Appearance of a novel Ca2+ influx pathway in Sf9 insect cells following expression of the transient receptor potential-like (trpl) protein of Drosophila

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.

Stromal Interaction Molecule 1 Promotes the Replication of vvIBDV by Mobilizing Ca2+ in the ER

Infectious bursal disease virus (IBDV) is one of the main threats to the poultry industry worldwide. Very virulent IBDV (vvIBDV) is a fatal virus strain that causes heavy mortality in young chicken flocks. Ca²⁺ is one of the most universal and versatile signalling molecules and is involved in almost every aspect of cellular processes. Clinical examination showed that one of the characteristics of vvIBDV-infected chickens was severe metabolic disorders, and the chemical examination showed that their serum Ca²⁺ level decreased significantly. However, there are limited studies on how vvIBDV infection modulates the cellular Ca²⁺ level and the effect of Ca²⁺ level changes on vvIBDV replication. In our study, we found Ca²⁺ levels in the endoplasmic reticulum (ER) of vvIBDV-infected B cells were higher than that of mock-infected cells, and the expression level of stromal interaction molecule 1 (STIM1), an ER Ca²⁺ sensor, was significantly upregulated due to vvIBDV infection. The knock-down expression of STIM1 led to decreased Ca²⁺ level in the ER and suppressed vvIBDV replication, while the over-expressed STIM1 led to ER Ca²⁺ upregulation and promoted vvIBDV replication. We also showed that the inhibition of Ca²⁺-release-activated-Ca²⁺ (CRAC) channels could reduce vvIBDV infection by blocking Ca²⁺ from entering the ER. This study suggests a new mechanism that STIM1 promotes the replication of vvIBDV by mobilizing Ca²⁺ in the ER.

The Role of Membrane Lipids in Light-Activation of Drosophila TRP Channels

Transient Receptor Potential (TRP) channels constitute a large superfamily of polymodal channel proteins with diverse roles in many physiological and sensory systems that function both as ionotropic and metabotropic receptors. From the early days of TRP channel discovery, membrane lipids were suggested to play a fundamental role in channel activation and regulation. A prominent example is the Drosophila TRP and TRP-like (TRPL) channels, which are predominantly expressed in the visual system of Drosophila. Light activation of the TRP and TRPL channels, the founding members of the TRP channel superfamily, requires activation of phospholipase Cβ (PLC), which hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP₂) into Diacylglycerol (DAG) and Inositol 1, 4,5-trisphosphate (IP₃). However, the events required for channel gating downstream of PLC activation are still under debate and led to several hypotheses regarding the mechanisms by which lipids gate the channels. Despite many efforts, compelling evidence of the involvement of DAG accumulation, PIP₂ depletion or IP₃-mediated Ca²⁺ release in light activation of the TRP/TRPL channels are still lacking. Exogeneous application of poly unsaturated fatty acids (PUFAs), a product of DAG hydrolysis was demonstrated as an efficient way to activate the Drosophila TRP/TRPL channels. However, compelling evidence for the involvement of PUFAs in physiological light-activation of the TRP/TRPL channels is still lacking. Light-induced mechanical force generation was measured in photoreceptor cells prior to channel opening. This mechanical force depends on PLC activity, suggesting that the enzymatic activity of PLC converting PIP₂ into DAG generates membrane tension, leading to mechanical gating of the channels. In this review, we will present the roles of membrane lipids in light activation of Drosophila TRP channels and present the many advantages of this model system in the exploration of TRP 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 channels: Structure, function, regulation and recent advances in small molecular probes

Transient receptor potential canonical (TRPC) channels constitute a group of receptor-operated calcium-permeable nonselective cation channels of the TRP superfamily. The seven mammalian TRPC members, which can be further divided into four subgroups (TRPC1, TRPC2, TRPC4/5, and TRPC3/6/7) based on their amino acid sequences and functional similarities, contribute to a broad spectrum of cellular functions and physiological roles. Studies have revealed complexity of their regulation involving several components of the phospholipase C pathway, G_i and G_o proteins, and internal Ca²⁺ stores. Recent advances in cryogenic electron microscopy have provided several high-resolution structures of TRPC channels. Growing evidence demonstrates the involvement of TRPC channels in diseases, particularly the link between genetic mutations of TRPC6 and familial focal segmental glomerulosclerosis. Because TRPCs were discovered by the molecular identity first, their pharmacology had lagged behind. This is rapidly changing in recent years owning to great efforts from both academia and industry. A number of potent tool compounds from both synthetic and natural products that selective target different subtypes of TRPC channels have been discovered, including some preclinical drug candidates. This review will cover recent advancements in the understanding of TRPC channel regulation, structure, and discovery of novel TRPC small molecular probes over the past few years, with the goal of facilitating drug discovery for the study of TRPCs and therapeutic development.

