or does not, accurately reflect, is key to interpret findings appropriately. To help researchers use and design anatomically accurate culture models, this review both outlines our current understanding of placental development, and critically considers the range of established and emerging culture models used to study this, with a focus on those derived from primary tissue.

TRPP2 ion channels: The roles in various subcellular locations

TRPP2 (PC2, PKD2 or Polycytin-2), encoded by PKD2 gene, belongs to the nonselective cation channel TRP family. Recently, the three-dimensional structure of TRPP2 was constructed. TRPP2 mainly functions in three subcellular compartments: endoplasmic reticulum, plasma membrane and primary cilia. TRPP2 can act as a calcium-activated intracellular calcium release channel on the endoplasmic reticulum. TRPP2 also interacts with other Ca²⁺ release channels to regulate calcium release, like IP3R and RyR2. TRPP2 acts as an ion channel regulated by epidermal growth factor through activation of downstream factors in the plasma membrane. TRPP2 binding to TRPC1 in the plasma membrane or endoplasmic reticulum is associated with mechanosensitivity. In cilium, TRPP2 was found to combine with PKD1 and TRPV4 to form a complex related to mechanosensitivity. Because TRPP2 is involved in regulating intracellular ion concentration, TRPP2 mutations often lead to autosomal dominant polycystic kidney disease, which may also be associated with cardiovascular disease. In this paper, we review the molecular structure of TRPP2, the subcellular localization of TRPP2, the related functions and mechanisms of TRPP2 at different sites, and the diseases related to TRPP2.

Polycystin-2 (TRPP2): Ion channel properties and regulation

Polycystin-2 (TRPP2, PKD2, PC2) is the product of the PKD2 gene, whose mutations cause Autosomal Dominant Polycystic Kidney Disease (ADPKD). PC2 belongs to the superfamily of TRP (Transient Receptor Potential) proteins that generally function as Ca²⁺-permeable nonselective cation channels implicated in Ca²⁺ signaling. PC2 localizes to various cell domains with distinct functions that likely depend on interactions with specific channel partners. Functions include receptor-operated, nonselective cation channel activity in the plasma membrane, intracellular Ca²⁺ release channel activity in the endoplasmic reticulum (ER), and mechanosensitive channel activity in the primary cilium of renal epithelial cells. Here we summarize our current understanding of the properties of PC2 and how other transmembrane and cytosolic proteins modulate this activity, providing functional diversity and selective regulatory mechanisms to its role in the control of cellular Ca²⁺ homeostasis.

Computational modeling of the interactions between the maternal and fetal circulations in human pregnancy

In pregnancy, fetal growth is supported by its placenta. In turn, the placenta is nourished by maternal blood, delivered from the uterus, in which the vasculature is dramatically transformed to deliver this blood an ever increasing volume throughout gestation. A healthy pregnancy is thus dependent on the development of both the placental and maternal circulations, but also the interface where these physically separate circulations come in close proximity to exchange gases and nutrients between mum and baby. As the system continually evolves during pregnancy, our understanding of normal vascular anatomy, and how this impacts placental exchange function is limited. Understanding this is key to improve our ability to understand, predict, and detect pregnancy pathologies, but presents a number of challenges, due to the inaccessibility of the pregnant uterus to invasive measurements, and limitations in the resolution of imaging modalities safe for use in pregnancy. Computational approaches provide an opportunity to gain new insights into normal and abnormal pregnancy, by connecting observed anatomical changes from high-resolution imaging to function, and providing metrics that can be observed by routine clinical ultrasound. Such advanced modeling brings with it challenges to scale detailed anatomical models to reflect organ level function. This suggests pathways for future research to provide models that provide both physiological insights into pregnancy health, but also are simple enough to guide clinical focus. We the review evolution of computational approaches to understanding the physiology and pathophysiology of pregnancy in the uterus, placenta, and beyond focusing on both opportunities and challenges. This article is categorized under: Reproductive System Diseases >Computational Models.

