Time course and magnitude of the calcium release induced by bright light in salamander rods

A calcium-relay mechanism in vertebrate phototransduction

Calcium-signaling in cells requires a fine-tuned system of calcium-transport proteins involving ion channels, exchangers, and ion-pumps but also calcium-sensor proteins and their targets. Thus, control of physiological responses very often depends on incremental changes of the cytoplasmic calcium concentration, which are sensed by calcium-binding proteins and are further transmitted to specific target proteins. This Review will focus on calcium-signaling in vertebrate photoreceptor cells, where recent physiological and biochemical data indicate that a subset of neuronal calcium sensor proteins named guanylate cyclase-activating proteins (GCAPs) operate in a calcium-relay system, namely, to make gradual responses to small changes in calcium. We will further integrate this mechanism in an existing computational model of phototransduction showing that it is consistent and compatible with the dynamics that are characteristic for the precise operation of the phototransduction pathways.

The cone-specific calcium sensor guanylate cyclase activating protein 4 from the zebrafish retina

Guanylate cyclase activating proteins (GCAPs) serve as neuronal Ca(2+)-sensor proteins in vertebrate rod and cone photoreceptor cells. Zebrafish express in their retina a variety of six different GCAPs, of which four are specific for cone cells. One isoform, zGCAP4, is mainly expressed in double cones and long single cones. We cloned the zGCAP4 gene, purified non-myristoylated and myristoylated forms of the protein after heterologous expression in Escherichia coli and studied its properties: zGCAP4 was a strong activator of membrane-bound guanylate cyclases from bovine and zebrafish retina, showing half-maximal activation at 520-570 nM free Ca(2+) concentration. Furthermore, the Ca(2+)-sensitive activation properties of non-myristoylated and myristoylated zGCAP4 were similar, indicating no influence of the myristoyl moiety on Ca(2+)-sensor function. Myristoylated zGCAP4 showed low affinity for membranes and did not exhibit a Ca(2+)-myristoyl switch, a feature typical of some but not all neuronal Ca(2+)-sensor proteins. However, tryptophan fluorescence studies and Ca(2+)-dependent differences in protease accessibility revealed Ca(2+)-induced conformational changes in myristoylated and non-myristoylated zGCAP4, indicating the operation as a Ca(2+) sensor. Thus, expression and biochemical properties of zGCAP4 are in agreement with its function as an efficient Ca(2+)-sensitive regulator of guanylate cyclase activity in cone vision.

Zebrafish: a model system for the study of eye genetics

Over the last decade, the use of the zebrafish as a genetic model has moved beyond the proof-of-concept for the analysis of vertebrate embryonic development to demonstrated utility as a mainstream model organism for the understanding of human disease. The initial identification of a variety of zebrafish mutations affecting the eye and retina, and the subsequent cloning of mutated genes have revealed cellular, molecular and physiological processes fundamental to visual system development. With the increasing development of genetic manipulations, sophisticated techniques for phenotypic characterization, behavioral approaches and screening strategies, the identification of novel genes or novel gene functions will have important implications for our understanding of human eye diseases, pathogenesis, and treatment.

Centrins, gatekeepers for the light-dependent translocation of transducin through the photoreceptor cell connecting cilium

Centrins are members of a highly conserved subgroup of the EF-hand superfamily of Ca(2+)-binding proteins commonly associated with centrosome-related structures. In the retina, centrins are also prominent components of the photoreceptor cell ciliary apparatus. Centrin isoforms are differentially localized at the basal body and in the lumen of the connecting cilium. All molecular exchanges between the inner and outer segments occur through this narrow connecting cilium. Ca(2+)-activated centrin isoforms bind to the visual heterotrimeric G-protein transducin via an interaction with the betagamma-subunit. Ca(2+)-dependent assemblies of centrin/G-protein complexes may regulate the transducin movement through the connecting cilium. Formation of this complex represents a novel mechanism in regulation of translocation of signaling proteins in sensory cells, as well as a potential link between molecular trafficking and signal transduction in general.

