The PDZ-interacting domain of TRPC4 controls its localization and surface expression in HEK293 cells

TRPCs as MS Channels

This chapter reviews recent evidence indicating that canonical or classical transient receptor potential (TRPC) channels are directly or indirectly mechanosensitive (MS) and can therefore be designated as mechano-operated channels (MOCs). The MS functions of TRPCs may be mechanistically related to their better known functions as store-operated and receptor-operated channels (SOCs and ROCs). Mechanical forces may be conveyed to TRPC channels through the "conformational coupling" mechanism that transmits information regarding the status of internal Ca(2+) stores. All TRPCs are regulated by receptors coupled to phospholipases that are themselves MS and can regulate channels via lipidic second messengers. Accordingly, there may be several nonexclusive mechanisms by which mechanical forces may regulate TRPC channels, including direct sensitivity to bilayer mechanics, physical coupling to internal membranes and/or cytoskeletal proteins, and sensitivity to lipidic second messengers generated by MS enzymes. Various strategies that can be used for separating out different MS-gating mechanisms and their possible role in specific TRPCs are discussed.

Intracellular trafficking of TRP channels

Thirteen years ago, it was suggested that exocytotic insertion of store-operated channels into the plasma membrane lead to increased Ca(2+) entry in non-excitable cells upon G protein-coupled or tyrosine kinase receptor stimulation. Since the discovery of the TRP channel superfamily and their involvement in receptor-induced Ca(2+) entry, many studies have shown that different members of the TRP superfamily translocate into the plasma membrane upon stimulation. While the exact molecular mechanism by which TRP channels insert into the plasma membrane is unknown, TRP-binding proteins have been shown to directly regulate this trafficking. This review summarizes recent advances related to the mechanism of TRP channel trafficking, focusing on the role of TRP-binding proteins.

Characterization of Xenopus egg membrane microdomains containing uroplakin Ib/III complex: roles of their molecular interactions for subcellular localization and signal transduction

A single-transmembrane protein uroplakin III (UPIII) and its tetraspanin binding-partner uroplakin Ib (UPIb) are members of the UP proteins that were originally identified in mammalian urothelium. In Xenopus laevis eggs, these proteins: xUPIII and xUPIb, are components of the cholesterol-enriched membrane microdomains or "rafts" and involved in the sperm-egg membrane interaction and subsequent egg activation signaling via Src tyrosine kinase at fertilization. Here, we investigate whether the xUPIII-xUPIb complex is in close proximity to CD9, a tetraspanin that has been implicated in the sperm-egg fusion in the mouse and GM1, a ganglioside typically enriched in egg rafts. Preparation of the egg membrane microdomains using different non-ionic detergents (Brij 98 and Triton X-100), chemical cross-linking, co-immunoprecipitation, in vitro kinase assay and in vitro fertilization experiments demonstrated that GM1, but not CD9, is in association with the xUPIII-xUPIb complex and contributes to the sperm-dependent egg activation. Transfection experiments using HEK293 cells demonstrated that xUPIII and xUPIb localized efficiently to the cholesterol-dependent membrane microdomains when they were co-expressed, whereas co-expression of xUPIII and CD9, instead of xUPIb, did not show this effect. Furthermore, xUPIII and xUPIb were shown to suppress kinase activity of the wild type, but not a constitutively active form of, Xenopus Src protein co-expressed in HEK293 cells. These results provide novel insight into the molecular architecture of the egg membrane microdomains containing xUPIII, xUPIb and Src, which may contribute to the understanding of sperm-egg interaction and signaling during Xenopus fertilization.

The NHERF1 PDZ2 domain regulates PKA-RhoA-p38-mediated NHE1 activation and invasion in breast tumor cells

Understanding the signal transduction systems governing invasion is fundamental for the design of therapeutic strategies against metastasis. Na(+)/H(+) exchanger regulatory factor (NHERF1) is a postsynaptic density 95/disc-large/zona occludens (PDZ) domain-containing protein that recruits membrane receptors/transporters and cytoplasmic signaling proteins into functional complexes. NHERF1 expression is altered in breast cancer, but its effective role in mammary carcinogenesis remains undefined. We report here that NHERF1 overexpression in human breast tumor biopsies is associated with metastatic progression, poor prognosis, and hypoxia-inducible factor-1alpha expression. In cultured tumor cells, hypoxia and serum deprivation increase NHERF1 expression, promote the formation of leading-edge pseudopodia, and redistribute NHERF1 to these pseudopodia. This pseudopodial localization of NHERF1 was verified in breast biopsies and in three-dimensional Matrigel culture. Furthermore, serum deprivation and hypoxia stimulate the Na(+)/H(+) exchanger, invasion, and activate a protein kinase A (PKA)-gated RhoA/p38 invasion signal module. Significantly, NHERF1 overexpression was sufficient to induce these morphological and functional changes, and it potentiated their induction by serum deprivation. Functional experiments with truncated and binding groove-mutated PDZ domain constructs demonstrated that NHERF1 regulates these processes through its PDZ2 domain. We conclude that NHERF1 overexpression enhances the invasive phenotype in breast cancer cells, both alone and in synergy with exposure to the tumor microenvironment, via the coordination of PKA-gated RhoA/p38 signaling.

TRPC channels: interacting proteins

TRP channels, in particular the TRPC and TRPV subfamilies, have emerged as important constituents of the receptor-activated Ca2+ influx mechanism triggered by hormones, growth factors, and neurotransmitters through activation ofphospholipase C (PLC). Several TRPC channels are also activated by passive depletion of endoplasmic reticulum (ER) Ca2+. Although in several studies the native TRP channels faithfully reproduce the respective recombinant channels, more often the properties of Ca2+ entry and/or the store-operated current are strikingly different from that of the TRP channels expressed in the same cells. The present review aims to discuss this disparity in the context of interaction of TRPC channels with auxiliary proteins that may alter the permeation and regulation of TRPC channels.

Ionic channels formed by TRPC4

TRPC4 (transient receptor potential canonical 4) is a member of the TRPC sub-family and, within this sub-family, TRPC4 is most closely related to TRPC5. A number of splice variants of TRPC4 have been identified, whereby TRPC4alpha and TRPC4beta appear to be the most abundant isoforms in various species. TRPC4alpha comprises six transmembrane segments and the N- and C-termini are located intracellularly. Additionally, TRPC4alpha shares other structural features with members of the TRPC sub-group, including ankyrin-like repeats, coiled-coil regions and binding sites for calmodulin and IP3 receptors. Three calmodulin-binding domains have been identified in the C-terminus of TRPC4alpha. TRPC4beta lack 84 amino acids in the C-terminus, which correspond to the last two calmodulin-binding sites of TRPCalpha. The first and last calmodulin-binding domains of TRPC4alpha overlap with binding sites for the N- and C-termini of IP3 receptors. The ionic channels formed by TRPC4 appear to be Ca(2+)-permeable, although there is a considerably discrepancy in the degree of Ca2+ selectivity. Studies with mice lacking TRPC4 (TRPC4(-/-)) suggest an important role for TRPC4 in supporting Ca2+ entry. The defect in Ca2+ entry in TRPC4(-/-) mice appears to be associated with a reduction of the vasorelaxation of arteries, vascular permeability in the lung and neurotransmitter release from thalamic dendrites.

