Coexpression of Drosophila TRP and TRP-like proteins in Xenopus oocytes reconstitutes capacitative Ca2+ entry

Transient Receptor Potential Channels in the Healthy and Diseased Blood-Brain Barrier

The large family of transient receptor potential (TRP) channels are integral membrane proteins that function as environmental sensors and act as ion channels after activation by mechanical (touch), physical (heat, pain), and chemical stimuli (pungent compounds such as capsaicin). Most TRP channels are localized in the plasma membrane of cells but some of them are localized in membranes of organelles and function as intracellular Ca2+-ion channels. TRP channels are involved in neurological disorders but their precise role(s) and relevance in these disorders are not clear. Endothelial cells of the blood-brain barrier (BBB) express TRP channels such as TRP vanilloid 1-4 and are involved in thermal detection by regulating BBB permeability. In neurological disorders, TRP channels in the BBB are responsible for edema formation in the brain. Therefore, drug design to modulate locally activity of TRP channels in the BBB is a hot topic. Today, the application of TRP channel antagonists against neurological disorders is still limited.

TRP (transient receptor potential) ion channel family: structures, biological functions and therapeutic interventions for diseases

Transient receptor potential (TRP) channels are sensors for a variety of cellular and environmental signals. Mammals express a total of 28 different TRP channel proteins, which can be divided into seven subfamilies based on amino acid sequence homology: TRPA (Ankyrin), TRPC (Canonical), TRPM (Melastatin), TRPML (Mucolipin), TRPN (NO-mechano-potential, NOMP), TRPP (Polycystin), TRPV (Vanilloid). They are a class of ion channels found in numerous tissues and cell types and are permeable to a wide range of cations such as Ca²⁺, Mg²⁺, Na⁺, K⁺, and others. TRP channels are responsible for various sensory responses including heat, cold, pain, stress, vision and taste and can be activated by a number of stimuli. Their predominantly location on the cell surface, their interaction with numerous physiological signaling pathways, and the unique crystal structure of TRP channels make TRPs attractive drug targets and implicate them in the treatment of a wide range of diseases. Here, we review the history of TRP channel discovery, summarize the structures and functions of the TRP ion channel family, and highlight the current understanding of the role of TRP channels in the pathogenesis of human disease. Most importantly, we describe TRP channel-related drug discovery, therapeutic interventions for diseases and the limitations of targeting TRP channels in potential clinical applications.

Diacylglycerol Activates the Drosophila Light Sensitive Channel TRPL Expressed in HEK Cells

Physiological activation by light of the Drosophila TRP and TRP-like (TRPL) channels requires the activation of phospholipase Cβ (PLC). The hydrolysis of phosphatidylinositol 4,5, bisphosphate (PIP2) by PLC is a crucial step in the still-unclear light activation, while the generation of Diacylglycerol (DAG) by PLC seems to be involved. In this study, we re-examined the ability of a DAG analogue 1-oleoyl-2-acetyl-sn-glycerol (OAG) to activate the TRPL channels expressed in HEK cells. Unlike previous studies, we added OAG into the cytosol via a patch-clamp pipette and observed robust activation of the expressed TRPL channels. However, TRPL channel activation was much slower than the physiologically activated TRPL by light. Therefore, we used a picosecond-fast optically activated DAG analogue, OptoDArG. Inactive OptoDArG was added into the intracellular solution with the patch-clamp pipette, and it slowly accumulated on the surface membrane of the recorded HEK cell in the dark. A fast application of intense UV light to the recorded cell resulted in a robust and relatively fast TRPL-dependent current that was greatly accelerated by the constitutively active TRPLF557I pore-region mutation. However, this current of the mutant channel was still considerably slower than the native light-induced TRPL current, suggesting that DAG alone is not sufficient for TRPL channel activation under physiological conditions.

Transient Receptor Potential Canonical (TRPC) Channels: Then and Now

Twenty-five years ago, the first mammalian Transient Receptor Potential Canonical (TRPC) channel was cloned, opening the vast horizon of the TRPC field. Today, we know that there are seven TRPC channels (TRPC1-7). TRPCs exhibit the highest protein sequence similarity to the Drosophila melanogaster TRP channels. Similar to Drosophila TRPs, TRPCs are localized to the plasma membrane and are activated in a G-protein-coupled receptor-phospholipase C-dependent manner. TRPCs may also be stimulated in a store-operated manner, via receptor tyrosine kinases, or by lysophospholipids, hypoosmotic solutions, and mechanical stimuli. Activated TRPCs allow the influx of Ca2+ and monovalent alkali cations into the cytosol of cells, leading to cell depolarization and rising intracellular Ca2+ concentration. TRPCs are involved in the continually growing number of cell functions. Furthermore, mutations in the TRPC6 gene are associated with hereditary diseases, such as focal segmental glomerulosclerosis. The most important recent breakthrough in TRPC research was the solving of cryo-EM structures of TRPC3, TRPC4, TRPC5, and TRPC6. These structural data shed light on the molecular mechanisms underlying TRPCs' functional properties and propelled the development of new modulators of the channels. This review provides a historical overview of the major advances in the TRPC field focusing on the role of gene knockouts and pharmacological tools.

TRPC Channel Structure and Properties

TRPC channels are the first identified members in the TRP family. They function as either homo- or heterotetramers regulating intracellular Ca²⁺ concentration in response to numerous physiological or pathological stimuli. TRPC channels are nonselective cation channels permeable to Ca²⁺. The properties and the functional domains of TRPC channels have been identified by electrophysiological and biochemical methods. However, due to the large size, instability, and flexibility of their complexes, the structures of the members in TRPC family remain unrevealed. More efforts should be made on structure analysis and generating good tools, including specific antibodies, agonist, and antagonist.

Alternative in vitro assays to assess the potency of sensory irritants-Is one TRP channel enough?

One important function of the peripheral nervous system is the detection of noxious chemicals in the environment as well as the recognition of tissue damage throughout the body. Transient receptor potential (TRP) ion channels are able to sense a multitude of signaling factors involved in these processes. Via the sensory ganglia these sentinels convey information to the central nervous system, where perceptions of nociception or sensory irritation are generated. From the 28 members of the 6 subfamilies present in mammals, researchers in toxicology paid special attention to TRPA1 and TRPV1 channels. Various xenobiotics (e.g., acrolein, formaldehyde) can open these channels causing sensory irritations and defense mechanisms like sneezing, coughing and lacrimation. Heterologous expression of these two channels and the subsequent investigation of ion fluxes have been proposed as in vitro models for the assessment of sensory irritation. In a series of experiments using acetophenone, isophorone, and 2-ethylhexanol (2-EH) we investigated the effects of these irritants on heterologously expressed TRP channels in comparison to a primary cell culture of trigeminal ganglia neurons of mice. We confirmed acetophenone as a specific TRPA1 agonist that activates the receptor in concentrations >3mM, whereas isophorone specifically activates TRPV1 in concentrations >100μM. 2-EH can activate heterologously expressed TRPA1 concentration-dependently (1 mM-10mM). In Ca²⁺ imaging we observed 2-EH as an agonist of multiple channels (TRPA1, TRPV1, GPCRs) that activates the trigeminal neurons by application of μM 2-EH concentrations. The convergent results of our experiments further support the specificity of acetophenone and isophorone to activate only one of these investigated TRP channels and a more unspecific activation in the case of 2-EH. However, the results of the two different in vitro systems also showed that both TRPA1 and TRPV1 channel activation is important for the perception of irritants and only the combined and tiered testing might lead to precise estimates describing the potency of a xenobiotic to cause sensory irritation or pain.