Brain transcriptome of honey bees (Apis mellifera) exhibiting impaired olfactory learning induced by a sublethal dose of imidacloprid

Declines in honey bee populations represent a worldwide concern. The widespread use of neonicotinoid insecticides has been one of the factors linked to these declines. Sublethal doses of a neonicotinoid insecticide, imidacloprid, has been reported to cause olfactory learning deficits in honey bees via impairment of the target organ, the brain. In the present study, olfactory learning of honey bees was compared between controls and imidacloprid-treated bees. The brains of imidacloprid-treated and control bees were used for comparative transcriptome analysis by RNA-Seq to elucidate the effects of imidacloprid on honey bee learning capacity. The results showed that the learning performance of imidacloprid-treated bees was significantly impaired in comparison with control bees after chronic oral exposure to imidacloprid (0.02 ng/μl) for 11 days. Gene expression profiles between imidacloprid treatment and the control revealed that 131 genes were differentially expressed, of which 130 were downregulated in imidacloprid-treated bees. Validation of the RNA-Seq data using qRT-PCR showed that the results of qRT-PCR and RNA-Seq exhibited a high level of agreement. Gene ontology annotation indicated that the oxidation-reduction imbalance might exist in the brain of honey bees due to oxidative stress induced by imidacloprid exposure. KEGG and ingenuity pathway analysis revealed that transient receptor potential and Arrestin 2 in the phototransduction pathway were significantly downregulated in imidacloprid-treated bees, and that five downregulated genes have causal effects on behavioral response inhibition in imidacloprid-treated bees. Our results suggest that downregulation of brain genes involved in immune, detoxification and chemosensory responses may result in decreased olfactory learning capabilities in imidacloprid-treated bees.

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.

Evolutionarily conserved, multitasking TRP channels: lessons from worms and flies

The Transient Receptor Potential (TRP) channel family is comprised of a large group of cation-permeable channels, which display an extraordinary diversity of roles in sensory signaling. TRPs allow animals to detect chemicals, mechanical force, light, and changes in temperature. Consequently, these channels control a plethora of animal behaviors. Moreover, their functions are not limited to the classical senses, as they are cellular sensors, which are critical for ionic homeostasis and metabolism. Two genetically tractable invertebrate model organisms, Caenorhabditis elegans and Drosophila melanogaster, have led the way in revealing a wide array of sensory roles and behaviors that depend on TRP channels. Two overriding themes have emerged from these studies. First, TRPs are multitasking proteins, and second, many functions and modes of activation of these channels are evolutionarily conserved, including some that were formerly thought to be unique to invertebrates, such as phototransduction. Thus, worms and flies offer the potential to decipher roles for mammalian TRPs, which would otherwise not be suspected.

Food experience-induced taste desensitization modulated by the Drosophila TRPL channel

Animals tend to reject bitter foods. However, long-term exposure to some unpalatable tastants increases acceptance of these foods. Here we show that dietary exposure to an unappealing but safe additive, camphor, caused the fruit fly Drosophila melanogaster to decrease camphor rejection. The transient receptor potential-like (TRPL) cation channel was a direct target for camphor in gustatory receptor neurons, and long-term feeding on a camphor diet led to reversible downregulation of TRPL protein concentrations. The turnover of TRPL was controlled by an E3 ubiquitin ligase, Ube3a. The decline in TRPL levels and increased acceptance of camphor reversed after returning the flies to a camphor-free diet long term. We propose that dynamic regulation of taste receptors by ubiquitin-mediated protein degradation comprises an important molecular mechanism that allows an animal to alter its taste behavior in response to a changing food environment.

Drosophila TRP channels and animal behavior

Multiple classes of cell surface receptors and ion channels participate in the detection of changes in environmental stimuli, and thereby influence animal behavior. Among the many classes of ion channels, Transient Receptor Potential (TRP) cation channels are notable in contributing to virtually every sensory modality, and in controlling a daunting array of behaviors. TRP channels appear to be conserved in all metazoan organisms including worms, insects and humans. Flies encode 13 TRPs, most of which are expressed and function in sensory neurons, and impact behaviors ranging from phototaxis to thermotaxis, gravitaxis, the avoidance of noxious tastants and smells and proprioception. Multiple diseases result from defects in TRPs, and flies provide an excellent animal model for dissecting the mechanisms underlying "TRPopathies." Drosophila TRPs also function in the sensation of botanically derived insect repellents, and related TRPs in insect pests are potential targets for the development of improved repellents to combat insect-borne diseases.

A brief history of trp: commentary and personal perspective

The history of the discovery of the transient receptor potential (TRP) cation channel superfamily began in 1969 with Cosens and Manning's isolation of the Drosophila transient receptor potential mutant, in which the photoreceptor response decays during continuous illumination. Early studies from Minke found that the elementary light response was unaffected in trp mutants, and he attributed the defect to an intermediate stage of phototransduction. Montell and Rubin cloned the trp gene in 1989: they recognised it as a transmembrane protein, but also concluded that it did not encode the light-sensitive channels. In 1991, Minke and Selinger proposed that TRP represented a Ca2+ transporter required for refilling intracellular InsP3-sensitive Ca2+ stores, in turn required for activation of the light-sensitive channels. Also in 1991, after developing a photoreceptor patch clamp preparation, I showed that the light-sensitive channels themselves were highly permeable to Ca2+, questioning the need for such a dedicated Ca2+ transporter. In 1992, in collaboration with Minke, I resolved this paradox by showing there were two classes of light-sensitive channels, one highly Ca2+ permeable and eliminated in trp mutants. This represented the first and compelling evidence that TRP represented a light-sensitive channel and was supported by the cloning of the second light-sensitive channel, TRPL, by Kelly's lab. Three years later, in 1995, the labs of Montell and Birnbaumer independently cloned TRPC1, the first of 29 vertebrate TRP isoforms distributed amongst seven subfamilies.

Synthetic modulators of TRP channel activity

In humans, 27 TRP channels from 6 related families contribute to a broad spectrum of cellular functions, such as thermo-, pressure-, volume-, pain- and chemosensation. Pain and inflammation-inducing compounds represent potent plant and animal defense mechanisms explaining the great variety of the naturally occurring, TRPV1-, TRPM8-, and TRPA1-activating ligands. The discovery of the first vanilloid receptor (TRPV1) and its involvement in nociception triggered the euphoria and the hope in novel therapeutic strategies treating pain, and this clear-cut indication inspired the development of TRPV1-selective ligands. On the other hand the nescience in the physiological role and putative clinical indication hampered the development of a selective drug in the case of the other TRP channels. Therefore, currently only a handful of mostly un-selective blocker is available to target TRP channels. Nevertheless, there is an ongoing quest for new, natural or synthetic ligands and modulators. In this chapter, we will give an overview on available broad-range blocker, as well as first TRP channel-selective compounds.