Trafficking of TRP channels: determinants of channel function

Transient receptor potential (TRP) channels are members of a relatively newly described family of cation channels that display a wide range of properties and mechanisms of activation. The exact physiological function and regulation of most of these channels have not yet been conclusively determined. Studies over the past decade have revealed important features of the channels that contribute to their function. These include homomeric interactions between TRP monomers, selective heteromeric interactions within members of the same subfamily, interactions of TRPs with accessory proteins and assembly into macromolecular signaling complexes, and regulation within functionally distinct cellular microdomains. Further, distinct constitutive and regulated vesicular trafficking mechanisms have a critical role not only in controlling the surface expression of TRP channels but also their activation in response to stimuli. A number of cellular components such as cytoskeletal and scaffolding proteins also contribute to TRP channel trafficking. Thus, mechanisms involved in the assembly and trafficking of TRP channels control their plasma membrane expression and critically impact their function and regulation.

Trafficking and assembly of the cold-sensitive TRPM8 channel

TRPM (transient receptor potential melastatin-like) channels are distinct from many other members of the transient receptor potential family in regard to their overall size (>1000 amino acids), the lack of N-terminal ankyrin-like repeats, and hydrophobicity predictions that may allow for more than six transmembrane regions. Common to each TRPM member is a prominent C-terminal coiled coil region. Here we have shown that TRPM8 channels assemble as multimers using the putative coiled coil region within the intracellular C terminus and that this assembly can be disturbed by a single point mutation within the coiled coil region. This mutant neither gives rise to functional channels nor do its subunits interact or form protein complexes that correspond to a multimer. However, they are still transported to the plasma membrane. Furthermore, wild-type currents can be suppressed by expressing the membrane-attached C-terminal region of TRPM8. To separate assembly from trafficking, we investigated the maturation of TRPM8 protein by identifying and mutating the relevant N-linked glycosylation site and showing that glycosylation is neither essential for multimerization nor for transport to the plasma membrane per se but appears to facilitate efficient multimerization and transport.

Evidence that TRPC4 supports the calcium selective I(CRAC)-like current in human gingival keratinocytes

We previously demonstrated that high external [Ca(2+)] activated two Ca(2+) currents in human gingival keratinocytes (HGKs): an initial small I(CRAC)-like current and a second large nonspecific cation current (Fatherazi S, Belton CM, Cai S, Zarif S, Goodwin PC, Lamont RJ, Izutsu KT; Pflugers Arch 448:93-104, 2004). It was recently shown that TRPC1, a member of the transient receptor potential protein family, is a component of the store-operated calcium entry mechanism in keratinocytes. To further elucidate the molecular identity of these channels, we investigated the expression of TRPC4 in gingival tissue and in cultured keratinocytes, and the effect of knockdown of TRPC4 expression on the Ca(2+) currents and influx. Immunohistochemistry showed TRPC4 was present in gingival epithelium as well as in HGKs cultured in different [Ca(2+)]s. Results from tissue and cultured HGKs demonstrated TRPC4 expression decreased with differentiation. Knockdown of TRPC4 in proliferating HGKs with antisense oligonucleotides significantly reduced the intracellular [Ca(2+)] increase obtained upon exposure to high external [Ca(2+)]. Antisense knockdown of TRPC4 expression was confirmed by reverse transcriptase polymerase chain reaction, Western blot, and immunofluorescence microscopy of transfected HGKs. Immunofluorescence microscopy and patch clamp measurements in Lucifer-yellow-tagged, antisense-treated HGKs showed attenuation of TRPC4 expression levels as well as attenuation of the I(CRAC)-like current in the same cell, whereas the large nonspecific cation current was unchanged but significantly delayed. Cells transfected with a scrambled TRPC4 oligonucleotide showed no change in either the I(CRAC)-like or nonspecific currents. The results indicate that TRPC4 is an important component of the I(CRAC)-like channel in HGKs.

Functional organization of TRPC-Ca2+ channels and regulation of calcium microdomains

TRP family of proteins are components of unique cation channels that are activated in response to diverse stimuli ranging from growth factor and neurotransmitter stimulation of plasma membrane receptors to a variety of chemical and sensory signals. This review will focus on members of the TRPC sub-family (TRPC1-TRPC7) which currently appear to be the strongest candidates for the enigmatic Ca(2+) influx channels that are activated in response to stimulation of plasma membrane receptors which result in phosphatidyl inositol-(4,5)-bisphosphate (PIP(2)) hydrolysis, generation of IP(3) and DAG, and IP(3)-induced Ca(2+) release from the intracellular Ca(2+) store via inositol trisphosphate receptor (IP(3)R). Homomeric or selective heteromeric interactions between TRPC monomers generate distinct channels that contribute to store-operated as well as store-independent Ca(2+) entry mechanisms. The former is regulated by the emptying/refilling of internal Ca(2+) store(s) while the latter depends on PIP(2) hydrolysis (due to changes in PIP(2) per se or an increase in diacylglycerol, DAG). Although the exact physiological function of TRPC channels and how they are regulated has not yet been conclusively established, it is clear that a variety of cellular functions are controlled by Ca(2+) entry via these channels. Thus, it is critical to understand how cells coordinate the regulation of diverse TRPC channels to elicit specific physiological functions. It is now well established that segregation of TRPC channels mediated by interactions with signaling and scaffolding proteins, determines their localization and regulation in functionally distinct cellular domains. Furthermore, both protein and lipid components of intracellular and plasma membranes contribute to the organization of these microdomains. Such organization serves as a platform for the generation of spatially and temporally dictated [Ca(2+)](i) signals which are critical for precise control of downstream cellular functions.

Transient receptor potential channels and caveolin-1: good friends in tight spaces

Caveolae formation has raised the concept of energy efficiency to new heights. The ultimate purpose of caveolae formation is to colocalize signaling proteins with membrane microdomains in order to facilitate their interaction and improve signal transduction efficiency. Although we know that the main structural protein of caveolae is caveolin, how caveolin interacts with membrane proteins to facilitate their integration into lipid raft domains is unclear. A caveolin-scaffolding domain (CSD) on caveolin itself can associate with membrane proteins such as G proteins and endothelial nitric oxide synthase. In this issue, Kwiatek et al. (p. 1174) report that the TRPC1 channel protein contains a C-terminal CSD-consensus binding sequence that allows for its physical and functional interaction with caveolin-1 in the caveolae of human pulmonary artery endothelial cells (PAEC). Competitive interaction with a CSD-conjugated peptide attenuates thrombin- and thapsigargin-induced Ca2+ influx via store-operated TRPC1 channels. Their data suggest that caveolin-1 can directly regulate TRPC1 function, extending its already ascribed role as a structural protein.