Diacylglycerol activates the light-dependent channel TRP in the photosensitive microvilli of Drosophila melanogaster photoreceptors

Drosophila light-dependent channels, TRP and TRPL, reside in the light-sensitive microvilli of the photoreceptor's rhabdomere. Phospholipase C mediates TRP/TRPL opening, but the gating process remains unknown. Controversial evidence has suggested diacylglycerol (DAG), polyunsaturated fatty acids (PUFAs, a DAG metabolite), phosphatidylinositol bisphosphate (PIP2), and H(+) as possible channel activators. We tested each of them directly in inside-out TRP-expressing patches excised from the rhabdomere, making use of mutants and pharmacology. When patches were excised in darkness TRP remained closed, while when excised under illumination it stayed constitutively active. TRP was opened by DAG and silenced by ATP, suggesting DAG-kinase (DGK) involvement. The ATP effect was abolished by inhibiting DGK and in the rdgA mutant, lacking functional DGK, implicating DGK. DAG activated TRP even in the presence of a DAG-lipase inhibitor, inconsistent with a requirement of PUFAs in opening TRP. PIP2 had no effect and acidification, pH 6.4, activated TRP irreversibly, unlike the endogenous activator. Complementary liquid-chromatography/mass-spectrometry determinations of DAG and PUFAs in membranes enriched in rhabdomere obtained from light- and dark-adapted eyes showed light-dependent increment in six DAG species and no changes in PUFAs. The results strongly support DAG as the endogenous TRP agonist, as some of its vertebrate TRPC homologs of the same channel family.

Photosensitive TRPs

The Drosophila "transient receptor potential" channel is the prototypical TRP channel, belonging to and defining the TRPC subfamily. Together with a second TRPC channel, trp-like (TRPL), TRP mediates the transducer current in the fly's photoreceptors. TRP and TRPL are also implicated in olfaction and Malpighian tubule function. In photoreceptors, TRP and TRPL are localised in the ~30,000 packed microvilli that form the photosensitive "rhabdomere"-a light-guiding rod, housing rhodopsin and the rest of the phototransduction machinery. TRP (but not TRPL) is assembled into multimolecular signalling complexes by a PDZ-domain scaffolding protein (INAD). TRPL (but not TRP) undergoes light-regulated translocation between cell body and rhabdomere. TRP and TRPL are also found in photoreceptor synapses where they may play a role in synaptic transmission. Like other TRPC channels, TRP and TRPL are activated by a G protein-coupled phospholipase C (PLCβ4) cascade. Although still debated, recent evidence indicates the channels can be activated by a combination of PIP2 depletion and protons released by the PLC reaction. PIP2 depletion may act mechanically as membrane area is reduced by cleavage of PIP2's bulky inositol headgroup. TRP, which dominates the light-sensitive current, is Ca(2+) selective (P Ca:P Cs >50:1), whilst TRPL has a modest Ca(2+) permeability (P Ca:P Cs ~5:1). Ca(2+) influx via the channels has profound positive and negative feedback roles, required for the rapid response kinetics, with Ca(2+) rapidly facilitating TRP (but not TRPL) and also inhibiting both channels. In trp mutants, stimulation by light results in rapid depletion of microvillar PIP2 due to lack of Ca(2+) influx required to inhibit PLC. This accounts for the "transient receptor potential" phenotype that gives the family its name and, over a period of days, leads to light-dependent retinal degeneration. Gain-of-function trp mutants with uncontrolled Ca(2+) influx also undergo retinal degeneration due to Ca(2+) cytotoxicity. In vertebrate retina, mice knockout studies suggest that TRPC6 and TRPC7 mediate a PLCβ4-activated transducer current in intrinsically photosensitive retinal ganglion cells, expressing melanopsin. TRPA1 has been implicated as a "photo-sensing" TRP channel in human melanocytes and light-sensitive neurons in the body wall of Drosophila.

Different uptake of gentamicin through TRPV1 and TRPV4 channels determines cochlear hair cell vulnerability

Hair cells at the base of the cochlea appear to be more susceptible to damage by the aminoglycoside gentamicin than those at the apex. However, the mechanism of base-to-apex gradient ototoxicity by gentamicin remains to be elucidated. We report here that gentamicin caused rodent cochlear hair cell damages in a time- and dose-dependent manner. Hair cells at the basal turn were more vulnerable to gentamicin than those at the apical turn. Gentamicin-conjugated Texas Red (GTTR) uptake was predominant in basal turn hair cells in neonatal rats. Transient receptor potential vanilloid 1 (TRPV1) and 4 (TRPV4) expression was confirmed in the cuticular plate, stereocilia and hair cell body of inner hair cells and outer hair cells. The involvement of TRPV1 and TRPV4 in gentamicin trafficking of hair cells was confirmed by exogenous calcium treatment and TRPV inhibitors, including gadolinium and ruthenium red, which resulted in markedly inhibited GTTR uptake and gentamicin-induced hair cell damage in rodent and zebrafish ototoxic model systems. These results indicate that the cytotoxic vulnerability of cochlear hair cells in the basal turn to gentamicin may depend on effective uptake of the drug, which was, in part, mediated by the TRPV1 and TRPV4 proteins.

STIM proteins: dynamic calcium signal transducers

Stromal interaction molecule (STIM) proteins function in cells as dynamic coordinators of cellular calcium (Ca(2+)) signals. Spanning the endoplasmic reticulum (ER) membrane, they sense tiny changes in the levels of Ca(2+) stored within the ER lumen. As ER Ca(2+) is released to generate primary Ca(2+) signals, STIM proteins undergo an intricate activation reaction and rapidly translocate into junctions formed between the ER and the plasma membrane. There, STIM proteins tether and activate the highly Ca(2+)-selective Orai channels to mediate finely controlled Ca(2+) signals and to homeostatically balance cellular Ca(2+). Details are emerging on the remarkable organization within these STIM-induced junctional microdomains and the identification of new regulators and alternative target proteins for STIM.

The history of the Drosophila TRP channel: the birth of a new channel superfamily

Transient receptor potential (TRP) channels are polymodal cellular sensors involved in a wide variety of cellular processes, mainly by changing membrane voltage and increasing cellular Ca(2+). This review outlines in detail the history of the founding member of the TRP family, the Drosophila TRP channel. The field began with a spontaneous mutation in the trp gene that led to a blind mutant during prolonged intense light. It was this mutant that allowed for the discovery of the first TRP channels. A combination of electrophysiological, biochemical, Ca(2+) measurements, and genetic studies in flies and in other invertebrates pointed to TRP as a novel phosphoinositide-regulated and Ca(2+)-permeable channel. The cloning and sequencing of the trp gene provided its molecular identity. These seminal findings led to the isolation of the first mammalian homologues of the Drosophila TRP channels. We now know that TRP channel proteins are conserved through evolution and are found in most organisms, tissues, and cell-types. The TRP channel superfamily is classified into seven related subfamilies: TRPC, TRPM, TRPV, TRPA, TRPP, TRPML, and TRPN. A great deal is known today about participation of TRP channels in many biological processes, including initiation of pain, thermoregulation, salivary fluid secretion, inflammation, cardiovascular regulation, smooth muscle tone, pressure regulation, Ca(2+) and Mg(2+) homeostasis, and lysosomal function. The native Drosophila photoreceptor cells, where the founding member of the TRP channels superfamily was found, is still a useful preparation to study basic features of this remarkable channel.

