Polyunsaturated fatty acids activate the Drosophila light-sensitive channels TRP and TRPL

Simultaneous roles for Ca2+ in excitation and adaptation of Limulus ventral photoreceptors

The ventral photoreceptors of Limulus have been one of the main preparations for the study of invertebrate phototransduction. The study of ventral photoreceptors has revealed that they have remarkable performance characteristics, most notably the very large amplification of the transduction process. This amplification is critically dependent upon the coupling of photoactivated rhodopsin to the phosphoinositide cascade, resulting in the release of Ca2+ from intracellular stores. The consequent elevation of Ca2+ within the photoreceptor's cytosol is amongst the most rapid and dramatic known to be activated by the phosphoinositide cascade. This review summarizes the evidence that intracellular Ca2+ is a key regulator of transduction in Limulus photoreceptors. The mechanisms that regulate Ca2+ as well as the possible targets of the action of Ca2+ are reviewed. Ca2+ elevation is critical for triggering both excitation and adaptation processes in the photoreceptor. The question of how a single second messenger can produce these two opposing effects is of obvious interest and is a topic dealt with throughout this review.

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.

Ca(2+)-dependent and Ca2+ release-dependent excitation in leech photoreceptors: evidence from a novel "inside-out" cell model

We have developed a novel, electrophysiologically intact and light-sensitive "inside-out" cell model (IOCM) of microvillar photoreceptors of the leech Hirudo medicinalis. Light responses recorded from the IOCM with sharp microelectrodes are depolarizations with amplitudes of up to 50-60 mV. In darkness, graded elevations of the free Ca(2+) concentration in the "intracellular medium" (ICM) reversibly increase the conductance of the microvillar membrane leading to Ca(2+)-induced graded voltage changes up to approximately 50 mV. The threshold for Ca(2+)-induced voltage changes is approximately 0.06 microM, EC(50) is approximately 1.2 microM, and saturation occurs at approximately 20 microM free Ca(2+). Small Ca(2+) elevations (<0.6 microM) produce discrete waves of depolarization resembling quantum bumps. Stimulating IOCMs with short (20-ms) and long (5-s) light stimuli produces transient light responses (repolarization within ca. 200 ms) in an ICM containing only 10nM free Ca(2+). At 0.44 microM free Ca(2+) in the ICM, the microvillar membrane depolarizes by 10-20 mV and responses to 5-s light steps have an initial transient component and a plateau component, similar to responses in intact cells. Generation of the plateau component in IOCMs is suppressed by heparin and cyclopiazonic acid (CPA), agents that block inositol 1,4,5-trisphosphate (Ins(1,4,5)P(3))-induced Ca(2+) release from and Ca(2+) uptake into the endoplasmic reticulum (ER). These results indicate that there is a Ca(2+)-dependent conductance in the microvillar membrane and that the light-induced Ins(1,4,5)P(3)- and Ca(2+) release-mediated intracellular Ca(2+) elevation in leech photoreceptors contributes to the generation of the receptor potential, particularly the plateau component of responses to long steps of light.

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.

Calcium signalling in lymphocytes

The modulation of intracellular calcium ion concentration, [Ca(2+)](i), is a common signalling mechanism used in many biological systems. B and T lymphocytes rely on Ca(2+) signalling to initiate both developmental and activation programs. Recent data has shed new light on the initiation of this signalling pathway, the connection between the release of intracellular Ca(2+) stores and the influx of extracellular Ca(2+), and the molecular identity of the elusive Ca(2+) release-activated Ca(2+) (CRAC) channel. In addition, recent gene profiling of T lymphocytes has identified the genes that are controlled by [Ca(2+)](i) and the Ca(2+)-dependent phosphatase calcineurin.

Rescue of light responses in the Drosophila "null" phospholipase C mutant, norpAP24, by the diacylglycerol kinase mutant, rdgA, and by metabolic inhibition

Light responses in Drosophila are reportedly abolished in severe mutants of the phospholipase C (PLC) gene, norpA. However, on establishing the whole-cell recording configuration in photoreceptors of the supposedly null allele, norpAP24, we detected a small ( approximately 15 pA) inward current that represented spontaneous light channel activity. The current decayed during approximately 20 min, after which tiny residual responses (<2 pA) were elicited by intense flashes. Both spontaneous currents and light responses appeared to be mediated by residual PLC activity, because they were enhanced by impairing diacylglycerol (DAG) kinase function by mutation (rdgA) or by restricting ATP but were reduced or abolished by a mutation of the PLC-specific Gq alpha subunit. It was reported recently that metabolic inhibition activated the light-sensitive transient receptor potential and transient receptor potential-like channels, even in norpAP24, leading to the conclusion that this action was independent of PLC (Agam, K., von Campenhausen, M., Levy, S., Ben-Ami, H. C., Cook, B., Kirschfeld, K., and Minke, B. (2000) J. Neurosci. 20, 5748-5755). However, we found that channel activation by metabolic inhibitors in norpAP24 was strictly dependent on the residual PLC activity underlying the spontaneous current, because the inhibitors failed to activate any channels after the spontaneous current had decayed. By contrast, polyunsaturated fatty acids invariably activated the channels independently of PLC. The results strongly support the obligatory requirement for PLC and DAG in Drosophila phototransduction, suggest that activation by metabolic inhibition is primarily because of the failure of diacylglycerol kinase, and are consistent with the proposal that polyunsaturated fatty acids, which are potential DAG metabolites, act directly on the channels.