Calcium-axonemal microtubuli interactions underlie mechanism(s) of primary cilia morphological changes

We have used cell culture of astrocytes aligned within microchannels to investigate calcium effects on primary cilia morphology. In the absence of calcium and in the presence of flow of media (10 μL.s^-1) the majority (90%) of primary cilia showed reversible bending with an average curvature of 2.1 ± 0.9 × 10^-4 nm^-1. When 1.0 mM calcium was present, 90% of cilia underwent bending. Forty percent of these cilia demonstrated strong irreversible bending, resulting in a final average curvature of 3.9 ± 1 × 10^-4 nm^-1, while 50% of cilia underwent bending similar to that observed during calcium-free flow. The average length of cilia was shifted toward shorter values (3.67 ± 0.34 μm) when exposed to excess calcium (1.0 mM), compared to media devoid of calcium (3.96 ± 0.26 μm). The number of primary cilia that became curved after calcium application was reduced when the cell culture was pre-incubated with 15 μM of the microtubule stabilizer, taxol, for 60 min prior to calcium application. Calcium caused single microtubules to curve at a concentration ≈1.0 mM in vitro, but at higher concentration (≈1.5 mM) multiple microtubule curving occurred. Additionally, calcium causes microtubule-associated protein-2 conformational changes and its dislocation from the microtubule wall at the location of microtubule curvature. A very small amount of calcium, that is 1.45 × 10¹¹ times lower than the maximal capacity of TRPPs calcium channels, may cause gross morphological changes (curving) of primary cilia, while global cytosol calcium levels are expected to remain unchanged. These findings reflect the non-linear manner in which primary cilia may respond to calcium signaling, which in turn may influence the course of development of ciliopathies and cancer.

CXCL1 activates TRPV1 via Gi/o protein and actin filaments

AIMS

CXCL1 is a chemokine with pleiotropic effects, including pain and itch. Itch, an unpleasant sensation that elicits the desire or reflex to scratch, it is evoked mainly from the skin and implicates activation of a specific subset of IB4+, C-type primary afferents. In previous studies we showed that acute application of CXCL1 induced a Ca²⁺ influx of low amplitude and slow kinetics in a subpopulation of transient receptor potential vanilloid type 1 (TRPV1)+/isolectin B4 (IB4)+dorsal root ganglia neurons which also responded to other itch-inducing agents. In this study we explored the mechanism behind the Ca²⁺ influx to better understand how CXCL1 acts on primary sensitive neurons to induce itch.

MATERIALS AND METHODS

Intracellular Ca²⁺ imaging and patch-clamp recordings on dorsal root ganglia neurons primary cultures and HEK293T cell transiently transfected with TRPV1 and CXCR2 plasmids were used to investigate the acute effect (12min application) of 4nM CXCL1. In primary cultures, the focus was on TRPV1+/IB4+ cells to which the itch-sensitive neurons belong.

KEY FINDINGS

The results showed that the Ca²⁺ influx induced by the acute application of CXCL1 is mediated mainly by TRPV1 receptors and depends on extracellular Ca²⁺ not on intracellular stores. TRPV1 was activated, not sensitized by CXCL1, in a CXCR2 receptors- and actin filaments-dependent manner, since specific blockers and actin depolymerizing agents disrupted the CXCL1 effect.

SIGNIFICANCE

This study brings additional data about the itch inducing mechanism of CXCL1 chemokine and about a new mechanism of TRPV1 activation via actin filaments.

The ClC-3 chloride channel associated with microtubules is a target of paclitaxel in its induced-apoptosis

Recent evidences show that cationic fluxes play a pivotal role in cell apoptosis. In this study, the roles of Cl⁻ channels in paclitaxel-induced apoptosis were investigated in nasopharyngeal carcinoma CNE-2Z cells. Chloride current and apoptosis were induced by paclitaxel and inhibited by chloride channel blockers. Paclitaxel-activated current possessed similar properties to volume-activated chloride current. After ClC-3 was knocked-down by ClC-3-siRNA, hypotonicity-activated and paclitaxel-induced chloride currents were obviously decreased, indicating that the chloride channel involved in paclitaxel-induced apoptosis may be ClC-3. In early apoptotic cells, ClC-3 was up-regulated significantly; over-expressed ClC-3 was accumulated in cell membrane to form intercrossed filaments, which were co-localized with α-tubulins; changes of ultrastructures and decrease of flexibility in cell membrane were detected by atomic force microscopy. These suggest that ClC-3 is a critical target of paclitaxel and the involvement of ClC-3 in apoptosis may be associated with its accumulation with membrane microtubules and its over activation.