Heteromerization of TRP channel subunits: extending functional diversity

Transient receptor potential (TRP) channels are widely found throughout the animal kingdom. By serving as cellular sensors for a wide spectrum of physical and chemical stimuli, they play crucial physiological roles ranging from sensory transduction to cell cycle modulation. TRP channels are tetrameric protein complexes. While most TRP subunits can form functional homomeric channels, heteromerization of TRP channel subunits of either the same subfamily or different subfamilies has been widely observed. Heteromeric TRP channels exhibit many novel properties compared to their homomeric counterparts, indicating that co-assembly of TRP channel subunits has an important contribution to the diversity of TRP channel functions.

Structure-functional intimacies of transient receptor potential channels

Although a unifying characteristic common to all transient receptor potential (TRP) channel functions remains elusive, they could be described as tetramers formed by subunits with six transmembrane domains and containing cation-selective pores, which in several cases show high calcium permeability. TRP channels constitute a large superfamily of ion channels, and can be grouped into seven subfamilies based on their amino acid sequence homology: the canonical or classic TRPs, the vanilloid receptor TRPs, the melastatin or long TRPs, ankyrin (whose only member is the transmembrane protein 1 [TRPA1]), TRPN after the nonmechanoreceptor potential C (nonpC), and the more distant cousins, the polycystins and mucolipins. Because of their role as cellular sensors, polymodal activation and gating properties, many TRP channels are activated by a variety of different stimuli and function as signal integrators. Thus, how TRP channels function and how function relates to given structural determinants contained in the channel-forming protein has attracted the attention of biophysicists as well as molecular and cell biologists. The main purpose of this review is to summarize our present knowledge on the structure of channels of the TRP ion channel family. In the absence of crystal structure information for a complete TRP channel, we will describe important protein domains present in TRP channels, structure-function mutagenesis studies, the few crystal structures available for some TRP channel modules, and the recent determination of some TRP channel structures using electron microscopy.

TRPC channels and store-operated Ca(2+) entry

Store-operated calcium entry (SOCE) is a major mechanism for Ca(2+) influx. Since SOCE was first proposed two decades ago many techniques have been used in attempting to identify the nature of store-operated Ca(2+) (SOC) channels. The first identified and best-characterised store-operated current is I(CRAC), but a number of other currents activated by Ca(2+) store depletion have also been described. TRPC proteins have long been proposed as SOC channel candidates; however, whether any of the TRPCs function as SOC channels remains controversial. This review attempts to provide an overview of the arguments in favour and against the role of TRPC proteins in the store-operated mechanisms of agonist-activated Ca(2+) entry.

TRP channels in cancer

The progression of cells from a normal differentiated state in which rates of proliferation and apoptosis are balanced to a tumorigenic and metastatic state involves the accumulation of mutations in multiple key signalling proteins and the evolution and clonal selection of more aggressive cell phenotypes. These events are associated with changes in the expression of numerous other proteins. This process of tumorigenesis involves the altered expression of one or more TRP proteins, depending on the nature of the cancer. The most clearly described changes are those involving TRPM8, TRPV6 and TRPM1. Expression of TRPM8 is substantially increased in androgen-dependent prostate cancer cells, but is decreased in androgen independent and metastatic prostate cancer. TRPM8 expression is regulated, in part, by androgens, most likely through androgen response elements in the TRPM8 promoter region. TRPM8 channels are involved in the regulation of cell proliferation and apoptosis. Expression of TRPV6 is also increased in prostate cancer and in a number of other cancers. In contrast to TRPM8, expression of TRPV6 is not directly regulated by androgens. TRPM1 is highly expressed in early stage melanomas but its expression declines with increases in the degree of aggressiveness of the melanoma. The expression of TRPV1, TRPC1, TRPC6, TRPM4, and TRPM5 is also increased in some cancers. The level of expression of TRPM8 and TRPV6 in prostate cancer, and of TRPM1 in melanomas, potentially provides a good prognostic marker for predicting the course of the cancer in individuals. The Drosophila melanogaster, TRPL, and the TRPV1 and TRPM8 proteins, have been used to try to develop strategies to selectively kill cancer cells by activating Ca(2+) and Na(+) entry, producing a sustained increase in the cytoplasmic concentration of these ions, and subsequent cell death by apoptosis and necrosis. TRPV1 is expressed in neurones involved in sensing cancer pain, and is a potential target for pharmacological inhibition of cancer pain in bone metastases, pancreatic cancer and most likely in other cancers. Further studies are required to assess which other TRP proteins are associated with the development and progression of cancer, what roles TRP proteins play in this process, and to develop further knowledge of TRP proteins as targets for pharmaceutical intervention and targeting in cancer.

The Alphavirus 6K Protein Activates Endogenous Ionic Conductances when Expressed in Xenopus Oocytes

The Alphavirus Sindbis 6K protein is involved in several functions. It contributes to the processing and membrane insertion of E1 and PE2 viral envelope glycoproteins and to virus budding. It also permeabilizes Escherichia coli and mammalian cells. These viroporin-like properties have been proposed to help virus budding by modifying membrane permeabilities. We expressed Sindbis virus 6K cRNA in Xenopus oocytes to further characterize the effect of 6K on membrane conductances and permeabilization. Although no intrinsic channel properties were seen, cell shrinkage was observed within 24 h. Voltage-clamp experiments showed that 6K upregulated endogenous currents: a hyperpolarization-activated inward current (I (in)) and a calcium-dependent chloride current (I (Cl)). 6K was located at both the plasma and the endoplasmic reticulum membranes. The plasma membrane current upregulation likely results from disruption of the calcium homeostasis of the cell at the endoplasmic reticulum level. Indeed, 6K cRNA expression induced reticular calcium store depletion and capacitative calcium entry activation. By experimental modifications of the incubation medium, we showed that downstream of these events cell shrinkage resulted from a 6K -induced KCl efflux (I (Cl) upregulation leads to chloride efflux, which itself electrically drives potassium efflux), which was responsible for an osmotic water efflux. Our data confirm that 6K specifically triggers a sequential cascade of events that leads to cytoplasmic calcium elevation and cell permeabilization, which likely play a role in the Sindbis virus life cycle.

Regulation of eosinophil adhesion by lysophosphatidylcholine via a non-store-operated Ca2+ channel

We examined the mechanism by which lysophosphatidylcholine (LPC) regulates beta2-integrin-mediated adhesion of eosiniophils. Eosinophils were isolated from blood of mildly atopic volunteers by negative immunomagnetic selection. beta2-integrin-dependent adhesion of eosinophils to plated bovine serum albumin (BSA) was measured by residual eosinophil peroxidase activity. LPC caused maximal adhesion of eosinophils to plated BSA at 4 microM. Lysophosphatidylinositol, which has a similar molecular shape, mimicked the effect of LPC on eosinophil adhesion, while neither lysophosphatidylserine nor lysophosphatidylethanolamine had any effect. Phosphatidylethanolamine, a lipid that has a molecular orientation that is the inverse of LPC, blocked eosinophil adhesion caused by LPC. Unlike platelet-activating factor, a G-protein-coupled receptor agonist, LPC did not cause Ca2+-store depletion, but caused increased Ca2+ influx upon addition of Ca2+ to extracellular medium. This influx was not inhibited by U73122, a phospholipase C inhibitor, demonstrating independence from the G protein-activated phospholipase C pathway. Ca2+ influx was inhibited by either preincubation of phosphotidylethanolamine or La3+, a broad spectrum blocker of cation channels. LPC induced up-regulation of the active conformation of CD11b, which was blocked by preincubation with phosphatidylethanolamine. These data suggest that LPC causes a non-store-operated Ca2+ influx into eosinophils, which subsequently activates CD11b/CD18 to promote eosinophil adhesion.

