Origin of reproducibility in the responses of retinal rods to single photons

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.

Open the loop: dissecting feedback regulation of a second messenger transduction cascade

Single photon responses were compared in wild-type and transgenic retinal rods with and without guanylate cyclase activating protein (GCAP) to disrupt Ca(2+)-dependent feedback regulation of guanylate cyclase (see Burns et al. in this issue of Neuron). The results provided new insights into the molecular mechanisms underlying phototransduction.

Mechanisms regulating variability of the single photon responses of mammalian rod photoreceptors

Variability in the single photon responses of rod photoreceptors limits the accuracy with which the number and timing of photon absorptions are encoded. We investigated how much single photon responses of mammalian rods fluctuate and what mechanisms control these fluctuations. Mammalian rods, like those of toads, generated responses to single photons with trial-to-trial fluctuations 3-4 times smaller than other familiar signals produced by single molecules. We used the properties of the measured fluctuations to constrain models for how the single photon responses are regulated. Neither feedback control of rhodopsin's activity nor saturation within the transduction cascade were consistent with experiment. The measured responses, however, could be explained by multistep shutoff of rhodopsin or a combination of multistep shutoff and saturation.

G proteins and phototransduction

Phototransduction is the process by which a photon of light captured by a molecule of visual pigment generates an electrical response in a photoreceptor cell. Vertebrate rod phototransduction is one of the best-studied G protein signaling pathways. In this pathway the photoreceptor-specific G protein, transducin, mediates between the visual pigment, rhodopsin, and the effector enzyme, cGMP phosphodiesterase. This review focuses on two quantitative features of G protein signaling in phototransduction: signal amplification and response timing. We examine how the interplay between the mechanisms that contribute to amplification and those that govern termination of G protein activity determine the speed and the sensitivity of the cellular response to light.

Signal amplification: let's turn down the lights

G protein activation by membrane-bound receptors initiates a chain reaction that amplifies cellular responses to external signals. In retinal photoreceptors, amplification by the phototransduction cascade is regulated by quickly switching off the visual pigment that acts as the receptor for light.

Rhodopsin phosphorylation: from terminating single photon responses to photoreceptor dark adaptation

Rhodopsin phosphorylation has provided one of the first examples of the ubiquitous regulatory pattern of specific kinases downregulating the activity of G-protein-coupled receptors. However, only recently have studies in living animals allowed us to consider the role of rhodopsin phosphorylation in a broader spectrum of visual functions, ranging from the ability of rods to generate reproducible electrical responses to their ability to adapt to darkness after substantial light exposures.

Activation, deactivation, and adaptation in vertebrate photoreceptor cells

Visual transduction captures widespread interest because its G-protein signaling motif recurs throughout nature yet is uniquely accessible for study in the photoreceptor cells. The light-activated currents generated at the photoreceptor outer segment provide an easily observed real-time measure of the output of the signaling cascade, and the ease of obtaining pure samples of outer segments in reasonable quantity facilitates biochemical experiments. A quiet revolution in the study of the mechanism has occurred during the past decade with the advent of gene-targeting techniques. These have made it possible to observe how transduction is perturbed by the deletion, overexpression, or mutation of specific components of the transduction apparatus.

Role of guanylate cyclase-activating proteins (GCAPs) in setting the flash sensitivity of rod photoreceptors

The retina's photoreceptor cells adjust their sensitivity to allow photons to be transduced over a wide range of light intensities. One mechanism thought to participate in sensitivity adjustments is Ca(2+) regulation of guanylate cyclase (GC) by guanylate cyclase-activating proteins (GCAPs). We evaluated the contribution of GCAPs to sensitivity regulation in rods by disrupting their expression in transgenic mice. The GC activity from GCAPs-/- retinas showed no Ca(2+) dependence, indicating that Ca(2+) regulation of GCs had indeed been abolished. Flash responses from dark-adapted GCAPs-/- rods were larger and slower than responses from wild-type rods. In addition, the incremental flash sensitivity of GCAPs-/- rods failed to be maintained at wild-type levels in bright steady light. GCAP2 expressed in GCAPs-/- rods restored maximal light-induced GC activity but did not restore normal flash response kinetics. We conclude that GCAPs strongly regulate GC activity in mouse rods, decreasing the flash sensitivity in darkness and increasing the incremental flash sensitivity in bright steady light, thereby extending the rod's operating range.