Interaction of epithelial ion channels with the actin-based cytoskeleton

The interaction of ion channels with the actin-based cytoskeleton in epithelial cells not only maintains the polarized expression of ion channels within specific membrane domains, it also functions in the intracellular trafficking and regulation of channel activity. Initial evidence supporting an interaction between epithelial ion channels and the actin-based cytoskeleton came from patch-clamp studies examining the effects of cytochalasins on channel activity. Cytochalasins were shown to either activate or inactivate epithelial ion channels. An interaction between the actin-based cytoskeleton and epithelial ion channels was further supported by the fact that the addition of monomeric or filamentous actin to excised patches had an effect on channel activity comparable to that of cytochalasins. Through the recent application of molecular and proteomic approaches, we now know that the interactions between epithelial ion channels and actin can either be direct or indirect, the latter being mediated through scaffolding or actin-binding proteins that serve as links between the channels and the actin-based cytoskeleton. This review discusses recent advances in our understanding of the interactions between epithelial ion channels and the actin-based cytoskeleton, and the roles these interactions play in regulating the cell surface expression, activity, and intracellular trafficking of epithelial ion channels.

An introduction to TRP channels

The aim of this review is to provide a basic framework for understanding the function of mammalian transient receptor potential (TRP) channels, particularly as they have been elucidated in heterologous expression systems. Mammalian TRP channel proteins form six-transmembrane (6-TM) cation-permeable channels that may be grouped into six subfamilies on the basis of amino acid sequence homology (TRPC, TRPV, TRPM, TRPA, TRPP, and TRPML). Selected functional properties of TRP channels from each subfamily are summarized in this review. Although a single defining characteristic of TRP channel function has not yet emerged, TRP channels may be generally described as calcium-permeable cation channels with polymodal activation properties. By integrating multiple concomitant stimuli and coupling their activity to downstream cellular signal amplification via calcium permeation and membrane depolarization, TRP channels appear well adapted to function in cellular sensation. Our review of recent literature implicating TRP channels in neuronal growth cone steering suggests that TRPs may function more widely in cellular guidance and chemotaxis. The TRP channel gene family and its nomenclature, the encoded proteins and alternatively spliced variants, and the rapidly expanding pharmacology of TRP channels are summarized in online supplemental material.

Renal Ca2+ handling in sgk1 knockout mice

Coexpression studies in Xenopus oocytes revealed the ability of the serum- and glucocorticoid-inducible kinase 1 (SGK1) to stimulate the renal epithelial Ca(2+) channel TRPV5. SGK1 increases the abundance of the channel protein in the plasma membrane, an effect requiring the participation of the Na(+)/H(+) exchanger regulating factor 2 (NHERF2). The present study was performed to explore the role of SGK1 in the regulation of renal Ca(2+) handling in vivo. To this end, TRPV5, calbindin D-28K abundance, and renal Ca(2+) excretion were analyzed in gene-targeted mice lacking functional SGK1 (sgk1( -/- )) and their age- and sex-matched littermates (sgk1( +/+ )). Immunohistochemistry revealed lower abundance of TRPV5 and calbindin D-28K protein in sgk1( -/- ) mice than in sgk1( +/+ ) mice, both fed with control diet. Feeding the mice a Ca(2+)-deficient diet marked ly increased TRPV5 protein abundance in both genotypes. Renal Ca(2+) excretion under control diet was significantly lower in sgk1 ( -/- ) than in sgk1( +/+ ) mice. The Ca(2+)-deficient diet decreased renal excretion of Ca(2+) to the same levels in both phenotypes. Furosemide increased fractional Ca(2+) excretion and dissipated the difference between phenotypes. We conclude that lack of SGK1 may lead to decrease in TRPV5 abundance in connecting tubules but does not abrogate TRPV5 regulation. The decrease in abundance of TRPV5 in connecting tubules of sgk1( -/- ) mice is presumably compensated for by enhanced Ca(2+) reabsorption in upstream nephron segments such as the loop of Henle, which may indirectly result from impaired SGK1-dependent Na(+) reabsorption in the aldosterone-sensitive distal part of the nephron, salt loss, and enhanced Na(+) (and Ca(2+)) reabsorption in those upstream nephron segments.

Interaction of the epithelial Ca2+ channels TRPV5 and TRPV6 with the intestine- and kidney-enriched PDZ protein NHERF4

The epithelial Ca(2+) channels TRPV5 and TRPV6 constitute the apical Ca(2+) influx pathway in epithelial Ca(2+) transport. PDZ proteins have been demonstrated to play a crucial role in the targeting or anchoring of ion channels and transporters in the apical domain of the cell. In this study, we describe the identification of NHERF4 (Na-P(i) Cap2/IKEPP/PDZK2) as a novel TRPV5- and TRPV6-associated PDZ protein. NHERF4 was identified using two separate yeast two-hybrid screens with the carboxyl termini of TRPV5 and TRPV6 as bait. Binding of the carboxyl termini of TRPV5 and TRPV6 with NHERF4 was confirmed by GST pull-down assays using in-vitro-translated NHERF4 or lysates of Xenopus laevis oocytes expressing NHERF4. Furthermore, the interaction was confirmed by GST pull-down and co-immunoprecipitation assays using in-vitro-translated full-length TRPV5 and Xenopus oocytes or HEK293 cells co-expressing NHERF4 and TRPV5/TRPV6, respectively. The fourth PDZ domain of NHERF4 was sufficient for the interaction, although PDZ domain 1 also contributed to the binding. The binding site for NHERF4 localized in a conserved region in the carboxyl terminus of TRPV5 and was distinct from the binding site of the PDZ protein NHERF2. NHERF4 predominantly localized at the plasma membrane of X. laevis oocytes and HeLa cells. This localization was independent of the presence of TRPV5. Therefore, we hypothesize a role for this novel PDZ protein as a putative plasma membrane scaffold for the epithelial Ca(2+) channels.

Insights on TRP channels from in vivo studies in Drosophila

Transient receptor potential (TRP) channels mediate responses in a large variety of signaling mechanisms. Most studies on mammalian TRP channels rely on heterologous expression, but their relevance to in vivo tissues is not entirely clear. In contrast, Drosophila TRP and TRP-like (TRPL) channels allow direct analyses of in vivo function. In Drosophila photoreceptors, activation of TRP and TRPL is mediated via the phosphoinositide cascade, with both Ca2+ and diacylglycerol (DAG) essential for generating the light response. In tissue culture cells, TRPL channels are constitutively active, and lipid second messengers greatly facilitate this activity. Inhibition of phospholipase C (PLC) completely blocks lipid activation of TRPL, suggesting that lipid activation is mediated via PLC. In vivo studies in mutant Drosophila also reveal an acute requirement for lipid-producing enzyme, which may regulate PLC activity. Thus, PLC and its downstream second messengers, Ca2+ and DAG, constitute critical mediators of TRP/TRPL gating in vivo.