Mechanism and functional significance of TRPC channel multimerization

Ca(2+) signaling regulates many important physiological events within a diverse set of living organisms. In particular, sustained Ca(2+) signals play an important role in controlling cell proliferation, cell differentiation and the activation of immune cells. Two key elements for the generation of sustained Ca(2+) signals are store-operated and receptor-operated Ca(2+) channels that are activated downstream of phospholipase C (PLC) stimulation, in response to G-protein-coupled receptor or growth factor receptor stimulation. One goal of this review is to help clarify the role of canonical transient receptor potential (TRPC) proteins in the formation of native store-operated and native receptor-operated channels. Toward that end, data from studies of endogenous TRPC proteins will be reviewed in detail to highlight the strong case for the involvement of certain TRPC proteins in the formation of one subtype of store-operated channel, which exhibits a low Ca(2+)-selectivity, in contrast to the high Ca(2+)-selectivity exhibited by the CRAC subtype of store-operated channel. A second goal of this review is to highlight the growing body of evidence indicating that native store-operated and native receptor-operated channels are formed by the heteromultimerization of TRPC subunits. Furthermore, evidence will be provided to argue that some TRPC proteins are able to form multiple channel types.

TRP channels and lipids: from Drosophila to mammalian physiology

The transient receptor potential (TRP) ion channel family was the last major ion channel family to be discovered. The prototypical member (dTRP) was identified by a forward genetic approach in Drosophila, where it represents the transduction channel in the photoreceptors, activated downstream of a Gq-coupled PLC. In the meantime 29 vertebrate TRP isoforms are recognized, distributed amongst seven subfamilies (TRPC, TRPV, TRPM, TRPML, TRPP, TRPA, TRPN). They subserve a wide range of functions throughout the body, most notably, though by no means exclusively, in sensory transduction and in vascular smooth muscle. However, their precise physiological roles and mechanism of activation and regulation are still only gradually being revealed. Most TRP channels are subject to multiple modes of regulation, but a common theme amongst the TRPC/V/M subfamilies is their regulation by lipid messengers. Genetic evidence supports an excitatory role of diacylglycerol (DAG) for the dTRP's, although curiously only DAG metabolites (PUFAs) have been found to activate the Drosophila channels. TRPC2,3,6 and 7 are widely accepted as DAG-activated channels, although TRPC3 can also be regulated via a store-operated mechanism. More recently PIP2 has been shown to be required for activity of TRPV5, TRPM4,5,7 and 8, whilst it may inhibit TRPV1 and the dTRPs. Although compelling evidence for a direct interaction of DAG with the TRPC channels is lacking, mutagenesis studies have identified putative PIP2-interacting domains in the C-termini of several TRPV and TRPM channels.

Coiled coils direct assembly of a cold-activated TRP channel

Transient receptor potential (TRP) channels mediate numerous sensory transduction processes and are thought to function as tetramers. TRP channel physiology is well studied; however, comparatively little is understood regarding TRP channel assembly. Here, we identify an autonomously folded assembly domain from the cold- and menthol-gated channel TRPM8. We show that the TRPM8 cytoplasmic C-terminal domain contains a coiled coil that is necessary for channel assembly and sufficient for tetramer formation. Cell biological experiments indicate that coiled-coil formation is required for proper channel maturation and trafficking and that the coiled-coil domain alone can act as a dominant-negative inhibitor of functional channel expression. Our data define an authentic TRP modular assembly domain, establish a clear role for coiled coils in ion channel assembly, demonstrate that coiled-coil assembly domains are a general feature of TRPM channels, and delineate a new tool that should be of general use in dissecting TRPM channel function.

TRPV1 regulators mediate gentamicin penetration of cultured kidney cells

Transient receptor potential (TRP) receptors are, typically, calcium-permeant cation channels that transduce environmental stimuli. Both kidney epithelial and inner ear sensory cells express TRPV1, are mechanosensors and accumulate the aminoglycoside antibiotic gentamicin. Recently, we showed that Texas Red-conjugated gentamicin (GTTR) enters kidney cells via an endosome-independent pathway. Here, we used GTTR to investigate this non-endocytotic mechanism of gentamicin uptake. In serum-free buffers, GTTR penetrated MDCK cells within 30 s and uptake was modulated by extracellular, multivalent cations (Ca2+, La3+, Gd3+) or protons. We verified the La3+ modulation of GTTR uptake using immunocytochemical detection of unconjugated gentamicin. Membrane depolarization, induced by high extracellular K+ or valinomycin, also reduced GTTR uptake, suggesting electrophoretic permeation through ion channels. GTTR uptake was enhanced by the TRPV1 agonists, resiniferatoxin and anandamide, in Ca2+-free media. Competitive antagonists of the TRPV1 cation current, iodo-resiniferatoxin and SB366791, also enhanced GTTR uptake independently of Ca2+, reinforcing these antagonists' potential as latent agonists in specific situations. Ruthenium Red blocked GTTR uptake in the presence or absence of these TRPV1-agonists and antagonists. In addition, GTTR uptake was blocked by RTX in the presence of more physiological levels (2 mM) of Ca2+. Thus gentamicin enters cells via cation channels, and gentamicin uptake can be modulated by regulators of the TRPV1 channel.

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.

Human TRPV4 channel splice variants revealed a key role of ankyrin domains in multimerization and trafficking

The TRPV4 cation channel exhibits a topology consisting of six predicted transmembrane domains (TM) with a putative pore loop between TM5 and TM6 and intracellular N- and C-tails, the former containing at least three ankyrin domains. Functional transient receptor potential (TRP) channels are supposed to result following the assembly of four subunits. However, the rules governing subunit assembly and protein domains implied in this process are only starting to emerge. The ankyrin, TM, and the C-tail domains have been identified as important determinants of the oligomerization process. We now describe the maturation and oligomerization of five splice variants of the TRPV4 channel. The already known TRPV4-A and TRPV4-B (delta384-444) variants and the new TRPV4-C (delta237-284), TRPV4-D (delta27-61), and TRPV4-E (delta237-284 and delta384-444) variants. All alternative spliced variants involved deletions in the cytoplasmic N-terminal region, affecting (except for TRPV4-D) the ankyrin domains. Subcellular localization, fluorescence resonance energy transfer, co-immunoprecipitation, glycosylation profile, and functional analysis of these variants permitted us to group them into two classes: group I (TRPV4-A and TRPV4-D) and group II (TRPV4-B, TRPV4-C, and TRPV4-E). Group I, unlike group II variants, were correctly processed, homo- and heteromultimerized in the endoplasmic reticulum, and were targeted to the plasma membrane where they responded to typical TRPV4 stimuli. Our results suggest that: 1) TRPV4 biogenesis involves core glycosylation and oligomerization in the endoplasmic reticulum followed by transfer to the Golgi apparatus for subsequent maturation; 2) ankyrin domains are necessary for oligomerization of TRPV4; and 3) lack of TRPV4 oligomerization determines its accumulation in the endoplasmic reticulum.