Vertebrate photoreceptors

The basis of the duplex theory of vision is examined in view of the dazzling array of data on visual pigment sequences and the pigments they form, on the microspectrophotometry measurements of single photoreceptor cells, on the kinds of photoreceptor cascade enzymes, and on the electrophysiological properties of photoreceptors. The implications of the existence of five distinct visual pigment families are explored, especially with regard to what pigments are in what types of photoreceptors, if there are different phototransduction enzymes associated with different types of photoreceptors, and if there are electrophysiological differences between different types of cones.

Adaptation in vertebrate photoreceptors

When light is absorbed within the outer segment of a vertebrate photoreceptor, the conformation of the photopigment rhodopsin is altered to produce an activated photoproduct called metarhodopsin II or Rh(*). Rh(*) initiates a transduction cascade similar to that for metabotropic synaptic receptors and many hormones; the Rh(*) activates a heterotrimeric G protein, which in turn stimulates an effector enzyme, a cyclic nucleotide phosphodiesterase. The phosphodiesterase then hydrolyzes cGMP, and the decrease in the concentration of free cGMP reduces the probability of opening of channels in the outer segment plasma membrane, producing the electrical response of the cell. Photoreceptor transduction can be modulated by changes in the mean light level. This process, called light adaptation (or background adaptation), maintains the working range of the transduction cascade within a physiologically useful region of light intensities. There is increasing evidence that the second messenger responsible for the modulation of the transduction cascade during background adaptation is primarily, if not exclusively, Ca(2+), whose intracellular free concentration is decreased by illumination. The change in free Ca(2+) is believed to have a variety of effects on the transduction mechanism, including modulation of the rate of the guanylyl cyclase and rhodopsin kinase, alteration of the gain of the transduction cascade, and regulation of the affinity of the outer segment channels for cGMP. The sensitivity of the photoreceptor is also reduced by previous exposure to light bright enough to bleach a substantial fraction of the photopigment in the outer segment. This form of desensitization, called bleaching adaptation (the recovery from which is known as dark adaptation), seems largely to be due to an activation of the transduction cascade by some form of bleached pigment. The bleached pigment appears to activate the G protein transducin directly, although with a gain less than Rh(*). The resulting decrease in intracellular Ca(2+) then modulates the transduction cascade, by a mechanism very similar to the one responsible for altering sensitivity during background adaptation.

Engineering aspects of enzymatic signal transduction: photoreceptors in the retina

Identifying the basic module of enzymatic amplification as an irreversible cycle of messenger activation/deactivation by a "push-pull" pair of opposing enzymes, we analyze it in terms of gain, bandwidth, noise, and power consumption. The enzymatic signal transduction cascade is viewed as an information channel, the design of which is governed by the statistical properties of the input and the noise and dynamic range constraints of the output. With the example of vertebrate phototransduction cascade we demonstrate that all of the relevant engineering parameters are controlled by enzyme concentrations and, from functional considerations, derive bounds on the required protein numbers. Conversely, the ability of enzymatic networks to change their response characteristics by varying only the abundance of different enzymes illustrates how functional diversity may be built from nearly conserved molecular components.

