Cyclic GMP diffusion coefficient in rod photoreceptor outer segments

Apo-Opsin and Its Dark Constitutive Activity across Retinal Cone Subtypes

Retinal rod and cone photoreceptors mediate vision in dim and bright light, respectively, by transducing absorbed photons into neural electrical signals. Their phototransduction mechanisms are essentially identical. However, one difference is that, whereas a rod visual pigment remains stable in darkness, a cone pigment has some tendency to dissociate spontaneously into apo-opsin and retinal (the chromophore) without isomerization. This cone-pigment property is long known but has mostly been overlooked. Importantly, because apo-opsin has weak constitutive activity, it triggers transduction to produce electrical noise even in darkness. Currently, the precise dark apo-opsin contents across cone subtypes are mostly unknown, as are their dark activities. We report here a study of goldfish red (L), green (M), and blue (S) cones, finding with microspectrophotometry widely different apo-opsin percentages in darkness, being ∼30% in L cones, ∼3% in M cones, and negligible in S cones. L and M cones also had higher dark apo-opsin noise than holo-pigment thermal isomerization activity. As such, given the most likely low signal amplification at the pigment-to-transducin/phosphodiesterase phototransduction step, especially in L cones, apo-opsin noise may not be easily distinguishable from light responses and thus may affect cone vision near threshold.

Position of rhodopsin photoisomerization on the disk surface confers variability to the rising phase of the single photon response in vertebrate rod photoreceptors

Retinal rods function as accurate photon counters to provide for vision under very dim light. To do so, rods must generate highly amplified, reproducible responses to single photons, yet outer segment architecture and randomness in the location of rhodopsin photoisomerization on the surface of an internal disk introduce variability to the rising phase of the photon response. Soon after a photoisomerization at a disk rim, depletion of cGMP near the plasma membrane closes ion channels and hyperpolarizes the rod. But with a photoisomerization in the center of a disk, local depletion of cGMP is distant from the channels in the plasma membrane. Thus, channel closure is delayed by the time required for the reduction of cGMP concentration to reach the plasma membrane. Moreover, the local fall in cGMP dissipates over a larger volume before affecting the channels, so response amplitude is reduced. This source of variability increases with disk radius. Using a fully space-resolved biophysical model of rod phototransduction, we quantified the variability attributable to randomness in the location of photoisomerization as a function of disk structure. In mouse rods that have small disks bearing a single incisure, this variability was negligible in the absence of the incisure. Variability was increased slightly by the incisure, but randomness in the shutoff of rhodopsin emerged as the main source of single photon response variability at all but the earliest times. Variability arising from randomness in the transverse location of photoisomerization increased in magnitude and persisted over a longer period in the photon response of large salamander rods. A symmetric arrangement of multiple incisures in the disks of salamander rods greatly reduced this variability during the rising phase, but the incisures had the opposite effect on variability arising from randomness in rhodopsin shutoff at later times.

Origins of the phototransduction delay as inferred from stochastic and deterministic simulation of the amplification cascade

PURPOSE

To identify steps of the phototransduction cascade responsible for the delay of the photoresponse.

METHODS

Electrical responses of fish (Carassius) cones and Rana ridibunda frog rods and cones were recorded with a suction pipette technique and as an aspartate-isolated mass receptor potential from isolated perfused retinas. Special attention was paid to sufficiently high temporal resolution (1-ms flash, 700 Hz amplification bandpass). Stochastic simulation of the activation steps from photon absorption to the formation of catalytically active phosphodiesterase (PDE) was performed. In addition, a deterministic mathematical model was fit to the experimental responses. The model included a detailed description of the activation steps of the cascade that enabled identification of the role of individual transduction stages in shaping the initial part of the response.

RESULTS

We found that the apparent delay of the photoresponse gets shorter with increasing stimulus intensity and reaches an asymptotic value of approximately 3 ms in cones and greater than or equal to 10 ms in rods. The result seems paradoxical since it is suggested that the delay occurs in the chain of steps from photon absorption to the formation of active transducin (T*) which in cones is, on average, slower than in rods. Stochastic simulation shows that actually the steps from photon absorption to T* may not contribute perceptibly to the delay. Instead, the delay occurs at the stage that couples the cycle of repetitive activation of T by rhodopsin (R*) with the activation of PDE. These steps include formation of T* (= T _α GTP) out of T _αβγ GTP released from the activation cycle and the subsequent interaction of T* with PDE. This poses a problem. The duration of an average cycle of activation of T in rods is approximately 5 ms and is determined by the frequency of collisions between R* and T in the photoreceptor membrane. The frequency is roughly proportional to the surface packing density of T in the membrane. As the packing density of PDE is approximately 12 times lower than that of T, it could be expected that the rate of the T*-PDE interaction were an order of magnitude slower than that of R* and T. As modeling shows, this is the case in rods. However, the delay in cones is approximately 3 ms which could be achieved only at a T*-PDE interaction time of less than or equal to 5 ms. This means that either the frequency of the collisions of T* and PDE, or the efficiency of collisions, or both in cones are approximately ten times higher than in rods. This may be a challenge to the present model of the molecular organization of the photoreceptor membrane.

CONCLUSIONS

The delay of the photoresponse is mainly set by the rate of interaction of T* with PDE. In cones, the delay is shorter than in rods and, moreover, shorter than the duration of the cycle of repetitive activation of T by R*. This poses a problem for the present model of diffusion interaction of phototransduction proteins in the photoreceptor membrane.

