Injection of inositol trisphosphorothioate into Limulus ventral photoreceptors causes oscillations of free cytosolic calcium

The excitation cascade of Limulus ventral photoreceptors: guanylate cyclase as the link between InsP3-mediated Ca2+ release and the opening of cGMP-gated channels

Background

Early stages in the excitation cascade of Limulus photoreceptors are mediated by activation of G_q by rhodopsin, generation of inositol-1,4,5-trisphosphate by phospholipase-C and the release of Ca²⁺. At the end of the cascade, cGMP-gated channels open and generate the depolarizing receptor potential. A major unresolved issue is the intermediate process by which Ca²⁺ elevation leads to channel opening.

Results

To explore the role of guanylate cyclase (GC) as a potential intermediate, we used the GC inhibitor guanosine 5'-tetraphosphate (GtetP). Its specificity in vivo was supported by its ability to reduce the depolarization produced by the phosphodiesterase inhibitor IBMX. To determine if GC acts subsequent to InsP₃ production in the cascade, we examined the effect of intracellular injection of GtetP on the excitation caused by InsP₃ injection. This form of excitation and the response to light were both greatly reduced by GtetP, and they recovered in parallel. Similarly, GtetP reduced the excitation caused by intracellular injection of Ca²⁺. In contrast, this GC inhibitor did not affect the excitation produced by injection of a cGMP analog.

Conclusion

We conclude that GC is downstream of InsP3-induced Ca2+ release and is the final enzymatic step of the excitation cascade. This is the first invertebrate rhabdomeric photoreceptor for which transduction can be traced from rhodopsin photoisomerization to ion channel opening.

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.

Inositol 1,4,5-trisphosphate-induced calcium release is necessary for generating the entire light response of limulus ventral photoreceptors

The experiments reported here were designed to answer the question of whether inositol 1,4,5-trisphosphate (IP3)-induced calcium release is necessary for generating the entire light response of Limulus ventral photoreceptors. For this purpose the membrane-permeable IP3 receptor antagonist 2-aminoethoxydiphenyl borate (2APB) (Maruyama, T., T. Kanaji, S. Nakade, T. Kanno, and K. Mikoshiba. 1997. J. Biochem. (Tokyo). 122:498-505) was used. Previously, 2APB was found to inhibit the light activated current of Limulus ventral photoreceptors and reversibly inhibit both light and IP3 induced calcium release as well as the current activated by pressure injection of calcium into the light sensitive lobe of the photoreceptor (Wang, Y., M. Deshpande, and R. Payne. 2002. Cell Calcium. 32:209). In this study 2APB was found to inhibit the response to a flash of light at all light intensities and to inhibit the entire light response to a step of light, that is, both the initial transient and the steady-state components of the response to a step of light were inhibited. The light response in cells injected with the calcium buffer 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) was reversibly inhibited by 2APB, indicating that these light responses result from IP3-mediated calcium release giving rise to an increase in Cai. The light response obtained from cells after treatment with 100 microM cyclopiazonic acid (CPA), which acts to empty intracellular calcium stores, was reversibly inhibited by 2APB, indicating that the light response after CPA treatment results from IP3-mediated calcium release and a consequent rise in Cai. Together these findings imply that IP3-induced calcium release is necessary for generating the entire light response of Limulus ventral photoreceptors.

The inositol trisphosphate receptor regulates a 50-second behavioral rhythm in C. elegans

The C. elegans defecation cycle is characterized by the contraction of three distinct sets of muscles every 50 s. Our data indicate that this cycle is regulated by periodic calcium release mediated by the inositol trisphosphate receptor (IP3 receptor). Mutations in the IP3 receptor slow down or eliminate the cycle, while overexpression speeds up the cycle. The IP3 receptor controls these periodic muscle contractions nonautonomously from the intestine. In the intestinal cells, calcium levels oscillate with the same period as the defecation cycle and peak calcium levels immediately precede the first muscle contraction. Mutations in the IP3 receptor slow or eliminate these calcium oscillations. Thus, the IP3 receptor is an essential component of the timekeeper for this cycle and represents a novel mechanism for the control of behavioral rhythms.