TRP gating is linked to the metabolic state and maintenance of the Drosophila photoreceptor cells

The Drosophila light-activated channel TRP is the founding member of a large and diverse family of channel proteins that is conserved throughout evolution. In spite of much progress, the gating mechanism of TRP channels is still unknown. However, recent studies have shown multi-faceted functions of the Drosophila light-sensitive TRP channel that may shed light on TRP gating. Accordingly, metabolic stress, which leads to depletion of cellular ATP, reversibly activates the Drosophila TRP and TRPL channels in the dark in a constitutive manner. In several Drosophila mutants, constitutive activity of TRP channels lead to a rapid retinal degeneration in the dark, while genetic elimination of TRP protects the cells from degeneration. Additional studies have shown that TRPL translocates in a light-dependent manner between the signaling membranes and the cell body. This light-activated translocation is accompanied by reversible morphological changes leading to partial and reversible collapse of the microvillar signaling membranes into the cytosol, which allows turnover of signaling molecules. These morphological changes are also blocked by genetic elimination of TRP channels. The link of TRP gating to the metabolic state and maintenance of cells makes cells expressing TRP extremely vulnerable to metabolic stress via a mechanism that may underlie retinal degeneration and neuronal cell death upon malfunction.

Making sense with TRP channels: store-operated calcium entry and the ion channel Trpm5 in taste receptor cells

The sense of taste plays a critical role in the life and nutritional status of organisms. During the last decade, several molecules involved in taste detection and transduction have been identified, providing a better understanding of the molecular physiology of taste receptor cells. However, a comprehensive catalogue of the taste receptor cell signaling machinery is still unavailable. We have recently described the occurrence of calcium signaling mechanisms in taste receptor cells via apparent store-operated channels and identified Trpm5, a novel candidate taste transduction element belonging to the mammalian family of transient receptor potential channels. Trpm5 is expressed in a tissue-restricted manner, with high levels in gustatory tissue. In taste cells, Trpm5 is co-expressed with taste-signaling molecules such as alpha-gustducin, Ggamma(13), phospholipase C beta(2) and inositol 1,4,5-trisphosphate receptor type III. Biophysical studies of Trpm5 heterologously expressed in Xenopus oocytes and mammalian CHO-K1 cells indicate that it functions as a store-operated channel that mediates capacitative calcium entry. The role of store-operated channels and Trpm5 in capacitative calcium entry in taste receptor cells in response to bitter compounds is discussed.

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.

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

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

Proteins modulating TRP channel function

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

CRAC channels: activation, permeation, and the search for a molecular identity

The Ca2+ release-activated Ca2+ (CRAC) channel is a highly Ca2+-selective store-operated channel that is expressed in T lymphocytes, mast cells, and other hematopoietic cells. In T cells, CRAC channels are essential for generating the prolonged intracellular Ca2+ ([Ca2+](i)) elevation required for the expression of T-cell activation genes. Here we review recent work addressing CRAC channel regulation, pore properties, and the search for CRAC channel genes. Of the current models for CRAC current (I(CRAC)) activation, several new studies argue against a conformational coupling mechanism in which IP(3) receptors communicate store depletion to CRAC channels through direct physical interaction. The study of CRAC channels has been complicated by the fact that they lose activity in the absence of extracellular Ca2+. Attempts to maintain current size by removing intracellular Mg2+ have been found to unmask Mg2+-inhibited cation (MIC/MagNuM/TRPM7) channels, which have been mistaken in several studies for the CRAC channel. Recent studies under conditions that prevent MIC activation reveal that CRAC channels use high-affinity binding of Ca2+ in the pore to achieve high Ca2+ selectivity but have a surprisingly low conductance for both Ca2+ (approximately 10fS) and Na+ (approximately 0.2pS). Pore properties provide a unique fingerprint that provides a stringent test for potential CRAC channel genes and suggest models for the ion selectivity mechanism.

TRP channels in Drosophila photoreceptors: the lipid connection

The light-sensitive current in Drosophila photoreceptors is mediated by transient receptor potential (TRP) channels, at least two members of which (TRP and TRPL) are activated downstream of phospholipase C (PLC) in response to light. Recent evidence is reviewed suggesting that Drosophila TRP channels are activated by one or more lipid products of PLC activity: namely diacylglycerol (DAG), its metabolites (polyunsaturated fatty acids) or the reduction in phosphatidylinositol 4,5-bisphosphate (PIP(2)). The most compelling evidence for this view comes from analysis of rdgA mutants which are unable to effectively metabolise DAG due to a defect in DAG kinase. The rdgA mutation leads to constitutive activation of both TRP and TRPL channels and dramatically increases sensitivity to light in hypomorphic mutations of PLC and G protein.

Role of gustation in the recognition of oleate and triolein in anosmic rats

Recent studies suggest a chemical perception of dietary fat in the oral cavity. To examine the role of gustation for the recognition of oleate and triolein, very short-term (5-min), two-bottle preference tests were conducted in anosmic rats. To minimize the effects of olfaction, texture and postingestive effects, rats were rendered anosmic with intranasal zinc sulfate, test substances were suspended in 0.3% xanthan gum solution and test fluids were offered for 5 min. Rats preferred oleate fluid but not triolein fluid to the control of 0.3% xanthan gum solution. The preference threshold for oleate in the rat oral cavity was between 0.2% and 0.5%. In the two-bottle preference tests between oleate and triolein, rats preferred oleate fluid to triolein fluid, showing discrimination of oleate and triolein. The results suggest that rat recognizes oleate by a gustatory cue and that fatty acid but not triglyceride is important for gustatory recognition of fat.

TRP channels: formation of signal complex and regulation of cellular functions

Cellular stimulation from the surrounding extracellular environment via receptors and other pathways evoke activation of Ca(2+)-permeable cation channels. An important clue to understand the molecular mechanisms underlying these receptor-activated cation channels (RACC) was first provided through molecular studies of the transient receptor potential (trp) protein (TRP), which controls light-induced deporlarization in Drosophila photoreceptor cells. Recent studies have revealed that these TRP channels are also activated by diverse stimuli such as heat, osmotic stress, and oxidative stress. Furthermore, involvement of TRP channels has been demonstrated in signaling pathways essential for biological responses, such as proliferation, differentiation, and cell death. These findings encourage usage of TRP channels and their signalplexes as powerful tools for the development of novel pharmaceutical targets.