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.

Interaction between Calcium Ions and Bacillus thuringiensis Toxin Activity against Sf9 Cells (Spodoptera frugiperda, Lepidoptera)

The effects of calcium ions and modulators of calcium movement on Bacillus thuringiensis insecticidal protein toxicity were investigated with Sf9 cells (Spodoptera frugiperda, fall armyworm) by a new B. thuringiensis toxicity assay based on measurement of fluorescence of ethidium homodimer, a high-affinity DNA stain. CryIC toxicity was substantially stimulated by extracellular calcium in a dose-dependent way (in the millimolar range), while toxicity enhancement could not be replicated when calcium was replaced by barium. This incremental toxicity was reduced by cobalt and lanthanum ions, two inorganic-calcium transport inhibitors. Methoxyverapamil, a voltage-dependent calcium channel blocker, and nifedipine, an inhibitor of dihydropyridine-sensitive L-type calcium channels, had no effect on CryIC toxin activity, but BAY K 8644, an L-type calcium channel activator, increased CryIC activity at high concentrations of extracellular calcium. While A23187, a calcium ionophore, and TMB-8, an inhibitor of intracellular-calcium mobilization, did not change CryIC-induced mortality, thapsigargin, an inhibitor of calcium uptake in intracellular stores, and more particularly trifluoperazine, which inhibits calcium-calmodulin-dependent processes, increased CryIC-mediated toxicity. The incremental effect of extracellular calcium on CryIC-induced toxicity was consistent with an increased concentration of intracellular calcium.

Regulation of the cellular localization and function of human transient receptor potential channel 1 by other members of the TRPC family

Members of the Canonical Transient Receptor Potential (TRPC) family of ionic channels are able to form homo- and heterotetrameric channels. Depending on the study, TRPC1 has been detected on both the surface and inside the cell, probably in the endoplasmic reticulum (ER). Likewise, TRPC1 has been described both as a store-operated channel and as one unable to function when forming a homotetramer. It is possible that the apparent differences in the expression and function of TRPC1 are due to its association with other proteins, possibly from the same TRPC family. In the present study we used confocal microscopy and a fluorescently tagged TRPC1 to examine the localization of this protein when co-expressed with other members of the TRPC family. Whole-cell and single channel electrophysiological recordings were conducted to study the function of TRPC1 expressed alone or co-expressed with other members of the TRPC family. A FRET-based calcium sensor fused to TRPC1 was used to assess the functionality of the intracellular TRPC1. Our results showed that TRPC4 and TRPC5 were able to increase the amount of membrane-expressed TRPC1 as evaluated by confocal microscopy and patch clamp recordings. The FRET-based calcium sensor fused to TRPC1 strongly suggests that this protein forms ER-expressed functional homotetrameric channels activated by agonists coupled to the IP(3) cascade. These results indicate that TRPC1 is a multifunctional protein able to form intracellular calcium release channels when expressed alone, and plasma membrane channels when co-expressed with TRPC4 or TRPC5, but not TRPC3 or TRPC6. Both (ER and plasma membrane) forms of the channel are activated upon addition of agonists coupled to the IP(3) cascade.

Metabotropic glutamate receptors in the lateral superior olive activate TRP-like channels: age- and experience-dependent regulation

The lateral superior olive (LSO) is the primary auditory nucleus for processing of interaural sound level differences, which is one of the major cues for sound localization. During development, survival and maturation of LSO neurons critically depend on synaptic activity and intracellular calcium signaling. Before hearing onset, glutamatergic synaptic inputs from the cochlear nucleus (CN) to the LSO activate group I metabotropic glutamate receptors (mGluRs), which leads to calcium release from intracellular stores and large calcium influx from the extracellular milieu. Here, we investigated the nature of the mGluR-activated membrane channel that mediates the influx of extracellular calcium. Using Fura-2 calcium imaging in brain stem slices of neonatal and juvenile mice, we found that this calcium channel is blocked by Ni(2+), La(3+), and 2-aminoethoxydiphenylborane (2-APB), known antagonists of transient receptor potential (TRP) channels. During postnatal development, the contribution of extracellular calcium influx to mGluR-mediated Ca(2+) responses gradually decreased and was almost abolished by the end of the third postnatal week. Over this period, the contribution of Ca(2+) release from internal stores remained unchanged. The developmental decrease of TRP-like channel-mediated calcium influx was significantly less in congenitally deaf waltzer mice, suggesting that early auditory experience is necessary for the normal age-dependent downregulation of functional TRP channels.

Regulation of Drosophila TRPC channels by protein and lipid interactions

The Drosophila TRPC channels TRP and TRPL are the founding members of the TRP superfamily of ion channels, proteins likely to be important components of calcium influx pathways. The activation of these channels in the context of fly phototransduction is one of the few in vivo models for TRPC channel activation and has served as a paradigm for understanding TRPC function. TRP and TRPL are activated by G-protein coupled PI(4,5)P(2) hydrolysis through a mechanism in which IP(3) receptor mediated calcium release seems dispensable. Recent analysis has provided compelling evidence that the accurate turnover of PI(4,5)P(2) generated lipid messengers in essential for regulating TRP and TRPL activity. TRP channels also appear to exist in the context of a macromolecular complex containing key components involved in activation such as phospholipase Cbeta and protein kinase C. This complex may be important for activation. The role of these protein and lipid elements in regulating TRP and TRPL activity is discussed in this review.