The Limulus ventral photoreceptor: light response and the role of calcium in a classic preparation

The ventral nerve photoreceptor of the horseshoe crab Limulus polyphemus has been used for many years to investigate basic mechanisms of invertebrate phototransduction. The activation of rhodopsin leads in visual cells of invertebrates to an enzyme cascade at the end of which ion channels in the plasma membrane are transiently opened. This allows an influx of cations resulting in a depolarization of the photoreceptor cell. The receptor current of the Limulus ventral photoreceptor consists of three components which differ in several aspects, such as the time course of activation, the time course of recovery from light adaptation, and the reversal potential. Each component is influenced in a different, characteristic way by various pharmacological manipulations. In addition, at least two types of single photon-evoked events (bumps) and three elementary channel conductances are observed in this photoreceptor cell. These findings suggest that the receptor current components are controlled by three different light-activated enzymatic pathways using three different ligands to increase membrane conductance. Probably one of these ligands is cyclic GMP, another one is activated via the IP3-cascade and calcium, the third one might be cyclic AMP. Calcium ions are very important for the excitation and adaptation of visual cells in invertebrates. The extracellular and intracellular calcium concentrations determine the functional state of the visual cell. A rise in the cytosolic calcium concentration appears to be an essential step in the excitatory transduction cascade. Cytosolic calcium is the major intracellular mediator of adaptation. If the cytosolic calcium level exceeds a certain threshold value after exposure to light it causes the desensitization of the visual cell. On the other hand, from a slight rise in cytosolic calcium facilitation results, i.e. increased sensitivity of the photoreceptor.

A role for hydrolysis of inositol 1,4,5-trisphosphate in terminating the response to inositol 1,4,5-trisphosphate and to a flash of light in Limulus ventral photoreceptors

Injection of inositol 1,4,5-trisphosphate (Ins 1,4,5-P3) into Limulus ventral photoreceptors produces excitation similar to that produced by light. One process which might contribute to rapid termination of the responses to Ins 1,4,5-P3 and to light is the hydrolysis of Ins 1,4,5-P3 by an InsP3-5-phosphatase to form inositol 1,4-bisphosphate. Inositol 2,4,5-trisphosphate (Ins 2,4,5-P3) is known to be less hydrolysable by the InsP3-5-phosphatase than is Ins 1,4,5-P3. Whereas ventral photoreceptors respond to an injection of Ins 1,4,5-P3 with a single wave of depolarization, the response to Ins 2,4,5-P3 is a burst of waves of depolarization. Our hypothesis is that it is the resistance to hydrolysis by the InsP3-5-phosphatase which accounts for the burst of waves produced by Ins 2,4,5-P3. To test this idea we injected ventral photoreceptors with Ins 1,4,5-P3 in the presence of the non-specific phosphatase inhibitors, vanadate and fluoride, which prolong the response to a flash of light in ventral photoreceptors (D.W. Corson, A. Fein, W.W. Walthall, J. Gen. Physiol. 82 (1983) 659-677). In the presence of fluoride or vanadate the response to Ins 1,4,5-P3 was composed of a burst of waves rather than a single wave of depolarization. We conclude that hydrolysis of Ins 1,4,5-P3 by the InsP3-5-phosphatase plays a role in terminating the ventral photoreceptors response to Ins 1,4,5-P3 and also to light.

Alteration of cytosolic calcium induced by angiotensin II and norepinephrine in mesangial cells from diabetic rats

To evaluate functional alterations of mesangial cells induced by diabetes (DMC), we observed the changes of cytosolic calcium ([Ca]i) in response to the vasoconstrictor agonists angiotensin II (Ang II) and norepinephrine (NOR). DMC were obtained from rats with streptozotocin-induced diabetes, cultured in normal medium and identified as mesangial cells (MC) in the third subculture. [Ca]i was measured using fura-2 as a fluorophore. Basal calcium levels (60 to 80 nM) in DMC were not different from control mesangial cells (CMC). The high glucose (30 mM) medium concentration reduced the response of CMC and DMC to Ang II and NOR. This was not an osmotic effect since mannitol did not alter these responses. When DMC were stimulated with Ang II, a desensitized response was always observed, with a transient variation of [Ca]i (N = 6, P < 0.05). In contrast, a non-desensitized response with a sustained pattern of [Ca]i increases was obtained in NOR-stimulated DMC. Therefore, the present results suggest that DMC show a modified response to stimulation of the Ang II receptor, which is expressed phenotypically in culture by desensitization. Furthermore, these alterations induced by diabetes environment in MC in vivo were maintained in vitro despite a long period (approximately 5 months) in which the cells were grown in normal culture medium.