NorpA and itpr mutants reveal roles for phospholipase C and inositol (1,4,5)- trisphosphate receptor in Drosophila melanogaster renal function

Mutants of norpA, encoding phospholipase C beta (PLC beta), and itpr, encoding inositol (1,4,5)-trisphosphate receptor (IP(3)R), both attenuate response to diuretic peptides of Drosophila melanogaster renal (Malpighian) tubules. Intact tubules from norpA mutants severely reduced diuresis stimulated by the principal cell- and stellate cell-specific neuropeptides, CAP(2b) and Drosophila leucokinin (Drosokinin), respectively, suggesting a role for PLC beta in both these cell types. Measurement of IP(3) production in wild-type tubules and in Drosokinin-receptor-transfected S2 cells stimulated with CAP(2b) and Drosokinin, respectively, confirmed that both neuropeptides elevate IP(3) levels. In itpr hypomorphs, basal IP(3) levels are lower, although CAP(2b)-stimulated IP(3) levels are not significantly reduced compared with wild type. However, CAP(2b)-stimulated fluid transport is significantly reduced in itpr alleles. Rescue of the itpr(90B.0) allele with wild-type itpr restores CAP(2b)-stimulated fluid transport levels to wild type. Drosokinin-stimulated fluid transport is also reduced in homozygous and heteroallelic itpr mutants. Measurements of cytosolic calcium levels in intact tubules of wild-type and itpr mutants using targeted expression of the calcium reporter, aequorin, show that mutations in itpr attenuated both CAP(2b)- and Drosokinin-stimulated calcium responses. The reductions in calcium signals are associated with corresponding reductions in fluid transport rates. Thus, we describe a role for norpA and itpr in renal epithelia and show that both CAP(2b) and Drosokinin are PLC beta-dependent, IP(3)-mobilising neuropeptides in Drosophila. IP(3)R contributes to the calcium signalling cascades initiated by these peptides in both principal and stellate cells.

Deficiencies in C20 polyunsaturated fatty acids cause behavioral and developmental defects in Caenorhabditis elegans fat-3 mutants

Arachidonic acid and other long-chain polyunsaturated fatty acids (PUFAs) are important structural components of membranes and are implicated in diverse signaling pathways. The Delta6 desaturation of linoleic and linolenic acids is the rate-limiting step in the synthesis of these molecules. C. elegans fat-3 mutants lack Delta6 desaturase activity and fail to produce C20 PUFAs. We examined these mutants and found that development and behavior were affected as a consequence of C20 PUFA deficiency. While fat-3 mutants are viable, they grow slowly, display considerably less spontaneous movement, have an altered body shape, and produce fewer progeny than do wild type. In addition, the timing of an ultradian rhythm, the defecation cycle, is lengthened compared to wild type. Since all these defects can be ameliorated by supplementing the nematode diet with gamma-linolenic acid or C20 PUFAs of either the n6 or the n3 series, we can establish a causal link between fatty acid deficiency and phenotype. Similar epidermal tissue defects and slow growth are hallmarks of human fatty acid deficiency.

Pharmacological characterization of mammalian D1 and D2 dopamine receptors expressed in Drosophila Schneider-2 cells

Mammalian D1 and D2 dopamine receptors were stably expressed in Drosophila Schneider-2 (S2) cells and screened for their pharmacological properties. Saturable, dose-dependent, high affinity binding of the D1-selective antagonist [3H]SCH-23390 was detected only in membranes from S2 cells induced to express rat dopamine D1 receptors, while saturable, dose-dependent, high affinity binding of the D2-selective antagonist [3H]methylspiperone was detected only in membranes from S2 cells induced to express rat dopamine D2 receptors. No specific binding of either radioligand could be detected in membranes isolated from uninduced or untransfected S2 cells. Both dopamine D1 and D2 receptor subtypes displayed the appropriate stereoselective binding of enantiomers of the nonselective antagonist butaclamol. Each receptor subtype also displayed the appropriate agonist stereoselectivities. The dopamine D1 receptor bound the (+)-enantiomer of the D1-selective agonist SKF38393 with higher affinity than the (-)-enantiomer, while the dopamine D2 receptor bound the (-)-enantiomer of the D2-selective agonist norpropylapomorphine with higher affinity than the (+)-enantiomer. At both receptor subtypes, dopamine binding was best characterized as occurring to a single low affinity site. In addition, the low affinity dopamine binding was also found to be insensitive to GTPgammaS and magnesium ions. Overall, the pharmacological profiles of mammalian dopamine D1 and D2 receptors expressed in Drosophila S2 cells is comparable to those observed for these same receptors when they are expressed in mammalian cell lines. A notable distinction is that there is no evidence for the coupling of insect G proteins to mammalian dopamine receptors. These results suggest that the S2 cell insect G system may provide a convenient source of pharmacologically active mammalian D1 and D2 dopamine receptors free of promiscuous G protein contaminants.

What is the role of alpha-linolenic acid for mammals?

This review examines the data pertaining to an important and often underrated EFA, alpha-linolenic acid (ALA). It examines its sources, metabolism, and biological effects in various population studies, in vitro, animal, and human intervention studies. The main role of ALA was assumed to be as a precursor to the longer-chain n-3 PUFA, EPA and DHA, and particularly for supplying DHA for neural tissue. This paper reveals that the major metabolic route of ALA metabolism is beta-oxidation. Furthermore, ALA accumulates in specific sites in the body of mammals (carcass, adipose, and skin), and only a small proportion of the fed ALA is converted to DHA. There is some evidence that ALA may be involved with skin and fur function. There is continuing debate regarding whether ALA has actions of its own in relation to the cardiovascular system and neural function. Cardiovascular disease and cancer are two of the major burdens of disease in the 21st century, and emerging evidence suggests that diets containing ALA are associated with reductions in total deaths and sudden cardiac death. There may be aspects of the action and, more importantly, the metabolism of ALA that need to be elucidated, and these will help us understand the biological effects of this compound better. Additionally, we must not forget that ALA is part of the whole diet and should be seen in this context, not in isolation.