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.

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.

Calmodulin and calcium interplay in the modulation of TRPC5 channel activity. Identification of a novel C-terminal domain for calcium/calmodulin-mediated facilitation

TRPC5 forms Ca2+-permeable nonselective cation channels important for neurite outgrowth and growth cone morphology of hippocampal neurons. Here we studied the activation of mouse TRPC5 expressed in Chinese hamster ovary and human embryonic kidney 293 cells by agonist stimulation of several receptors that couple to the phosphoinositide signaling cascade and the role of calmodulin (CaM) on the activation. We showed that exogenous application of 10 microM CaM through patch pipette accelerated the agonist-induced channel activation by 2.8-fold, with the time constant for half-activation reduced from 4.25 +/- 0.4 to 1.56 +/- 0.85 min. We identified a novel CaM-binding site located at the C terminus of TRPC5, 95 amino acids downstream from the previously determined common CaM/IP3R-binding (CIRB) domain for all TRPC proteins. Deletion of the novel CaM-binding site attenuated the acceleration in channel activation induced by CaM. However, disruption of the CIRB domain from TRPC5 rendered the channel irresponsive to agonist stimulation without affecting the cell surface expression of the channel protein. Furthermore, we showed that high (>5 microM) intracellular free Ca2+ inhibited the current density without affecting the time course of TRPC5 activation by receptor agonists. These results demonstrated that intracellular Ca2+ has dual and opposite effects on the activation of TRPC5. The novel CaM-binding site is important for the Ca2+/CaM-mediated facilitation, whereas the CIRB domain is critical for the overall response of receptor-induced TRPC5 channel activation.

TRP channels in Drosophila photoreceptor cells

TRP cation channels are conserved throughout animal phylogeny and include many members that function in sensory physiology. The founding TRP is required for Drosophila phototransduction and has served as a paradigm for unravelling the roles and macromolecular organizations of TRP channels in native tissues. Two other TRPC channels, TRPL and TRPgamma, are expressed in photoreceptor cells and form heteromultimers with TRP and with each other. TRP is a member of a supramolecular signalling complex, the signalplex, which includes the PDZ scaffold protein, INAD, and two other core members that remain bound and depend on INAD for localization. Other INAD binding proteins are proposed to interact dynamically with INAD, one of which, TRPL, undergoes light-dependent translocation in photoreceptor cells. Surprisingly, TRP has non-channel functions, including an anchoring role necessary for retaining INAD in the rhabdomeres. Loss of TRP function or constitutive TRP activity results in retinal degeneration, which can be suppressed by disruption or overexpression of the Na+/Ca2+ exchanger, CalX, respectively. Given that hypoxia-induced constitutive activity of some mammalian TRPs leads to neuronal cell death, interventions that increase Na+/Ca2+ exchanger or decrease TRP function have the potential to reduce the severity of cell death due to ischaemia.

Activation of vanilloid receptor type I in the endoplasmic reticulum fails to activate store-operated Ca2+ entry

To evaluate interaction of vanilloid receptor type 1 (TRPV1) with endogenous Ca(2+) signalling mechanisms, TRPV1 was expressed in Spodoptera frugiperda (Sf 9) insect cells using recombinant baculovirus. Stimulation of TRPV1-expressing cells, but not control Sf 9 cells, with resiniferatoxin (RTX), capsaicin or anandamide, produced an increase in cytosolic free Ca(2+) concentration ([Ca(2+)](i)), with EC(50) values of 166 pM, 24.5 nM and 3.89 microM respectively. In the absence of extracellular Ca(2+), both capsaicin and RTX caused an increase in [Ca(2+)](i) with EC(50) values of approx. 10 microM and 10 nM respectively. This TRPV1-induced release of Ca(2+) from intracellular stores was not blocked by U73122, suggesting that phospholipase C was not involved. Substantial overlap was found between the thapsigargin- and RTX-sensitive internal Ca(2+) pools, and confocal imaging showed that intracellular TRPV1 immunofluorescence co-localized with the endoplasmic reticulum targeting motif KDEL. To determine if TRPV1-induced mobilization of intracellular Ca(2+) activates endogenous store-operated Ca(2+) entry, the effect of 2-aminoethoxydiphenyl borate (2-APB) on Ba(2+) influx was examined. 2-APB blocked thapsigargin-induced Ba(2+) influx, but not RTX-induced Ba(2+) entry. In the combined presence of thapsigargin and a store-releasing concentration of RTX, the 2-APB-sensitive component was essentially identical with the thapsigargin-induced component. Similar results were obtained in HEK-293 cells stably expressing TRPV1. These results suggest that TRPV1 forms agonist-sensitive channels in the endoplasmic reticulum, which when activated, release Ca(2+) from internal stores, but fail to activate endogenous store-operated Ca(2+) entry. Selective activation of intracellular TRPV1, without concomitant involvement of plasmalemmal Ca(2+) influx mechanisms, could play an important role in Ca(2+) signalling within specific subcellular microdomains.

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.