Kinetics of cytosolic Ca2+ concentration after photolytic release of 1-D-myo-inositol 1,4-bisphosphate 5-phosphorothioate from a caged derivative in guinea pig hepatocytes

The influence of 1-D-myo-inositol 1,4,5-trisphosphate (InsP3) breakdown by InsP3 5-phosphatase in determining the time course of Ca2+ release from intracellular stores was investigated with flash photolytic release of a stable InsP3 derivative, 5-thio-InsP3, from a photolabile caged precursor. The potency and Ca(2+)-releasing properties of the biologically active D isomers of 5-thio-InsP3 and InsP3 itself were compared by photolytic release in guinea pig hepatocytes. After a light flash, cytosolic free calcium concentration ([Ca2+]i) showed an initial delay before rising quickly to a peak and declining more slowly to resting levels, with time course and amplitude generally similar to those seen with photolytic release of InsP3. Differences were a three- to eightfold lower potency of 5-thio-InsP3 in producing Ca2+ release, much longer delays between photolytic release and Ca2+ efflux with low concentrations of 5-thio-InsP3 than with InsP3, and persistent reactivation of Ca2+ release, producing periodic fluctuations of cytosolic [Ca2+]i with high concentrations of 5-thio-InsP3 but not InsP3 itself. The lower potency of 5-thio-InsP3 may be a result of a lower affinity for closed receptor/channels or a lower open probability of liganded receptor/channels. The longer delays with 5-thio-InsP3 at low concentration suggest that metabolism of InsP3 by 5-phosphatase may reduce the concentration sufficiently to prevent receptor activation and may have a similar effect on InsP3 concentration during hormonal activation. The maximal rate of rise of [Ca2+]i, the duration of the period of high Ca2+ efflux, and the initial decline of [Ca2+]i are similar with5-thio-lnsP3 and lnsP3, indicating that lnsP3 breakdown is not important in terminating Ca2+ release. The second activation ofInsP3 receptors with 5-thio-lnsP3 and particularly the sustained periodic fluctuations of [Ca2+]i indicate persistence of 5-thio-lnsP3,suggesting that InsP3 breakdown prevents reactivation of InsP3 receptors. The photochemical properties of 1-(2-nitrophenyl)-ethyl caged 5-thio-lnsP3 are photolytic quantum yield = 0.57 (cf. 0.65 for caged InsP3) and rate of photolysis = 87 s-I (half-life approximately 8 ms; cf. 3 ms for caged lnsP3; pH7.1; ionic strength, 0.2 M; 21 OC). Caged 5-thio-lnsP3 at concentrations up to 360 pM did not activate lnsP3 receptors to produce Ca2+ release or block Ca2+ release by free 5-thio-lnsP3.

Caffeine- and ryanodine-sensitive Ca(2+)-induced Ca2+ release from the endoplasmic reticulum in honeybee photoreceptors