Molecular basis of amplification in Drosophila phototransduction: roles for G protein, phospholipase C, and diacylglycerol kinase

In Drosophila photoreceptors, the amplification responsible for generating quantum bumps in response to photoisomerization of single rhodopsin molecules has been thought to be mediated downstream of phospholipase C (PLC), since bump amplitudes were reportedly unaffected in mutants with greatly reduced levels of either G protein or PLC. We now find that quantum bumps in such mutants are reduced approximately 3- to 5-fold but are restored to near wild-type values by mutations in the rdgA gene encoding diacylglycerol kinase (DGK) and also by depleting intracellular ATP. The results demonstrate that amplification requires activation of multiple G protein and PLC molecules, identify DGK as a key enzyme regulating amplification, and implicate diacylglycerol as a messenger of excitation in Drosophila phototransduction.

New aspects of calcium signaling in skeletal muscle cells: implications in Duchenne muscular dystrophy

Calcium is the most ubiquitous second messenger. Its concentration inside the cell is tightly regulated by a series of mechanisms, among which some have been extensively studied in nonmuscle cells. This is the case of the "store-operated entry of Ca(2+)", the uptake of Ca(2+) by mitochondria and the inositol 1,4,5-trisphosphate (IP(3)) cascade. These processes were recently found to be also present in skeletal muscle and are reviewed here. The "store-operated entry of Ca(2+)" allows the refilling of the stores after muscle fiber depolarization and is activated even after a partial depletion of the sarcoplasmic reticulum (SR). The uptake of Ca(2+) by mitochondria accelerates muscle relaxation and allows the adaptation of ATP supply to the increased energy demand. IP(3) receptors are found in the nuclear envelope and are involved in Ca(2+) waves propagating from one nucleus to another. This pathway is possibly involved in gene expression regulation. Finally, cytosolic Ca(2+) buffers like parvalbumins modify [Ca(2+)](i) transients and, therefore, muscle mechanics. The importance of these regulation mechanisms is also evaluated in Duchenne muscular dystrophy (DMD), a disease in which impairment of [Ca(2+)](i) homeostasis has been postulated but remains, however, controversial. This genetic disease is indeed characterized by the absence of a cytoskeletal protein called dystrophin, a situation leading to a disorganization of the cytoskeleton and to an abnormal influx of Ca(2+). How this increased entry of Ca(2+) affects the local concentration of Ca(2+) in subcellular compartments and whether this process is involved in the development of the disease are still unclear.

The cellular and molecular basis of store-operated calcium entry

The impact of calcium signalling on so many areas of cell biology reflects the crucial role of calcium signals in the control of diverse cellular functions. Despite the precision with which spatial and temporal details of calcium signals have been resolved, a fundamental aspect of the generation of calcium signals -- the activation of 'store-operated channels' (SOCs) -- remains a molecular and mechanistic mystery. Here we review new insights into the exchange of signals between the endoplasmic reticulum (ER) and plasma membrane that result in activation of calcium entry channels mediating crucial long-term calcium signals.

Arachidonic acid plays a role in rat vomeronasal signal transduction

Sensory neurons of the vomeronasal organ (VNO) detect volatile chemicals that are released by conspecific animals and convey information about social and reproductive behavior. The signal transduction pathway in vomeronasal receptor neurons (VRNs) is not known in detail, but is believed to be distinct from that of the sensory neurons of the main olfactory system. Many of the identified olfactory transduction components are not expressed by VRNs. Using Ca2+ imaging and electrophysiological recordings, we investigated the signal transduction pathway of urine perception and the possible role of polyunsaturated fatty acids (PUFAs) as intracellular messengers in freshly dissociated rat VNO neurons. We found that application of urine induced a transient increase in intracellular Ca2+ that was dependent on the activity of phospholipase C and diacylglycerol (DAG) lipase. The Ca2+ transient was not dependent on depletion of intracellular Ca2+ stores but was dependent on the presence of extracellular Ca2+. Furthermore, the urine response was not sensitive to modulators of adenylate cyclase and inhibitors of inositol 1,4,5-trisphosphate receptors. Application of PUFAs (linolenic acid and arachidonic acid, synthesized in living cells from DAG) also elicited Ca2+ transients in fura 2 measurements and inward currents in whole-cell voltage-clamp recordings. Pharmacological inhibition of lipoxygenase and cyclooxygenase induced a transient increase in intracellular Ca2+, possibly by increasing the endogenous level of PUFAs, leading to activation of transduction channels. These data provide evidence for a role of PUFAs in rat vomeronasal signal transduction.

Ca(2+)-calmodulin regulates receptor-operated Ca(2+) entry activity of TRPC6 in HEK-293 cells

Mammalian homologues of the Drosophila transient receptor potential channel (TRPC) are involved in Ca(2+) entry following agonist stimulation of nonexcitable cells. Seven mammalian TRPCs have been cloned but their mechanisms of activation and/or regulation are still the subject of intense research efforts. It has already been shown that calmodulin (CaM) can regulate the activity of Drosophila TRP and TRPL and, more recently, CaM has been shown to interact with mammalian TRPCs. In this study, TRPC6 stably transfected into HEK-293 cells was used to investigate the possible influence of CaM on TRPC6-dependent Ca(2+) entry. Overexpression of TRPC6 in mammalian cells is known to enhance agonist-induced Ca(2+) entry, but not thapsigargin-induced Ca(2+) entry. Here, we show that CaM inhibitors (calmidazolium and trifluoperazine) abolish receptor-operated Ca(2+) entry (ROCE) without affecting thapsigargin-operated Ca(2+) entry and that the activity of CaM is dependent on complexation with Ca(2+). We also show that Ca(2+)-CaM binds to TRPC6 and that the binding can be abolished by CaM inhibitors. These results indicate that CaM is involved in the modulation of ROCE.