From genes to integrative physiology: ion channel and transporter biology in Caenorhabditis elegans

The stunning progress in molecular biology that has occurred over the last 50 years drove a powerful reductionist approach to the study of physiology. That same progress now forms the foundation for the next revolution in physiological research. This revolution will be focused on integrative physiology, which seeks to understand multicomponent processes and the underlying pathways of information flow from an organism's "parts" to increasingly complex levels of organization. Genetically tractable and genomically defined nonmammalian model organisms such as the nematode Caenorhabditis elegans provide powerful experimental advantages for elucidating gene function and the molecular workings of complex systems. This review has two main goals. The first goal is to describe the experimental utility of C. elegans for investigating basic physiological problems. A detailed overview of C. elegans biology and the experimental tools, resources, and strategies available for its study is provided. The second goal of this review is to describe how forward and reverse genetic approaches and direct behavioral and physiological measurements in C. elegans have generated novel insights into the integrative physiology of ion channels and transporters. Where appropriate, I describe how insights from C. elegans have provided new understanding of the physiology of membrane transport processes in mammals.

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.

Regulation of Drosophila TRPL channels by immunophilin FKBP59

Transient receptor potential and transient receptor potential-like (TRPL) are Ca(2+)-permeable cation channels found in Drosophila photoreceptor cells associated with large multimeric signaling complexes held together by the scaffolding protein, INAD. To identify novel proteins involved in channel regulation, Drosophila INAD was used as bait in a yeast two-hybrid screen of a Drosophila head cDNA library. Sequence analysis of one identified clone showed it to be identical to the Drosophila homolog of human FK506-binding protein, FKBP52 (previously known as FKBP59). To determine the function of dFKBP59, TRPL channels and dFKBP59 were co-expressed in Sf9 cells. Expression of dFKBP59 produced an inhibition of Ca(2+) influx via TRPL in fura-2 assays. Likewise, purified recombinant dFKBP59 produced a graded inhibition of TRPL single channel activity in excised inside-out patches when added to the cytoplasmic membrane surface. Immunoprecipitations from Sf9 cell lysates using recombinant tagged dFKBP59 and TRPL showed that these proteins directly interact with each other and with INAD. Addition of FK506 prior to immunoprecipitation resulted in a temperature-dependent dissociation of dFKBP59 and TRPL. Immunoprecipitations from Drosophila S2 cells and from fly head lysates demonstrated that dFKBP59, but not dFKBP12, interacts with TRPL in vivo. Likewise, INAD immunoprecipitates with dFKBP59 from S2 cell and head lysates. Immunocytochemical evaluation of thin sections of fly heads revealed specific FKBP immunoreactivity associated with the eye. Site-directed mutagenesis showed that mutations of P702Q or P709Q in the highly conserved TRPL sequence (701)LPPPFNVLP(709) eliminated interaction of the TRPL with dFKBP59. These results provide strong support for the hypothesis that immunophilin dFKBP59 is part of the TRPL-INAD signaling complex and plays an important role in modulation of channel activity via interaction with conserved leucyl-prolyl dipeptides located near the cytoplasmic mouth of the channel.

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.

Physiology, phylogeny, and functions of the TRP superfamily of cation channels

The transient receptor potential (TRP) protein superfamily consists of a diverse group of Ca(2+) permeable nonselective cation channels that bear structural similarities to Drosophila TRP. TRP-related proteins play important roles in nonexcitable cells, as demonstrated by the recent finding that a mammalian TRPC protein is expressed in endothelial cells and functions in vasorelaxation. 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 can be divided into six subfamilies, the first of which is composed of the "classical TRPs" (TRPC subfamily). These proteins all share the common features of three to four ankryin repeats, >/=30% amino acid homology over >/=750 amino acids, and a gating mechanism that operates through phospholipase C. Some classical TRPs may be store-operated channels (SOCs), which are activated by release of Ca(2+) from internal stores. The mammalian TRPC proteins are also expressed in the central nervous system, and several are highly enriched in the brain. One TRPC protein has been implicated in the pheromone response. The archetypal TRP, Drosophila TRP, is predominantly expressed in the visual system and is required for phototransduction. Many members of a second subfamily (TRPV) function in sensory physiology. These include VR1 and OSM-9, which respond to heat, osmolarity, odorants, and mechanical stimuli. A third subfamily, TRPN, includes proteins with many ankyrin repeats, one of which, NOMPC, participates in mechanotransduction. Among the members of a fourth subfamily, TRPM, is a putative tumor suppressor termed melastatin, and a bifunctional protein, TRP-PLIK, consisting of a TRPM channel fused to a protein kinase. PKD2 and mucolipidin are the founding members of the TRPP and TRPML subfamilies, respectively. Mutations in PKD2 are responsible for polycystic kidney disease, and mutations in mucolipidin result in a severe neurodegenerative disorder. Recent studies suggest that alterations in the activities of SOC and TRP channels may be at the heart of several additional neurodegenerative diseases. Thus, TRP channels may prove to be important new targets for drug discovery.

Mechanisms mediating pituitary adenylate cyclase-activating polypeptide depolarization of rat sympathetic neurons

The direct effects of pituitary adenylate cyclase-activating polypeptides (PACAP) on sympathetic neurons were investigated using rat superior cervical ganglion neurons. Electrophysiological and pharmacological analyses were used to evaluate PACAP modulation of sympathetic neuron membrane potentials and to investigate potential ionic and intracellular signaling mechanisms mediating the responses. More than 90% of the sympathetic neurons were depolarized by the PACAP peptides even when stimulated release was blocked, indicating that the PACAP peptides elicited primary responses in the postganglionic neurons. The response profile was consistent for activation of PACAP-selective PAC(1) receptors: nanomolar concentrations of PACAP27 and PACAP38 were required to stimulate depolarization, whereas vasoactive intestinal peptide failed to evoke any response. Furthermore, depolarizations elicited by PACAP27 were reduced by the PAC(1) receptor antagonist PACAP(6-38). Both sodium influx and inhibition of a potassium current contributed to the peptide-induced depolarizations. Activation of neither pertussis toxin- nor cholera toxin-sensitive G-proteins was required for generation of the depolarizations. cAMP and diacylglycerol production and activation of protein kinase A or protein kinase C also were not requisite for the responses. By contrast, phospholipase C (PLC)-dependent inositol 1,4,5-triphosphate (IP(3)) synthesis was crucial to the PACAP-mediated depolarizations. Although calcium release from IP(3)-sensitive stores was not required for the PACAP-induced responses, inhibition of IP(3) receptors reduced the depolarizations. Thus, among the many signal transduction pathways coupled to the PAC(1) receptor, the PACAP-induced depolarization of sympathetic neurons appears to require activation of PLC and subsequent generation of IP(3).

