The characteristics of chemically induced noise inMusca photoreceptors

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.

Adaptation of single photon responses in photoreceptors of the housefly, Musca domestica: a novel spectral analysis

The absorption of a photon by a photoreceptor triggers a small voltage fluctuation termed the 'bump'. Here, in the housefly, I introduce the bispectrum of photoreceptor noise to characterise the bump under dim light. The bispectrum provides explicit phase information and is not contaminated by Gaussian background noise. Over the photon rates examined (<10(4) s(-1)), I show that bumps are minimum-phase, noise spectra are little affected by natural variations in bump shape and bumps adapt such that amplitude is approximately proportional to duration squared. In the dark exists a 'dark event', which I suggest represents spontaneous activation of G-protein.

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.

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.

Phospholipase C and termination of G-protein-mediated signalling in vivo

In Drosophila photoreceptors, phospholipase C (PLC) and other signalling components form multiprotein structures through the PDZ scaffold protein INAD. Association between PLC and INAD is important for termination of responses to light; the underlying mechanism is, however, unclear. Here we report that the maintenance of large amounts of PLC in the signalling membranes by association with INAD facilitates response termination, and show that PLC functions as a GTPase-activating protein (GAP). The inactivation of the G protein by its target, the PLC, is crucial for reliable production of single-photon responses and for the high temporal and intensity resolution of the response to light.

The visual G protein of fly photoreceptors interacts with the PDZ domain assembled INAD signaling complex via direct binding of activated Galpha(q) to phospholipase cbeta

Visual transduction in the compound eye of flies is a well-established model system for the study of G protein-coupled transduction pathways. Pivotal components of this signaling pathway, including the principal light-activated Ca(2+) channel transient receptor potential, an eye-specific protein kinase C, and the norpA-encoded phospholipase Cbeta, are assembled into a supramolecular signaling complex by the modular PDZ domain protein INAD. We have used immunoprecipitation assays to study the interaction of the heterotrimeric visual G protein with this INAD signaling complex. Light-activated Galpha(q)- guanosine 5'-O-(thiotriphosphate) and AlF(4)(-)-activated Galpha(q), but not Gbetagamma, form a stable complex with the INAD signaling complex. This interaction requires the presence of norpA-encoded phospholipase Cbeta, indicating that phospholipase Cbeta is the target of activated Galpha(q). Our data establish that the INAD signaling complex is a light-activated target of the phototransduction pathway, with Galpha(q) forming a molecular on-off switch that shuttles the visual signal from activated rhodopsin to INAD-linked phospholipase Cbeta.

Excitation of Drosophila photoreceptors by BAPTA and ionomycin: evidence for capacitative Ca2+ entry?

It has been suggested that excitation in Drosophila photoreceptors may be mediated by the depletion of intracellular Ca2+ stores (capacitative Ca2+ entry). To investigate this hypothesis, simultaneous whole-cell recordings and Indo-1 Ca2+ measurements were made from dissociated Drosophila photoreceptors, whilst testing the effects of Ca2+ releasing agents. In Ca2+ free Ringer's solution, thapsigargin raised cytosolic Ca2+ by approximately 80 nM; subsequent application of ionomycin released further Ca2+ (approximately 100 nM). A possible third compartment was indicated by the ability of monensin to mobilize further Ca2+ after saturating doses of ionomycin. Under most conditions, none of these agents activated an inward conductance, and their effects on the light response were consistent with their effects on cytosolic Ca2+. However, in the absence of both external Ca2+ and Mg2+ (to relieve a Mg2+ block of the light-sensitive channels), and after loading cells with BAPTA buffering cytosolic free Ca2+ at approximately 10 nM, ionomycin (but not thapsigargin) activated inward currents of approximately 800 pA. The response to ionomycin was enhanced (10 nA) by buffering cytosolic Ca2+ at 250 nM. A similar current also developed after approximately 3 min in cells loaded with Ca-BAPTA without any ionomycin application. The current-voltage relationships of currents activated by Ca-BAPTA or ionomycin were indistinguishable from that of the light-activated conductance and were similarly affected by a null mutation of the transient receptor potential (trp) gene which is believed to encode a subunit of the light-sensitive channels. These experiments provide some evidence for the suggestion that the light-activated and trp-dependent conductance in Drosophila photoreceptors can be activated by depletion of internal stores. However, activation by Ca-BAPTA and ionomycin had an absolute requirement for cytosolic Ca2+ as no currents could be activated by ionomycin in cells loaded with BAPTA and no Ca2+.