Fatty acid augmentation of the cardiac slowly activating delayed rectifier current (IKs) is conferred by hminK

The mechanism by which dietary fatty acids confer protection against cardiac arrhythmias and sudden cardiac death is not resolved. Here, we study the effects of several known cardio-protective and arrhythmogenic fatty acids on the slowly activating delayed rectifier potassium current (IKs), which is responsible for the repolarization phase of the cardiac action potential. cRNAs encoding either or both of the two subunits, KvLQT1 and hminK, that together produce IKs, were injected into Xenopus oocytes, and the effects of various fatty acids were determined. Docosahexaenoic acid (DHA) significantly augmented IKs as did the short-chained fully saturated lauric acid, and to a lesser extent the cis-unsaturated oleic acid. Eicosapentaenoic acid (EPA) was without significant effect on current magnitude, although it reduced the rate of activation. These results suggest that not all "antiarrhythmic" fatty acids target the same channel. To examine the role of hminK in this response, KvLQT1 was expressed alone. In this case, DHA, lauric acid, and oleic acid did not augment current, suggesting that hminK confers fatty acid sensitivity to IKs.

Combinatorial expression of TRPV channel proteins defines their sensory functions and subcellular localization in C. elegans neurons

C. elegans OSM-9 is a TRPV channel protein involved in sensory transduction and adaptation. Here, we show that distinct sensory functions arise from different combinations of OSM-9 and related OCR TRPV proteins. Both OSM-9 and OCR-2 are essential for several forms of sensory transduction, including olfaction, osmosensation, mechanosensation, and chemosensation. In neurons that express both OSM-9 and OCR-2, tagged OCR-2 and OSM-9 proteins reside in sensory cilia and promote each other's localization to cilia. In neurons that express only OSM-9, tagged OSM-9 protein resides in the cell body and acts in sensory adaptation rather than sensory transduction. Thus, alternative combinations of TRPV proteins may direct different functions in distinct subcellular locations. Animals expressing the mammalian TRPV1 (VR1) channel in ASH nociceptor neurons avoid the TRPV1 ligand capsaicin, allowing selective, drug-inducible activation of a specific behavior.

Current understanding of mammalian TRP homologues

Calcium influx into the cell from the extracellular medium is crucial for important processes including muscle contraction, secretion and gene expression. This calcium influx is mainly mediated through calcium influx channels, which on the basis of their activation mechanism can be subdivided in voltage-gated calcium channels, which have already been thoroughly characterized and non-voltage-gated calcium permeable channels. This latter group includes ion channels activated by binding of extra and intracellular messengers, mechanical stress or depletion of intracellular calcium stores. Currently little molecular data is available concerning this class of calcium influx channels. However, recent studies have indicated that members of the transient receptor potential (TRP) family of ion channels can function as calcium influx channels both in excitable and non-excitable tissues. On the basis of structural information the TRP family is subdivided in three main subfamilies: the TRPC (canonical) group, the TRPV (vanilloid) group and the TRPM (melastatin) group. The cloning and characterization of members of this cation channel family has exploded during recent years, leading to a plethora of data concerning TRPs in a variety of tissues and species, including mammals, insects and yeast. This review summarizes the currently available information concerning members of the TRP family expressed in mammalian tissues.

Vanilloid and TRP channels: a family of lipid-gated cation channels

The emergence of the TRP (C) and vanilloid (TRPV) receptor family of Ca(2+) permeable channels has started to provide molecular focus to a linked group of ion channels whose common feature is activation primarily by intracellular ligands. These channels have a central role in Ca(2+) homeostasis in virtually all cells and in particular those that lack voltage-gated Ca(2+) channels. We will discuss recent work that is more precisely defining both molecular form and physiological function of this important group of Ca(2+) permeable channels with particular focus on the intracellular ligands that gate and modulate channel activity.

The TRPM7 channel is inactivated by PIP(2) hydrolysis

TRPM7 (ChaK1, TRP-PLIK, LTRPC7) is a ubiquitous, calcium-permeant ion channel that is unique in being both an ion channel and a serine/threonine kinase. The kinase domain of TRPM7 directly associates with the C2 domain of phospholipase C (PLC). Here, we show that in native cardiac cells and heterologous expression systems, G alpha q-linked receptors or tyrosine kinase receptors that activate PLC potently inhibit channel activity. Numerous experimental approaches demonstrated that phosphatidylinositol 4,5-bisphosphate (PIP(2)), the substrate of PLC, is a key regulator of TRPM7. We conclude that receptor-mediated activation of PLC results in the hydrolysis of localized PIP(2), leading to inactivation of the TRPM7 channel.

Genetic dissection of polyunsaturated fatty acid synthesis in Caenorhabditis elegans

Polyunsaturated fatty acids (PUFAs) are important membrane components and precursors of signaling molecules. To investigate the roles of these fatty acids in growth, development, and neurological function in an animal system, we isolated Caenorhabditis elegans mutants deficient in PUFA synthesis by direct analysis of fatty acid composition. C. elegans possesses all the desaturase and elongase activities to synthesize arachidonic acid and eicosapentaenoic acid from saturated fatty acid precursors. In our screen we identified mutants with defects in each fatty acid desaturation and elongation step of the PUFA biosynthetic pathway. The fatty acid compositions of the mutants reveal the substrate preferences of the desaturase and elongase enzymes and clearly demarcate the steps of this pathway. The mutants show that C. elegans does not require n3 or Delta5-unsaturated PUFAs for normal development under laboratory conditions. However, mutants with more severe PUFA deficiencies display growth and neurological defects. The mutants provide tools for investigating the roles of PUFAs in membrane biology and cell function in this animal model.

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.