Regulation of Drosophila transient receptor potential-like (TrpL) channels by phospholipase C-dependent mechanisms

Patch clamp and fura-2 fluorescence were employed to characterize receptor-mediated activation of recombinant Drosophila TrpL channels expressed in Sf9 insect cells. TrpL was activated by receptor stimulation and by exogenous application of diacylglycerol (DAG) or poly-unsaturated fatty acids (PUFAs). Activation of TrpL was blocked more than 70% by U73122, suggesting that the effect of these agents was dependent upon phospholipase C (PLC). In fura-2 assays, extracellular application of bacterial phosphatidylinositol (PI)-PLC or phosphatidylcholine (PC)-PLC caused a transient increase in TrpL channel activity, the magnitude of which was significantly less than that observed following receptor stimulation. TrpL channels were also activated in excised inside-out patches by cytoplasmic application of mammalian PLC-b2, bacterial PI-PLC and PC-PLC, but not by phospholipase D (PLD). The phospholipases had little or no effect when examined in either whole-cell or cell-attached configurations.TrpL activity was inhibited by addition of phosphatidylinositol-4,5-bisphosphate (PIP2) to excised inside-out membrane patches exhibiting spontaneous channel activity or to patches pre-activated by treatment with PLC. The effect was reversible, specific for PIP2, and was not observed with phosphatidylethanolamine (PE), PI, PC or phosphatidylserine (PS). However, antibodies against PIP2 consistently failed to activate TrpL in inside-out patches. It is concluded that both the hydrolysis of PIP2 and the generation of DAG are required to rapidly activate TrpL following receptor stimulation, or that some other PLC-dependent mechanism plays a crucial role in the activation process.

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).

TRPgamma, a drosophila TRP-related subunit, forms a regulated cation channel with TRPL

TRP and TRPL are two light-sensitive cation channel subunits required for the Drosophila photoresponse; however, our understanding of the identities, subunit composition, and function of the light-responsive channels is incomplete. To explain the residual photoresponse that remains in the trp mutant, a third TRP-related subunit has previously been proposed to function with TRPL. Here, we identify such a subunit, TRPgamma. We show that TRPgamma is highly enriched in photoreceptor cells and preferentially heteromultimerizes with TRPL in vitro and in vivo. The N-terminal domain of TRPgamma dominantly suppressed the TRPL-dependent photoresponse, indicating that TRPgamma-TRPL heteromultimers contribute to the photoresponse. While TRPL and TRPgamma homomultimers are constitutively active, we demonstrate that TRPL-TRPgamma heteromultimers form a regulated phospholipase C- (PLC-) stimulated channel.

Normal phototransduction in Drosophila photoreceptors lacking an InsP(3) receptor gene

The Drosophila light-sensitive channels TRP and TRPL are prototypical members of an ion channel family responsible for a variety of receptor-mediated Ca(2+) influx phenomena, including store-operated calcium influx. While phospholipase Cbeta is essential, downstream events leading to TRP and TRPL activation remain unclear. We investigated the role of the InsP(3) receptor (InsP(3)R) by generating mosaic eyes homozygous for a deficiency of the only known InsP(3)R gene in Drosophila. Absence of gene product was confirmed by RT-PCR, Western analysis, and immunocytochemistry. Mutant photoreceptors underwent late onset retinal degeneration; however, whole-cell recordings from young flies demonstrated that phototransduction was unaffected, quantum bumps, macroscopic responses in the presence and absence of external Ca(2+), light adaptation, and Ca(2+) release from internal stores all being normal. Using the specific TRP channel blocker La(3+) we demonstrated that both TRP and TRPL channel functions were unaffected. These results indicate that InsP(3)R-mediated store depletion does not underlie TRP and TRPL activation in Drosophila photoreceptors.

Rotavirus infection induces an increase in intracellular calcium concentration in human intestinal epithelial cells: role in microvillar actin alteration

Rotaviruses, which infect mature enterocytes of the small intestine, are recognized as the most important cause of viral gastroenteritis in young children. We have previously reported that rotavirus infection induces microvillar F-actin disassembly in human intestinal epithelial Caco-2 cells (N. Jourdan, J. P. Brunet, C. Sapin, A. Blais, J. Cotte-Laffitte, F. Forestier, A. M. Quero, G. Trugnan, and A. L. Servin, J. Virol. 72:7228-7236, 1998). In this study, to determine the mechanism responsible for rotavirus-induced F-actin alteration, we investigated the effect of infection on intracellular calcium concentration ([Ca(2+)](i)) in Caco-2 cells, since Ca(2+) is known to be a determinant factor for actin cytoskeleton regulation. As measured by quin2 fluorescence, viral replication induced a progressive increase in [Ca(2+)](i) from 7 h postinfection, which was shown to be necessary and sufficient for microvillar F-actin disassembly. During the first hours of infection, the increase in [Ca(2+)](i) was related only to an increase in Ca(2+) permeability of plasmalemma. At a late stage of infection, [Ca(2+)](i) elevation was due to both extracellular Ca(2+) influx and Ca(2+) release from the intracellular organelles, mainly the endoplasmic reticulum (ER). We noted that at this time the [Ca(2+)](i) increase was partially related to a phospholipase C (PLC)-dependent mechanism, which probably explains the Ca(2+) release from the ER. We also demonstrated for the first time that viral proteins or peptides, released into culture supernatants of rotavirus-infected Caco-2 cells, induced a transient increase in [Ca(2+)](i) of uninfected Caco-2 cells, by a PLC-dependent efflux of Ca(2+) from the ER and by extracellular Ca(2+) influx. These supernatants induced a Ca(2+)-dependent microvillar F-actin alteration in uninfected Caco-2 cells, thus participating in rotavirus pathogenesis.