Phosphoinositide-mediated phototransduction in Drosophila photoreceptors: the role of Ca2+ and trp

Drosphoinate photoreceptors, represent a paradigm for the genetic dissection of phototransduction and, more generally for Ca2+ signaling. As in most invertebrates, phototransduction in Drosophila is mediated by the phosphoinositide (PI) cascade and is completely blocked by null mutations of the norpA gene which encodes a phospholipase C-beta isoform. The light-activated conductance in Drosophila is normally highly permeable to Ca2+, but in null mutants of the trp gene Ca2+ permeability is greatly reduced. Furthermore, the trp gene sequence shows homologies with voltage gated Ca2+ channels, suggesting that trp encodes a light-sensitive channel subunit. Ca2+ influx via these channels is instrumental in light adaptation, and profoundly influences phototransduction via positive and negative feedback at multiple molecular targets including protein kinase C. The mechanism of activation of the light-sensitive channels remains unresolved. A requirement for Ca2+ release from internal stores is suggested by the finding that Drosophila photoreceptors cannot sustain a maintained response under various conditions which might be expected to result in depletion of Ca2+ stores. However, Ca2+ release cannot be detected by Ca2+ indicator dyes and raising Ca2+ by photorelease of caged Ca2+ fails to mimic excitation. Recent studies, both in situ and with heterologously expressed trp protein, suggest that the trp-dependent channels may be activated by a process analogous to 'capacitative Ca2+ entry', a widespread, but poorly understood mode of PI-regulated Ca2+ influx in vertebrate cells.

Phototransduction: different mechanisms in vertebrates and invertebrates

The photoreceptor cells of invertebrate animals differ from those of vertebrates in morphology and physiology. Our present knowledge of the different structures and transduction mechanisms of the two animal groups is described. In invertebrates, rhodopsin is converted by light into a meta-rhodopsin which is thermally stable and is usually re-isomerized by light. In contrast, photoisomerization in vertebrates leads to dissociation of the chromophore from opsin, and a metabolic process is necessary to regenerate rhodopsin. The electrical signals of visual excitation have opposite character in vertebrates and invertebrates: the vertebrate photoreceptor cell is hyperpolarized because of a decrease in conductance and invertebrate photoreceptors are depolarized owing to an increase in conductance. Single-photon-evoked excitatory events, which are believed to be a result of concerted action (the opening in invertebrates and the closing in vertebrates) of many light-modulated cation channels, are very different in terms of size and time course of photoreceptors for invertebrates and vertebrates. In invertebrates, the single-photon events (bumps) produced under identical conditions vary greatly in delay (latency), time course and size. The multiphoton response to brighter stimuli is several times as long as a response evoked by a single photon. The single-photon response of vertebrates has a standard size, a standard latency and a standard time course, all three parameters showing relatively small variations. Responses to flashes containing several photons have a shape and time scale that are similar to the single-photon-evoked events, varying only by an amplitude scaling factor, but not in latency and time course. In both vertebrate and invertebrate photoreceptors the single-photon-evoked events become smaller (in size) and faster owing to light adaptation. Calcium is mainly involved in these adaptation phenomena. All light adaptation in vertebrates is primarily, or perhaps exclusively, attributable to calcium feedback. In invertebrates, cyclic AMP (cAMP) is apparently another controller of sensitivity in dark adaptation. The interaction of photoexcited rhodopsin with a G-protein is similar in both vertebrate and invertebrate photoreceptors. However, these G-proteins activate different photoreceptor enzymes (phosphodiesterases): phospholipase C in invertebrates and cGMP phosphodiesterase in vertebrates. In the photoreceptors of vertebrates light leads to a rapid hydrolysis of cGMP which results in closing of cation channels. At present, the identity of the internal terminal messenger in invertebrate photoreceptors is still unsolved.(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanisms of signal transduction in mouse spermatozoa