Transient receptor potential 1 regulates capacitative Ca(2+) entry and Ca(2+) release from endoplasmic reticulum in B lymphocytes

Capacitative Ca(2+) entry (CCE) activated by release/depletion of Ca(2+) from internal stores represents a major Ca(2+) influx mechanism in lymphocytes and other nonexcitable cells. Despite the importance of CCE in antigen-mediated lymphocyte activation, molecular components constituting this mechanism remain elusive. Here we demonstrate that genetic disruption of transient receptor potential (TRP)1 significantly attenuates both Ca(2+) release-activated Ca(2+) currents and inositol 1,4,5-trisphosphate (IP(3))-mediated Ca(2+) release from endoplasmic reticulum (ER) in DT40 B cells. As a consequence, B cell antigen receptor-mediated Ca(2+) oscillations and NF-AT activation are reduced in TRP1-deficient cells. Thus, our results suggest that CCE channels, whose formation involves TRP1 as an important component, modulate IP(3) receptor function, thereby enhancing functional coupling between the ER and plasma membrane in transduction of intracellular Ca(2+) signaling in B lymphocytes.

The TRP family of cation channels: probing and advancing the concepts on receptor-activated calcium entry

Stimulation of membrane receptors linked to a phospholipase C and the subsequent production of the second messengers diacylglycerol and inositol-1,4,5-trisphosphate (InsP(3)) is a signaling pathway of fundamental importance in eukaryotic cells. Signaling downstream of these initial steps involves mobilization of Ca(2+) from intracellular stores and Ca(2+) influx through the plasma membrane. For this influx, several contrasting mechanisms may be responsible but particular relevance is attributed to the induction of Ca(2+) influx as consequence of depletion of intracellular calcium stores. This phenomenon (frequently named store-operated calcium entry, SOCE), in turn, may be brought about by various signals, including soluble cytosolic factors, interaction of proteins of the endoplasmic reticulum with ion channels in the plasma membrane, and a secretion-like coupling involving translocation of channels to the plasma membrane. Experimental approaches to analyze these mechanisms have been considerably advanced by the discovery of mammalian homologs of the Drosophila cation channel transient receptor potential (TRP). Some members of the TRP family can be expressed to Ca(2+)-permeable channels that enable SOCE; other members form channels activated independently of stores. TRP proteins may be an essential part of endogenous Ca(2+) entry channels but so far expression of most TRP cDNAs has not resulted in restitution of channels found in any mammalian cells, suggesting the requirement for further unknown subunits. A major exception is CaT1, a TRP channel demonstrated to provide Ca(2+)-selective, store-operated currents identical to those characterized in several cell types. Ongoing and future research on TRP channels will be crucial to understand the molecular basis of receptor-mediated Ca(2+) entry, with respect to the structure of the entry channels as well as to the mechanisms of its activation and regulation.

Effects of inhibitors of the lipo-oxygenase family of enzymes on the store-operated calcium current I(CRAC) in rat basophilic leukaemia cells

In non-excitable cells, the major Ca2+ entry pathway is the store-operated pathway in which emptying of intracellular Ca2+ stores activates Ca2+ channels in the plasma membrane. In many cell types, store-operated influx gives rise to a Ca2+-selective current called I(CRAC) (Ca2+ release-activated Ca2+ current). Using both the whole-cell patch clamp technique to measure I(CRAC) directly and fluorescent Ca2+ imaging, we have examined the role of the lipo-oxygenase pathway in the activation of store-operated Ca2+ entry in the RBL-1 rat basophilic leukaemia cell-line. Pretreatment with a variety of structurally distinct lipo-oxygenase inhibitors all reduced the extent of I(CRAC), whereas inhibition of the cyclo-oxygenase enzymes was without effect. The inhibition was still seen in the presence of the broad protein kinase blocker staurosporine, or when Na+ was used as the charge carrier through CRAC channels. The lipo-oxygenase blockers released Ca2+ from intracellular stores but this was not associated with subsequent Ca2+ entry. Lipo-oxygenase blockers also reduced both the amount of Ca2+ that could subsequently be released by the combination of thapsigargin and ionomycin in Ca2+-free solution and the Ca2+ influx component that occurred when external Ca2+ was re-admitted. The inhibitors were much less effective if applied after I(CRAC) had been activated. This inhibition of I(CRAC) could not be rescued by dialysis with 5(S)-hydroxyperoxyeicosa-6E,8Z,11Z,14Z,tetraenoic acid (5-HPETE), the first product of the 5-lipo-oxygenase pathway. Our findings indicate that exposure to pharmacological tools that inhibit the lipo-oxygenase enzymes all decrease the extent of activation of the current. Our results raise the possibility that a lipo-oxygenase might be involved in the activation of I(CRAC). Alternative explanations are also discussed.

Molecular basis for species-specific sensitivity to "hot" chili peppers

Chili peppers produce the pungent vanilloid compound capsaicin, which offers protection from predatory mammals. Birds are indifferent to the pain-producing effects of capsaicin and therefore serve as vectors for seed dispersal. Here, we determine the molecular basis for this species-specific behavioral response by identifying a domain of the rat vanilloid receptor that confers sensitivity to capsaicin to the normally insensitive chicken ortholog. Like its mammalian counterpart, the chicken receptor is activated by heat or protons, consistent with the fact that both mammals and birds detect noxious heat and experience thermal hypersensitivity. Our findings provide a molecular basis for the ecological phenomenon of directed deterence and suggest that the capacity to detect capsaicin-like inflammatory substances is a recent acquisition of mammalian vanilloid receptors.

Loss of the phospholipase C gene product induces massive endocytosis of rhodopsin and arrestin in Drosophila photoreceptors

Previously we have shown that a subset of visual transduction mutants in Drosophila melanogaster induce the formation of stable complexes between rhodopsin and arrestin. One such mutant is in a visual system-specific phospholipase C (PLC). The rhodopsin/arrestin complexes generated in PLC mutants induce massive retinal degeneration. Here we demonstrate that both arrestin and rhodopsin undergo light-dependent endocytosis in a PLC mutant background. Interestingly, the internalized rhodopsin is rapidly degraded, but the arrestin is fully stable. The data are discussed with respect to mechanisms of arrestin-mediated endocytosis and human retinal disease.