Light stimulation of invertebrate microvillar photoreceptors causes a large rapid elevation in Cai, shown previously to modulate the adaptational state of the cells. Cai rises, at least in part, as a result of Ins(1,4,5)P3-induced Ca2+ release from the submicrovillar endoplasmic reticulum (ER). Here, we provide evidence for Ca(2+)-induced Ca2+ release (CICR) in an insect photoreceptor. In situ microphotometric measurements of Ca2+ fluxes across the ER membrane in permeabilized slices of drone bee retina show that (a) caffeine induces Ca2+ release from the ER; (b) caffeine and Ins(1,4,5)P3 open distinct Ca2+ release pathways because only caffeine-induced Ca2+ release is ryanodine sensitive and heparin insensitive, and because caffeine and Ins(1,4,5)P3 have additive effects on the rate of Ca2+ release; (c) Ca2+ itself stimulates release of Ca2+ via a ryanodine-sensitive pathway; and (d) cADPR is ineffective in releasing Ca2+. Microfluorometric intracellular Ca2+ measurements with fluo-3 indicate that caffeine induces a persistent elevation in Cai. Electrophysiological recordings demonstrate that caffeine mimics all aspects of Ca(2+)-mediated facilitation and adaptation in drone photoreceptors. We conclude that the ER in drone photoreceptors contains, in addition to the Ins(1,4,5)P3-sensitive release pathway, a CICR pathway that meets key pharmacological criteria for a ryanodine receptor. Coexpression of both release mechanisms could be required for the production of rapid light-induced Ca2+ elevations, because Ca2+ amplifies its own release through both pathways by a positive feedback. CICR may also mediate the spatial spread of Ca2+ release from the submicrovillar ER toward more remote ER subregions, thereby activating Ca(2+)-sensitive cell processes that are not directly involved in phototransduction.

Facilitation of the responses to injections of inositol 1,4,5-trisphosphate analogs in Limulus ventral photoreceptors

Injection of inositol 1,4,5-trisphosphate and its metabolically resistant analogs InsP3S3 and L-chiro-2,3,5-InsP3 into the ventral photoreceptors of Limulus results in the release of calcium from internal stores and in a current flow into the cells. We show here that the dependence of the current response on the amount of analog injected is supralinear. The injections also facilitate the responses to subsequent injections. We analyze the kinetics of the responses either by very slow application of the analogs directly into the lobe that is sensitive to InsP3 and light or by delivering a pulse into the nonsensitive lobe of the cell, in both cases creating a ramp of rising concentration in the sensitive region. Typically, a long latent period was followed by a strong brief inward current. The ratio between the latency and the duration of the response, defined as twice the time from half-amplitude to the peak of the response, reaches values greater than 10. Our analysis shows that this value cannot be attained within realistic models whose only nonlinearity is the cooperative binding of the ligand to its receptor. The observed ratio, however, can be achieved with a positive feedback model. Treatments that lead to partial depletion of calcium stores reversibly increase the latency of the response. We conclude that the mechanisms of the response of Limulus ventral eye to the metabolically resistant analogs of InsP3 probably involves a positive feedback mechanism and that the carrier of the feedback is likely to be Ca2+.

Role of cytosolic Ca2+ in inhibition of InsP3-evoked Ca2+ release in Xenopus oocytes

1. Calcium liberation induced in Xenopus oocytes by flash photorelease of inositol 1,4,5-trisphosphate (InsP3) from a caged precursor was monitored by confocal microfluorimetry. The object was to determine whether inhibition of Ca2+ release seen with paired flashes arose as a direct consequence of elevated cytosolic free [Ca2+]. 2. Responses evoked by just-suprathreshold test flashes were not inhibited by subthreshold conditioning flashes, but were strongly suppressed when conditioning flashes were raised above threshold. 3. Inhibition at first increased progressively as the inter-flash interval was lengthened to about 2 s and thereafter declined, with a half-recovery at about 4 s. 4. Intracellular injections of Ca2+ caused relatively slight inhibition of InsP3-evoked signals, even when cytosolic free [Ca2+] was elevated to levels similar to those at which strong inhibition was seen in paired-flash experiments. 5. Recovery from inhibition was not appreciably slowed when Ca2+ was injected to raise the free Ca2+ level between paired flashes. 6. We conclude that inhibition of InsP3-evoked Ca2+ liberation is not directly proportional to cytosolic free Ca2+ level and that recovery from inhibition in paired-pulse experiments involves factors other than the decline of cytosolic [Ca2+] following a conditioning response.