TRPC channel expression during calcium-induced differentiation of human gingival keratinocytes

BACKGROUND

Extracellular calcium is an important regulator of keratinocyte differentiation. An increase in intracellular calcium ion concentration is required for activation of calcium-induced keratinocyte differentiation. The signaling elements in this differentiation response include the calcium sensing receptor, phospholipase C, release of calcium ions from intracellular stores, and store-operated calcium channels. Nothing is currently known about the calcium-entry channels activated by the increase in external calcium. However, canonical transient receptor potential (TRPC) channels have been identified as store-operated calcium channels in several tissues.

OBJECTIVE

To examine the expression of TRPC channels in human gingival keratinocytes (HGKs) in primary culture under both low calcium (basal) and high calcium (differentiating) conditions, and in gingival tissue.

METHODS

TRPC channel expression was evaluated via RT-PCR, Western blots, and immunohistology.

RESULTS

TRPC1, TRPC5, TRPC6 and TRPC7 mRNAs were detected in undifferentiated keratinocytes. Their levels initially increased, then decreased during calcium-induced differentiation. TRPC1 and TRPC6 protein expression reflected these changes.

CONCLUSION

TRPC channels are present in both proliferating and differentiating keratinocytes in primary culture and in gingival tissue. The above expression patterns suggest that these channels may be involved in calcium-induced differentiation of keratinocytes.

Non-stimulated Ca2+ leak pathway in cerebellar granule neurones

The aim of this study was to investigate the pathways of calcium influx routes in non-stimulated cerebellar granule neurones by use of standard microspectrofluorimetric techniques. Repetitive application of Ca2+-free solutions for various time intervals induced decreases of resting cytosolic free Ca2+ concentration ([Ca2+]i) which were followed, on Ca2+ readmission, by a full recovery, always to the initial resting [Ca2+]i levels. Use of drugs to deplete calcium stores (thapsigargin, alone or combined with low levels of ionomycin) did not cause release of Ca2+ from the intracellular stores nor enhanced the activity of the Ca2+ entry pathway. This influx was mainly independent of voltage operated calcium channels, since both L-type channel blockers (nitrendipine) and the hyperpolarizing agent pinacidil (a K+-channel opener) were without effect. Contribution from glutamate receptors to this influx was eliminated since a combination of blockers of NMDA and AMPA glutamate receptors (NBQX and D-AP5) did not affect the properties of the Ca2+ response. The Ca2+ leak pathway was sensitive to micromolar levels of lanthanum and gadolinium, and to the compound 2-APB, features shared by several channels of the TRP superfamily. In summary, our results show the presence of a Ca2+ permeable pathway, active and patent in resting conditions in cerebellar granule neurones, and which is different from the voltage-operated calcium channels and not operated by depletion of the stores.

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.

Overexpression ofTRPC1enhances pulmonary vasoconstriction induced by capacitative Ca2+entry

Transient receptor potential (TRP) cation channels are a critical pathway for Ca2+entry during pulmonary artery (PA) smooth muscle contraction. However, whether canonical TRP (TRPC) subunits and which TRP channel isoforms are involved in store depletion-induced pulmonary vasoconstriction in vivo remain unclear. This study was designed to test whether overexpression of the human TRPC1 gene ( hTRPC1) in rat PA enhances pulmonary vasoconstriction due to store depletion-mediated Ca2+influx. The hTRPC1 was infected into rat PA rings with an adenoviral vector. RT-PCR and Western blot analyses confirmed the mRNA and protein expression of hTRPC1 in the arterial rings. The amplitude of active tension induced by 40 mM K+(40K) in PA rings infected with an empty adenoviral vector (647 ± 88 mg/mg) was similar to that in PA rings infected with hTRPC1 (703 ± 123 mg/mg, P = 0.3). However, the active tension due to capacitative Ca2+entry (CCE) induced by cyclopiazonic acid was significantly enhanced in PA rings overexpressing hTRPC1 (91 ± 13% of 40K-induced contraction) compared with rings infected with an empty adenoviral vector (61 ± 14%, P < 0.001). Endothelial expression of hTRPC1 was not involved since the CCE-induced vasoconstriction was also enhanced in endothelium-denuded PA rings infected with the adenoviral vector carrying hTRPC1. These observations demonstrate that hTRPC1 is an important Ca2+-permeable channel that mediates pulmonary vasoconstriction when PA smooth muscle cell intracellular Ca2+stores are depleted.

Expression of Drosophila Ca2+ permeable transient receptor potential-like channel protein in a prostate cancer cell line decreases cell survival

The effects of expression of Drosophila melanoga ster Ca(2+) permeable transient receptor potential-like (TRPL) channels, under the control of the cytomegalovirus (CMV) or prostate cell-specific promoters, on cell survival and apoptosis in the androgen-sensitive LNCaP prostate cancer cell line were investigated. A prostate-specific antigen (PSA) promoter construct (designated PSAEn/PSAPr) composed of a 0.6 kb region of the promoter and a 1.45 kb region of the enhancer resulted in androgen-dependent and prostate-specific expression of a luciferase reporter gene in transiently transfected LNCaP cells. Expression of the enhanced green fluorescence protein-TRPL chimeric protein under the control of the CMV promoter was confirmed by Western blot. Whereas the majority of the expressed protein was located in the cytoplasmic space, confocal microscopy with the CD-9 protein as a plasma membrane marker demonstrated that approximately 10% of the expressed TRPL protein was located in a band in the plasma membrane. Using recombinant adenoviruses, expression of the TRPL protein was associated with an increase in both the initial and sustained rates of Ca(2+) inflow. Expression of TRPL under the control of the CMV promoter for 96 hours decreased cell number and increased the number of cells undergoing apoptosis by 23 and 27%, respectively. Apoptosis was inhibited by a caspase-3 inhibitor, Z-DEVD-fmk. It is concluded that, when heterologously expressed in LNCaP cells, the TRPL protein leads to a reduction in cell survival due, in part, to the induction of apoptosis. The effects of TRPL are likely caused by enhanced Na(+) and Ca(2+) inflow to the cells. This finding suggests a novel approach to modify the growth of prostate cancer cells that fail to undergo apoptosis following androgen ablation therapy.

Calcium homeostasis in fly photoreceptor cells

In fly photoreceptor cells, two processes dominate the Ca2+ homeostasis: light-induced Ca2+ influx through members of the TRP family of ion channels, and Ca2+ extrusion by Na+/Ca2+ exchange. Ca2+ release from intracellular stores is quantitatively insignificant. Both, the light-activated channels and the Ca2+-extruding exchangers are located in or close to the rhabdomeric microvilli, small protrusions of the plasma membrane. The microvilli also contain the molecular machinery necessary for generating quantum bumps, short electrical responses caused by the absorption of a single photon. Due to this anatomical arrangement, the light-induced Ca2+ influx results in two separate Ca2+ signals that have different functions: a global, homogeneous increase of the Ca2+ concentration in the cell body, and rapid but large amplitude Ca2+ transients in the microvilli. The global rise of the Ca2+ concentration mediates light adaptation, via regulatory actions on the phototransduction cascade, the voltage-gated K+ channels and small pigment granules controlling the light intensity. The local Ca2+ transients in the microvilli are responsible for shaping the quantum bumps into fast, all-or-nothing events. They achieve this by facilitating strongly the phototransduction cascade at early stages ofthe light response and subsequently inhibiting it. Many molecular targets of these feedback mechanisms have been identified and characterized due to the availability of numerous Drosophila mutant showing defects in the phototransduction.