Rapid and reproducible deactivation of rhodopsin requires multiple phosphorylation sites

Efficient single-photon detection by retinal rod photoreceptors requires timely and reproducible deactivation of rhodopsin. Like other G protein-coupled receptors, rhodopsin contains multiple sites for phosphorylation at its COOH-terminal domain. Transgenic and electrophysiological methods were used to functionally dissect the role of the multiple phosphorylation sites during deactivation of rhodopsin in intact mouse rods. Mutant rhodopsins bearing zero, one (S338), or two (S334/S338) phosphorylation sites generated single-photon responses with greatly prolonged, exponentially distributed durations. Responses from rods expressing mutant rhodopsins bearing more than two phosphorylation sites declined along smooth, reproducible time courses; the rate of recovery increased with increasing numbers of phosphorylation sites. We conclude that multiple phosphorylation of rhodopsin is necessary for rapid and reproducible deactivation.

Mechanisms of single-photon detection in rod photoreceptors

Rod photoreceptors detect and encode incident photons exceptionally well. They collect sparse photons with high efficiency, maintain a low dark noise, and generate reproducible responses to each absorbed photon. The mechanisms involved in single-photon detection--control of the effective lifetime of a single active receptor molecule, amplification of the activity of this single molecule by a second-messenger cascade, and reliable transmission of small synaptic signals--recur throughout the nervous system. Indeed, several other sensory systems reach or approach limits set by quantization of their input signals. For example, olfactory receptors can detect single odorant molecules. Although our understanding of visual transduction and signal processing has advanced rapidly during the past 10-15 years, fundamental questions still remain: What mechanisms are responsible for the reproducibility of the rod's elementary response? What are the tradeoffs of speed and sensitivity in the transduction cascade? How are the rod single-photon responses reliably transmitted to the rest of the visual system? Future technical innovations, particularly better methods to monitor the activity of intermediate steps in transduction, will play an important role in providing answers.

Phosphorylation modulates the affinity of light-activated rhodopsin for G protein and arrestin

Reduced effector activity and binding of arrestin are widely accepted consequences of GPCR phosphorylation. However, the effect of receptor multiphosphorylation on G protein activation and arrestin binding parameters has not previously been quantitatively examined. We have found receptor phosphorylation to alter both G protein and arrestin binding constants for light-activated rhodopsin in proportion to phosphorylation stoichiometry. Rod disk membranes containing different average receptor phosphorylation stoichiometries were combined with G protein or arrestin, and titrated with a series of brief light flashes. Binding of G(t) or arrestin to activated rhodopsin augmented the 390 nm MII optical absorption signal by stabilizing MII as MII.G or MII.Arr. The concentration of active arrestin or G(t) and the binding constant of each to MII were determined using a nonlinear least-squares (Simplex) reaction model analysis of the titration data. The binding affinity of phosphorylated MII for G(t) decreased while that for arrestin increased with each added phosphate. G(t) binds more tightly to MII at phosphorylation levels less than or equal to two phosphates per rhodopsin; at higher phosphorylation levels, arrestin binding is favored. However, arrestin was found to bind much more slowly than G(t) at all phosphorylation levels, perhaps allowing time for phosphorylation to gradually reduce receptor-G protein interaction before arrestin capping of rhodopsin. Sensitivity of the binding constants to ionic strength suggests that a strong membrane electrostatic component is involved in both the reduction of G(t) binding and the increase of arrestin binding with increasing rhodopsin phosphorylation.

Single photon responses in Drosophila photoreceptors and their regulation by Ca2+