Expression and functional role of bTRPC1 channels in native endothelial cells

We have analyzed the expression and localization of bovine transient receptor potential-C1 (bTRPC1) in bovine aortic endothelial cells, and its possible involvement in the store-independent calcium influx induced by basic fibroblast growth factor (bFGF). RT-PCR experiments confirmed the existence of two btrpc1 mRNA isoforms; conversely, the btrpc3 gene was not transcribed. Anti-TRPC1 antibody revealed the presence of the protein in the membrane-rich compartment only. Application of anti-TRPC1 during the response to bFGF caused a partial but significant reduction of calcium entry. This is the first evidence of TRP channel involvement in a non-capacitative calcium influx induced by a biologically relevant agonist such as the angiogenic factor bFGF in native endothelial cells.

A(2) adenosine receptors regulate CFTR through PKA and PLA(2)

We investigated adenosine (Ado) activation of the cystic fibrosis transmembrane conductance regulator (CFTR) in vitro and in vivo. A(2B) Ado receptors were identified in Calu-3, IB-3-1, COS-7, and primary human airway cells. Ado elevated cAMP in Calu-3, IB-3-1, and COS-7 cells and activated protein kinase A-dependent halide efflux in Calu-3 cells. Ado promoted arachidonic acid release from Calu-3 cells, and phospholipase A(2) (PLA(2)) inhibition blocked Ado-activated halide efflux in Calu-3 and COS-7 cells expressing CFTR. Forskolin- and beta(2)-adrenergic receptor-stimulated efflux were not affected by the same treatment. Cytoplasmic PLA(2) (cPLA(2)) was identified in Calu-3, IB-3-1, and COS-7 cells, but cPLA(2) inhibition did not affect Ado-stimulated cAMP concentrations. In cftr(+) and cftr(-/-) mice, Ado stimulated nasal Cl(-) secretion that was CFTR dependent and sensitive to A(2) receptor and PLA(2) blockade. In COS-7 cells transiently expressing DeltaF508 CFTR, Ado activated halide efflux. Ado also activated G551D CFTR-dependent halide efflux when combined with arachidonic acid and phosphodiesterase inhibition. In conclusion, PLA(2) and protein kinase A both contribute to A(2) receptor activation of CFTR, and components of this signaling pathway can augment wild-type and mutant CFTR activity.

LTRPC2 Ca2+-permeable channel activated by changes in redox status confers susceptibility to cell death

Redox status changes exert critical impacts on necrotic/apoptotic and normal cellular processes. We report here a widely expressed Ca2+-permeable cation channel, LTRPC2, activated by micromolar levels of H2O2 and agents that produce reactive oxygen/nitrogen species. This sensitivity of LTRPC2 to redox state modifiers was attributable to an agonistic binding of nicotinamide adenine dinucleotide (beta-NAD+) to the MutT motif. Arachidonic acid and Ca2+ were important positive regulators for LTRPC2. Heterologous LTRPC2 expression conferred susceptibility to death on HEK cells. Antisense oligonucleotide experiments revealed physiological involvement of "native" LTRPC2 in H2O2- and TNFalpha-induced Ca2+ influx and cell death. Thus, LTRPC2 represents an important intrinsic mechanism that mediates Ca2+ and Na+ overload in response to disturbance of redox state in cell death.

The complex and intriguing lives of PIP2 with ion channels and transporters

Phosphatidylinositol-4,5-bisphosphate (PIP(2)), the precursor of several signaling molecules in eukayotic cells, is itself also used by cells to signal to membrane-associated proteins. PIP(2) anchors numerous signaling molecules and cytoskeleton at the cell membrane, and the metabolism of PIP(2) is closely connected to membrane trafficking. Recently, ion transporters and channels have been discovered to be regulated by PIP(2). Systems reported to be activated by PIP(2) include (i) plasmalemmal calcium pumps (PMCA), (ii) cardiac sodium-calcium exchangers (NCX1), (iii) sodium-proton exchangers (NHE1-4), (iv) a sodium-magnesium exchanger of unknown identity, (v) all inward rectifier potassium channels (KATP, IRK, GIRK, and ROMK channels), (vi) epithelial sodium channels (ENaC), and (vii) ryanodine-sensitive calcium release channels (RyR). Systems reported to be inhibited by PIP(2) include (i) cyclic nucleotide-gated channels of the rod (CNG), (ii) transient receptor potential-like (TRPL) Drosophila phototransduction channels, (iii) capsaicin-activated transient receptor potential (TRP) channels (VR1), and (iv) IP(3)-gated calcium release channels (IP3R). Systems that appear to be completely insensitive to PIP(2) include (i) voltage-gated sodium channels, (ii) most voltage-gated potassium channels, (iii) sodium-potassium pumps, (iv) several neurotransmitter transporters, and (v) cystic fibrosis transmembrane receptor (CFTR)-type chloride channels. Presumably, local changes of the concentration of PIP(2) in the plasma membrane represent cell signals to those mechanisms sensitive to PIP(2) changes. Unfortunately, our understanding of how local PIP(2) concentrations are regulated remains very limited. One important complexity is the probable existence of phospholipid microdomains, or lipid rafts. Such domains may serve to localize PIP(2) and thereby PIP(2) signaling, as well as to organize PIP(2) binding partners into signaling complexes. A related biological role of PIP(2) may be to control the activity of ion transporters and channels during biosynthesis or vesicle trafficking. Low PIP(2) concentrations in the secretory pathway would inactivate all of the systems that are stimulated by PIP(2). How, in detail, is PIP(2) used by cells to control ion channel and transporter activities? Further progress requires an improved understanding of lipid kinases and phosphatases, how they are regulated, where they are localized in cells, and with which ion channels and transporters they might localize.

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.

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.