Sperm from all species studied thus far contain G-proteins. The presence of such signal-transducing proteins in these cells suggests that the regulation of sperm function might have control elements that are similar to ligand:receptor:G-protein:second messenger systems common to many somatic cells. This hypothesis is supported by experiments that demonstrate a potential intermediary role for the mouse sperm Gi-like protein in the acrosome reaction induced by ZP3. The specific function of this Gi-like protein in this important physiologic event is not known at this time, although possible roles in regulating ionic movements, cyclic nucleotide metabolism, and polyphosphoinositide turnover are possible candidates. Studies directed at the localization and biochemical identity of the mouse sperm Gi-like protein, as well as the nature of the second messenger system(s) modulated by this protein, are in progress and should help to delineate the sequence of events involved in some of the early steps of sperm-ZP interaction.

Chemically induced retinal degeneration in the rdgB (retinal degeneration B) mutant of Drosophila

Chemicals which affect different steps of the phototransduction cascade were used to identify the site of action of the rdgB gene product of Drosophila. In the rdgB mutant, the photoreceptor cells degenerate after several days of exposure to light, whereas raising the flies in the dark largely prevents the degeneration. In the rdgBKS222 mutant, which was used in the present studies, the light-induced degeneration is characterized by (1) selective degeneration of the peripheral but not the central photoreceptor cells; (2) random distribution of degenerated cells among ommatidia; and (3) the degeneration is specific to the rdgB but not the wild-type photoreceptor cells. In the present study, we show that application of specific chemical agents to the eyes of rdgBKS222 flies in the dark mimics the effects of light and causes retinal degeneration indistinguishable from light. The agents used in these studies are the metabolically stable GTP analogs GTP gamma S and Gpp(NH)p as well as fluoride ions, which are known to activate the transducing guanine nucleotide binding protein (G-protein of fly photoreceptors). It is unlikely that the chemically induced retinal degeneration is mediated by effects on energy metabolism, since application of the metabolic inhibitors CN- and 2-deoxy-D-glucose did not increase the extent of retinal degeneration over that observed in control flies treated with Ringer solution. The GDP analog GDP beta S, which inhibits G-protein activity, greatly reduced the extent of retinal degeneration in the dark, over that observed in control flies treated with Ringer solution. These results suggest that activation of the G-protein precedes the step in the transduction cascade that leads to retinal degeneration and provides a powerful tool to investigate the molecular mechanism of light-induced degeneration in the rdgB mutant.

On the interpretation of voltage noise in small cells

Noise analysis is a powerful technique for studying membrane conductance mechanisms, but it can be applied straightforwardly only to current noise recorded under voltage clamp. Many small cells, however, such as some sensory receptors and interneurons, are not suited for voltage clamping. In such cases it is of great interest to be able to interpret recordings of voltage noise. Voltage noise must be corrected for the effects of: (1) changes in membrane potential, (2) the cells' input impedance, and (3) noise contributed by conductances other than the one of interest. This paper describes in detail the factors--including in particular voltage-dependent K+-conductances--that affect voltage-noise recordings, and describes how these factors may be measured and corrected for in practice. A novel method allows membrane impedance to be measured at the same time as voltage noise. An example is given, showing how this approach may be applied to voltage noise from an insect photoreceptor.