1. Discrete events (quantum bumps) elicited by dim light were analysed in whole-cell voltage clamp of photoreceptors from dissociated Drosophila ommatidia. Bumps were automatically detected and analysed for amplitude, rise and decay times, and latency. 2. The bump interval and amplitude distributions, and the 'frequency of seeing' curve conformed to Poisson predictions for the absorption of single photons. 3. At resting potential (-70 mV), bumps averaged 10 pA in peak amplitude with a half-width of ca 20 ms, representing simultaneous activation of ca 15 channels. 4. The macroscopic response to flashes containing up to at least 750 photons were predicted by the linear summation of quantum bumps convolved with their latency dispersion. 5. Bump duration was unaffected by lowering the extracellular Ca2+ concentration ([Ca2+]o) from 1.5 to 0.5 mM, but increased >10-fold between 0.5 mM Ca2+ and 0 Ca2+. Bump amplitude was constant over the range 1.5-100 microM, but decreased ca 5- to 10-fold at lower Ca2+ concentrations. Bump latency increased by ca 50 % between 1.5 mM and 100 microM Ca2+o but returned to near control levels in Ca2+-free solutions. At intermediate [Ca2+]o bumps were biphasic with a slow rising phase followed by rapid amplification and inactivation. This behaviour was mimicked in high [Ca2+]o by internal buffering with BAPTA, but not EGTA. This suggests that Ca2+ influx through the light-sensitive channels must first raise cytosolic Ca2+ to a threshold level before initiating a cycle of positive and negative feedback mediated by molecular targets within the same microvillus. Quantum bumps in trp mutants lacking the major class of light-sensitive channel were reduced in size (mean 3.5 pA) representing simultaneous activation of only one or two channels; however, a second rarer (10 %) class of large bump had an amplitude similar to wild-type (WT) bumps. Bumps in trpl mutants lacking the second class of light-sensitive channel were very similar to WT bumps, but with slightly slower decay times. In InaDP215 mutants, in which the association of the TRP channels with the INAD scaffolding molecule is disrupted, bumps showed a defect in quantum bump termination, but their amplitudes and latencies were near normal.

Properties of the evoked spatio-temporal electrical activity in neuronal assemblies

Properties of neural computation were studied in two types of neuronal networks: isolated leech ganglia and neuronal cultures of dissociated cortical neurons from neonatal rats. With appropriate experimental set-ups it was possible to obtain a precise description of the spread of excitation induced by specific inputs. The evoked spatio-temporal electrical activity was characterized by large variability and the electrical activity of neurons activated by the same stimulation was found to be statistically independent to a high degree. The variability presumably originates from basic properties of synaptic transmission, which is stochastic in nature. As a consequence, the large variability of the evoked spatio-temporal electrical activity appears to be a general property of neural computation and a typical feature of neuronal assemblies. It is shown, however, that the observed statistical independence of co-activated neurons may be used to reduce the effects of variability by appropriately averaging or pooling the electrical activity.

Variability in the time course of single photon responses from toad rods: termination of rhodopsin's activity

We examined the responses of toad rod photoreceptors to single photons of light. To minimize the effects of variability in the early rising phase, we selected sets of responses that closely matched the rise of the mean single photon response. Responses selected in this way showed substantial variations in kinetics, appearing to peel off from a common time course after different delays. Following incorporation of the calcium buffer BAPTA, the time to peeling off was retarded. Our analysis indicates that it is not necessary to invoke a long series of reaction steps to explain the shutoff of rhodopsin activity. Instead, our results suggest that the observed behavior is explicable by the presently known shutoff reactions of activated rhodopsin, modulated by feedback.

Abnormal photoresponses and light-induced apoptosis in rods lacking rhodopsin kinase

Phosphorylation is thought to be an essential first step in the prompt deactivation of photoexcited rhodopsin. In vitro, the phosphorylation can be catalyzed either by rhodopsin kinase (RK) or by protein kinase C (PKC). To investigate the specific role of RK, we inactivated both alleles of the RK gene in mice. This eliminated the light-dependent phosphorylation of rhodopsin and caused the single-photon response to become larger and longer lasting than normal. These results demonstrate that RK is required for normal rhodopsin deactivation. When the photon responses of RK-/- rods did finally turn off, they did so abruptly and stochastically, revealing a first-order backup mechanism for rhodopsin deactivation. The rod outer segments of RK-/- mice raised in 12-hr cyclic illumination were 50% shorter than those of normal (RK+/+) rods or rods from RK-/- mice raised in constant darkness. One day of constant light caused the rods in the RK-/- mouse retina to undergo apoptotic degeneration. Mice lacking RK provide a valuable model for the study of Oguchi disease, a human RK deficiency that causes congenital stationary night blindness.