Spontaneous activation of light-sensitive channels in Drosophila photoreceptors

Calmodulin regulation of light adaptation and store-operated dark current in Drosophila photoreceptors

Phototransduction in Drosophila occurs through inositol lipid signaling that results in Ca2+ mobilization. In this system, we investigate the hitherto unknown physiological roles of calmodulin (CaM) in light adaptation and in regulation of the inward current that is brought about by depletion of cellular Ca2+ stores. To see the effects of a decreased Ca-CaM content in photoreceptor cells, we used several methods. Transgenic Drosophila P[ninaCDeltaB] flies, which have CaM-deficient photoreceptors, were studied. The peptide inhibitor M5, which binds to Ca-CaM and prevents its action, was applied. A Ca2+-free medium, which prevents Ca2+ influx and thereby diminishes the generation of Ca-CaM, was used. The decrease in the Ca-CaM level caused the following effects. (i) Fluorescence of Ca2+ indicator revealed an enhanced light-induced Ca2+ release from internal stores. (ii) Measurements of the light-induced current in P[ninaCDeltaB] cells showed a reduced light adaptation. (iii) Internal dialysis of M5 initially enhanced excitation and subsequently disrupted the light-induced current. (iv) An inward dark current appeared after depletion of the Ca2+ stores with ryanodine and caffeine. Importantly, application of Ca-CaM into the photoreceptor cells prevented all of the above effects. We propose that negative feedback of Ca-CaM on Ca2+ release from ryanodine-sensitive stores mediates light adaptation, is essential for light excitation, and keeps the store-operated inward current under a tight control.

Properties of single Drosophila Trpl channels expressed in Sf9 insect cells

The transient receptor potential (trp)-like (trpl) gene is thought to encode an ion channel important for signal transduction in Drosophila photoreceptor cells. Consistent with this hypothesis, heterologous expression of the trpl-encoded protein (Trpl) is associated with the appearance of an outwardly rectifying, nonselective cation current. In the present study, single channels were recorded in cell-attached, inside-out, and outside-out membrane patches from Sf9 insect cells infected with recombinant baculovirus-containing trpl cDNA under control of the polyhedrin promoter. The single-channel current-voltage relationship was linear from -100 to +80 mV with a slope conductance of 89-110 pS. The probability of opening was voltage sensitive, increasing at positive potentials contributing to the outwardly rectifying properties of the whole cell currents. The single channels 1) were never observed in Sf9 cells infected with recombinant baculovirus containing the B2 bradykinin receptor cDNA or in noninfected Sf9 cells; 2) appear at the same time postinfection as the Trpl whole cell current; 3) were nonselective with respect to Na+, Ca2+, and Ba2+; 4) were blocked by 1-2 mM La3+ and Gd3+ (but not 10 microM); and 5) were blocked by 4-8 mM Mg2+. The single Trpl channel activity increased spontaneously with time after patch formation, and the activity was further increased by application of bradykinin to cells expressing both the B2 bradykinin receptor and the Trpl protein. These results suggest that this single-channel activity reflects expression of the Trpl protein and provides conclusive evidence that trpl encodes a nonselective cation channel consistent with its proposed role in Drosophila phototransduction.

Differential effects of ninaC proteins (p132 and p174) on light-activated currents and pupil mechanism in Drosophila photoreceptors

The Drosophila ninaC locus encodes two retinal specific proteins (p132 and p174) both consisting of a protein kinase joined to a myosin head domain and a C terminal with a calmodulin-binding domain. The role of p132 and p174 was studied via whole-cell recording and through measurements of the pupil mechanism, i.e. the pigment migration in the photoreceptor cells, in the ninaC mutants, P[ninaC delta 132] (p132 absent), P[ninaC delta 174] (p174 absent), and ninaCP235 (null mutant). Voltage-clamped flash responses in P[ninaC delta 174] and ninaCP235 showed delayed response termination. In response to steady light, plateau responses in both P[ninaC delta 174] and ninaCP235 were also large. In both cases the defect was significantly more severe in ninaCP235. Responses in P[ninaC delta 132] were apparently normal. P[ninaC delta 174] and ninaCP235 were also characterized by spontaneous quantum bump-like activity in the dark and by larger and longer light-induced quantum bumps. The turn-off of the pupil mechanism in P[ninaC delta 174] and ninaCP235 was also defective, although in this case the rate of return to baseline in both mutants was more or less the same. In all ninaC mutants, the amplitudes of the pupillary pigment migration were distinctly smaller than that in the wild type. The reduction of the amplitude was largest in P[ninaC delta 174]. The light sensitivity of the pupil mechanism of P[ninaC delta 174] compared to that of wild type was reduced by 1.3 log units. Remarkably, the light sensitivity of P[ninaC delta 132] and ninaCP235 was ca. 0.5 log units higher than that of the wild type. The results suggest that the p174 protein is required for normal termination of the transduction cascade. The diverse phenotypes observed may suggest multiple roles calmodulin distribution for controlling response termination and regulating pigment migration in Drosophila photoreceptors.