Glycosylation of the osmoresponsive transient receptor potential channel TRPV4 on Asn-651 influences membrane trafficking

We identified a consensus N-linked glycosylation motif within the pore-forming loop between the fifth and sixth transmembrane segments of the osmoresponsive transient receptor potential (TRP) channel TRPV4. Mutation of this residue from Asn to Gln (i.e., TRPV4(N651Q)) resulted in loss of a slower migrating band on anti-TRPV4 immunoblots and a marked reduction in lectin-precipitable TRPV4 immunoreactivity. HEK293 cells transiently transfected with the mutant TRPV4(N651Q) exhibited increased calcium entry in response to hypotonic stress relative to wild-type TRPV4 transfectants. This increase in hypotonicity responsiveness was associated with an increase in plasma membrane targeting of TRPV4(N651Q) relative to wild-type TRPV4 in both HEK293 and COS-7 cells but had no effect on overall channel abundance in whole cell lysates. Residue N651 of TRPV4 is immediately adjacent to the pore-forming loop. Although glycosylation in this vicinity has not been reported for a TRP channel, the structurally related hexahelical hyperpolarization-activated cyclic nucleotide-gated channel, HCN2, and the voltage-gated potassium channel, human ether-a-go-go-related (HERG), share a nearly identically situated and experimentally confirmed N-linked glycosylation site which promotes rather than limits channel insertion into the plasma membrane. These data point to a potentially conserved structural and functional feature influencing membrane trafficking across diverse members of the voltage-gated-like ion channel superfamily.

Ca2+ signaling microdomains:platforms for the assembly and regulation of TRPC channels

The transient receptor potential canonical family (TRPC1-TRPC7) of ion channel proteins, which are activated in response to agonist-stimulated phosphatidylinositol (4,5)-bisphosphate [PtdIns(4,5)P(2)] hydrolysis, are proposed components of the elusive store-operated Ca(2+) (SOC) channel. TRPC channels display distinct properties and interact to form homomeric or heteromeric channels that differ in their function and regulation. Although the exact function of TRPC channels and how they are regulated has not been established, increasing data suggest that they are localized and regulated within Ca(2+) signaling microdomains. TRPC channels contribute to store-operated and store-independent Ca(2+) entry mechanisms, both of which are activated by agonist-stimulated PtdIns(4,5)P(2) hydrolysis. Elucidation of how cells achieve specificity and precise temporal and spatial coordination of channel activation is crucial for understanding the molecular basis of agonist-mediated stimulation of Ca(2+) entry and identifying downstream physiological functions. This review will address the assembly and localization of TRPC channels and how these processes impact their function.

Receptor-operated cation channels formed by TRPC4 and TRPC5

TRPC4 and TRPC5 form cation channels that contribute to phospholipase C-dependent Ca(2+) entry following stimulation of G-protein-coupled receptors or receptor tyrosine kinases. Surprisingly, in different studies, TRPC4 and TRPC5 have been shown to form either store-operated channels with a relatively high Ca(2+) permeability, or nonselective cation channels activated independently of store depletion. In this review, we summarize and discuss data on the regulation and permeability properties of TRPC4 and TRPC5, and data on native channels that might be composed of these isoforms.

Structure-function analysis of TRPV channels

In recent years many new members of the family of TRP ion channels have been identified. These channels are classified into several subgroups and participate in many sensory and physiological functions. TRPV channels are important for the perception of pain, temperature sensing, osmotic regulation, and maintenance of calcium homeostasis, and much recent research concerns the identification of protein domains involved in mediating specific channel functions. Recent literature on TRPV channel subunit composition, protein domains required for subunit assembly, trafficking, and regulation will be reviewed and discussed.

The epithelial calcium channels TRPV5 and TRPV6: regulation and implications for disease

The epithelial Ca(2+) channels TRPV5 and TRPV6 represent a new family of Ca(2+) channels that belongs to the superfamily of transient receptor potential channels. TRPV5 and TRPV6 constitute the apical Ca(2+) entry mechanism in active Ca(2+) transport in kidney and intestine. The central role of TRPV5 and TRPV6 in active Ca(2+) (re)absorption makes it a prime target for regulation to maintain Ca(2+) balance. This review covers the hormonal regulation, interaction with accessory proteins and (patho)physiological implications of these epithelial Ca(2+) channels.

Epidermal Growth Factor Induces Tyrosine Phosphorylation, Membrane Insertion, and Activation of Transient Receptor Potential Channel 4

Various members of the canonical family of transient receptor potential channels (TRPCs) exhibit increased cation influx following receptor stimulation or Ca(2+) store depletion. Tyrosine phosphorylation of TRP family members also results in increased channel activity; however, the link between the two events is unclear. We report that two tyrosine residues in the C terminus of human TRPC4 (hTRPC4), Tyr-959 and Tyr-972, are phosphorylated following epidermal growth factor (EGF) receptor stimulation of COS-7 cells. This phosphorylation was mediated by Src family tyrosine kinases (STKs), with Fyn appearing to be the dominant kinase. In addition, EGF receptor stimulation induced the exocytotic insertion of hTRPC4 into the plasma membrane dependent on the activity of STKs and was accompanied by a phosphorylation-dependent increase in the association of hTRPC4 with Na(+)/H(+) exchanger regulatory factor. Furthermore, this translocation and association was defective upon mutation of Tyr-959 and Tyr-972 to phenylalanine. Significantly, inhibition of STKs was concomitant with a reduction in Ca(2+) influx in both native COS-7 cells and hTRPC4-expressing HEK293 cells, with cells expressing the Y959F/Y972F mutant exhibiting a reduced EGF response. These findings represent the first demonstration of a mechanism for phosphorylation to modulate TRPC channel function.

Activation of the endothelial store-operated ISOC Ca2+ channel requires interaction of protein 4.1 with TRPC4

Store-operated calcium (SOC) entry represents the principal Ca2+ entry pathway into nonexcitable cells. Despite intensive investigation, mechanisms underlying activation of SOC entry have remained elusive. The endothelial ISOC channel is a Ca2+-selective SOC entry channel to which the transient receptor potential (TRP) proteins TRPC1 and TRPC4 contribute subunits. Activation of ISOC is specifically regulated by the spectrin-actin membrane skeleton; however, the nature of coupling between the ISOC channel and membrane skeleton is unknown. Here we demonstrate that protein 4.1 is an essential component of the ISOC channel gating mechanism. Protein 4.1 interacts with TRPC4 and the membrane skeleton. Deletion of the protein 4.1 binding domain on TRPC4 or peptide competition to the protein 4.1 binding domain prevents ISOC activation. These findings reveal that interaction of protein 4.1 with TRPC4 is required for activation of the endothelial ISOC channel.

Light-regulated interaction of Dmoesin with TRP and TRPL channels is required for maintenance of photoreceptors

Recent studies in Drosophila melanogaster retina indicate that absorption of light causes the translocation of signaling molecules and actin from the photoreceptor's signaling membrane to the cytosol, but the underlying mechanisms are not fully understood. As ezrin-radixin-moesin (ERM) proteins are known to regulate actin-membrane interactions in a signal-dependent manner, we analyzed the role of Dmoesin, the unique D. melanogaster ERM, in response to light. We report that the illumination of dark-raised flies triggers the dissociation of Dmoesin from the light-sensitive transient receptor potential (TRP) and TRP-like channels, followed by the migration of Dmoesin from the membrane to the cytoplasm. Furthermore, we show that light-activated migration of Dmoesin results from the dephosphorylation of a conserved threonine in Dmoesin. The expression of a Dmoesin mutant form that impairs this phosphorylation inhibits Dmoesin movement and leads to light-induced retinal degeneration. Thus, our data strongly suggest that the light- and phosphorylation-dependent dynamic association of Dmoesin to membrane channels is involved in maintenance of the photoreceptor cells.