Simultaneous measurement of current and calcium in the ultraviolet-sensitive cones of zebrafish

In rods and visible cone photoreceptors, multiple measurements cannot be made of intracellular Ca2+ concentration from the same cell using fluorescent dyes, because a single exposure of the measuring light bleaches too large a fraction of the rod or cone photopigment. We have therefore identified and characterized UV-sensitive cones of the zebrafish, whose wavelength of maximum sensitivity is at 360 nm which is far enough from the wavelength of our measuring light (514.5 nm) so that it has been possible to make multiple determinations of photocurrent and Ca2+ concentration from the same cells. We show that for a limited number of measurements, for which the bleaching of the cone photopigment is too small to affect flash kinetics, the outer segment Ca2+ concentration closely follows the wave form of the flash response convolved with the dominant time constant for Ca2+ removal by Na+-Ca2+-K+ exchange. For a larger number of measurements, significant acceleration of the response kinetics by pigment bleaching inevitably occurs, but the Ca2+ concentration nevertheless rises and falls in approximate agreement with the flash wave form. During exposure to steady background light, the Ca2+ concentration falls in proportion to the steady-state current for dim backgrounds at all times and for bright backgrounds at steady state. At early times following the onset of bright backgrounds, however, the Ca2+ concentration is markedly higher than expected from the current of the cone. We show this to be the result of light-dependent Ca2+ release by bright background light, which can be abolished by pre-exposure of the cone to the membrane-permeant acetoxymethyl ester of the Ca2+ chelator BAPTA. Our results therefore demonstrate that the cone outer segment Ca2+ concentration is predominantly a function of the rate of influx and efflux of Ca2+ across the plasma membrane, but that a release of Ca2+ in bright light most probably from buffer sites within the cell can transiently elevate the Ca2+ concentration above the level expected from the open probability of the light-dependent channels.

Serca isoform expression in the mammalian retina

The sarcoplasmic-endoplasmic reticulum calcium ATPase (SERCA) is a key intracellular calcium transporter, which regulates cellular calcium concentration [Ca2+] by transporting Ca2+ ions from the cytosol into the endoplasmic reticulum. SERCA-mediated Ca2+ sequestration controls proper folding of newly synthesized proteins within the ER as well as the timing and spatial patterning of depolarization-evoked Ca2+ responses in the cytoplasm. To understand the spatial and temporal properties of Ca2+ homeostasis in retinal neurons better, I studied expression and distribution of all three SERCA isoforms in the mouse retina using isoform-specific antibodies. No immunostaining was observed with the SERCA1 antibody. SERCA2 was expressed in photoreceptor inner segments, amacrine and ganglion cells of the mouse retina. Similar SERCA2 localization was observed in adult rat, macaque and ground squirrel retinas. Analysis of distribution of SERCA2 immunofluorescence in the developing mouse retina revealed prominent SERCA2 signals throughout postnatal development. The N89 antibodys used to identify the SERCA3 isoforms labelled cone outer segments, inner segments of photoreceptors and cell processes in the inner nuclear layer of the mouse retina. These results imply that the SERCA2 isoform controls Ca2+ sequestration into the endoplasmic reticulum in most classes of retinal neuron. A potential role for SERCA3 in cone function is suggested.

Light-induced Ca2+ release in the visible cones of the zebrafish

We used suction-pipette recording and fluo-4 fluorescence to study light-induced Ca2+ release from the visible double cones of zebrafish. In Ringer, light produces a slow decrease in fluorescence which can be fitted by the sum of two decaying exponentials with time constants of 0.5 and 3.8 s. In 0Ca2+-0Na+ solution, for which fluxes of Ca2+ across the outer segment plasma membrane are greatly reduced, light produces a slow increase in fluorescence. Both the decrease and increase are delayed after incorporation of the Ca2+ chelator BAPTA, indicating that both are produced by a change in Ca2+. If the Ca2+ pool is first released by bright light in 0Ca2+-0Na+ solution and the cone returned to Ringer, the time course of Ca2+ decline is much faster than in Ringer without previous light exposure. This indicates that the time constants of 0.5 and 3.8 s actually reflect a sum of Na+/Ca2+-K+ exchange and light-induced release of Ca2+. The Ca2+ released by light appears to come from at least two sites, the first comprising 66% of the total pool and half-released by bleaching 4.8% of the pigment. Release of the remaining Ca2+ from the second site requires the bleaching of nearly all of the pigment. If, after release, the cone is maintained in darkness, a substantial fraction of the Ca2+ returns to the release pool even in the absence of pigment regeneration. The light-induced release of Ca2+ can produce a modulation of the dark current as large as 0.75 pA independently of the normal transduction cascade, though the rise time of the current is considerably slower than the normal light response. These experiments show that Ca2+ can be released within the cone outer segment by light intensities within the physiological range of photopic vision. The role this Ca2+ release plays remains unresolved.