Visual transduction in Drosophila

The Drosophila phototransduction cascade has emerged as an attractive paradigm for understanding the molecular mechanisms underlying visual transduction, as well as other G protein-coupled signaling cascades that are activated and terminated with great rapidity. A large collection of mutants affecting the fly visual cascade have been isolated, and the nature and function of many of the affected gene products have been identified. Virtually all of the proteins, including those that were initially classified as novel, are highly related to vertebrate homologs. Recently, it has become apparent that most of the proteins central to Drosophila phototransduction are coupled into a supramolecular signaling complex, signalplex, through association with a PDZ-containing scaffold protein. The characterization of this complex has led to a re-evaluation of the mechanisms underlying the activation and deactivation of the phototransduction cascade.

Ca2+ entry channels in rat thoracic aortic smooth muscle cells activated by endothelin-1

The contraction of the rat aorta induced by endothelin-1 (ET-1) requires entry of extracellular Ca2+, but involvement of voltage-operated Ca2+ channel is minor. Using whole-cell recordings of patch-clamp and monitoring of the intracellular free Ca2+ concentration ([Ca2+]i), we characterized Ca2+ entry channels in A7r5 cells activated by ET-1. ET-1 activates three types of voltage-independent Ca2+ entry channels: two types of Ca2+-permeable nonselective cation channels (designated NSCC-1 and NSCC-2) and a store-operated Ca2+ channel (SOCC). Furthermore, it was found that these channels can be pharmacologically discriminated using Ca2+ channel blockers such as SK&F 96365 and LOE 908. NSCC-1 is resistant to SK&F 96365, but sensitive to LOE 908, whereas NSCC-2 is sensitive to both SK&F 96365 and LOE 908. SOCC is sensitive to SK&F 96365, but resistant to LOE 908. Using these channel blockers, we analyzed Ca2+ entry channels involved in the ET-1-induced contractions of rat thoracic aorta and increases in [Ca2+]i of single smooth muscle cells. The responses to lower concentrations of ET-1 (< or = 0.1 nM) were abolished by either SK&F 96365 or LOE 908 alone. In contrast, the responses to higher concentrations of ET-1 (> or = 1 nM) were suppressed by SK&F 96365 or LOE 908 to about 10% and 35% of controls, respectively, and abolished by combined treatment with SK&F 96365 and LOE 908. These results show that the responses of rat aorta to lower concentrations of ET-1 involve only one Ca2+ channel that is sensitive to SK&F 96365 and LOE 908 (NSCC-2), whereas those to higher concentrations of ET-1 involve NSCC-1, NSCC-2 and SOCC, contributing 10%, 55% and 35%, respectively, to total Ca2+ entry.

Ca2+-dependent interaction of the trpl cation channel and calmodulin

The transient receptor potential-like ion channel from Drosophila melanogaster was originally identified as a calmodulin binding protein (Philips et al., 1992) involved in the dipterian phototransduction process. We used a series of fusion proteins and an epitope expression library of transient receptor potential-like fusion proteins to characterize calmodulin binding regions in the transient receptor potential-like channel through the use of [125I]calmodulin and biotinylated calmodulin and identified two distinct sites at the C-terminus of the transient receptor potential-like ion channel. Calmodulin binding site 1, predicted from searching of the primary structure for amphiphilic helices (Philips et al., 1992), covers a 16 amino acid sequence (S710-I725) and could only be detected through biotinylated calmodulin. Calmodulin binding site 2 comprises at least 13 amino acids (K859ETAKERFQRVAR871) and binds both [125I]calmodulin and biotinylated calmodulin. Both sites (i) bind calmodulin at least in a one to one stoichiometry, (ii) differ in their affinity for calmodulin revealing apparent Ki values of 12.3 nM (calmodulin binding site 1) and 1.7 nM (calmodulin binding site 2), respectively, (iii) bind calmodulin only in the presence of Ca2+ with 50% of site 1 and site 2, respectively, occupied by calmodulin in the presence of 0.1 microM (calmodulin binding site 1) and 3.3 microM Ca2+ (calmodulin binding site 2) and give evidence that (iv) a Ca2+-calmodulin-dependent mechanism contributes to transient receptor potential-like cation channel modulation when expressed in CHO cells.

Capacitative calcium entry channels

In the phospholipase C signaling system, Ca(2+) is mobilized from intracellular stores by an action of inositol 1,4,5-trisphosphate. The depletion of intracellular calcium stores activates a calcium entry mechanism at the plasma membrane called capacitative calcium entry. The signal for activating the entry is unknown but likely involves either the generation or release, or both, from the endoplasmic reticulum of some diffusible signal. Recent research has focused on mammalian homologues of the Drosophila TRP protein as potential candidates for capacitative calcium entry channels. This review summarizes current knowledge about the nature of capacitative calcium entry signals, as well as the potential role of mammalian TRP proteins as capacitative calcium entry channel molecules.

