Effects of intracellular Ca2+ chelation on the light response in Drosophila photoreceptors

Functional cooperation between the IP3 receptor and phospholipase C secures the high sensitivity to light of Drosophila photoreceptors in vivo

Drosophila phototransduction is a model system for the ubiquitous phosphoinositide signaling. In complete darkness, spontaneous unitary current events (dark bumps) are produced by spontaneous single Gqα activation, while single-photon responses (quantum bumps) arise from synchronous activation of several Gqα molecules. We have recently shown that most of the spontaneous single Gqα activations do not produce dark bumps, because of a critical phospholipase Cβ (PLCβ) activity level required for bump generation. Surpassing the threshold of channel activation depends on both PLCβ activity and cellular [Ca(2+)], which participates in light excitation via a still unclear mechanism. We show here that in IP3 receptor (IP3R)-deficient photoreceptors, both light-activated Ca(2+) release from internal stores and light sensitivity were strongly attenuated. This was further verified by Ca(2+) store depletion, linking Ca(2+) release to light excitation. In IP3R-deficient photoreceptors, dark bumps were virtually absent and the quantum-bump rate was reduced, indicating that Ca(2+) release from internal stores is necessary to reach the critical level of PLCβ catalytic activity and the cellular [Ca(2+)] required for excitation. Combination of IP3R knockdown with reduced PLCβ catalytic activity resulted in highly suppressed light responses that were partially rescued by cellular Ca(2+) elevation, showing a functional cooperation between IP3R and PLCβ via released Ca(2+). These findings suggest that in contrast to the current dogma that Ca(2+) release via IP3R does not participate in light excitation, we show that released Ca(2+) plays a critical role in light excitation. The positive feedback between PLCβ and IP3R found here may represent a common feature of the inositol-lipid signaling.

Elementary and macroscopic light-induced currents and their Ca(2+)-dependence in the photoreceptors of Periplaneta americana

In a microvillar photoreceptor, absorption of an incident photon initiates a phototransduction reaction that generates a depolarizing light-induced current (LIC) in the microvillus. Although in-depth knowledge about these processes in photoreceptors of the fruitfly Drosophila is available, not much is known about their nature in other insect species. Here, we present description of some basic properties of both elementary and macroscopic LICs and their Ca(2+)-dependence in the photoreceptors of a dark-active species, the cockroach Periplaneta americana. Cockroach photoreceptors respond to single photon absorptions by generating quantum bumps with about 5-fold larger amplitudes than in Drosophila. At the macroscopic current level, cockroach photoreceptors responded to light with variable sensitivity and current waveform. This variability could be partially attributed to differences in whole-cell capacitance. Transient LICs, both elementary and macroscopic, showed only moderate dependence on extracellular Ca(2+). However, with long light pulses, response inactivation was largely abolished and the overall size of LICs increased when extracellular Ca(2+) was omitted. Finally, by determining relative ionic permeabilities from reversals of LICs, we demonstrate that when compared to Drosophila, cockroach light-gated channels are only moderately Ca(2+)-selective.

The Drosophila SK channel (dSK) contributes to photoreceptor performance by mediating sensitivity control at the first visual network

The contribution of the SK (small-conductance calcium-activated potassium) channel to neuronal functions in complex circuits underlying sensory processing and behavior is largely unknown in the absence of suitable animal models. Here, we generated a Drosophila line that lacks the single highly conserved SK gene in its genome (dSK). In R1-R6 photoreceptors, dSK encodes a slow Ca²⁺-activated K(+) current similar to its mammalian counterparts. Compared with wild-type, dSK(-) photoreceptors and interneurons showed accelerated oscillatory responses and adaptation. These enhanced kinetics were accompanied with more depolarized dSK(-) photoreceptors axons, assigning a role for dSK in network gain control during light-to-dark transitions. However, compensatory network adaptation, through increasing activity between synaptic neighbors, overcame many detriments of missing dSK current enabling dSK(-) photoreceptors to maintain normal information transfer rates to naturalistic stimuli. While demonstrating important functional roles for dSK channel in the visual circuitry, these results also clarify how homeostatically balanced network functions can compensate missing or faulty ion channels.