Bleaching desensitization: background and current challenges

"Bleaching desensitization" in rod photoreceptors refers to the prolonged depression of phototransduction sensitivity exhibited by rods after their exposure to bright light, i.e., after photolysis (bleaching) of a substantial fraction of rhodopsin in the outer segments. Rod recovery from bleaching desensitization depends critically on operation of the retinoid visual cycle: in particular, on the removal of all-trans retinal bleaching product from opsin and on the delivery of 11-cis retinal to opsin's chromophore binding site. The present paper summarizes representative findings that address the mechanism of bleaching desensitization.

The effect of light on outer segment calcium in salamander rods

Calcium acts as a second messenger in vertebrate rods, regulating the recovery phase of the light response and modulating sensitivity during light-adaptation. Since light not only decreases the outer segment calcium concentration ([Ca2+]i) by closing cyclic nucleotide-gated channels but can also increase [Ca2+]i by releasing Ca2+ from buffer sites or intracellular stores, we examined in detail the effect of light and circulating current on [Ca2+]i by making simultaneous measurements of suction pipette current and [Ca2+]i from isolated rods of the salamander Ambystoma tigrinum after incorporation of the fluorescent dye fluo-5F. When the release of Ca2+ is measured in 0 Ca2+-0 Na+ solution, minimising fluxes of Ca2+ across the plasma membrane, it is substantial only for light bright enough to bleach a significant fraction of the photopigment and is restricted to the part of the outer segment in which the bleach occurred. It is unlikely, therefore, to make a large contribution to [Ca2+]i for most of the physiological operating range of the rod. Nevertheless, since release is half-maximal for a bleach of less than 10 %, it cannot be produced by a simple mechanism such as a change in the affinity of a binding site on rhodopsin itself but must instead require some more complex interaction. In Ringer solution, the Ca2+ in the light-releasable pool can be discharged merely by the decrease in [Ca2+]i that occurs as the outer segment channels close. In steady background light or after exposure to saturating illumination, the fraction of Ca2+ in the pool decreases essentially in proportion to [Ca2+]i as if Ca2+ were being removed from a buffer site within the cytoplasm. Furthermore, [Ca2+]i itself changes in proportion to the circulating current, with little evidence for a contribution from Ca2+ release or other mechanisms of Ca2+ homeostasis. This indicates that flux of Ca2+ across the plasma membrane is the major determinant of outer segment Ca2+ concentration within the rod's normal operating light intensity range. Once Ca2+ has been discharged from the releasable pool, it is restored following dim illumination apparently as the simple result of the subsequent restoration of dark [Ca2+]i and the rebinding of Ca2+ to its release site, but after brighter light perhaps also as a consequence of regeneration of the photopigment.

Spontaneous activity of opsin apoprotein is a cause of Leber congenital amaurosis

Mutations in Rpe65 disrupt synthesis of the opsin chromophore ligand 11-cis-retinal and cause Leber congenital amaurosis (LCA), a severe, early-onset retinal dystrophy. To test whether light-independent signaling by unliganded opsin causes the degeneration, we used Rpe65-null mice, a model of LCA. Dark-adapted Rpe65-/- mice behaved as if light adapted, exhibiting reduced circulating current, accelerated response turn-off, and diminished intracellular calcium. A genetic block of transducin signaling completely rescued degeneration irrespective of an elevated level of retinyl ester. These studies clearly show that activation of sensory transduction by unliganded opsin, and not the accumulation of retinyl esters, causes light-independent retinal degeneration in LCA. A similar mechanism may also be responsible for degeneration induced by vitamin A deprivation.

Free magnesium concentration in salamander photoreceptor outer segments

Magnesium ions (Mg2+) play an important role in biochemical functions. In vertebrate photoreceptor outer segments, numerous reactions utilize MgGTP and MgATP, and Mg2+ also regulates several of the phototransduction enzymes. Although Mg2+ can pass through light-sensitive channels under certain conditions, no clear extrusion mechanism has been identified and removing extracellular Mg2+ has no significant effect on the light sensitivity or the kinetics of the photoresponse. We have used the fluorescent Mg2+ dye Furaptra to directly measure and monitor the free Mg2+ concentration in photoreceptor outer segments and examine whether the free Mg2+ concentration changes under physiological conditions. Resting free Mg2+ concentrations in bleached salamander rod and cone photoreceptor cell outer segments were 0.86 +/- 0.06 and 0.81 +/- 0.09 mM, respectively. The outer segment free Mg2+ concentration was not significantly affected by changes in extracellular pH, Ca2+ and Na+, excluding a significant role for the respective exchangers in the regulation of Mg2+ homeostasis. The resting free Mg2+ concentration was also not significantly affected by exposure to 0 Mg2+, suggesting the lack of significant basal Mg2+ flux. Opening the cGMP-gated channels led to a significant increase in the Mg2+ concentration in the absence of Na+ and Ca2+, but not in their presence, indicating that depolarization can cause a significant Mg2+ influx only in the absence of other permeant ions, but not under physiological conditions. Finally, light stimulation did not change the Mg2+ concentration in the outer segments of dark-adapted photoreceptors. The results suggest that there are no influx and efflux pathways that can significantly affect the Mg2+ concentration in the outer segment under physiological conditions. Therefore, it is unlikely that Mg2+ plays a significant role in the dynamic modulation of phototransduction.