Regulation of TRP channels via lipid second messengers

In Drosophila photoreceptors, the light-sensitive current is mediated downstream of phospholipase C by TRP (transient receptor potential) channels. Recent evidence suggests that Drosophila TRP channels are activated by diacylglycerol (DAG) or its metabolites (polyunsaturated fatty acids), possibly in combination with the reduction in phosphatidyl inositol 4,5 bisphosphate (PIP2). Consistent with this view, diacylglycerol kinase is identified as a key enzyme required for response termination. Signaling is critically dependent upon efficient PIP2 synthesis; mutants of this pathway in combination with genetically targeted PIP2 reporters provide unique insights into the kinetics and regulation of PIP2 turnover. Recent evidence indicates that a growing number of mammalian TRP homologues are also regulated by lipid messengers, including DAG, arachidonic acid, and PIP2.

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.

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.

Mg2+ blocks myogenic tone but not K+-induced constriction: role for SOCs in small arteries

The effects of Mg(2+) and nifedipine (Nif) on vasoconstriction and Ca(2+) transients were studied in intact, pressurized rat mesenteric arteries with myogenic tone. Changes in cytosolic Ca(2+) concentration ([Ca(2+)](cyt)) were measured with confocal microscopy in fluo 4-AM loaded, individual myocytes. Myogenic tone was abolished by 10 mM Mg(2+) or 0.3 microM Nif. Contractions induced by 75 mM K(+) depolarization were blocked by 0.3 microM Nif, but not by 10 mM Mg(2+). Phenylephrine (PE; 5 microM) evoked sustained [Ca(2+)](cyt) elevation and vasoconstriction with superimposed Ca(2+) oscillations and vasomotion. The subsequent addition of 10 mM Mg(2+) or 0.3 microM Nif reduced [Ca(2+)](cyt) and abolished plateau vasoconstriction. When added before PE, both Mg(2+) and Nif abolished the PE-evoked Ca(2+) oscillations and vasomotion. Mg(2+) dilated the PE-constricted arteries after a brief (< or =180-240 s) vasoconstriction, but Nif did not. Both agents also abolished the vasoconstriction attributed to Ca(2+) entry through store-operated channels (SOCs) during internal Ca(2+) store refilling that followed store depletion. The data suggest that Ca(2+) entry through SOCs helps maintain both myogenic tone and alpha(1)-adrenoceptor-induced tonic vasoconstriction.

A transient receptor potential channel expressed in taste receptor cells

We used differential screening of cDNAs from individual taste receptor cells to identify candidate taste transduction elements in mice. Among the differentially expressed clones, one encoded Trpm5, a member of the mammalian family of transient receptor potential (TRP) channels. We found Trpm5 to be expressed in a restricted manner, with particularly high levels in taste tissue. In taste cells, Trpm5 was coexpressed with taste-signaling molecules such as alpha-gustducin, Ggamma13, phospholipase C-beta2 (PLC-beta2) and inositol 1,4,5-trisphosphate receptor type III (IP3R3). Our heterologous expression studies of Trpm5 indicate that it functions as a cationic channel that is gated when internal calcium stores are depleted. Trpm5 may be responsible for capacitative calcium entry in taste receptor cells that respond to bitter and/or sweet compounds.

The PDZ scaffold protein INAD abolishes apparent store-dependent regulation of the light-activated cation channel TRP

In fly photoreceptor cells, light initiates a G protein-coupled phospholipase Cb-dependent signaling cascade that results in the depolarization of the cell membrane, which is mediated by the cation channels TRP and TRPL. Together with phospholipase Cb and an eye-specific protein kinase C, TRP is tethered to the scaffolding protein INAD, which forms a multimolecular signaling complex. Divergent data from expressed TRP and studies from photoreceptor cells have brought up a controversy whether or not a capacitative calcium entry (CCE) mechanism is involved in the Drosophila phototransduction pathway. Our initial characterization of TRP from photoreceptors of Calliphora vicina supported the hypothesis of a CCE mechanism, as heterologously expressed TRP was stimulated after application of thapsigargin. The situation changed when the PDZ domain protein INAD was coexpressed with TRP. In cells coexpressing TRP and INAD, no calcium entry was detectable on application of store depletion protocols. Suppression of CCE by INAD was not observed when the described interaction was disrupted by mutations in TRP and INAD. Our data show that apparent activation of TRP by CCE is abolished by INAD. Within the complex, the proteins necessary for phototransduction mutually influence their activities. The results support the hypothesis of a store-independent activation of TRP.

TRP channel proteins and signal transduction

TRP channel proteins constitute a large and diverse family of proteins that are expressed in many tissues and cell types. This family was designated TRP because of a spontaneously occurring Drosophila mutant lacking TRP that responded to a continuous light with a transient receptor potential (hence TRP). In addition to responses to light, TRPs mediate responses to nerve growth factor, pheromones, olfaction, mechanical, chemical, temperature, pH, osmolarity, vasorelaxation of blood vessels, and metabolic stress. Furthermore, mutations in several members of TRP-related channel proteins are responsible for several diseases, such as several tumors and neurodegenerative disorders. TRP-related channel proteins are found in a variety of organisms, tissues, and cell types, including nonexcitable, smooth muscle, and neuronal cells. The large functional diversity of TRPs is also reflected in their diverse permeability to ions, although, in general, they are classified as nonselective cationic channels. The molecular domains that are conserved in all members of the TRP family constitute parts of the transmembrane domains and in most members also the ankyrin-like repeats at the NH2 terminal of the protein and a "TRP domain" at the COOH terminal, which is a highly conserved 25-amino acid stretch with still unknown function. All of the above features suggest that members of the TRP family are "special assignment" channels, which are recruited to diverse signaling pathways. The channels' roles and characteristics such as gating mechanism, regulation, and permeability are determined by evolution according to the specific functional requirements.

Dissecting receptor-mediated Ca2+ influx pathways: TRP channels and their native counterparts

Cellular stimulation from the surrounding extracellular environment via receptors and other pathways evoke activation of Ca2+-permeable cation channels that form essential signaling pathways in controlling biological responses. An important clue to understand the molecular mechanisms underlying these cation channels (tentatively termed as receptor-mediated cation channels (RMCC)) was first provided through molecular studies of the transient receptor potential (trp) protein (TRP), which controls light-induced depolarization in Drosophila photoreceptor cells. Use of the genetic information and recombinant expression technique lead to the discovery of numerous mammalian TRP homologues revealing novel RMCCs. In this review, we focus on the dramatic progress in the molecular investigation of RMCC in mammalian systems. The recent findings should provide powerful tools for the development of novel pharmaceutical targets.

Scaffolding proteins organize multimolecular protein complexes for sensory signal transduction

Scaffolding proteins composed of protein-protein interaction domains have emerged as organizers of multiprotein complexes in diverse cellular compartments, including neuronal synapses, cell-cell junctions of epithelial cells, and the stimulus perceiving structures of sensory neurons. This review focuses on the INAD-assembled signalling complex of Drosophila photoreceptors, which organizes key components of the phototransduction cascade into a multiprotein signal transduction unit. The structure, the physiological consequences, and the assembly and targeting of the members of the INAD signalling complex will be described. In addition, the existence of signalling complexes in vertebrate photoreceptors, olfactory neurons and mechanosensitive hair cells will be discussed.