The roles of trp and calcium in regulating photoreceptor function in Drosophila

Invertebrate photoreceptors use the ubiquitous inositol-lipid signaling pathway for phototransduction. This pathway depends on Ca2+ release from internal stores and on Ca2+ entry via light-activated channels to replenish the loss of Ca2+ in those stores. The Drosophila transient receptor potential (TRP) protein is essential for the high Ca2+ permeability and other biophysical properties of these light-activated channels, which affect both excitation and adaptation in photoreceptor cells. Physiological and heterologous expression studies indicate that TRP is a putative subunit of a surface membrane channel that can be activated by depletion of internal Ca2+ stores. Furthermore, trp is an archetypal member of a multigene family whose products share a structure that is highly conserved throughout evolution, from worms to humans.

Role of Drosophila TRP in inositide-mediated Ca2+ entry

Inositol lipid signaling relies on an InsP3-induced Ca2+ release from intracellular stores and on extracellular Ca2+ entry, which takes place when the Ca2+ stores become depleted of Ca2+. This interplay between Ca2+ release and Ca2+ entry has been termed capacitative Ca2+ entry and the inward current calcium release activated current (CRAC) to indicate gating of Ca2+ entry by Ca2+-store depletion. The signaling pathway and the gating mechanism of capacitative Ca2+ entry, however, are largely unknown and the molecular participants in this process have not been identified. In this article we review genetic, molecular, and functional studies of wild-type and mutant Drosophila photoreceptors, suggesting that the transient receptor potential mutant (trp) is the first putative capacitative Ca2+ entry mutant. Furthermore, several lines of evidence suggest that the trp gene product TRP is a candidate subunit of the plasma membrane channel that is activated by Ca2+ store depletion.

The biology of vision of Drosophila

Phototransduction systems in vertebrates and invertebrates share a great deal of similarity in overall strategy but differ significantly in the underlying molecular machinery. Both are rhodopsin-based G protein-coupled signaling cascades displaying exquisite sensitivity and broad dynamic range. However, light activation of vertebrate photoreceptors leads to activation of a cGMP-phosphodiesterase effector and the generation of a hyperpolarizing response. In contrast, activation of invertebrate photoreceptors, like Drosophila, leads to stimulation of phospholipase C and the generation of a depolarizing receptor potential. The comparative study of these two systems of phototransduction offers the opportunity to understand how similar biological problems may be solved by different molecular mechanisms of signal transduction. The study of this process in Drosophila, a system ideally suited to genetic and molecular manipulation, allows us to dissect the function and regulation of such a complex signaling cascade in its normal cellular environment. In this manuscript I review some of our recent findings and the strategies used to dissect this process.

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

In order to test the hypothesis that excitation in Drosophila photoreceptors is mediated by Ca2+ released from internal stores, the Ca2+ buffers EGTA, BAPTA and di-bromo-BAPTA (DBB) were introduced into dissociated photoreceptors via whole-cell recording pipettes. All buffers were preloaded with Ca2+ to provide the same free Ca2+ concentration (250 nM). EGTA (up to 18 mM free buffer) had only weak effects upon voltage-clamped flash responses in normal Ringer's solution (1.5 mM Ca (2 0+)), and no effect in Ca2+ -free solution. The maximum BAPTA concentration tested (14.4 mM free BAPTA) reduced the initial rate of rise by ca. 5000-fold in normal Ringer's solution; by ca. 500-fold in Ca2+ free solution; and only ca. 60-fold in the absence of Mg2+, which preferentially blocks one component of the light-sensitive current. Although BAPTA delayed the time-to-peak in normal Ringer's solution, responses in Ca2+ free Ringer's solution were accelerated. These results support the role of Ca2+ influx in regulating sensitivity and response kinetics; however, in view of the high concentrations required to attenuate responses in Ca2+ free Ringer's solution, the role of Ca2+ release in excitation remains unclear. DBB was ca. 2-3 fold more potent than BAPTA, and at concentrations > 5 mM had a qualitatively different action, greatly delaying the time-to-peak. This suggests DBB may have distinct pharmacological actions or access to compartments inaccessible to BAPTA. The only current activated by introducing 5-500 microM Ca2+ (buffered with nitrilo-triacetic acid) was electrogenic Na+/Ca2+ exchange. When this was blocked by removing Nao0+, a novel cationic conductance was activated. However, its properties did not resemble those the light-activated conductance, and thus do not support the hypothesis that Ca2+ is sufficient for excitation.