Light stimulates a transducin-independent increase of cytoplasmic Ca2+ and suppression of current in cones from the zebrafish mutant nof

Transducins couple visual pigments to cGMP hydrolysis, the only recognized phototransduction pathway in vertebrate photoreceptors. Here we describe a zebrafish mutant, no optokinetic response f(w21) (nof), with a nonsense mutation in the gene encoding the alpha subunit of cone transducin. Retinal morphology and levels of phototransduction enzymes are normal in nof retinas, but cone transducin is undetectable. Dark current in nof cones is also normal, but it is insensitive to moderate intensity light. The nof cones do respond, however, to bright light. These responses are produced by a light-stimulated, but transducin-independent, release of Ca2+ into the cone cytoplasm. Thus, in addition to stimulating transducin, light also independently induces release of Ca2+ into the photoreceptor cytoplasm.

Light stimulates a transducin-independent increase of cytoplasmic Ca2+ and suppression of current in cones from the zebrafish mutant nof

Transducins couple visual pigments to cGMP hydrolysis, the only recognized phototransduction pathway in vertebrate photoreceptors. Here we describe a zebrafish mutant, no optokinetic response f(w21) (nof), with a nonsense mutation in the gene encoding the alpha subunit of cone transducin. Retinal morphology and levels of phototransduction enzymes are normal in nof retinas, but cone transducin is undetectable. Dark current in nof cones is also normal, but it is insensitive to moderate intensity light. The nof cones do respond, however, to bright light. These responses are produced by a light-stimulated, but transducin-independent, release of Ca2+ into the cone cytoplasm. Thus, in addition to stimulating transducin, light also independently induces release of Ca2+ into the photoreceptor cytoplasm.

Measurement of cytoplasmic calcium concentration in the rods of wild-type and transducin knock-out mice

A 10 microm spot of argon laser light was focused onto the outer segments of intact mouse rods loaded with fluo-3, fluo-4 or fluo-5F, to estimate dark, resting free Ca(2+) concentration ([Ca(2+)](i)) and changes in [Ca(2+)](i) upon illumination. Dye concentration was adjusted to preserve the normal physiology of the rod, and the laser intensity was selected to minimise bleaching of the fluorescent dye. Wild-type mouse rods illuminated continuously with laser light showed a progressive decrease in fluorescence well fitted by two exponentials with mean time constants of 154 and 540 ms. Rods from transducin alpha-subunit knock-out (Tralpha-/-) animals showed no light-dependent decline in fluorescence but exhibited an initial rapid component of fluorescence increase which could be fitted with a single exponential (tau~1-4 ms). This fluorescence increase was triggered by rhodopsin bleaching, since its amplitude was reduced by pre-exposure to bright bleaching light and its time constant decreased with increasing laser intensity. The rapid component was however unaffected by incorporation of the calcium chelator BAPTA and seemed therefore not to reflect an actual increase in [Ca(2+)](i). A similar rapid increase in fluorescence was also seen in the rods of wild-type mice just preceding the fall in fluorescence produced by the light-dependent decrease in [Ca(2+)](i). Dissociation constants were measured in vitro for fluo-3, fluo-4 and fluo-5F with and without 1 mM Mg(2+) from 20 to 37 degrees C. All three dyes showed a strong temperature dependence, with the dissociation constant changing by a factor of 3-4 over this range. Values at 37 degrees C were used to estimate absolute levels of rod [Ca(2+)](i). All three dyes gave similar values for [Ca(2+)](i) in wild-type rods of 250 +/- 20 nM in darkness and 23 +/- 2 nM after exposure to saturating light. There was no significant difference in dark [Ca(2+)](i) between wild-type and Tralpha-/- animals.