Plasma membrane localization and function of TRPC1 is dependent on its interaction with beta-tubulin in retinal epithelium cells

Mammalian homologues of the Drosophila canonical Transient Receptor Potential (TRPC) protein have been proposed to encode the store-operated Ca2+ influx (SOC) channel(s). This study examines the role of TRPC1 in the SOC mechanism of retinal cells. htrpc1 transcript was detected in bovine retinal and in human adult retinal pigment epithelial (ARPE) cells. Western blot analysis also confirmed the expression of TRPC1 protein in neuronal cells including retina and ARPE cells. To determine the role of TRPC1 protein in retinal cells, TRPC1 was recombinantly expressed in ARPE cells and changes in intracellular Ca2+ were analyzed. ARPE cells stably transfected with htrp1 cDNA displayed 2-fold higher Ca2+ influx with no significant increase in the basal influx. Consistent with this the overexpressed TRPC1 protein was localized in the plasma membrane region of ARPE cells. Interestingly, both bovine retinal tissues and ARPE cells showed that TRPC1 protein co-localizes and could be co-immunoprecipitated with beta-tubulin. Disruption of tubulin by colchicine significantly decreased both plasma membrane staining of the TRPC1 protein and Ca2+ influx in ARPE cells. These results suggest that TRPC1 channel protein is expressed in retinal cells, further, targeting/retention of the TRPC1 protein to the plasma membrane in retinal cells is mediated via its interaction with beta-tubulin.

Polarized localization of vitamin C transporters, SVCT1 and SVCT2, in epithelial cells

Messenger RNA of homologous sodium-vitamin C cotransporters, SVCT1 and SVCT2, were found in the intestine. Studies using cultured intestinal cells suggested an apical presence of SVCT1 but the function of SVCT2 was unknown. Here, we showed that enterocytes from heterozygous SVCT2-knockout mice had lower sodium-dependent vitamin C accumulation compared to those from the wildtype. Thus, SVCT2 appears to be functional in enterocytes. We then tested whether SVCT2 could have a redundant function as SVCT1 by constructing and expressing EGFP-tagged SVCTs in intestinal Caco-2 and kidney MDCK cells. In confluent epithelial cells, SVCT1 protein expressed predominantly on the apical membrane. SVCT2, in contrast, accumulated at the basolateral surface. Functionally, SVCT1 expression led to more transport activity from the apical membrane, while SVCT2 expression only increased the uptake under the condition when basolateral membrane was exposed. This differential epithelial membrane distribution and function suggests non-redundant functions of these two isoforms.

TRPMs and neuronal cell death

Death of CNS neurons during acute injury occurs as a result of a complex combination of excitotoxicity, necrosis, apoptosis, oedema and inflammatory reactions. Neuroprotection via glutamate receptor blockade or antioxidant or anti-inflammatory therapy have not proven effective in the clinical treatment of brain damage due to narrow therapeutic windows, poor pharmacokinetics or blockade of the signalling essential for normal excitatory neurotransmission and neuronal survival. Recent work in neuronal biochemistry, genomics and proteomics has increased understanding of the molecular organization of the excitatory synapse and the neuronal postsynaptic density. Transient receptor potential (TRP) channels are an exciting new family of cation channels that are highly expressed in the brain. Several members can be induced by oxidative stress and oxygen free radicals, both of which play important roles in neurodegeneration. Recent work has indicated that members of the melastatin subfamily (TRPM) of TRP proteins, particularly TRPM7 and TRPM2, may play key roles in neuronal death that is activated by oxidative stress and downstream from excitotoxic signal pathways. This discovery provides an exiting new avenue for research into the pathophysiology and treatment of acute neurodegeneration.

Protein-protein interaction and functionTRPC channels

Since their identification in the concluding years of the last century, the mammalian transient receptor potential (canonical) (TRPC) channels have remained in the limelight as the primary candidates for the Ca(2+) entry pathway activated by the hormones, growth factors, and neurotransmitters that exert their effect through activation of PLC. Although TRPC channels have been shown clearly to mediate, at least in part, receptor-activated Ca(2+) entry in literally all cell types, several of their central characteristics, as recorded in expression systems using recombinant channels, differ from those of the native receptor-dependent Ca(2+) influx channels. The present review attempts to highlight the interaction of TRPC channels with other proteins, which may explain the variability of TRPC channel activation and regulatory mechanisms observed with the native and recombinant channels. These include the homologous and heterotopous interactions of TRPC channel isoforms, the interaction of TRPC channels with calmodulin, PLCgamma, IP(3) receptors, and with scaffolding proteins like InaD, EBP50/NEHRF, caveolin, Janctate and Homers.

Identification of a protein that interacts with the vanilloid receptor

The vanilloid receptor (VR1 or TRPV1) is a capsaicin (CAP)-sensitive non-selective cation channel. Although its channel activity is reportedly modulated through protein-protein interactions, to date very few VR1 interacting proteins have been identified. To address this issue, a yeast two-hybrid screening technique using the C-terminus of rVR1 as bait was employed. Upon interrogation of a mouse brain library, one gene product that interacts with VR1 and is highly homologous to human eferin was found. Its interaction with VR1 was confirmed by GST-pull-down and co-immunoprecipitation. When cotransfected into HEK cells, VR1 and eferin largely colocalize. Furthermore, in rat dorsal root ganglion cells, the rat eferin homologue also colocalizes with rVR1. However, this protein had no significant effect on VR1 channel activity in response to CAP. This was determined by two-electrode recording of oocytes and whole cell recording of HEK cells that were cotransfected with VR1 and human eferin.

Formation of a physiological complex between TRPV2 and RGA protein promotes cell surface expression of TRPV2

The transient receptor potential, sub-family Vanilloid (TRPV)(2) cation channel is activated in response to extreme temperature elevations in sensory neurons. However, TRPV2 is widely expressed in tissues with no sensory function, including cells of the immune system. Regulation of GRC, the murine homolog of TRPV2 has been studied in insulinoma cells and myocytes. GRC is activated in response to certain growth factors and neuropeptides, via a mechanism that involves regulated access of the channel to the plasma membrane. This is likely to be an important primary control mechanism for TRPV2 outside the CNS. Here, we report that a regulated trafficking step controls the access of TRPV2 to the cell surface in mast cells. In mast cells, elevations in cytosolic cAMP are sufficient to drive plasma membrane localization of TRPV2. We have previously proposed that the recombinase gene activator protein (RGA), a four-transmembrane domain, intracellular protein, associates with TRPV2 during the biosynthesis and early trafficking of the channel. We use a polyclonal antibody to RGA to confirm the formation of a physiological complex between RGA and TRPV2. Finally, we show that over-expression of the RGA protein potentiates the basal surface localization of TRPV2. We propose that trafficking and activation mechanisms intersect for TRPV2, and that cAMP mobilizing stimuli may regulate TRPV2 localization in non-sensory cells. RGA participates in the control of TRPV2 surface levels, and co-expression of RGA may be a key component of experimental systems that seek to study TRPV2 physiology.