Transposon activation mutagenesis as a screening tool for identifying resistance to cancer therapeutics

Background

The development of resistance to chemotherapies represents a significant barrier to successful cancer treatment. Resistance mechanisms are complex, can involve diverse and often unexpected cellular processes, and can vary with both the underlying genetic lesion and the origin or type of tumor. For these reasons developing experimental strategies that could be used to understand, identify and predict mechanisms of resistance in different malignant cells would be a major advance.

Methods

Here we describe a gain-of-function forward genetic approach for identifying mechanisms of resistance. This approach uses a modified piggyBac transposon to generate libraries of mutagenized cells, each containing transposon insertions that randomly activate nearby gene expression. Genes of interest are identified using next-gen high-throughput sequencing and barcode multiplexing is used to reduce experimental cost.

Results

Using this approach we successfully identify genes involved in paclitaxel resistance in a variety of cancer cell lines, including the multidrug transporter ABCB1, a previously identified major paclitaxel resistance gene. Analysis of co-occurring transposons integration sites in single cell clone allows for the identification of genes that might act cooperatively to produce drug resistance a level of information not accessible using RNAi or ORF expression screening approaches.

Conclusion

We have developed a powerful pipeline to systematically discover drug resistance in mammalian cells in vitro. This cost-effective approach can be readily applied to different cell lines, to identify canonical or context specific resistance mechanisms. Its ability to probe complex genetic context and non-coding genomic elements as well as cooperative resistance events makes it a good complement to RNAi or ORF expression based screens.

Cytoskeletal and scaffolding proteins as structural and functional determinants of TRP channels

Transient receptor potential (TRP) channels are six transmembrane-spanning proteins, with variable selectivity for cations, that play a relevant role in intracellular Ca(2+) homeostasis. There is a large body of evidence that shows association of TRP channels with the actin cytoskeleton or even the microtubules and demonstrating the functional importance of this interaction for TRP channel function. Conversely, cation currents through TRP channels have also been found to modulate cytoskeleton rearrangements. The interplay between TRP channels and the cytoskeleton has been demonstrated to be essential for full activation of a variety of cellular functions. Furthermore, TRP channels have been reported to take part of macromolecular complexes including different signal transduction proteins. Scaffolding proteins play a relevant role in the association of TRP proteins with other signaling molecules into specific microdomains. Especially relevant are the roles of the Homer family members for the regulation of TRPC channel gating in mammals and INAD in the modulation of Drosophila TRP channels. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

Polycystins and cellular Ca2+ signaling

The cystic phenotype in autosomal dominant polycystic kidney disease is characterized by a profound dysfunction of many cellular signaling patterns, ultimately leading to an increase in both cell proliferation and apoptotic cell death. Disturbance of normal cellular Ca²⁺ signaling seems to be a primary event and is clearly involved in many pathways that may lead to both types of cellular responses. In this review, we summarize the current knowledge about the molecular and functional interactions between polycystins and multiple components of the cellular Ca²⁺-signaling machinery. In addition, we discuss the relevant downstream responses of the changed Ca²⁺ signaling that ultimately lead to increased proliferation and increased apoptosis as observed in many cystic cell types.

TRPV1-tubulin complex: involvement of membrane tubulin in the regulation of chemotherapy-induced peripheral neuropathy