Depletion of Ca2+ in the sarcoplasmic reticulum stimulates Ca2+ entry into mouse skeletal muscle fibres

To examine whether a capacitative Ca2+ entry pathway is present in skeletal muscle, thin muscle fibre bundles were isolated from extensor digitorum longus (EDL) muscle of adult mice, and isometric tension and fura-2 signals were simultaneously measured. The sarcoplasmic reticulum (SR) in the muscle fibres was successfully depleted of Ca2+ by repetitive treatments with high-K+ solutions, initially in the absence and then in the presence of a sarcoplasmic/endoplasmic reticulum Ca2+-ATPase (SERCA) inhibitor. Depletion of the SR of Ca2+ enabled us for the first time to show convincingly that the vast majority of the voltage-sensitive Ca2+ store overlaps the caffeine-sensitive Ca2+ store in intact fibres from mouse EDL muscle. This conclusion was based on the observation that both high-K+ solution and caffeine failed to cause a contracture in the depleted muscle fibres. The existence of a Ca2+ influx pathway active enough to refill the depleted SR within several minutes was shown in skeletal muscle fibres. Ca2+ entry was sensitive to Ni2+, but resistant to nifedipine and was suppressed by plasma membrane depolarisation. Evidence for store-operated Ca2+ entry was provided by measurements of Mn2+ entry. Significant acceleration of Mn2+ entry was observed only when the SR was severely depleted of Ca2+. The Mn2+ influx, which was blocked by Ni2+ but not by nifedipine, was inwardly rectifying, as is the case with the Ca2+ entry. These results indicate that the store-operated Ca2+ entry is similar to the Ca2+ release-activated Ca2+ channel (CRAC) current described in other preparations.

Non-synaptic ion channels in insects--basic properties of currents and their modulation in neurons and skeletal muscles

Insects are favoured objects for studying information processing in restricted neuronal networks, e.g. motor pattern generation or sensory perception. The analysis of the underlying processes requires knowledge of the electrical properties of the cells involved. These properties are determined by the expression pattern of ionic channels and by the regulation of their function, e.g. by neuromodulators. We here review the presently available knowledge on insect non-synaptic ion channels and ionic currents in neurons and skeletal muscles. The first part of this article covers genetic and structural informations, the localization of channels, their electrophysiological and pharmacological properties, and known effects of second messengers and modulators such as neuropeptides or biogenic amines. In a second part we describe in detail modulation of ionic currents in three particularly well investigated preparations, i.e. Drosophila photoreceptor, cockroach DUM (dorsal unpaired median) neuron and locust jumping muscle. Ion channel structures are almost exclusively known for the fruitfly Drosophila, and most of the information on their function has also been obtained in this animal, mainly based on mutational analysis and investigation of heterologously expressed channels. Now the entire genome of Drosophila has been sequenced, it seems almost completely known which types of channel genes--and how many of them--exist in this animal. There is much knowledge of the various types of channels formed by 6-transmembrane--spanning segments (6TM channels) including those where four 6TM domains are joined within one large protein (e.g. classical Na+ channel). In comparison, two TM channels and 4TM (or tandem) channels so far have hardly been explored. There are, however, various well characterized ionic conductances, e.g. for Ca2+, Cl- or K+, in other insect preparations for which the channels are not yet known. In some of the larger insects, i.e. bee, cockroach, locust and moth, rather detailed information has been established on the role of ionic currents in certain physiological or behavioural contexts. On the whole, however, knowledge of non-synaptic ion channels in such insects is still fragmentary. Modulation of ion currents usually involves activation of more or less elaborate signal transduction cascades. The three detailed examples for modulation presented in the second part indicate, amongst other things, that one type of modulator usually leads to concerted changes of several ion currents and that the effects of different modulators in one type of cell may overlap. Modulators participate in the adaptive changes of the various cells responsible for different physiological or behavioural states. Further study of their effects on the single cell level should help to understand how small sets of cells cooperate in order to produce the appropriate output.

A common mechanism underlies vertebrate calcium signaling and Drosophila phototransduction

Drosophila phototransduction is an important model system for studies of inositol lipid signaling. Light excitation in Drosophila photoreceptors depends on phospholipase C, because null mutants of this enzyme do not respond to light. Surprisingly, genetic elimination of the apparently single inositol trisphosphate receptor (InsP(3)R) of Drosophila has no effect on phototransduction. This led to the proposal that Drosophila photoreceptors do not use the InsP(3) branch of phospholipase C (PLC)-mediated signaling for phototransduction, unlike most other inositol lipid-signaling systems. To examine this hypothesis we applied the membrane-permeant InsP(3)R antagonist 2-aminoethoxydiphenyl borate (2-APB), which has proved to be an important probe for assessing InsP(3)R involvement in various signaling systems. We first examined the effects of 2-APB on Xenopus oocytes. We found that 2-APB is efficient at reversibly blocking the robust InsP(3)-mediated Ca(2+) release and store-operated Ca(2+) entry in Xenopus oocytes at a stage operating after production of InsP(3) but before the opening of the surface membrane Cl(-) channels by Ca(2+). We next demonstrated that 2-APB is effective at reversibly blocking the response to light of Drosophila photoreceptors in a light-dependent manner at a concentration range similar to that effective in Xenopus oocytes and other cells. We show furthermore that 2-APB does not directly block the light-sensitive channels, indicating that it operates upstream in the activation of these channels. The results indicate an important link in the coupling mechanism of vertebrate store-operated channels and Drosophila TRP channels, which involves the InsP(3) branch of the inositol lipid-signaling pathway.

A store-operated nonselective cation channel in lymphocytes is activated directly by Ca(2+) influx factor and diacylglycerol

Agonist-receptor interactions at the plasma membrane often lead to activation of store-operated channels (SOCs) in the plasma membrane, allowing for sustained Ca(2+) influx. While Ca(2+) influx is important for many biological processes, little is known about the types of SOCs, the nature of the depletion signal, or how the SOCs are activated. We recently showed that in addition to the Ca(2+) release-activated Ca(2+) (CRAC) channel, both Jurkat T cells and human peripheral blood mononuclear cells express novel store-operated nonselective cation channels that we termed Ca(2+) release-activated nonselective cation (CRANC) channels. Here we demonstrate that activation of both CRAC and CRANC channels is accelerated by a soluble Ca(2+) influx factor (CIF). In addition, CRANC channels in inside-out plasma membrane patches are directly activated upon exposure of their cytoplasmic side to highly purified CIF preparations. Furthermore, CRANC channels are also directly activated by diacylglycerol. These results strongly suggest that the Ca(2+) store-depletion signal is a diffusible molecule and that at least some SOCs may have dual activation mechanisms.