Phototransduction early steps model based on Beer-Lambert optical law

The amount of available rhodopsin on the photoreceptor outer segment and its change over time is not considered in classic models of phototransduction. Thus, those models do not take into account the absorptance variation of the outer segment under different brightness conditions. The relationship between the light absorbed by a medium and its absorptance is well described by the Beer-Lambert law. This newly proposed model implements the absorptance variation phenomenon in a set of equations that admit photons per second as input and results in active rhodopsins per second as output. This study compares the classic model of phototransduction developed by Forti et al. (1989) to this new model by using different light stimuli to measure active rhodopsin and photocurrent. The results show a linear relationship between light stimulus and active rhodopsin in the Forti model and an exponential saturation in the new model. Further, photocurrent values have shown that the new model behaves equivalently to the experimental and theoretical data as published by Forti in dark-adapted rods, but fits significantly better under light-adapted conditions. The new model successfully introduced a physics optical law to the standard model of phototransduction adding a new processing layer that had not been mathematically implemented before. In addition, it describes the physiological concept of saturation and delivers outputs in concordance to input magnitudes.

5D imaging approaches reveal the formation of distinct intracellular cAMP spatial gradients

Cyclic AMP (cAMP) is a ubiquitous second messenger known to differentially regulate many cellular functions. Several lines of evidence suggest that the distribution of cAMP within cells is not uniform. However, to date, no studies have measured the kinetics of 3D cAMP distributions within cells. This is largely due to the low signal-to-noise ratio of FRET-based probes. We previously reported that hyperspectral imaging improves the signal-to-noise ratio of FRET measurements. Here we utilized hyperspectral imaging approaches to measure FRET signals in five dimensions (5D) - three spatial (x, y, z), wavelength (λ), and time (t) - allowing us to visualize cAMP gradients in pulmonary endothelial cells. cAMP levels were measured using a FRET-based sensor (H188) comprised of a cAMP binding domain sandwiched between FRET donor and acceptor - Turquoise and Venus fluorescent proteins. We observed cAMP gradients in response to 0.1 or 1 μM isoproterenol, 0.1 or 1 μM PGE1, or 50 μM forskolin. Forskolin- and isoproterenol-induced cAMP gradients formed from the apical (high cAMP) to basolateral (low cAMP) face of cells. In contrast, PGE1-induced cAMP gradients originated from both the basolateral and apical faces of cells. Data suggest that 2D (x,y) studies of cAMP compartmentalization may lead to erroneous conclusions about the existence of cAMP gradients, and that 3D (x,y,z) studies are required to assess mechanisms of signaling specificity. Results demonstrate that 5D imaging technologies are powerful tools for measuring biochemical processes in discrete subcellular domains. This work was supported by NIH P01HL066299, R01HL058506, S10RR027535, AHA 16PRE27130004 and the Abraham Mitchell Cancer Research Fund.

Three dimensional measurement of cAMP gradients using hyperspectral confocal microscopy

Cyclic AMP (cAMP) is a ubiquitous second messenger known to differentially regulate many cellular functions over a wide range of timescales. Several lines of evidence have suggested that the distribution of cAMP within cells is not uniform, and that cAMP compartmentalization is largely responsible for signaling specificity within the cAMP signaling pathway. However, to date, no studies have experimentally measured three dimensional (3D) cAMP distributions within cells. Here we use both 2D and 3D hyperspectral microscopy to visualize cAMP gradients in endothelial cells from the pulmonary microvasculature (PMVECs). cAMP levels were measured using a FRET-based cAMP sensor comprised of a cAMP binding domain from EPAC sandwiched between FRET donors and acceptors - Turquoise and Venus fluorescent proteins. Data were acquired using either a Nikon A1R spectral confocal microscope or custom spectral microscopy system. Analysis of hyperspectral image stacks from a single confocal slice or from summed images of all slices (2D analysis) indicated little or no cAMP gradients were formed within PMVECs under basal conditions or following agonist treatment. However, analysis of hyperspectral image stacks from 3D cellular geometries (z stacks) demonstrate marked cAMP gradients from the apical to basolateral membrane of PMVECs. These results strongly suggest that 2D imaging studies of cAMP compartmentalization - whether epifluorescence or confocal microscopy - may lead to erroneous conclusions about the existence of cAMP gradients, and that 3D studies are required to assess mechanisms of signaling specificity.

How rods respond to single photons: Key adaptations of a G-protein cascade that enable vision at the physical limit of perception

Rod photoreceptors are among the most sensitive light detectors in nature. They achieve their remarkable sensitivity across a wide variety of species through a number of essential adaptations: a specialized cellular geometry, a G-protein cascade with an unusually stable receptor molecule, a low-noise transduction mechanism, a nearly perfect effector enzyme, and highly evolved mechanisms of feedback control and receptor deactivation. Practically any change in protein expression, enzyme activity, or feedback control can be shown to impair photon detection, either by decreasing sensitivity or signal-to-noise ratio, or by reducing temporal resolution. Comparison of mammals to amphibians suggests that rod outer-segment morphology and the molecules and mechanism of transduction may have evolved together to optimize light sensitivity in darkness, which culminates in the extraordinary ability of these cells to respond to single photons at the ultimate limit of visual perception.

The phototransduction machinery in the rod outer segment has a strong efficacy gradient

Rod photoreceptors consist of an outer segment (OS) and an inner segment. Inside the OS a biochemical machinery transforms the rhodopsin photoisomerization into electrical signal. This machinery has been treated as and is thought to be homogenous with marginal inhomogeneities. To verify this assumption, we developed a methodology based on special tapered optical fibers (TOFs) to deliver highly localized light stimulations. By using these TOFs, specific regions of the rod OS could be stimulated with spots of light highly confined in space. As the TOF is moved from the OS base toward its tip, the amplitude of saturating and single photon responses decreases, demonstrating that the efficacy of the transduction machinery is not uniform and is 5-10 times higher at the base than at the tip. This gradient of efficacy of the transduction machinery is attributed to a progressive depletion of the phosphodiesterase along the rod OS. Moreover we demonstrate that, using restricted spots of light, the duration of the photoresponse along the OS does not increase linearly with the light intensity as with diffuse light.

cGMP in mouse rods: the spatiotemporal dynamics underlying single photon responses

Vertebrate vision begins when retinal photoreceptors transduce photons into electrical signals that are then relayed to other neurons in the eye, and ultimately to the brain. In rod photoreceptors, transduction of single photons is achieved by a well-understood G-protein cascade that modulates cGMP levels, and in turn, cGMP-sensitive inward current. The spatial extent and depth of the decline in cGMP during the single photon response (SPR) have been major issues in phototransduction research since the discovery that single photons elicit substantial and reproducible changes in membrane current. The spatial profile of cGMP decline during the SPR affects signal gain, and thus may contribute to reduction of trial-to-trial fluctuations in the SPR. Here we summarize the general principles of rod phototransduction, emphasizing recent advances in resolving the spatiotemporal dynamics of cGMP during the SPR.