Homo- and heteromeric assembly of TRP channel subunits

Mammalian homologues of the Drosophila melanogaster transient receptor potential (TRP) channels are the second largest cation channel family within the superfamily of hexahelical cation channels. Most mammalian TRP channels function as homooligomers and mediate mono- or divalent cation entry upon activation by a variety of stimuli. Because native TRP channels may be multimeric proteins of possibly complex composition, it is difficult to compare cation conductances in native tissues to those of clearly defined homomeric TRP channel complexes in living cells. Therefore, the possibility of heteromeric TRP channel assembly has been investigated in recent years by several groups. As a major conclusion of these studies, most heteromeric TRP channel complexes appear to consist of subunit combinations only within relatively narrow confines of phylogenetic subfamilies. Although the general capability of heteromer formation between closely related TRP channel subunits is now clearly established, we are only beginning to understand whether these heteromeric complexes are of physiological significance. This review summarizes the current knowledge on the promiscuity and specificity of the assembly of channel complexes composed of TRPC-, TRPV- and TRPM-subunits of mammalian TRP channels.

Desensitization of canonical transient receptor potential channel 5 by protein kinase C

The classic type of transient receptor potential channel (TRPC) is a molecular candidate for Ca(2+)-permeable cation channel in mammalian cells. TRPC5 is desensitized rapidly after activation by G protein-coupled receptor. Herein we report our investigation into the desensitization of mTRPC5 and localization of the molecular determinants of this desensitization using mutagenesis. TRPC5 was initially activated by muscarinic stimulation using 100 microM carbachol (CCh) and then decayed rapidly even in the presence of CCh (desensitization). Increased EGTA or omission of MgATP in the pipette solution slowed the rate of this desensitization. The protein kinase C (PKC) inhibitors, 1 microM chelerythrine, 100 nM GF109203X, or PKC peptide inhibitor (19-36), inhibited this desensitization of TRPC5 activated by 100 microM CCh. When TRPC5 current was activated by intracellular GTPgammaS, PKC inhibitors prevented TRPC5 desensitization and the mutation of TRPC5 T972 to alanine slowed the desensitization process dramatically. We conclude that the desensitization of TRPC5 occurs via PKC phosphorylation and suggest that threonine at residue 972 of mouse TRPC5 might be required for its phosphorylation by PKC.

Expression of TRPC4 channel protein that interacts with NHERF-2 in rat descending vasa recta

The PDZ domain adaptor protein Na+/H+exchanger regulatory factor (NHERF)-2 is expressed in renal medullary descending vasa recta (DVR), although its function has not been defined. Transient receptor potential channels (TRPC) TRPC4 and TRPC5, nonselective cation channels that transport Ca2+, were recently demonstrated to complex with the NHERF proteins. We investigated whether TRPC4 and/or TRPC5 are associated with NHERF-2 in DVR. RT-PCR revealed mRNA for TRPC4 and NHERF-2, but not for TRPC5 or NHERF-1, in microdissected DVR. Immunohistochemical studies demonstrated expression of TRPC4 and NHERF-2 proteins in both the endothelial cells and pericytes. These proteins colocalized in some cells of the DVR. TRPC4 coimmunoprecipitated with NHERF-2 from renal medullary lysates, and NHERF-2 coimmunoprecipitated with TRPC4. TRPC5 was not detected in DVR with the use of immunohistochemistry or in NHERF-2 immunoprecipitates. We conclude that DVR pericytes and endothelia coexpress TRPC4 and NHERF-2 mRNA and protein and that these proteins colocalize and coimmunoprecipitate, indicating a possible physical association. These findings suggest that TRPC4 and NHERF-2 may play a role in interactions related to Ca2+signaling.

The TRP superfamily of cation channels

The transient receptor potential (TRP) protein superfamily consists of a diverse group of cation channels that bear structural similarities to Drosophila TRP. TRP channels play important roles in nonexcitable cells; however, an emerging theme is that many TRP-related proteins are expressed predominantly in the nervous system and function in sensory physiology. The TRP superfamily is divided into seven subfamilies, the first of which is composed of the "classical" TRPs" (TRPC subfamily). Some TRPCs may be store-operated channels, whereas others appear to be activated by production of diacylglycerol or regulated through an exocytotic mechanism. Many members of a second subfamily (TRPV) function in sensory physiology and respond to heat, changes in osmolarity, odorants, and mechanical stimuli. Two members of the TRPM family function in sensory perception and three TRPM proteins are chanzymes, which contain C-terminal enzyme domains. The fourth and fifth subfamilies, TRPN and TRPA, include proteins with many ankyrin repeats. TRPN proteins function in mechanotransduction, whereas TRPA1 is activated by noxious cold and is also required for the auditory response. In addition to these five closely related TRP subfamilies, which comprise the Group 1 TRPs, members of the two Group 2 TRP subfamilies, TRPP and TRPML, are distantly related to the group 1 TRPs. Mutations in the founding members of these latter subfamilies are responsible for human diseases. Each of the TRP subfamilies are represented by members in worms and flies, providing the potential for using genetic approaches to characterize the normal functions and activation mechanisms of these channels.

The mammalian TRPC cation channels

Transient Receptor Potential-Canonical (TRPC) channels are mammalian homologs of Transient Receptor Potential (TRP), a Ca(2+)-permeable channel involved in the phospholipase C-regulated photoreceptor activation mechanism in Drosophila. The seven mammalian TRPCs constitute a family of channels which have been proposed to function as store-operated as well as second messenger-operated channels in a variety of cell types. TRPC channels, together with other more distantly related channel families, make up the larger TRP channel superfamily. This review summarizes recent findings on the structure, regulation and function of the apparently ubiquitous TRPC cation channels.

Apparent cytosolic calcium gradients in T-lymphocytes due to fura-2 accumulation in mitochondria

Fura-2 is the most common dye to measure cytosolic Ca2+ concentrations ([Ca2+]i). To facilitate simultaneous imaging of many cells while preserving their cytosolic environment, fura-2 is often loaded into the cytosol in its membrane-permeant ester form. It has been reported that small amounts of fura-2 accumulate in intracellular compartments, an effect that is usually neglected. We show that either focal or non-focal stimulation methods induce large [Ca2+]i gradients in T-lymphocytes during both, Ca2+ release and Ca2+ influx across the plasma membrane. Interfering with mitochondrial Ca2+ homeostasis and by labeling mitochondria with MitoTracker, we demonstrate that [Ca2+]i gradients co-localize with mitochondria and are attributable to mitochondrial fura-2 sequestration. Gradients could not be avoided by different loading protocols, compromising measurements of "real" [Ca2+]i gradients following T-cell stimulation. They were observed in human blood and lamina propria lymphocytes, Jurkat T-cells, mast cells, but not to the same extent in HEK-293 cells. Finally, we show that T-lymphocytes can be efficiently loaded with the membrane-impermeant fura-2 salt by electroporation and by osmotic lysis of pinocytic vesicles, which result in the loss of [Ca2+]i gradients. These methods are therefore suitable to study localized Ca2+ signals in large populations of T-cells while preserving their cytosolic integrity.