Existence of microtubule cytoskeleton at the membrane and submembranous regions, referred as 'membrane tubulin' has remained controversial for a long time. Since we reported physical and functional interaction of Transient Receptor Potential Vanilloid Sub Type 1 (TRPV1) with microtubules and linked the importance of TRPV1-tubulin complex in the context of chemotherapy-induced peripheral neuropathy, a few more reports have characterized this interaction in in vitro and in in vivo condition. However, the cross-talk between TRPs with microtubule cytoskeleton, and the complex feedback regulations are not well understood. Sequence analysis suggests that other than TRPV1, few TRPs can potentially interact with microtubules. The microtubule interaction with TRPs has evolutionary origin and has a functional significance. Biochemical evidence, Fluorescence Resonance Energy Transfer analysis along with correlation spectroscopy and fluorescence anisotropy measurements have confirmed that TRPV1 interacts with microtubules in live cell and this interaction has regulatory roles. Apart from the transport of TRPs and maintaining the cellular structure, microtubules regulate signaling and functionality of TRPs at the single channel level. Thus, TRPV1-tubulin interaction sets a stage where concept and parameters of 'membrane tubulin' can be tested in more details. In this review, I critically analyze the advancements made in biochemical, pharmacological, behavioral as well as cell-biological observations and summarize the limitations that need to be overcome in the future.

Critical role of TRPC1-mediated Ca²⁺ entry in decidualization of human endometrial stromal cells

Decidualization is an ovarian steroid-induced remodeling/differentiation process of uterus essential for embryo implantation and placentation. Here, we investigated the possible involvement of enhanced Ca²⁺ dynamics in the decidualization process in human endometrial stromal cells (hESC) in its connection with a recently emerging nonvoltage-gated Ca²⁺ entry channel superfamily, the transient receptor potential (TRP) protein. Combined application of 17β-estradiol (E₂) (10 nM) and progesterone (P₄) (1 μM) for 7-14 d resulted in morphological changes of hESC characteristic of decidualization (i.e. cell size increase), whereas sole application of E₂ exerted little effects. A 7- to 14-d E₂/P₄ treatment greatly increased the expression level of decidualization markers IGF binding protein-1 (IGFBP-1) and prolactin and also up-regulated the expression of TRPC1, a canonical TRP subfamily member that has been implicated in store-operated Ca²⁺ influx (SOC) in other cell types. In parallel with this up-regulation, SOC activity in hESC, the nuclear translocation of phosphorylated cAMP responsive element binding protein (p-CREB) and the expression of Forkhead box protein 01 were enhanced significantly. Small interfering RNA knockdown of TRPC1 counteracted the E₂/P₄-induced up-regulation of IGFBP-1 and prolactin and enhancement of SOC activity together with the inhibition of hESC size increase, p-CREB nuclear translocation, and FOXO1 up-regulation. Coadministration of SOC inhibitors SK&F96365 or Gd³⁺ with E₂/P₄ also suppressed the up-regulation of IGFBP-1 and hESC size increase. Similar inhibitory effects were observed with extracellularly applied TRPC1 extracellular loop 3-directed antibody, which is known to bind a near-pore domain of TRPC1 channel and block its Ca²⁺ transporting activity. These results strongly suggest that up-regulation of TRPC1 protein and consequent enhancement of SOC-mediated Ca²⁺ influx may serve as a crucial step for the decidualization process of hESC probably via p-CREB-dependent transcriptional activity associated with FOXO1 activation.

Soluble levels of cytosolic tubulin regulate ciliary length control

We show that manipulation of either the microtubule or the actin cytoskeleton has unexpected influences on cilia length control.

The primary cilium is an evolutionarily conserved dynamic organelle important for regulating numerous signaling pathways, and, as such, mutations disrupting ciliogenesis result in a variety of developmental abnormalities and postnatal disorders. The length of the cilium is regulated by the cell through largely unknown mechanisms. Normal cilia length is important, as either shortened or elongated cilia have been associated with disease and developmental defects. Here we explore the importance of cytoskeletal dynamics in regulating cilia length. Using pharmacological approaches in different cell types, we demonstrate that actin depolymerization or stabilization and protein kinase A activation result in a rapid elongation of the primary cilium. The effects of pharmacological agents on cilia length are associated with a subsequent increase in soluble tubulin levels and can be impaired by depletion of soluble tubulin with taxol. In addition, subtle nocodazole treatment was able to induce ciliogenesis under conditions in which cilia are not normally formed and also increases cilia length on cells that have already established cilia. Together these data indicate that cilia length can be regulated through changes in either the actin or microtubule network and implicate a possible role for soluble tubulin levels in cilia length control.