Light-dependent changes in outer retinal water diffusion in rats in vivo

PURPOSE

To test the hypothesis that in rats, intraretinal light-dependent changes on diffusion-weighted magnetic resonance imaging (MRI) in vivo are consistent with known retinal layer-specific physiology.

METHODS

In male Sprague-Dawley rats, retinal morphology (thickness, extent, surface area, volume) and intraretinal profiles of the apparent diffusion coefficient (ADC, i.e., water mobility) parallel and perpendicular to the optic nerve were measured in vivo using quantitative MRI methods during light and dark stimulation.

RESULTS

The parallel ADC in the posterior half of the avascular, photoreceptor-dominated outer retina was significantly higher in light than dark, and this pattern was reversed (dark>light) in the anterior outer retina. The perpendicular ADC in the posterior outer retina was similar in light and dark, but was significantly higher in dark than light in the anterior outer retina. No light-dark changes in the inner retina were noted.

CONCLUSIONS

We identified light-dependent intraretinal diffusion changes that reflected established stimulation-based changes in outer retinal hydration. These findings are expected to motivate future applications of functional diffusion-based MRI in blinding disorders of the outer retina.

Spatiotemporal cGMP dynamics in living mouse rods

Signaling of single photons in rod photoreceptors decreases the concentration of the second messenger, cyclic GMP (cGMP), causing closure of cGMP-sensitive channels located in the plasma membrane. Whether the spatiotemporal profiles of the fall in cGMP are narrow and deep, or broad and shallow, has important consequences for the amplification and the fidelity of signaling. The factors that determine the cGMP profiles include the diffusion coefficient for cGMP, the spontaneous rate of cGMP hydrolysis, and the rate of cGMP synthesis, which is powerfully regulated by calcium feedback mechanisms. Here, using suction electrodes to record light-dependent changes in cGMP-activated current in living mouse rods lacking calcium feedback, we have determined the rate constant of spontaneous cGMP hydrolysis and the longitudinal cGMP diffusion coefficient. These measurements result in a fully constrained spatiotemporal model of phototransduction, which we used to determine the effect of feedback to cGMP synthesis in spatially constricting the fall of cGMP during the single-photon response of normal rods. We find that the spatiotemporal cGMP profiles during the single-photon response are optimized for maximal amplification and preservation of signal linearity, effectively operating within an axial signaling domain of ~2 μm.

Cyclic nucleotide phosphodiesterase 1A: a key regulator of cardiac fibroblast activation and extracellular matrix remodeling in the heart

Cardiac fibroblasts become activated and differentiate to smooth muscle-like myofibroblasts in response to hypertension and myocardial infarction (MI), resulting in extracellular matrix (ECM) remodeling, scar formation and impaired cardiac function. cAMP and cGMP-dependent signaling have been implicated in cardiac fibroblast activation and ECM synthesis. Dysregulation of cyclic nucleotide phosphodiesterase (PDE) activity/expression is also associated with various diseases and several PDE inhibitors are currently available or in development for treating these pathological conditions. The objective of this study is to define and characterize the specific PDE isoform that is altered during cardiac fibroblast activation and functionally important for regulating myofibroblast activation and ECM synthesis. We have found that Ca(2+)/calmodulin-stimulated PDE1A isoform is specifically induced in activated cardiac myofibroblasts stimulated by Ang II and TGF-β in vitro as well as in vivo within fibrotic regions of mouse, rat, and human diseased hearts. Inhibition of PDE1A function via PDE1-selective inhibitor or PDE1A shRNA significantly reduced Ang II or TGF-β-induced myofibroblast activation, ECM synthesis, and pro-fibrotic gene expression in rat cardiac fibroblasts. Moreover, the PDE1 inhibitor attenuated isoproterenol-induced interstitial fibrosis in mice. Mechanistic studies revealed that PDE1A modulates unique pools of cAMP and cGMP, predominantly in perinuclear and nuclear regions of cardiac fibroblasts. Further, both cAMP-Epac-Rap1 and cGMP-PKG signaling was involved in PDE1A-mediated regulation of collagen synthesis. These results suggest that induction of PDE1A plays a critical role in cardiac fibroblast activation and cardiac fibrosis, and targeting PDE1A may lead to regression of the adverse cardiac remodeling associated with various cardiac diseases.

Diffusion in narrow domains and application to phototransduction

The mean time for a Brownian particle to find a small target inside a narrow domain is a key parameter for many chemical reactions occurring in cellular microstructures. Although current estimations are given for a large class of domains, they cannot be used for narrow domains often encountered in cellular biology, such as the synaptic cleft, narrow compartments in the outer segment of vertebrate photoreceptors, or neuron-glia contact. We compute here the mean time for a Brownian particle to hit a small target placed on the surface of a narrow cylinder. We then use this result to estimate the rate constant of cyclic-GMP (cGMP) hydrolysis by the activated enzyme phosphodiesterase (PDE) in the narrow microdomains that build up the outer segment of a rod photoreceptor. By controlling the cGMP concentration, PDE activity is at the basis of the early photoresponse chemical reaction cascade. Our approach allows us to compute the cGMP rate constant as a function of biophysical parameters.