Requirement of PDZ Domains for the Stimulation of the Epithelial Ca2+ Channel TRPV5 by the NHE Regulating Factor NHERF2 and the Serum and Glucocorticoid Inducible Kinase SGK1

Renal calcium reabsorption involves the epithelial calcium channel ECaC1 (TRPV5) which is tightly regulated by 1,25(OH)2D3. As shown recently, TRPV5 is activated by the serum and glucocorticoid inducible kinase SGK1, a kinase transcriptionally upregulated by 1,25(OH)2D3. This stimulatory effect is due to enhanced TRPV5 abundance in the plasma membrane and requires the presence of the scaffold protein NHERF2 (sodium hydrogen exchanger regulating factor 2). The present study aims to define the molecular requirements for the interaction of TRPV5 with SGK1 and NHERF2. Pull-down experiments and overlay assays revealed that the TRPV5 C-tail interacts in a Ca2+-independent manner with NHERF2. Deletion of the second but not of the first PDZ domain in NHERF2 abrogates the stimulating effect of SGK1/NHERF2 on TRPV5 protein abundance in the plasma membrane as quantified by chemiluminescence and electrophysiology. Thus, the second PDZ domain in NHERF2 is required for stabilization at or TRPV5 targeting to the plasma membrane. The experiments demonstrate the significance of SGK1 and NHERF2 as TRPV5 modulators which are likely to participate in the regulation of calcium homeostasis by 1,25(OH)2D3.

TRPC5 activation kinetics are modulated by the scaffolding protein ezrin/radixin/moesin-binding phosphoprotein-50 (EBP50)

TRPC1-7 proteins are members of a family of mammalian non-specific cation channels that mediate receptor-operated, phospholipase Cbeta/Cgamma dependent Ca(2+) influx in various cell types. TRPC4 and TRPC5 form a subfamily within TRPCs. Uniquely in the TRPC family, these channels possess a C-terminal "VTTRL" motif that binds to PDZ-domains of the scaffolding protein, EBP50 (NHERF1; Tang et al., J Biol Chem 275:37559-37564). The functional effects of EBP50 on TRPC4/5 activity have not been investigated. We have cloned rat TRPC5 (rTRPC5), functionally expressed it in HEK293 cell, and studied channel regulation with patch-clamp techniques. Both rTRPC5 and its VTTRL deletion mutant (r5dV) were localized to the plasma membrane. rTRPC5 did not display any significant basal activity in unstimulated HEK293 cells. In cells co-expressing rTRPC5 and H1 histamine receptor, rTRPC5 current evoked by GTPgammaS or histamine developed in two phases: a slowly developing, small inward current was followed by a rapidly developing, transient, large inward current. Each phase had a characteristic non-linear current-voltage (I-V) relationship. Deletion of the VTTRL motif had no detectable effect on the biophysical properties of the channel. Co-expression of EBP50 with rTRPC5 caused a significant delay in the time-to-peak of the histamine-evoked, transient large inward current. EBP50 did not modify the activation kinetics of the VTTRL-deletion mutant. We conclude that the VTTRL motif is not necessary for activation of TRPC5, but may mediate the modulatory effect of EBP50 on TRPC5 activation kinetics.

Ca2+-selective Transient Receptor Potential V Channel Architecture and Function Require a Specific Ankyrin Repeat

Transient receptor potential (TRP) proteins form cation-conducting ion channels with currently 28 known genes encoding TRP channel monomers in mammals. These monomers are thought to coassemble to form homo- or heterotetrameric channels, but the signals governing their assembly are unknown. Within the TRPV subgroup, TRPV5 and TRPV6 show exclusive calcium selectivity and play an important role in calcium uptake. To identify signals that mediate assembly of functional TRPV6, we screened domains for self-association using co-immunoprecipitation, sucrose gradient centrifugation, bacterial two-hybrid assays, and patch clamp analysis. Of the two identified interaction domains within the N-terminal region, we showed that the first domain encompassing the third ankyrin repeat is the stringent requirement for physical assembly of TRPV6 subunits and when transferred to an unrelated protein enables its interaction with TRPV6. Deletion of this repeat or mutation of critical residues within this repeat rendered nonfunctional channels that do not co-immunoprecipitate or form tetramers. Suppression of dominant-negative inhibitors of TRPV6-specific currents was achieved by deletion of ankyrin (ANK) 3. We propose that the third ANK repeat initiates a molecular zippering process that proceeds past the fifth ANK repeat and creates an intracellular anchor that is necessary for functional subunit assembly.

Stimulation by Parathyroid Hormone of a NHERF-1-assembled Complex Consisting of the Parathyroid Hormone I Receptor, Phospholipase Cβ, and Actin Increases Intracellular Calcium in Opossum Kidney Cells

Parathyroid hormone (PTH) binds its cognate G-protein-coupled receptor (PTH1R) and signals through both adenylyl cyclase and phospholipase C (PLC). C-terminal determinants of the PTH1R interact with the Na+/H+ exchanger regulatory factor 1 (NHERF-1) by binding the first of two PDZ (psd95, discs-large, ZO-1) domains. Compared with wild-type opossum kidney (OK) cells, OKH cells, a sub-clone, do not display PTH-mediated increases of [Ca2+]i and express NHERF-1 at markedly lower levels. Stable expression of NHERF-1 in the OKH parent (OKH-N1) restores the PTH-mediated increase of [Ca2+]i that arises from an influx of extracellular calcium and is both PLC-dependent and pertussis toxin-sensitive. From a morphological perspective, NHERF-1 and the PTH1R co-localize to apical patches of OKH-N1 cells, an expression pattern that is absent in OKH cells and depends on a direct NHERF-1-PTH1R interaction in OKH-N1 cells. Actin and PLCbeta1 and -beta3 co-localize with NHERF-1 and the PTH1R in OKH-N1 cell apical patches. Actin is also an integral component of the NHERF-1-assembled complex because cytochalasin D disrupts apical localization of both NHERF-1 and the PTH1R and inhibits the PTH-mediated increase of [Ca2+]i. Expression of the first PDZ domain of NHERF-1 acts as a dominant-negative interactor by blocking apical localization of the PTH1R and inhibiting PTH-elicited increases of [Ca2+]i. Thus, NHERF-1 assembles a signaling complex in the apical domains of OK cells that contains the PTH1R, PLCbeta, and the actin cytoskeleton. Disruption of this complex blocks the PTH mediated increases of intracellular calcium.

Heterogeneous nuclear ribonuclear protein U associates with YAP and regulates its co-activation of Bax transcription

Although initially described as a cytosolic scaffolding protein, YAP (Yes-associated protein of 65 kDa) is known to associate with multiple transcription factors in the nucleus. Using affinity chromatography and mass spectrometry, we show that YAP interacts with heterogeneous nuclear ribonuclear protein U (hnRNP U), an RNA- and DNA-binding protein enriched in the nuclear matrix that also plays a role in the regulation of gene expression. hnRNP U interacts specifically with the proline-rich amino terminus of YAP, a region of YAP that is not found in the related protein TAZ. Although hnRNP U and YAP localize to both the nucleus and the cytoplasm, YAP does not translocate to the nucleus in an hnRNP U-dependent manner. Furthermore, hnRNP U and YAP only interact in the nucleus, suggesting that the association between the two proteins is regulated. Co-expression of hnRNP U attenuates the ability of YAP to increase the activity of a p73-driven Bax-luciferase reporter plasmid. In contrast, hnRNP U has no effect when co-expressed with a truncated YAP protein lacking the hnRNP U-binding site. Because YAP is distinguished from the homologue TAZ by its proline-rich amino terminus, the YAP-hnRNP U interaction may uniquely regulate the nuclear function(s) of YAP. The YAP-hnRNP U interaction provides another mechanism of YAP transcriptional regulation.