Inhibition of phospholipase C activity in Drosophila photoreceptors by 1,2-bis(2-aminophenoxy)ethane N,N,N',N'-tetraacetic acid (BAPTA) and di-bromo BAPTA

In vivo light-induced and basal hydrolysis of phosphatidyl inositol 4,5-bisphosphate (PIP2) by phospholipase C (PLC) were monitored in Drosophila photoreceptors using genetically targeted PIP2-sensitive ion channels (Kir2.1) as electrophysiological biosensors for PIP2. In cells loaded via patch pipettes with varying concentrations of Ca2+ buffered by 4 mM free BAPTA, light-induced PLC activity, showed an apparent bell-shaped dependence on free Ca2+ (maximum at "100 nM", approximately 10-fold inhibition at <10nM or approximately 1 microM). However, experiments where the total BAPTA concentration was varied whilst free [Ca2+] was maintained constant indicated that inhibition of PLC at higher (>100 nM) nominal Ca2+ concentrations was independent of Ca2+ and due to inhibition by BAPTA itself (IC50 approximately 8 mM). Di-bromo BAPTA (DBB) was yet more potent at inhibiting PLC activity (IC50 approximately 1mM). Both BAPTA and DBB also appeared to induce a modest, but less severe inhibition of basal PLC activity. By contrast, EGTA, failed to inhibit PLC activity when pre-loaded with Ca2+, but like BAPTA, inhibited both basal and light-induced PLC activity when introduced without Ca2+. The results indicate that both BAPTA and DBB inhibit PLC activity independently of their role as Ca2+ chelators, whilst non-physiologically low (<100 nM) levels of Ca2+ suppress both basal and light-induced PLC activity.

In Vivo Light-induced and Basal Phospholipase C Activity in Drosophila Photoreceptors Measured with Genetically Targeted Phosphatidylinositol 4,5-Bisphosphate-sensitive Ion Channels (Kir2.1)

The phosphatidylinositol 4,5-bisphosphate (PIP(2))-sensitive inward rectifier channel Kir2.1 was expressed in Drosophila photoreceptors and used to monitor in vivo PIP(2) levels. Since the wild-type (WT) Kir2.1 channel appeared to be saturated by the prevailing PIP(2) concentration, we made a single amino acid substitution (R228Q), which reduced the effective affinity for PIP(2) and yielded channels generating currents proportional to the PIP(2) levels relevant for phototransduction. To isolate Kir2.1 currents, recordings were made from mutants lacking both classes of light-sensitive transient receptor potential channels (TRP and TRPL). Light resulted in the effective depletion of PIP(2) by phospholipase C (PLC) in approximately three or four microvilli per absorbed photon at rates exceeding approximately 150% of total microvillar phosphoinositides per second. PIP(2) was resynthesized with a half-time of approximately 50 s. When PIP(2) resynthesis was prevented by depriving the cell of ATP, the Kir current spontaneously decayed at maximal rates representing a loss of approximately 40% loss of total PIP(2) per minute. This loss was attributed primarily to basal PLC activity, because it was greatly decreased in norpA mutants lacking PLC. We tried to confirm this by using the PLC inhibitor U73122; however, this was found to act as a novel inhibitor of the Kir2.1 channel. PIP(2) levels were reduced approximately 5-fold in the diacylglycerol kinase mutant (rdgA), but basal PLC activity was still pronounced, consistent with the suggestion that raised diacylglycerol levels are responsible for the constitutive TRP channel activity characteristic of this mutant.

Activation of the Drosophila TRP and TRPL channels requires both Ca2+ and protein dephosphorylation

The Transient Receptor Potential (TRP) proteins constitute a large and diverse family of channel proteins, which is conserved through evolution. TRP channel proteins have critical functions in many tissues and cell types, but their gating mechanism is an enigma. In the present study patch-clamp whole-cell recordings was applied to measure the TRP- and TRP-like (TRPL)-dependent currents in isolated Drosophila ommatidia. Also, voltage responses to light and to metabolic stress were recorded from the eye in vivo. We report new insight into the gating of the Drosophila light-sensitive TRP and TRPL channels, by which both Ca2+ and protein dephosphorylation are required for channel activation. ATP depletion or inhibition of protein kinase C activated the TRP channels, while photo-release of caged ATP or application of phorbol ester antagonized channels openings in the dark. Furthermore, Mg(2+)-dependent stable phosphorylation event by ATPgammaS or protein phosphatase inhibition by calyculin A abolished activation of the TRP and TRPL channels. While a high reduction of cellular Ca2+ abolished channel activation, subsequent application of Ca2+ combined with ATP depletion induced a robust dark current that was reminiscent of light responses. The results suggest that the combined action of Ca2+ and protein dephosphorylation activate the TRP and TRPL channels, while protein phosphorylation by PKC antagonized channels openings.