Estimating the rate constant of cyclic GMP hydrolysis by activated phosphodiesterase in photoreceptors

The early steps of light response occur in the outer segment of rod and cone photoreceptor. They involve the hydrolysis of cGMP, a soluble cyclic nucleotide, that gates ionic channels located in the outer segment membrane. We shall study here the rate by which cGMP is hydrolyzed by activated phosphodiesterase (PDE). This process has been characterized experimentally by two different rate constants beta(d) and beta(sub): beta(d) accounts for the effect of all spontaneously active PDE in the outer segment, and beta(sub) characterizes cGMP hydrolysis induced by a single light-activated PDE. So far, no attempt has been made to derive the experimental values of beta(d) and beta(sub) from a theoretical model, which is the goal of this work. Using a model of diffusion in the confined rod geometry, we derive analytical expressions for beta(d) and beta(sub) by calculating the flux of cGMP molecules to an activated PDE site. We obtain the dependency of these rate constants as a function of the outer segment geometry, the PDE activation and deactivation rates and the aqueous cGMP diffusion constant. Our formulas show good agreement with experimental measurements. Finally, we use our derivation to model the time course of the cGMP concentration in a transversally well-stirred outer segment.

Activation-dependent hindrance of photoreceptor G protein diffusion by lipid microdomains

The dynamics of G protein-mediated signal transduction depend on the two-dimensional diffusion of membrane-bound G proteins and receptors, which has been suggested to be rate-limiting for vertebrate phototransduction, a highly amplified G protein-coupled signaling pathway. Using fluorescence recovery after photobleaching (FRAP), we measured the diffusion of the G protein transducin alpha-subunit (Galpha(t)) and the G protein-coupled receptor rhodopsin on disk membranes of living rod photoreceptors from transgenic Xenopus laevis. Treatment with either methyl-beta-cyclodextrin or filipin III to disrupt cholesterol-containing lipid microdomains dramatically accelerated diffusion of Galpha(t) in its GTP-bound state and of the rhodopsin-Galphabetagamma(t) complex but not of rhodopsin or inactive GDP-bound Galphabetagamma. These results imply an activity-dependent sequestration of G proteins into cholesterol-dependent lipid microdomains, which limits diffusion and exclude the majority of free rhodopsin and the free G protein heterotrimer. Our data offer a novel demonstration of lipid microdomains in the internal membranes of living sensory neurons.

Diffusion of the second messengers in the cytoplasm acts as a variability suppressor of the single photon response in vertebrate phototransduction

The single photon response in vertebrate phototransduction is highly reproducible despite a number of random components of the activation cascade, including the random activation site, the random walk of an activated receptor, and its quenching in a random number of steps. Here we use a previously generated and tested spatiotemporal mathematical and computational model to identify possible mechanisms of variability reduction. The model permits one to separate the process into modules, and to analyze their impact separately. We show that the activation cascade is responsible for generation of variability, whereas diffusion of the second messengers is responsible for its suppression. Randomness of the activation site contributes at early times to the coefficient of variation of the photoresponse, whereas the Brownian path of a photoisomerized rhodopsin (Rh*) has a negligible effect. The major driver of variability is the turnoff mechanism of Rh*, which occurs essentially within the first 2-4 phosphorylated states of Rh*. Theoretically increasing the number of steps to quenching does not significantly decrease the corresponding coefficient of variation of the effector, in agreement with the biochemical limitations on the phosphorylated states of the receptor. Diffusion of the second messengers in the cytosol acts as a suppressor of the variability generated by the activation cascade. Calcium feedback has a negligible regulatory effect on the photocurrent variability. A comparative variability analysis has been conducted for the phototransduction in mouse and salamander, including a study of the effects of their anatomical differences such as incisures and photoreceptors geometry on variability generation and suppression.

The alpha2beta1 isoform of guanylyl cyclase mediates plasma membrane localized nitric oxide signalling

Nitric oxide (NO) is a mediator of copious biological processes, in many cases through the production of cGMP from the enzyme nitric oxide-sensitive guanylyl cyclase. Natriuretic peptides also elevate cGMP, often with distinct biological effects, raising the issue of how specificity is achieved. Here we show that a recently described alpha(2)beta(1) isoform of guanylyl cyclase is expressed in a number of epithelia, where it is localized to the apical plasma membrane. We measured the functional properties of the alpha(2)beta(1) isoform by utilizing the NO-dependent activation of the ion channel cystic fibrosis transmembrane conductance regulator (CFTR), which occurs by phosphorylation via the membrane-bound type II isoform of cGMP-dependent protein kinase. We found that cGMP generated by NO activation of the alpha(2)beta(1) isoform of guanylyl cyclase is an exceptionally efficient mediator of nitric oxide action on membrane targets, activating CFTR far more effectively than the cytoplasmically located alpha(1)beta(1) guanylyl cyclase isoform. Targeting the alpha(1)beta(1) isoform of guanylyl cyclase to the membrane also dramatically enhanced the effects of nitric oxide on CFTR within the membrane. This was not due to increased enzymatic activity of guanylyl cyclase in a membrane location, but to production of a localised membrane pool of cGMP by membrane-localized NO-dependent guanylyl cyclase that was resistant to degradation by phosphodiesterases. Selective effects of cGMP produced from this enzyme in response to NO are directed at membrane targets and suggest that drugs selectively activating or inhibiting this alpha(2)beta(1) isoform of guanylyl cyclase may have unique pharmacological properties.