TRP channels in female reproductive organs and placenta

TRP channel proteins are widely expressed in female reproductive organs. Based on studies detecting TRP transcripts and proteins in different parts of the female reproductive organs and placenta they are supposed to be involved in the transport of the oocyte or the blastocyte through the oviduct, implantation of the blastocyte, development of the placenta and transport processes across the feto-maternal barrier. Furthermore uterus contractility and physiological processes during labour and in mammary glands seem to be dependant on TRP channel expression.

Kif3a deficiency reverses the skeletal abnormalities in Pkd1 deficient mice by restoring the balance between osteogenesis and adipogenesis

Pkd1 localizes to primary cilia in osteoblasts and osteocytes. Targeted deletion of Pkd1 in osteoblasts results in osteopenia and abnormalities in Runx2-mediated osteoblast development. Kif3a, an intraflagellar transport protein required for cilia function, is also expressed in osteoblasts. To assess the relationship between Pkd1 and primary cilia function on bone development, we crossed heterozygous Pkd1- and Kif3a-deficient mice to create compound Pkd1 and Kif3a-deficient mice. Pkd1 haploinsufficiency (Pkd1(+/Δ)) resulted in osteopenia, characterized by decreased bone mineral density, trabecular bone volume, and cortical thickness. In addition, deficiency of Pkd1 resulted in impaired osteoblastic differentiation and enhanced adipogenesis in both primary osteoblasts and/or bone marrow stromal cell cultures. These changes were associated with decreased Runx2 expression, increased PPARγ expression, and impaired hedgehog signaling as evidenced by decreased Gli2 expression in bone and osteoblast cultures. In contrast, heterozygous Kif3a(+/Δ) mice display no abnormalities in skeletal development or osteoblast function, but exhibited decreased adipogenic markers in bone and impaired adipogenesis in vitro in association with decreased PPARγ expression and upregulation of Gli2. Superimposed Kif3a deficiency onto Pkd1(+/Δ) mice paradoxically corrected the effects of Pkd1 deficiency on bone mass, osteoblastic differentiation, and adipogenesis. In addition, Runx2, PPARγ and Gli2 expression in bone and osteoblasts were normalized in compound double Pkd1(+/Δ) and Kif3a(+/Δ) heterozygous mice. The administration of sonic hedgehog, overexpression of Gli2, and the PC1 C-tail construct all increased Gli2 and Runx2-II expression, but decreased PPARγ2 gene expression in C3H10T1/2 cells. Our findings suggest a role for Pkd1 and Kif3a to counterbalance the regulation of osteogenesis and adipogenesis through differential regulation of Runx2 and PPARγ by Gli2.

An outwardly rectifying chloride channel in BeWo choriocarcinoma cell line

In this study, an outwardly rectifying chloride channel was characterized in the trophoblastic cell line BeWo, a human hormone-synthesizing cell which displays many biochemical and morphological properties similar to those reported for the human cytotrophoblast. Ion channel activity was recorded in the cell attached and inside-out configurations with standard patch-clamp technology. In most of the BeWo cells studied, the channel under symmetrical N-methyl-d-glucamine (NMDG-Cl) concentration (Na(+) free solution) in both sides of the membrane exhibited spontaneous activity, an outwardly rectifying current/voltage relationship and single-channel conductances of 15 pS and 48 pS for inwards and outwards currents, respectively. The channel has a low permeability for gluconate with a relative permeability P(gluconate)/P(Cl) of 0.23, and a higher permeability to I(-). The open probability (Po) of the channel exhibited dependence with the applied membrane potential with greater activity at positive pulses. The channel activity was inhibited by the sulphonylurea hypoglycemic agent glibenclamide (50 μM) or by diphenylamine-2-carboxylate (DPC, 500 μM) added to the cytoplasmic side of the patch whereas conductances remained unchanged. The blockade with glibenclamide and DPC was independent of the applied membrane potential. All these results are characteristic of the outwardly rectifying Cl channel (ORCC) found in other types of cells. Neither Po, conductances nor reversal potential (Er) values were affected by the absence of intracellular Ca(2+), suggesting that the channel is not sensitive to Ca(2+).