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.

Role of protein kinase C in light adaptation of molluscan microvillar photoreceptors

The mechanisms by which Ca2+ regulates light adaptation in microvillar photoreceptors remain poorly understood. Protein kinase C (PKC) is a likely candidate, both because some sub-types are activated by Ca2+ and because of its association with the macromolecular 'light-transduction complex' in Drosophila. We investigated the possible role of PKC in the modulation of the light response in molluscan photoreceptors. Western blot analysis with isoform-specific antibodies revealed the presence of PKCalpha in retinal homogenates. Immunocytochemistry in isolated cell preparations confirmed PKCalpha localization in microvillar photoreceptors, preferentially confined to the light-sensing lobe. Light stimulation induced translocation of PKCalpha immunofluorescence to the photosensitive membrane, an effect that provides independent evidence for PKC activation by illumination; a similar outcome was observed after incubation with the phorbol ester PMA. Several chemically distinct activators of PKC, such as phorbol-12-myristate-13-acetate (PMA), (-)indolactam V and 1,2,-dioctanoyl-sn-glycerol (DOG) inhibited the light response of voltage-clamped microvillar photoreceptors, but were ineffective in ciliary photoreceptors, in which light does not activate the G(q)/PLC cascade, nor elevates intracellular Ca2+. Pharmacological inhibition of PKC antagonized the desensitization produced by adapting lights and also caused a small, but consistent enhancement of basal sensitivity. These results strongly support the involvement of PKC activation in the light-dependent regulation of response sensitivity. However, unlike adapting background light or elevation of [Ca2+]i, PKC activators did not speed up the photoresponse, nor did PKC inhibitors antagonize the accelerating effects of background adaptation, suggesting that modulation of photoresponse time course may involve a separate Ca2+-dependent signal.

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.

Evaluation of the effect of EDTAC, CDTA, and EGTA on radicular dentin microhardness

The effect of EDTAC (ethylenediamine tetra-acetic acid plus Cetavion, an ammonium surfactant), CDTA (cyclohexane-1,2-diaminetetra-acetic acid), and EGTA (ethylene glycol-bis-(beta-amino-ethyl ether) N,N,N',N'-tetra-acetic acid) on the microhardness of radicular dentin of the cervical third of human teeth was studied. Five newly extracted maxillary incisors were sectioned transversely at the cementoenamel junction, and the crowns were discarded. The roots were embedded in blocks of high-speed polymerized acrylic resin and cut transversely into 1-mm sections. The second slice of the cervical third of the root of each tooth was sectioned and divided into four parts. Each part was placed on an acrylic disc that was used as a base for microhardness measurement. Fifty microliters of 15% EDTAC, 1% CDTA, or 1% EGTA were applied to the dentin surface. Deionized and distilled water was used as control. Dentin microhardness was then measured with a Vicker's microhardness apparatus with a load of 50 g for 15 s. Statistical analysis showed that the three chelating solutions significantly reduced dentin microhardness when compared with water; however, there was no statistically significant difference among the three solutions.

Light adaptation in Drosophila photoreceptors: I. Response dynamics and signaling efficiency at 25 degrees C