Longitudinal diffusion of a polar tracer in the outer segments of rod photoreceptors from different species

Vertebrate rod photoreceptors are the ultimate light sensors, as they can detect a single photon. In darkness, rods maintain a high concentration of the intracellular messenger cyclic guanosine monophosphate (cGMP), which binds to and keeps open cationic channels on the plasma membrane of the outer segment. Absorption of a photon by the visual pigment of the rod, rhodopsin, initiates a biochemical amplification cascade that leads to a reduction in the concentration of cGMP and closure of the channels, thereby converting the incoming light to an electrical signal. Because the absorption of a photon and the ensuing reactions are localized events, the magnitude of the response of the rod to a single photon depends on the spread of the decrease in the cGMP concentration along the length of the outer segment. The longitudinal diffusion of cGMP depends on the structural parameters of the rod outer segment, specifically the area and the volume available for diffusion. To characterize the effect of rod outer segment cytoarchitecture on diffusion, we have used fluorescence recovery after photobleaching (FRAP) and examined the mobility of a fluorescent polar tracer, calcein, in the rod outer segments from three species with different outer segment structures: frog (Rana pipiens), mouse (Mus musculus domesticus) and gecko (Gekko gekko). We found that the diffusion coefficient is similar for all three species, in the order of 8-17 microm(2) s(-1), in broad agreement with the predictions by Holcman and Korenbrot (Biophys. J. 2004:86;2566-2582) based on the known cytoarchitecture of rod outer segments. Consequently, the results also support their prediction that the longitudinal spread of light excitation in rods is similar across species.

Modeling the role of incisures in vertebrate phototransduction

Phototransduction is mediated by a G-protein-coupled receptor-mediated cascade, activated by light and localized to rod outer segment (ROS) disk membranes, which, in turn, drives a diffusion process of the second messengers cGMP and Ca2+ in the ROS cytosol. This process is hindered by disks-which, however, bear physical cracks, known as incisures, believed to favor the longitudinal diffusion of cGMP and Ca2+. This article is aimed at highlighting the biophysical functional role and significance of incisures, and their effect on the local and global response of the photocurrent. Previous work on this topic regarded the ROS as well stirred in the radial variables, lumped the diffusion mechanism on the longitudinal axis of the ROS, and replaced the cytosolic diffusion coefficients by effective ones, accounting for incisures through their total patent area only. The fully spatially resolved model recently published by our group is a natural tool to take into account other significant details of incisures, including their geometry and distribution. Using mathematical theories of homogenization and concentrated capacity, it is shown here that the complex diffusion process undergone by the second messengers cGMP and Ca2+ in the ROS bearing incisures can be modeled by a family of two-dimensional diffusion processes on the ROS cross sections, glued together by other two-dimensional diffusion processes, accounting for diffusion in the ROS outer shell and in the bladelike regions comprised by the stack of incisures. Based on this mathematical model, a code has been written, capable of incorporating an arbitrary number of incisures and activation sites, with any given arbitrary distribution within the ROS. The code is aimed at being an operational tool to perform numerical experiments of phototransduction, in rods with incisures of different geometry and structure, under a wide spectrum of operating conditions. The simulation results show that incisures have a dual biophysical function. On the one hand, since incisures line up from disk to disk, they create vertical cytoplasmic channels crossing the disks, thus facilitating diffusion of second messengers; on the other hand, at least in those species bearing multiple incisures, they divide the disks into lobes like the petals of a flower, thus confining the diffusion of activated phosphodiesterase and localizing the photon response. Accordingly, not only the total area of incisures, but their geometrical shape and distribution as well, significantly influence the global photoresponse.

Longitudinal diffusion in retinal rod and cone outer segment cytoplasm: the consequence of cell structure

Excitation signals spread along photoreceptor outer segments away from the site of photon capture because of longitudinal diffusion of cGMP, a cytoplasmic second messenger. The quantitative features of longitudinal diffusion reflect the anatomical structure of the outer segment, known to be profoundly different in rod and cone photoreceptors. To explore how structural differences affect cytoplasmic diffusion and to assess whether longitudinal diffusion may contribute to the difference in signal transduction between photoreceptor types, we investigated, both theoretically and experimentally, the longitudinal diffusion of small, hydrophilic molecules in outer segments. We developed a new theoretical analysis to explicitly compute the longitudinal diffusion constant, Dl, in terms of outer segment structure. Using time-resolved fluorescence imaging we measured Dl of Alexa488 and lucifer yellow in intact, single cones and validated the theoretical analysis. We used numerical simulations of the theoretical model to investigate cGMP diffusion in outer segments of various species. At a given time interval, cGMP spreads further in rod than in cone outer segments of the same dimensions. Across all species, the spatial spread of cGMP at the peak of the dim light photocurrent is 3-5 microm in rod outer segments, regardless of their absolute size. Similarly the cGMP spatial spread is 0.7-1 microm in cone outer segments, independently of their dimensions.

Mathematical model of the spatio-temporal dynamics of second messengers in visual transduction

A model describing the role of transversal and longitudinal diffusion of cGMP and Ca(2+) in signaling in the rod outer segment of vertebrates is developed. Utilizing a novel notion of surface-volume reaction and the mathematical theories of homogenization and concentrated capacity, the diffusion of cGMP and Ca(2+) in the inter-disc spaces is shown to be reducible to a one-parameter family of diffusion processes taking place on a single rod cross section; whereas the diffusion in the outer shell is shown to be reducible to a diffusion on a cylindrical surface. Moreover, the exterior flux of the former serves as a source term for the latter, alleviating the assumption of a well-stirred cytosol. A previous model of visual transduction that assumes a well-stirred rod outer segment cytosol (and thus contains no spatial information) can be recovered from this model by imposing a "bulk" assumption. The model shows that upon activation of a single rhodopsin, cGMP changes are local, and exhibit both a longitudinal and a transversal component. Consequently, membrane current is also highly localized. The spatial spread of the single photon response along the longitudinal axis of the outer segment is predicted to be 3-5 microm, consistent with experimental data. This approach represents a tool to analyze point-wise signaling dynamics without requiring averaging over the entire cell by global Michaelis-Menten kinetics.