Model of ionic currents through microtubule nanopores and the lumen

It has been suggested that microtubules and other cytoskeletal filaments may act as electrical transmission lines. An electrical circuit model of the microtubule is constructed incorporating features of its cylindrical structure with nanopores in its walls. This model is used to study how ionic conductance along the lumen is affected by flux through the nanopores, both with and without an external potential applied across its two ends. Based on the results of Brownian dynamics simulations, the nanopores were found to have asymmetric inner and outer conductances, manifested as nonlinear IV curves. Our simulations indicate that a combination of this asymmetry and an internal voltage source arising from the motion of the C-terminal tails causes cations to be pumped across the microtubule wall and propagate in both directions down the microtubule through the lumen, returning to the bulk solution through its open ends. This effect is demonstrated to add directly to the longitudinal current through the lumen resulting from an external voltage source applied across the two ends of the microtubule. The predicted persistent currents directed through the microtubule wall and along the lumen could be significant in directing the dissipation of weak, endogenous potential gradients toward one end of the microtubule within the cellular environment.

Function and regulation of TRPP2 at the plasma membrane

The vast majority (approximately 99%) of all known cases of autosomal dominant polycystic kidney disease (ADPKD) are caused by naturally occurring mutations in two separate, but genetically interacting, loci, pkd1 and pkd2. pkd1 encodes a large multispanning membrane protein (PKD1) of unknown function, while pkd2 encodes a protein (TRPP2, polycystin-2, or PKD2) of the transient receptor potential (TRP) superfamily of ion channels. Biochemical, functional, and genetic studies support a model in which PKD1 physically interacts with TRPP2 to form an ion channel complex that conveys extracellular stimuli to ionic currents. However, the molecular identity of these extracellular stimuli remains elusive. Functional studies in cell culture show that TRPP2 can be activated in response to mechanical cues (fluid shear stress) and/or receptor tyrosine kinase (RTK) and G protein-coupled receptor (GPCR) activation at the cell surface. Recent genetic studies in Chlamydomonas reinhardtii show that CrPKD2 functions in a pathway linking cell-cell adhesion and Ca(2+) signaling. The mode of activation depends on protein-protein interactions with other channel subunits and auxiliary proteins. Therefore, understanding the mechanisms underlying the molecular makeup of TRPP2-containing complexes is critical in delineating the mechanisms of TRPP2 activation and, most importantly, the mechanisms by which naturally occurring mutations in pkd1 or pkd2 lead not only to ADPKD, but also to other defects reported in model organisms lacking functional TRPP2. This review focuses on the molecular assembly, function, and regulation of TRPP2 as a cell surface cation channel and discusses its potential role in Ca(2+) signaling and ADPKD pathophysiology.

A short carboxy-terminal domain of polycystin-1 reorganizes the microtubular network and the endoplasmic reticulum

Mutations of PKD1 cause autosomal dominant polycystic kidney disease (ADPKD), a syndrome characterized by kidney cysts and progressive renal failure. Polycystin-1, the protein encoded by PKD1, is a large integral membrane protein with a short carboxy-terminal cytoplasmic domain that appears to initiate multiple cellular programs. We report now that this polycystin-1 domain contains a novel motif responsible for rearrangements of intermediate filaments, microtubules and the endoplasmic reticulum (ER). This motif reveals homology to CLIMP-63, a microtubule-binding protein that rearranges the ER. Our findings suggest that polycystin-1 influences the shape and localization of both the microtubular network and the ER.

The multimeric structure of polycystin-2 (TRPP2): structural-functional correlates of homo- and hetero-multimers with TRPC1