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.

Receptor-mediated activation of recombinant Trpl expressed in Sf9 insect cells

The Drosophila proteins, Trp and Trpl, are suggested to be cation channels responsible for depolarization of the receptor potential associated with stimulation of insect photoreceptor cells by light. Consistent with this hypothesis, we recently showed that recombinant Trpl forms Ca(2+)- and Ba(2+)-permeable non-selective cation channels when expressed in Sf9 cells using the baculovirus expression vector. As Trpl may be activated in the photoreceptor cell after stimulation of phospholipase C, we hypothesized that a similar regulation of recombinant Trpl may be observed in the Sf9 cell after activation of heterologous membrane receptors linked to Ca(2+)-signal-transduction pathways. To test this hypothesis, Ca2+ signalling was examined in Fura-2-loaded Sf9 cells infected with baculovirus containing cDNA for the M5 muscarinic receptor alone (M5 cells) or in cells co-infected with both M5 and Trpl-containing baculoviruses (M5-Trpl cells). Addition of carbachol (100 microM) to M5 cells produced an increase in cytosolic free Ca2+ concentration ([Ca2+]i) (mean +/- S.D.; n = 17) from 101 +/- 20 to 762 +/- 178 nM which declined to a sustained elevated level of 384 +/- 102 nM after 3 min. The sustained component was eliminated by removal of extracellular Ca2+ or by addition of La3+ or Gd3+ (10 microM). In M5-Trpl cells, basal [Ca2+]i increased as a function of time after infection. To evaluate the contribution of Ca2+ influx to the overall profile observed, Ba2+, a Ca2+ surrogate that is not a substrate for the Ca2+ pump, was used. The increase in basal [Ca2+]i seen in M5-Trpl cells was associated with an increase in basal Ba2+ influx. Addition of carbachol to M5-Trpl cells at 30-36 h after infection produced a large increase in [Ca2+]i to a sustained value of 677 +/- 143 nM. This change in [Ca2+]i was (1) blocked by atropine, (2) attenuated in the absence of extracellular Ca2+, and (3) relatively insensitive to La3+, but blocked by Gd3+ in the 0.1-1 mM range. In the presence of 10 microM Gd3+ to block the endogenous-receptor-mediated Ca(2+)-influx in M5-Trpl cells. In sharp contrast increase in Ba2+ influx in M5-Trpl cells. In sharp contrast, neither Ca2+ nor Ba2+ influx through Trpl was affected by thapsigargin, a selective inhibitor of the endoplasmic reticulum Ca(2+)-ATPase pump.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium-dependent inactivation of light-sensitive channels in Drosophila photoreceptors

Whole-cell voltage clamp recordings were made from photoreceptors of dissociated Drosophila ommatidia under conditions when the light-sensitive channels activate spontaneously, generating a "rundown current" (RDC). The Ca2+ and voltage dependence of the RDC was investigated by applying voltage steps (+80 to -100 mV) at a variety of extracellular Ca2+ concentrations (0-10 mM). In Ca(2+)-free Ringer large currents are maintained tonically throughout 50-ms-long voltage steps. In the presence of external Ca2+, hyperpolarizing steps elicit transient currents which inactivate increasingly rapidly as Ca2+ is raised. On depolarization inactivation is removed with a time constant of approximately 10 ms at +80 mV. The Ca(2+)-dependent inactivation is suppressed by 10 mM internal BAPTA, suggesting it requires Ca2+ influx. The inactivation is absent in the trp mutant, which lacks one class of Ca(2+)-selective, light-sensitive channel, but appears unaffected by the inaC mutant which lacks an eye-specific protein kinase C. Hyperpolarizing voltage steps applied during light responses in wild-type (WT) flies before rundown induce a rapid transient facilitation followed by slower inhibition. Both processes accelerate as Ca2+ is raised, but the time constant of inhibition (12 ms with 1.5 mM external Ca2+ at -60 mV) is approximately 10 times slower than that of the RDC inactivation. The Ca(2+)-mediated inhibition of the light response recovers in approximately 50-100 ms on depolarization, recovery being accelerated with higher external Ca2+. The Ca2+ and voltage dependence of the light-induced current is virtually eliminated in the trp mutant. In inaC, hyperpolarizing voltage steps induced transient currents which appeared similar to those in WT during early phases of the light response. However, 200 ms after the onset of light, the currents induced by voltage steps inactivated more rapidly with time constants similar to those of the RDC. It is suggested that the Ca(2+)-dependent inactivation of the light-sensitive channels first occurs at some concentration of Ca2+ not normally reached during the moderate illumination regimes used, but that the defect in inaC allows this level to be reached.