Calcium and phototransduction

Visual phototransduction, the conversion of incoming light to an electrical signal, takes place in the outer segments of the rod and cone photoreceptor cells. Light reduces the concentration of cGMP, which, in darkness, keeps open cationic channels present in the plasma membrane of the outer segment. Ca2+ plays an important role in phototransduction by modulating the cGMP-gated channels as well as cGMP synthesis and breakdown. Ca2+ is involved in a negative feedback that is essential for photoreceptor adaptation to background illumination. The effects of Ca2+ on the different components of rod phototransduction have been characterized and can quantitatively account for the steady state responses of the rod cell to background illumination. The propagation of the Ca2+ feedback signal from the periphery toward the center of the outer segment depends on the Ca2+ diffusion coefficient, which has a value of 15 +/- 1 microm2 s(-1). This value shows that diffusion of Ca2+ in the radial direction is quite slow providing a significant barrier in the propagation of the feedback signal. Also, because the diffusion coefficient of Ca2+ is much smaller than that of cGMP, the decline of Ca2+ in the longitudinal direction lags behind the propagation of excitation by the decline of cGMP.

Dynamic behavior of rod photoreceptor disks

Eukaryotic cells use membrane organelles, like the endoplasmic reticulum or the Golgi, to carry out different functions. Vertebrate rod photoreceptors use hundreds of membrane sacs (the disks) for the detection of light. We have used fluorescent tracers and single cell imaging to study the properties of rod photoreceptor disks. Labeling of intact rod photoreceptors with membrane markers and polar tracers revealed communication between intradiskal and extracellular space. Internalized tracers moved along the length of the rod outer segment, indicating communication between the disks as well. This communication involved the exchange of both membrane and aqueous phase and had a time constant in the order of minutes. The communication pathway uses approximately 2% of the available membrane disk area and does not allow the passage of molecules larger than 10 kDa. It was possible to load the intradiskal space with fluorescent Ca(2+) and pH dyes, which reported an intradiskal Ca(2+) concentration in the order of 1 microM and an acidic pH 6.5, both of them significantly different than intracellular and extracellular Ca(2+) concentrations and pH. The results suggest that the rod photoreceptor disks are not discrete, passive sacs but rather comprise an active cellular organelle. The communication between disks may be important for membrane remodeling as well as for providing access to the intradiskal space of the whole outer segment.

Functional characterisation and subcellular localisation of HCN1 channels in rabbit retinal rod photoreceptors

Gating of voltage-dependent conductances in retinal photoreceptors is the first step of a process leading to the enhancement of the temporal performance of the visual system. The molecular components underlying voltage-dependent gating in rods are presently poorly defined. In the present work we have investigated the isoform composition and the functional characteristics of hyperpolarisation-activated cyclic nucleotide-gated channels (HCN) in rabbit rods. Using immunocytochemistry we show the expression in the inner segment and cell body of the isoform 1 (HCN1). Electrophysiological investigations show that hyperpolarisation-activated currents (I(h)) can be measured only from the cell regions where HCN1 is expressed. Half-activation voltage (-75.0 +/- 0.3 mV) and kinetics (t(1/2) of 101 +/- 8 ms at -110 mV and 20 degrees C) of the I(h) in rods are similar to those of the macroscopic current carried by homomeric rabbit HCN1 channels expressed in HEK 293 cells. The homomeric nature of HCN1 channels in rods is compatible with the observation that cAMP induces a small shift (2.3 +/- 0.8 mV) in the half-activation voltage of I(h). In addition, the observation that within the physiological range of membrane potentials, cAMP does not significantly affect the gain of the current-to-voltage conversion, may reflect the need to protect the first step in the processing of visual signals from changes in cAMP turnover.

Calcium diffusion coefficient in rod photoreceptor outer segments

Calcium (Ca(2+)) modulates several of the enzymatic pathways that mediate phototransduction in the outer segments of vertebrate rod photoreceptors. Ca(2+) enters the rod outer segment through cationic channels kept open by cyclic GMP (cGMP) and is pumped out by a Na(+)/Ca(2+),K(+) exchanger. Light initiates a biochemical cascade, which leads to closure of the cGMP-gated channels, and a concomitant decline in the concentration of Ca(2+). This decline mediates the recovery from stimulation by light and underlies the adaptation of the cell to background light. The speed with which the decline in the Ca(2+) concentration propagates through the rod outer segment depends on the Ca(2+) diffusion coefficient. We have used the fluorescent Ca(2+) indicator fluo-3 and confocal microscopy to measure the profile of the Ca(2+) concentration after stimulation of the rod photoreceptor by light. From these measurements, we have obtained a value of 15 +/- 1 microm(2)s(-1) for the radial Ca(2+) diffusion coefficient. This value is consistent with the effect of a low-affinity, immobile buffer reported to be present in the rod outer segment (L.Lagnado, L. Cervetto, and P.A. McNaughton, 1992, J. Physiol. 455:111-142) and with a buffering capacity of approximately 20 for rods in darkness(S. Nikonov, N. Engheta, and E.N. Pugh, Jr., 1998, J. Gen. Physiol. 111:7-37). This value suggests that diffusion provides a significant delay for the radial propagation of the decline in the concentration of Ca(2+). Also, because of baffling by the disks, the longitudinal Ca(2+) diffusion coefficient will be in the order of 2 microm(2)s(-1), which is much smaller than the longitudinal cGMP diffusion coefficient (30-60 microm(2)s(-1); ). Therefore, the longitudinal decline of Ca(2+) lags behind the longitudinal spread of excitation by cGMP.