Phototransduction in Drosophila melanogaster

As in most invertebrate microvillar photoreceptors, phototransduction in Drosophila melanogaster uses a G-protein-coupled phosphoinositide pathway, whereby hydrolysis of phosphatidyl inositol 4,5-bisphosphate (PIP2) by phospholipase C generates inositol 1,4,5-trisphosphate (InsP3) and diacyl glycerol (DAG), leading to activation of two classes of Ca2+-permeable light-sensitive channel, encoded by the trp and trpl genes. In some invertebrate photoreceptors, excitation is mediated by release of Ca2+ from intracellular stores by InsP3; however, in Drosophila melanogaster, recent evidence suggests instead that a lipid messenger, such as DAG, its metabolites and/or the reduction in PIP2 levels, may mediate excitation. Like vertebrate rods, Drosophila melanogaster photoreceptors generate quantum bumps in response to single photons, but their kinetics is approximately 10–100 times faster, and this reflects a fundamentally different strategy incorporating a threshold, positive and negative feedback by Ca2+ acting downstream of phospholipase C and a refractory period.

Soluble guanylate cyclase is required during development for visual system function in Drosophila

A requirement for nitric oxide (NO) in visual system development has been demonstrated in many model systems, but the role of potential downstream effector molecules has not been established. Developing Drosophila photoreceptors express an NO-sensitive soluble guanylate cyclase (sGC), whereas the optic lobe targets express NO synthase. Both of these molecules are expressed after photoreceptor outgrowth to the optic lobe, when retinal growth cones are actively selecting their postsynaptic partners. We have previously shown that inhibition of the NO-cGMP pathway in vitro leads to overgrowth of retinal axons. Here we examined flies mutant for the alpha subunit gene of the Drosophila sGC (Gcalpha1). This mutation severely reduced but did not abolish GCalpha1 protein levels and NO-stimulated sGC activity in the developing photoreceptors. Although few mutant individuals possessed a disorganized retinal projection pattern, pharmacological NOS inhibition during metamorphosis increased this disorganization in mutants to a greater degree than in the wild type. Adult mutants lacked phototactic behavior, and the off-transient component of electroretinograms was frequently absent or greatly reduced in amplitude. Normal phototaxis and off-transient amplitude were restored by heat shock-mediated Gcalpha1 expression applied during metamorphosis but not in the adult. We propose that diminished sGC activity in the visual system during development causes inappropriate or inadequate formation of first-order retinal synapses, leading to defects in visual system function and visually mediated behavior.

Ultrastructural localization of G-proteins and the channel protein TRP2 to microvilli of rat vomeronasal receptor cells

Microvilli of vomeronasal organ (VNO) sensory epithelium receptor cells project into the VNO lumen. This lumen is continuous with the outside environment. Therefore, the microvilli are believed to be the subcellular sites of VNO receptor cells that interact with incoming VNO-targeted odors, including pheromones. Candidate molecules, which are implicated in VNO signaling cascades, are shown to be present in VNO receptor cells. However, ultrastructural evidence that such molecules are localized within the microvilli is sparse. The present study provides firm evidence that immunoreactivity for several candidate VNO signaling molecules, notably the G-protein subunits G(ialpha2) and G(oalpha), and the transient receptor potential channel 2 (TRP2), is localized prominently and selectively in VNO receptor cell microvilli. Although G(ialpha2) and G(oalpha) are localized separately in the microvilli of two cell types that are otherwise indistinguishable in their apical and microvillar morphology, the microvilli of both cell types are TRP2(+). VNO topographical distinctions were also apparent. Centrally within the VNO sensory epithelium, the numbers of receptor cells with G(ialpha2)(+) and G(oalpha)(+) microvilli were equal. However, near the sensory/non-sensory border, cells with G(ialpha2)(+) microvilli predominated. Scattered ciliated cells in this transition zone resembled neither VNO nor main olfactory organ (MO) receptor cells and may represent the same ciliated cells as those found in the non-sensory part of the VNO. Thus, this study shows that, analogous to the cilia of MO receptor cells, microvilli of VNO receptor cells are enriched selectively in proteins involved putatively in signal transduction. This provides important support for the role of these molecules in VNO signaling.

Ion channels and their functional role in vascular endothelium

Endothelial cells (EC) form a unique signal-transducing surface in the vascular system. The abundance of ion channels in the plasma membrane of these nonexcitable cells has raised questions about their functional role. This review presents evidence for the involvement of ion channels in endothelial cell functions controlled by intracellular Ca(2+) signals, such as the production and release of many vasoactive factors, e.g., nitric oxide and PGI(2). In addition, ion channels may be involved in the regulation of the traffic of macromolecules by endocytosis, transcytosis, the biosynthetic-secretory pathway, and exocytosis, e.g., tissue factor pathway inhibitor, von Willebrand factor, and tissue plasminogen activator. Ion channels are also involved in controlling intercellular permeability, EC proliferation, and angiogenesis. These functions are supported or triggered via ion channels, which either provide Ca(2+)-entry pathways or stabilize the driving force for Ca(2+) influx through these pathways. These Ca(2+)-entry pathways comprise agonist-activated nonselective Ca(2+)-permeable cation channels, cyclic nucleotide-activated nonselective cation channels, and store-operated Ca(2+) channels or capacitative Ca(2+) entry. At least some of these channels appear to be expressed by genes of the trp family. The driving force for Ca(2+) entry is mainly controlled by large-conductance Ca(2+)-dependent BK(Ca) channels (slo), inwardly rectifying K(+) channels (Kir2.1), and at least two types of Cl( -) channels, i.e., the Ca(2+)-activated Cl(-) channel and the housekeeping, volume-regulated anion channel (VRAC). In addition to their essential function in Ca(2+) signaling, VRAC channels are multifunctional, operate as a transport pathway for amino acids and organic osmolytes, and are possibly involved in endothelial cell proliferation and angiogenesis. Finally, we have also highlighted the role of ion channels as mechanosensors in EC. Plasmalemmal ion channels may signal rapid changes in hemodynamic forces, such as shear stress and biaxial tensile stress, but also changes in cell shape and cell volume to the cytoskeleton and the intracellular machinery for metabolite traffic and gene expression.