Multiple channels mediate calcium leakage in the A7r5 smooth muscle-derived cell line

Ca2+ entry under resting conditions may be important for contraction of vascular smooth muscle, but little is known about the mechanisms involved. Ca2+ leakage was studied in the A7r5 smooth muscle-derived cell line by patch-clamp techniques. Two channels that could mediate calcium influx at resting membrane potentials were characterized. In 110 mM Ba2+, one channel had a slope conductance of 6.0 +/- 0.6 pS and an extrapolated reversal potential of +41 +/- 13 mV (mean +/- SD, n = 8). The current rectified strongly, with no detectable outward current, even at +90 mV. Channel gating was voltage independent. A second type of channel had a linear current-voltage relationship, a slope conductance of 17.0 +/- 3.2 pS, and a reversal potential of +7 +/- 4 mV (n = 9). The open probability increased e-fold per 44 +/- 10 mV depolarization (n = 5). Both channels were also observed in 110 mM Ca2+. Noise analysis of whole-cell currents indicates that approximately 100 6-pS channels and 30 17-pS channels are open per cell. These 6-pS and 17-pS channels may contribute to resting calcium entry in vascular smooth muscle cells.

A novel capacitative calcium entry channel expressed in excitable cells

In addition to voltage-gated calcium influx, capacitative calcium entry (CCE) represents a major pathway for calcium entry into the cell. Here we report the structure, expression and functional properties of a novel CCE channel, TRP5. This channel is a member of a new subfamily of mammalian homologues of the Drosophila transient receptor potential (TRP) protein, now comprising TRP5 (also CCE2) and the structurally related CCE1 (also TRP4). Like TRP4, TRP5 forms ion channels mainly permeable for Ca2+ which are not active under resting conditions but can be activated by manoeuvres known to deplete intracellular calcium stores. Accordingly, dialysis of TRP5-expressing cells with inositol-(1,4,5)-trisphosphate evokes inward rectifying currents which reversed polarity at potentials more positive than +30 mV. Ca2+ store depletion with thapsigargin induced TRP5-mediated calcium entry dependent on the concentration of extracellular calcium, as seen by dual wavelength fura-2 fluorescence ratio measurements. TRP5 transcripts are expressed almost exclusively in brain, where they are present in mitral cells of the olfactory bulb, in lateral cerebellar nuclei and, together with TRP4 transcripts, in CA1 pyramidal neurons of the hippocampus, indicating the presence of CCE channels in excitable cells and their participation in neuronal calcium homeostasis.

Signal transduction and regulation of lung endothelial cell permeability. Interaction between calcium and cAMP

Pulmonary endothelium forms a semiselective barrier that regulates fluid balance and leukocyte trafficking. During the course of lung inflammation, neurohumoral mediators and oxidants act on endothelial cells to induce intercellular gaps permissive for transudation of proteinaceous fluid from blood into the interstitium. Intracellular signals activated by neurohumoral mediators and oxidants that evoke intercellular gap formation are incompletely understood. Cytosolic Ca2+ concentration ([Ca2+]i) and cAMP are two signals that importantly dictate cell-cell apposition. Although increased [Ca2+]i promotes disruption of the macrovascular endothelial cell barrier, increased cAMP enhances endothelial barrier function. Furthermore, during the course of inflammation, elevated endothelial cell [Ca2+]i decreases cAMP to facilitate intercellular gap formation. Given the significance of both [Ca2+]i and cAMP in mediating cell-cell apposition, this review addresses potential sites of cross talk between these two intracellular signaling pathways. Emerging data also indicate that endothelial cells derived from different vascular sites within the pulmonary circulation exhibit distinct sensitivities to permeability-inducing stimuli; that is, elevated [Ca2+]i promotes macrovascular but not microvascular barrier disruption. Thus this review also considers the roles of [Ca2+]i and cAMP in mediating site-specific alterations in endothelial permeability.

Structure and mRNA expression of a bovine trp homologue related to mammalian trp2 transcripts

Mammalian homologues of the transient receptor potential (trp) gene product from Drosophila melanogaster function as Ca2+-selective or non-selective cation channels. Complementary DNA was isolated from a bovine testis cDNA library which encodes bovine trp2 (btrp2), a protein of 432 amino acid residues comprising four predicted transmembrane segments. Btrp2 mRNA is expressed in bovine testis, spleen and liver but not in brain, heart, adrenal gland or retina. In bovine testis expression of btrp2 mRNA is restricted to spermatocytes but is not present in spermatogonia, Leydig or Sertoli cells suggesting that btrp2 may contribute to the formation of ion channels in sperm cells.

TRP trapped in fly signaling web

In the Drosophila eye, photoactivation of rhodopsin leads to the opening of the light-sensitive cation influx channels TRP and TRPL. This response is extremely rapid and results in depolarization of the photoreceptor cells followed by Ca(2+)-mediated feedback regulation of the visual signaling cascade. The mechanisms that facilitate the rapid kinetics of activation and feedback regulation are poorly understood. However, the recent discovery that most of the proteins that function in fly phototransduction associate into a supramolecular complex permits a re-evaluation of the mechanisms underlying the activation and regulation of the cascade. The central player in the signaling complex is INAD, a protein with five protein-interaction motifs known as PDZ domains. The INAD complex does not appear to be a particle, but a massive signaling web composed of an INAD polymer with which some of the target proteins associate through complex multivalent interactions.