Cyclic nucleotide-gated channels colocalize with adenylyl cyclase in regions of restricted cAMP diffusion

Cyclic AMP is a ubiquitous second messenger that coordinates diverse cellular functions. Current methods for measuring cAMP lack both temporal and spatial resolution, leading to the pervasive notion that, unlike Ca(2+), cAMP signals are simple and contain little information. Here we show the development of adenovirus-expressed cyclic nucleotide-gated channels as sensors for cAMP. Homomultimeric channels composed of the olfactory alpha subunit responded rapidly to jumps in cAMP concentration, and their cAMP sensitivity was measured to calibrate the sensor for intracellular measurements. We used these channels to detect cAMP, produced by either heterologously expressed or endogenous adenylyl cyclase, in both single cells and cell populations. After forskolin stimulation, the endogenous adenylyl cyclase in C6-2B glioma cells produced high concentrations of cAMP near the channels, yet the global cAMP concentration remained low. We found that rapid exchange of the bulk cytoplasm in whole-cell patch clamp experiments did not prevent the buildup of significant levels of cAMP near the channels in human embryonic kidney 293 (HEK-293) cells expressing an exogenous adenylyl cyclase. These results can be explained quantitatively by a cell compartment model in which cyclic nucleotide-gated channels colocalize with adenylyl cyclase in microdomains, and diffusion of cAMP between these domains and the bulk cytosol is significantly hindered. In agreement with the model, we measured a slow rate of cAMP diffusion from the whole-cell patch pipette to the channels (90% exchange in 194 s, compared with 22-56 s for substances that monitor exchange with the cytosol). Without a microdomain and restricted diffusional access to the cytosol, we are unable to account for all of the results. It is worth noting that in models of unrestricted diffusion, even in extreme proximity to adenylyl cyclase, cAMP does not reach high enough concentrations to substantially activate PKA or cyclic nucleotide-gated channels, unless the entire cell fills with cAMP. Thus, the microdomains should facilitate rapid and efficient activation of both PKA and cyclic nucleotide-gated channels, and allow for local feedback control of adenylyl cyclase. Localized cAMP signals should also facilitate the differential regulation of cellular targets.

Longitudinal spread of second messenger signals in isolated rod outer segments of lizards

1. In vertebrate rods activation of the phototransduction cascade by light triggers changes in the concentrations of at least two diffusible intracellular second messengers (cGMP and Ca2+) whose actions depend on how far they spread from their site of production or entry. To address questions about their spatial spread, cell-attached patch current recording and fluorescence imaging of Calcium Green-dextran were used to measure the longitudinal spread of cGMP and Ca2+, respectively, in functionally intact isolated Gecko gecko lizard rod outer segments under whole-cell voltage clamp. 2. The light-evoked changes in cGMP and Ca2+ concentrations decayed with distance from a site of steady focal activation by two-photon absorption of 1064 nm light with similar decay lengths of approximately 3.5 microm. 3. These results can be understood on the basis of a quantitative model of coupled diffusible intracellular messengers, which is likely to have broad relevance for second messenger signalling pathways in general. 4. The decay length for the spread of adaptation from a site of steady local illumination was about 8 microm, i.e. substantially longer than the decay lengths measured for the spread of cGMP and Ca2+. There are a number of factors, however, that could broaden the apparent relationship between functional changes in the light response and the concentration of a diffusible messenger. For these reasons the measured decay length is an upper limit estimate of the spread of adaptation and does not rule out the possibility that Ca2+ and/or cGMP carry the adaptation signal.

Cyclic AMP diffusion coefficient in frog olfactory cilia

Cyclic AMP (cAMP) is one of the intracellular messengers that mediate odorant signal transduction in vertebrate olfactory cilia. Therefore, the diffusion coefficient of cAMP in olfactory cilia is an important factor in the transduction of the odorous signal. We have employed the excised cilium preparation from the grass frog (Rana pipiens) to measure the cAMP diffusion coefficient. In this preparation an olfactory cilium is drawn into a patch pipette and a gigaseal is formed at the base of the cilium. Subsequently the cilium is excised, allowing bath cAMP to diffuse into the cilium and activate the cyclic nucleotide-gated channels on the plasma membrane. In order to estimate the cAMP diffusion coefficient, we analyzed the kinetics of the currents elicited by step changes in the bath cAMP concentration in the absence of cAMP hydrolysis. Under such conditions, the kinetics of the cAMP-activated currents has a simple dependence on the diffusion coefficient. From the analysis we have obtained a cAMP diffusion coefficient of 2.7 +/- 0.2. 10(-6) cm2 s-1 for frog olfactory cilia. This value is similar to the expected value in aqueous solution, suggesting that there are no significant diffusional barriers inside olfactory cilia. At cAMP concentrations higher than 5 microM, diffusion slowed considerably, suggesting the presence of buffering by immobile cAMP binding sites. A plausible physiological function of such buffering sites would be to prolong the response of the cell to strong stimuli.

Molecular and pharmacological analysis of cyclic nucleotide-gated channel function in the central nervous system

Most functional studies of cyclic nucleotide-gated (CNG) channels have been confined to photoreceptors and olfactory epithelium, in which CNG channels are abundant and easy to study. The widespread distribution of CNG channels in tissues throughout the body has only recently been recognized and the functions of this channel family in many of these tissues remain largely unknown. The molecular biological and pharmacological properties of the CNG channel family are summarized in order to put in context studies aimed at probing CNG channel functions in these tissues using pharmacological and genetic methods. Compounds have now been identified that are useful in distinguishing CNG channel activated pathways from cAMP/cGMP dependent-protein kinases or other pathways. The ways in which these interact with CNG channels are understood and this knowledge is leading to the identification of more potent and more specific CNG channel subtype-specific agonists or antagonists. Recent molecular and genetic analyses have identified novel roles of CNG channels in neuronal development and plasticity in both invertebrates and vertebrates. Targeting CNG channels via specific drugs and genetic manipulation (such as knockout mice) will permit better understanding of the role of CNG channels in both basic and higher orders of brain function.