A common mechanism underlies vertebrate calcium signaling and Drosophila phototransduction

Drosophila phototransduction is an important model system for studies of inositol lipid signaling. Light excitation in Drosophila photoreceptors depends on phospholipase C, because null mutants of this enzyme do not respond to light. Surprisingly, genetic elimination of the apparently single inositol trisphosphate receptor (InsP(3)R) of Drosophila has no effect on phototransduction. This led to the proposal that Drosophila photoreceptors do not use the InsP(3) branch of phospholipase C (PLC)-mediated signaling for phototransduction, unlike most other inositol lipid-signaling systems. To examine this hypothesis we applied the membrane-permeant InsP(3)R antagonist 2-aminoethoxydiphenyl borate (2-APB), which has proved to be an important probe for assessing InsP(3)R involvement in various signaling systems. We first examined the effects of 2-APB on Xenopus oocytes. We found that 2-APB is efficient at reversibly blocking the robust InsP(3)-mediated Ca(2+) release and store-operated Ca(2+) entry in Xenopus oocytes at a stage operating after production of InsP(3) but before the opening of the surface membrane Cl(-) channels by Ca(2+). We next demonstrated that 2-APB is effective at reversibly blocking the response to light of Drosophila photoreceptors in a light-dependent manner at a concentration range similar to that effective in Xenopus oocytes and other cells. We show furthermore that 2-APB does not directly block the light-sensitive channels, indicating that it operates upstream in the activation of these channels. The results indicate an important link in the coupling mechanism of vertebrate store-operated channels and Drosophila TRP channels, which involves the InsP(3) branch of the inositol lipid-signaling pathway.

TRPC1 and TRPC5 form a novel cation channel in mammalian brain

TRP proteins are cation channels responding to receptor-dependent activation of phospholipase C. Mammalian (TRPC) channels can form hetero-oligomeric channels in vitro, but native TRPC channel complexes have not been identified to date. We demonstrate here that TRPC1 and TRPC5 are subunits of a heteromeric neuronal channel. Both TRPC proteins have overlapping distributions in the hippocampus. Coexpression of TRPC1 and TRPC5 in HEK293 cells resulted in a novel nonselective cation channel with a voltage dependence similar to NMDA receptor channels, but unlike that of any reported TRPC channel. TRPC1/TRPC5 heteromers were activated by G(q)-coupled receptors but not by depletion of intracellular Ca(2+) stores. In contrast to the more common view of the TRP family as comprising store-operated channels, we propose that many TRPC heteromers form diverse receptor-regulated nonselective cation channels in the mammalian brain.

Mechanism of store-operated calcium entry

Activation of receptors coupled to the phospholipase C/IP3 signalling pathway results in a rapid release of calcium from its intracellular stores, eventually leading to depletion of these stores. Calcium store depletion triggers an influx of extracellular calcium across the plasma membrane, a mechanism known as the store-operated calcium entry or capacitative calcium entry. Capacitative calcium current plays a key role in replenishing calcium stores and activating various physiological processes. Despite considerable efforts, very little is known about the molecular nature of the capacitative channel and the signalling pathway that activates it. This review summarizes our current knowledge about store operated calcium entry and suggests possible hypotheses for its mode of activation.

Association of mammalian trp4 and phospholipase C isozymes with a PDZ domain-containing protein, NHERF

Mammalian homologues of Drosophila Trp have been implicated to form channels that are activated following the depletion of Ca(2+) from internal stores. Recent studies indicate that actin redistribution is required for the activation of these channels. Here we show that murine Trp4 and Trp5, as well as phospholipase C beta1 and beta2 interact with the first PDZ domain of NHERF, regulatory factor of the Na(+)/H(+) exchanger. We demonstrated the association of Trp4 and phospholipase C-beta1 with NHERF in vivo in an HEK293 cell line expressing Trp4 and in adult mouse brain by immuno-coprecipitation. NHERF is a two PDZ domain-containing protein that associates with the actin cytoskeleton via interactions with members of ezrin/radixin/moesin family. Thus, store-operated channels involving Trp4 and Trp5 can form signaling complexes with phospholipase C isozymes via interactions with NHERF and thereby linking the lipase and the channels to the actin cytoskeleton. The interaction with the PDZ protein may constitute an important mechanism for distribution and regulation of store-operated channels.

A hyperpolarization- and acid-activated nonselective cation current in Xenopus oocytes

Heterologous expression of a variety of membrane proteins in Xenopus oocytes sometimes results in the appearance of a hyperpolarization-activated inward current. The nature of this current remains incompletely understood. Some investigators have suggested that this current is a Cl current, whereas others have identified it as a nonselective cation current. The purpose of this investigation was to characterize this current in more detail. The hyperpolarization-activated inward current (I(IN)) present in native oocytes was composed of a current carried at least partly by Ca and Mg under physiological ionic conditions plus a Ca-activated Cl current. The Ca-activated Cl current was blocked by chelation of cytosolic Ca with 1,2-bis(2-aminophenoxy)ethane-N,N,N', N'-tetraacetic acid. When Cl currents were blocked, the cation current could be carried by Ca, Mg, or Co, but not appreciably by Ba, Mn, or Cd. I(IN) was stimulated by intracellular acidification. The properties of I(IN) were quite different from those of the store-operated Ca current. Heterologous expression of transient receptor potential-like gene product (TRPL), one of the members of the transient receptor potential family of putative store-operated Ca channels, apparently resulted in alteration of the voltage sensitivity of the endogenous I(IN).

Phenotypes of trpl mutants and interactions between the transient receptor potential (TRP) and TRP-like channels in Drosophila

The trp and trpl genes are thought to encode two classes of light-activated ion channels in Drosophila. A previous report indicated that a null trpl mutant does not display any mutant phenotype. This lack of detectable mutant phenotypes made it difficult to suggest functions for the transient receptor potential-like (TRPL) channel in photoreceptor responses. Here, the properties of trpl photoreceptor responses were studied by using electroretinogram (ERG) and intracellular recording techniques in combination with light stimuli of relatively long durations. Distinct mutant phenotypes were detectable under these conditions. These consisted of a reduced sustained component, oscillations superimposed on the response, a poststimulus hyperpolarization, and altered adaptation properties to dim background light. Comparison of photoreceptor responses obtained from wild type, trp, and trpl showed that the responses obtained from the trp and trpl null mutants did not sum up to that of the wild-type response. To explain the nonlinear summation at the peak of the response, Reuss et al. (1997) proposed that Ca(2+) ions entering through the TRP channel modulate TRP and TRPL channel activities differentially. However, nonlinear summation was present not only at the peak but throughout the duration of response. Two lines of evidence are presented to suggest that, in addition to the interaction proposed by Reuss et al. (1997), there are other forms of interactions between TRP and TRPL channels, probably involving the channel proteins themselves.

Phenotypes of trpl mutants and interactions between the transient receptor potential (TRP) and TRP-like channels in Drosophila

The trp and trpl genes are thought to encode two classes of light-activated ion channels in Drosophila. A previous report indicated that a null trpl mutant does not display any mutant phenotype. This lack of detectable mutant phenotypes made it difficult to suggest functions for the transient receptor potential-like (TRPL) channel in photoreceptor responses. Here, the properties of trpl photoreceptor responses were studied by using electroretinogram (ERG) and intracellular recording techniques in combination with light stimuli of relatively long durations. Distinct mutant phenotypes were detectable under these conditions. These consisted of a reduced sustained component, oscillations superimposed on the response, a poststimulus hyperpolarization, and altered adaptation properties to dim background light. Comparison of photoreceptor responses obtained from wild type, trp, and trpl showed that the responses obtained from the trp and trpl null mutants did not sum up to that of the wild-type response. To explain the nonlinear summation at the peak of the response, Reuss et al. (1997) proposed that Ca(2+) ions entering through the TRP channel modulate TRP and TRPL channel activities differentially. However, nonlinear summation was present not only at the peak but throughout the duration of response. Two lines of evidence are presented to suggest that, in addition to the interaction proposed by Reuss et al. (1997), there are other forms of interactions between TRP and TRPL channels, probably involving the channel proteins themselves.