Canonical transient receptor potential channel 4 (TRPC4) co-localizes with the scaffolding protein ZO-1 in human fetal astrocytes in culture

Members of the mammalian transient receptor potential (TRP) family form cation-permeable channels at the plasma membrane implicated in capacitative calcium influx after activation by either second-messenger-mediated pathways or store depletion, or both. This study shows that with the use of RT-PCR, Western blotting, and immunohistochemistry, resting astrocytes express TRPC4 at the cell membrane, particularly at sites of cell-to-cell contact. By confocal imaging and immunoelectron microscopy, we detected co-localization of TRPC4 with the scaffolding protein zonula occludens 1 (ZO-1), and demonstrated that immunoprecipitation with antibodies to ZO-1 brought down TRPC4, and vice-versa. It has been proposed that the targeting of TRPC4 to the cell membrane is dependent on the interaction of the C-terminal TRL motif with PDZ domains. Using transfection of astrocytes with myc-tagged TRPC4 or TRL-motif truncated TRPC4 (deltaTRL), we found that deltaTRL localized predominantly to a juxtanuclear compartment, whereas the wild-type protein showed cell surface distribution. Deletion of the TRL motif also reduced plasma membrane expression as assessed by cell surface biotinylation experiments. Using GST fusion proteins, we found that TRPC4 interacted with the PDZ1 domain of ZO-1 and that this was also dependent on the TRL motif. Thus, our data demonstrate that the PDZ-interacting domain of TRPC4 controls its cell surface localization. These data implicate TRPC4 in the regulation of calcium homeostasis in astrocytes, particularly as part of a signaling complex that forms at junctional sites between astrocytes.

The venerable inveterate invertebrate TRP channels

The transient receptor potential (TRP) superfamily is subdivided into four main classes of cation channels, TRPC, TRPV, TRPM and TRPN, each of which includes members in worms, flies, mice and humans. While the biophysical features of many of the mammalian channels have been described, relatively little is known concerning the biological roles of these channels. Forward genetic screens in Drosophila melanogaster and Caenorhabditis elegans have led to the identification of the founding members of each of these four subfamilies. Moreover, phenotypic analyses of invertebrate mutants have contributed greatly to our understanding of the roles of TRP proteins. A recurring theme is that many of these proteins function in sensory signaling processes ranging from vision to olfaction, osmosensation, light touch, social feeding, and temperature- and mechanically-induced nociception. In addition, at least one invertebrate TRP protein is required for cell division. As many of these functions may be conserved among the mammalian TRPs, the invertebrate TRPs offer valuable genetic handles for characterizing the functions of these cation channels in vivo.

On the endothelial cell I(SOC)

Ca2+ store depletion activates both Ca2+ selective and non-selective currents in endothelial cells. Recently, considerable progress has been made in understanding the molecular make-up and regulation of an endothelial cell thapsigargin-activated Ca2+ selective current, I(SOC). Indeed, I(SOC) is a relatively small inward Ca2+ current that exhibits an approximate +40mV reversal potential and is strongly inwardly rectifying. This current is sensitive to organization of the actin-based cytoskeleton. Transient receptor potential (TRP) proteins 1 and 4 (TRPC1 and TRPC4, respectively) each contribute to the molecular basis of I(SOC), although it is TRPC4 that appears to be tethered to the cytoskeleton through a dynamic interaction with protein 4.1. Activation of I(SOC) requires association between protein 4.1 and the actin-based cytoskeleton (mediated through spectrin), suggesting protein 4.1 mediates the physical communication between Ca2+ store depletion and channel activation. Thus, at present findings indicate a TRPC4-protein 4.1 physical linkage regulates I(SOC) activation following Ca2+ store depletion.

TRPC4 and TRPC5: receptor-operated Ca2+-permeable nonselective cation channels

The seven mammalian channels from the classical (TRPC) subfamily of transient receptor potential (TRP) channels are thought to be receptor-operated cation channels activated in a phospholipase C (PLC)-dependent manner. Based on sequence similarity, TRPC channels can be divided into four subgroups. Group 4 comprises TRPC4 and TRPC5, and is most closely related to group 1 (TRPC1). The functional properties observed following heterologous expression of TRPC4 or TRPC5 in mammalian cells are contradictory and, therefore, controversial. In our hands, and in several independent studies, both channels, probably as homotetramers, form receptor-operated, Ca2+-permeable, nonselective cation channels activated independently of inositol 1,4,5-trisphosphate (InsP(3)) receptor activation or Ca2+ store-depletion. As heteromultimers with TRPC1, TRPC4 and TRPC5 form receptor-operated, Ca2+-permeable, nonselective cation channels with biophysical properties distinct from homomeric TRPC4 or TRPC5. In other studies, TRPC4 and TRPC5 have been shown to be store-operated channels, with moderate to high Ca2+ permeabilities. At present there is no clear explanation for these major differences in functional properties. To date, little is known as to which native cation channels are formed by TRPC4 and TRPC5. Endothelial cells from TRPC4(-/-) mice lack a highly Ca2+-permeable, store-dependent current, and data support a role for TRPC4 in endothelium-mediated vasorelaxation. A similar current in adrenal cortical cells is reduced by TRPC4 antisense. From similarities in the properties of the currents and expression of appropriate isoforms in the tissues, it is likely that heteromultimers of TRPC1 and TRPC4 or TRPC5 form receptor-operated nonselective cation channels in central neurones, and that TRPC4 contributes to nonselective cation channels in intestinal smooth muscle.

Proteins modulating TRP channel function

TRP channels are involved in different signaling cascades; TRP channels can be activated via hormones and neurotransmitter in a receptor/G-protein-mediated manner or by osmotic, thermic or mechanic stimuli. The overall functional role of TRP channels within these processes of hormonal cellular control, nociception or cellular calcium homeostasis is still unclear, as these complex processes often involve macromolecular structures. Whereas the integration of Drosophila TRP in the phototransduction process is becoming clear, the understanding of the participation of mammalian TRP channels in signal transduction complexes is only beginning. TRP channels have been demonstrated to interact with PDZ domain proteins, and both scaffold and regulatory function have been shown for INAD, the PDZ domain protein of the Drosophila phototransduction complex. In mammalian cells, the interaction of NHERF and TRPC4 has been shown and it is anticipated that NHERF may abolish the apparent store-dependent regulation of TRPC4 and TRPC5. Whereas TRP channels and PDZ domain proteins form permanent heterodimeric proteins, the interaction of calcium-binding proteins is dependent on the calcium concentration and is, therefore, dynamic. The prototype of calcium-binding protein used for experiments is calmodulin; whether or not calmodulin is also the natural interaction partner of TRP channels is an open question.