Polycystin-2 (PC2, TRPP2), the gene product of PKD2, whose mutations cause autosomal dominant polycystic kidney disease (ADPKD), belongs to the superfamily of TRP channels. PC2 is a non-selective cation channel, with multiple subconductance states. In this report, we explored structural and functional properties of PC2 and whether the conductance substates represent monomeric contributions to the channel complex. A kinetic analysis of spontaneous channel currents of PC2 showed that four intrinsic, non-stochastic subconductance states, which followed a staircase behavior, were both pH- and voltage-dependent. To confirm the oligomeric contributions to PC2 channel function, heteromeric PC2/TRPC1 channel complexes were also functionally assessed by single channel current analysis. Low pH inhibited the PC2 currents in PC2 homomeric complexes, but failed to affect PC2 currents in PC2/TRPC1 heteromeric complexes. Amiloride, in contrast, abolished PC2 currents in both the homomeric PC2 complexes and the heteromeric PC2/TRPC1 complexes, thus PC2/TRPC1 complexes have distinct functional properties from the homomeric complexes. The topological features of the homomeric PC2-, TRPC1- and heteromeric PC2/TRPC1 channel complexes, assessed by atomic force microscopy, were consistent with structural tetramers. TRPC1 homomeric channels had different average diameter and protruding height when compared with the PC2 homomers. The contribution of individual monomers to the PC2/TRPC1 hetero-complexes was easily distinguishable. The data support tetrameric models of both the PC2 and TRPC1 channels, where the overall conductance of a particular channel will depend on the contribution of the various functional monomers in the complex.

Mechanically induced osteogenic differentiation--the role of RhoA, ROCKII and cytoskeletal dynamics

Many biochemical factors regulating progenitor cell differentiation have been examined in detail; however, the role of the local mechanical environment on stem cell fate has only recently been investigated. In this study, we examined whether oscillatory fluid flow, an exogenous mechanical signal within bone, regulates osteogenic, adipogenic or chondrogenic differentiation of C3H10T1/2 murine mesenchymal stem cells by measuring Runx2, PPARgamma and SOX9 gene expression, respectively. Furthermore, we hypothesized that the small GTPase RhoA and isometric tension within the actin cytoskeleton are essential in flow-induced differentiation. We found that oscillatory fluid flow induces the upregulation of Runx2, Sox9 and PPARgamma, indicating that it has the potential to regulate transcription factors involved in multiple unique lineage pathways. Furthermore, we demonstrate that the small GTPase RhoA and its effector protein ROCKII regulate fluid-flow-induced osteogenic differentiation. Additionally, activated RhoA and fluid flow have an additive effect on Runx2 expression. Finally, we show RhoA activation and actin tension are negative regulators of both adipogenic and chondrogenic differentiation. However, an intact, dynamic actin cytoskeleton under tension is necessary for flow-induced gene expression.

Submembraneous microtubule cytoskeleton: biochemical and functional interplay of TRP channels with the cytoskeleton

Much work has focused on the electrophysiological properties of transient receptor potential channels. Recently, a novel aspect of importance emerged: the interplay of transient receptor potential channels with the cytoskeleton. Recent data suggest a direct interaction and functional repercussion for both binding partners. The bi-directionality of physical and functional interaction renders therefore, the cytoskeleton a potent integration point of complex biological signalling events, from both the cytoplasm and the extracellular space. In this minireview, we focus mostly on the interaction of the cytoskeleton with transient receptor potential vanilloid channels. Thereby, we point out the functional importance of cytoskeleton components both as modulator and as modulated downstream effector. The resulting implications for patho-biological situations are discussed.

Function and dynamics of PKD2 in Chlamydomonas reinhardtii flagella

To analyze the function of ciliary polycystic kidney disease 2 (PKD2) and its relationship to intraflagellar transport (IFT), we cloned the gene encoding Chlamydomonas reinhardtii PKD2 (CrPKD2), a protein with the characteristics of PKD2 family members. Three forms of this protein (210, 120, and 90 kD) were detected in whole cells; the two smaller forms are cleavage products of the 210-kD protein and were the predominant forms in flagella. In cells expressing CrPKD2-GFP, about 10% of flagellar CrPKD2-GFP was observed moving in the flagellar membrane. When IFT was blocked, fluorescence recovery after photobleaching of flagellar CrPKD2-GFP was attenuated and CrPKD2 accumulated in the flagella. Flagellar CrPKD2 increased fourfold during gametogenesis, and several CrPKD2 RNA interference strains showed defects in flagella-dependent mating. These results suggest that the CrPKD2 cation channel is involved in coupling flagellar adhesion at the beginning of mating to the increase in flagellar calcium required for subsequent steps in mating.