Molecular origin of continuous dark noise in rod photoreceptors

Noise in the rod photoreceptors limits the ability of the dark-adapted visual system to detect dim lights. We investigated the molecular mechanism of the continuous component of the electrical dark noise in toad rods. Membrane current was recorded from intact, isolated rods or truncated, internally dialyzed rod outer segments. The continuous noise was separated from noise due to thermal activation of rhodopsin and to transitions in the cGMP-activated channels. Selectively disabling different elements of the phototransduction cascade allowed examination of their contributions to the continuous noise. These experiments indicate that the noise is generated by spontaneous activation of cGMP phosphodiesterase (PDE) through a process that does not involve transducin. The addition of recombinant gamma, the inhibitory subunit of PDE, did not suppress the noise, indicating that endogenous gamma does not completely dissociate from the catalytic subunit of PDE during spontaneous activation. Quantitative analysis of the noise provided estimates of the rate constants for spontaneous PDE activation and deactivation and the catalytic activity of a single PDE molecule in situ.

Gain and kinetics of activation in the G-protein cascade of phototransduction

The guanine nucleotide binding protein (G protein) cascade underlying phototransduction is one of the best understood of all signaling pathways. The diffusional interactions of the proteins underlying the cascade have been analyzed, both at a macroscopic level and also in terms of the stochastic nature of the molecular contacts. In response to a single activated rhodopsin (R*) formed as a result of a single photon hit, it can be shown that molecules of the G-protein transducin will be activated approximately linearly with time. This, in turn, will cause the number of activated molecules of the effector protein (the phosphodiesterase) also to increase linearly with time. These kinetics of protein activation provide an accurate description of the time course of the rising phase of the photoreceptor's electrical response over a wide range of flash intensities. Recent estimates indicate that at room temperature each R* triggers activation of the phosphodiesterase at a rate of 1000-2000 subunits.s-1. Now that a quantitative description of the activation steps in transduction has been obtained, perhaps the greatest challenge for the future is to provide a comprehensive description of the shutoff reactions, so that a complete account of the photoreceptor's response to light can be achieved.

Diffusion coefficient of the cyclic GMP analog 8-(fluoresceinyl)thioguanosine 3',5' cyclic monophosphate in the salamander rod outer segment

Cyclic GMP (cGMP) is the intracellular messenger mediating phototransduction in retinal rods, with its longitudinal diffusion in the rod outer segment (ROS) likely to be a factor in determining light sensitivity. From the kinetics of cGMP-activated currents in the truncated ROS of the salamander (Ambystoma tigrinum), the cGMP diffusion coefficient was previously estimated to be approximately 60 x 10(-8) cm2 s-1. On the other hand, fluorescence measurements in intact salamander ROS using 8-(fluoresceinyl)thioguanosine 3',5'-cyclic monophosphate (Fl-cGMP) led to a diffusion coefficient for this compound of 1 x 10(-8) cm2 s-1; after corrections for differences in size and in binding to cellular components between cGMP and Fl-cGMP, this gave an upper limit of 11 x 10(-8) cm2 s-1 for the cGMP diffusion coefficient. To properly compare the two sets of measurements, we have examined the diffusion of Fl-cGMP in the truncated ROS. From the kinetics of Fl-cGMP-activated currents, we have obtained a diffusion coefficient of 3 x 10(-8) cm2 s-1 for this analog; the cGMP diffusion coefficient measured from the same truncated ROSs was approximately 80 x 10(-8) cm2 s-1. Thus, a factor of 27 appears appropriate for correcting differences in size and intracellular binding between cGMP and Fl-cGMP. Application of this correction factor to the Fl-cGMP diffusion coefficient measurements by Olson and Pugh (1993) gives a cGMP diffusion coefficient of approximately 30 x 10(-8) cm2 s-1, in reasonable agreement with the value measured from the truncated ROS.

Activation of light-dependent K+ channels in ciliary invertebrate photoreceptors involves cGMP but not the IP3/Ca2+ cascade

The activation of light-dependent K+ channels in ciliary photoreceptors from Pecten was investigated using intracellular dialysis of putative messengers and modulators. Neither elevated [Ca2+] nor BAPTA changed the membrane current in the dark or the light response. IP3 and the antagonists heparin and decavanadate were similarly ineffective, indicating that in these cells the IP3/Ca2+ signaling pathway is not crucial for phototransduction. By contrast, 8-Br-cGMP and cGMP induced an outward current accompanied by an increase in membrane conductance; 8-Br-cAMP was ineffective. The identity between the cGMP-induced and the light-induced currents is suggested by the following: both are carried by K+ and blocked by 4-AP, and both show outward rectification. In addition, guanine cyclic nucleotides depressed the photoresponse and induced single-channel currents in excised patches of light-sensitive membrane. These light-dependent channels therefore appear to represent a link between the families of cyclic nucleotide-gated channels and voltage-dependent K+ channels.

Ca2+ modulation of the cGMP-gated channel of bullfrog retinal rod photoreceptors

1. The outer segment of an isolated rod photoreceptor from the bullfrog retina was drawn into a pipette containing choline solution for recording membrane current. The rest of the cell was sheared off with a glass probe to allow internal dialysis of the outer segment with a bath potassium solution ('truncated rod outer segment' preparation). The potential between the inside and the outside of the pipette was held at 0 mV. 2. Application of bath cGMP, in the presence of 3-isobutyl-1-methylxanthine (IBMX), gave rise to an outward membrane current. At saturating cGMP concentrations, this current was insensitive to intracellular Ca2+ at concentrations between 0 and 10 microM. At subsaturating cGMP concentrations, however, this current was inhibited by intracellular Ca2+. This sensitivity to Ca2+ declined after dialysis with a low-Ca2+ solution, suggesting the involvement of a soluble factor. 3. At low (nominally 0) Ca2+, the half-maximal activation constant and Hill coefficient for the activation of the cGMP-gated current by cGMP were 27 microM and 2.0, respectively. At high (ca 10 microM) Ca2+, the corresponding values were 40 microM cGMP and 2.4. 4. The inhibition of the current by Ca2+ was characterized at 20 microM cGMP. Ca2+ inhibited the current by up to 60%, with half-maximal inhibition at 48 nM Ca2+ and a Hill coefficient of 1.6.