Besides the physical limits imposed on photon absorption, the coprocessing of visual information by the phototransduction cascade and photoreceptor membrane determines the fidelity of photoreceptor signaling. We investigated the response dynamics and signaling efficiency of Drosophila photoreceptors to natural-like fluctuating light contrast stimulation and intracellular current injection when the cells were adapted over a 4-log unit light intensity range at 25 degrees C. This dual stimulation allowed us to characterize how an increase in the mean light intensity causes the phototransduction cascade and photoreceptor membrane to produce larger, faster and increasingly accurate voltage responses to a given contrast. Using signal and noise analysis, this appears to be associated with an increased summation of smaller and faster elementary responses (i.e., bumps), whose latency distribution stays relatively unchanged at different mean light intensity levels. As the phototransduction cascade increases, the size and speed of the signals (light current) at higher adapting backgrounds and, in conjunction with the photoreceptor membrane, reduces the light-induced voltage noise, and the photoreceptor signal-to-noise ratio improves and extends to a higher bandwidth. Because the voltage responses to light contrasts are much slower than those evoked by current injection, the photoreceptor membrane does not limit the speed of the phototransduction cascade, but it does filter the associated high frequency noise. The photoreceptor information capacity increases with light adaptation and starts to saturate at approximately 200 bits/s as the speed of the chemical reactions inside a fixed number of transduction units, possibly microvilli, is approaching its maximum.

Light dependence of calcium and membrane potential measured in blowfly photoreceptors in vivo

Light adaptation in insect photoreceptors is caused by an increase in the cytosolic Ca2+ concentration. To better understand this process, we measured the cytosolic Ca2+ concentration in vivo as a function of adapting light intensity in the white-eyed blowfly mutant chalky. We developed a technique to measure the cytosolic Ca2+ concentration under conditions as natural as possible. The calcium indicator dyes Oregon Green 1, 2, or 5N (Molecular Probes, Inc., Eugene, OR) were iontophoretically injected via an intracellular electrode into a photoreceptor cell in the intact eye; the same electrode was also used to measure the membrane potential. The blue-induced green fluorescence of these dyes could be monitored by making use of the optics of the facet lens and the rhabdomere waveguide. The use of the different Ca2+-sensitive dyes that possess different affinities for Ca2+ allowed the quantitative determination of the cytosolic Ca2+ concentration in the steady state. Determining the cytosolic Ca2+ concentration as a function of the adapting light intensity shows that the Ca2+ concentration is regulated in a graded fashion over the whole dynamic range where a photoreceptor cell can respond to light. When a photoreceptor is adapted to bright light, the cytosolic Ca2+ concentration reaches stable values higher than 10 microM. The data are consistent with the hypothesis that the logarithm of the increase in cytosolic Ca2+ concentration is linear with the logarithm of the light intensity. From the estimated values of the cytosolic Ca2+ concentration, we conclude that the Ca2+-buffering capacity is limited. The percentage of the Ca2+ influx that is buffered gradually decreases with increasing Ca2+ concentrations; at cytosolic Ca2+ concentration levels above 10 microM, buffering becomes minimal.

TRP, TRPL and trouble in photoreceptor cells

Calcium influx through the TRP and TRPL light-activated channels triggers a complex regulatory hierarchy resulting in positive and negative feedback regulation of the phototransduction cascade. Recent studies have begun to elucidate the function of TRP and TRPL in vivo, and to examine their relationship to intracellular calcium changes during the light response.

A quantitative estimate of the maximum amount of light-induced Ca2+ release in Drosophila photoreceptors

Simultaneous measurements of the light-induced current (LIC) and cytosolic Ca2+ (using INDO-1) were made in Drosophila photoreceptors. In the presence of 1.5 mM Cao2+, the UV light used to measure INDO-1 fluorescence saturated the LIC and induced a large Ca2+ rise. In the absence of extracellular Ca2+ and with Na+ replaced by N-methyl-D-glucamine, the light-induced Ca2+ rise was virtually abolished. A residual rise of about 20 nM is regarded as an upper estimate of Ca2+ released from internal stores. To estimate the Ca2+ flux required to generate such a rise, Ca2+ influx signals in response to weak light steps (500 ms LED stimulus) were measured in the presence of external Ca2+. The relationship between [Ca(in)] and the total charge carried during the LIC had a slope of 2.7 nM pC-1. Assuming that 50% of the LIC is carried by Ca2+ and that the single-channel Ca2+ current carried by the InsP3 receptor is 0.04 pA, it was estimated that about 350 InsP3 receptors should have been sufficient to generate a Ca2+ rise of 20 nM within 500 ms. By contrast, the current activated by the UV measuring light was equivalent to the activation of at least 5000 quantum bumps, making it unlikely that InsP3-induced Ca2+ release could have been the causal event for excitation under these conditions.

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.