Cyclic 3',5'-nucleotide phosphodiesterase: cytochemical localization in photoreceptor cells of the fly Calliphora erythrocephala

The distribution of cyclic 3',5'-nucleotide phosphodiesterase activity was determined in photoreceptor cells of the fly Calliphora erythrocephala. With cAMP as substrate, staining was most intense within the phototransducing region of these cells, the rhabdomeral microvilli and also in the extracellular space surrounding the microvilli and in the mitochondria. With cGMP as substrate, the intensity within the rhabdomeres was less marked, while their extracellular surroundings were stained heavily. Thus, compared to cGMP, cAMP is the better substrate for the phosphodiesterase in the rhabdomeres of the fly. For comparison, the same cytochemical method was used to localize the well-known phosphodiesterase activity in retinal tissue of the mouse. Under the same conditions as used for fly photoreceptors, a very intense reaction product was obtained in rod outer segments. With regard to the conflicting reports concerning the light-stimulated changes of cyclic nucleotides in invertebrate photoreceptor cells, the results presented here further argue for an important role of a cyclic nucleotide in the process of phototransduction of invertebrates.

Intracellular injection of heparin and polyamines. Effects on phototransduction in limulus ventral photoreceptors

Heparin is thought to inhibit InsP3 binding to receptors involved in the intracellular release of Ca2+. Injection of heparin into Limulus ventral photoreceptors to high intracellular concentrations reduces the amplitude and slows the rate of rise of voltage-clamp currents induced by brief flashes, tends to make the responses to long flashes more "square," and tends to block the light-induced rise in [Ca2+]i detected by arsenazo III. In these ways, intracellular heparin mimics the effects of high concentrations of intracellular BAPTA or EGTA. In addition, the effects of heparin are attenuated by prior injection of BAPTA to high intracellular concentrations. Neomycin and spermine are thought to inhibit phospholipase C activity. Injections of spermine or neomycin to low intracellular concentrations largely mimic the effects of intracellular heparin. These findings suggest that the predominant effect of polyamines is to inhibit light-induced production of InsP3 by phospholipase C activity and thereby reduce the light-induced increase in [Ca2+]i. Our findings suggest that excitation can proceed in the absence of InsP3-induced increases in [Ca2+]i, but (a) the gain and speed of transduction are reduced and (b) adaptation is largely blocked.

Fast desensitization of the response to InsP3 in Limulus ventral photoreceptors

In Limulus ventral photoreceptor cells the time-course of the desensitization of InsP3 response was measured by an injection-pair paradigm. Pressure pulses of InsP3 were delivered into the cell with various interpulse intervals. The desensitization of the response to the second injection of each pair approached totality at 200 ms, which is the duration of the response to a single pressure pulse of InsP3. Lowering extracellular calcium did not affect the time-course of the desensitization. Lowering the temperature slowed down both the time-course of the response to InsP3 and the time-course of the desensitization to the same extent. These findings suggest that the desensitization is powerful enough and its onset fast enough to contribute to the transience of the InsP3 response. The time-course of the desensitization suggests it may influence light adaptation.

Non-independent quantum bumps in Limulus ventral nerve photoreceptors--a new insight in the light transduction mechanism

Single photon-induced transient currents, called quantum bumps were stimulated by short flashes in dark-adapted ventral nerve photoreceptors of Limulus. Flash intensities were set to activate 3 or more bumps. In most cases, current bumps were activated with a constant rate. The frequency of bump occurrence was between 9 and 17 Hz. Results show that consecutive bumps are not independent and that some of them are not activated by a photon. The periodic bump activation indicates a molecular mechanism which quantifies the transmitter release not only by a light quantum, but also by a late phase of the transduction cascade. A model is proposed, in which Ca2+ ions released from intracellular stores transiently block the further Ca2+ release by inositol trisphosphate in an all-or-none manner.

Oscillations and waves of cytosolic calcium: insights from theoretical models

Oscillations in cytosolic Ca2+ occur in a wide variety of cells, either spontaneously or as a result of external stimulation. This process is often accompanied by intracellular Ca2+ waves. A number of theoretical models have been proposed to account for the periodic generation and spatial propagation of Ca2+ signals. These models are reviewed and their predictions compared with experimental observations. Models for Ca2+ oscillations can be distinguished according to whether or not they rely on the concomitant, periodic variation in inositol 1,4,5-trisphosphate. Such a variation, however, is not required in models based on Ca(2+)-induced Ca2+ release. When Ca2+ diffusion is incorporated into these models, propagating waves of cytosolic Ca2+ arise, with profiles and rates comparable to those seen in the experiments.