Coupling of Conformational Dynamics and Inhibitor Binding in the Phosphodiesterase-5 Family

Phosphodiesterase-5 (PDE5) is responsible for regulating the concentration of the second messenger molecule cGMP by hydrolyzing it into 5'-GMP. PDE5 is implicated in erectile dysfunction and cardiovascular diseases. The substrate binding site in the catalytic domain of PDE5 is surrounded by several dynamic structural motifs (including the α14 helix, M-loop, and H-loop) that are known to switch between inactive and active conformational states via currently unresolved structural intermediates. We evaluated the conformational dynamics of these structural motifs in the apo state and upon binding of an allosteric inhibitor (evodamine) or avanafil, a competitive inhibitor. We employed enhanced sampling based replica exchange solute scaling (REST2) method, principal component analysis (PCA), time-lagged independent component analysis (tICA), molecular dynamics (MD) simulations, and well-tempered metadynamics simulations to probe the conformational changes in these structural motifs. Our results support a regulatory mechanism for PDE5, where the α14 helix alternates between an inward (lower activity) conformation and an outward (higher activity) conformation that is accompanied by the folding/unfolding of the α 8' and α 8″ helices of the H-loop. When the allosteric inhibitor evodamine is bound to PDE5, the inward (inactive) state of the α 14 helix is preferred, thus preventing substrate access to the catalytic site. In contrast, competitive inhibitors of PDE5 block catalysis by occupying the active site accompanied by stabilization of the outward conformation of the α 14 helix. Defining the conformational dynamics underlying regulation of PDE5 activation will be helpful in rational design of next-generation small molecules modulators of PDE5 activity. This article is protected by copyright. All rights reserved.

Reconstitution of Membrane-associated Components of a G-protein Signaling Pathway on Membrane-coated Nanoparticles (Lipobeads)

G-protein coupled signaling pathways are organized into multi-protein complexes called signalosomes that are located within and on cellular membranes. We describe the use of silica nanoparticles coated with a unilamellar phospholipid bilayer (lipobeads) to reconstitute the activated photoreceptor G-protein α-subunit (Gtα*) with its cognate effector (phosphodiesterase-6; PDE6) for biochemical and structural studies of the activation mechanism regulating this GPCR signaling pathway. Lipobeads are prepared by resuspending dried-down phospholipid mixtures with monodisperse 70 nm silica particles, followed by extrusion through a 100 nm membrane filter. This uniform and supported liposomal preparation is easily sedimented, permitting the separation of soluble from membrane-associated proteins. Upon loading lipobeads with Gtα* and PDE6, we find that activation of PDE6 catalysis by Gtα* occurs much more efficiently than in the absence of membranes. Chemical cross-linking of membrane-confined proteins allows detection of changes in protein-protein interactions, resulting from G-protein activation of PDE6. The advantages of using lipobeads over partially purified membrane preparations or traditional liposomal preparations are generally applicable to the study of other membrane-confined signal transduction pathways.

Structural Analysis of the Regulatory GAF Domains of cGMP Phosphodiesterase Elucidates the Allosteric Communication Pathway

Regulation of photoreceptor phosphodiesterase (PDE6) activity is responsible for the speed, sensitivity, and recovery of the photoresponse during visual signaling in vertebrate photoreceptor cells. It is hypothesized that physiological differences in the light responsiveness of rods and cones may result in part from differences in the structure and regulation of the distinct isoforms of rod and cone PDE6. Although rod and cone PDE6 catalytic subunits share a similar domain organization consisting of tandem GAF domains (GAFa and GAFb) and a catalytic domain, cone PDE6 is a homodimer whereas rod PDE6 consists of two homologous catalytic subunits. Here we provide the x-ray crystal structure of cone GAFab regulatory domain solved at 3.3 Å resolution, in conjunction with chemical cross-linking and mass spectrometric analysis of conformational changes to GAFab induced upon binding of cGMP and the PDE6 inhibitory γ-subunit (Pγ). Ligand-induced changes in cross-linked residues implicate multiple conformational changes in the GAFa and GAFb domains in forming an allosteric communication network. Molecular dynamics simulations of cone GAFab revealed differences in conformational dynamics of the two subunits forming the homodimer and allosteric perturbations on cGMP binding. Cross-linking of Pγ to GAFab in conjunction with solution NMR spectroscopy of isotopically labeled Pγ identified the central polycationic region of Pγ interacting with the GAFb domain. These results provide a mechanistic basis for developing allosteric activators of PDE6 with therapeutic implications for halting the progression of several retinal degenerative diseases.

The molecular architecture of photoreceptor phosphodiesterase 6 (PDE6) with activated G protein elucidates the mechanism of visual excitation

Photoreceptor phosphodiesterase 6 (PDE6) is the central effector of the visual excitation pathway in both rod and cone photoreceptors, and PDE6 mutations that alter PDE6 structure or regulation can result in several human retinal diseases. The rod PDE6 holoenzyme consists of two catalytic subunits (Pαβ) whose activity is suppressed in the dark by binding of two inhibitory γ-subunits (Pγ). Upon photoactivation of rhodopsin, the heterotrimeric G protein (transducin) is activated, resulting in binding of the activated transducin α-subunit (Gt_α) to PDE6, displacement of Pγ from the PDE6 active site, and enzyme activation. Although the biochemistry of this pathway is understood, a lack of detailed structural information about the PDE6 activation mechanism hampers efforts to develop therapeutic interventions for managing PDE6-associated retinal diseases. To address this gap, here we used a cross-linking MS-based approach to create a model of the entire interaction surface of Pγ with the regulatory and catalytic domains of Pαβ in its nonactivated state. Following reconstitution of PDE6 and activated Gt_α with liposomes and identification of cross-links between Gt_α and PDE6 subunits, we determined that the PDE6-Gt_α protein complex consists of two Gt_α-binding sites per holoenzyme. Each Gt_α interacts with the catalytic domains of both catalytic subunits and induces major changes in the interaction sites of the Pγ subunit with the catalytic subunits. These results provide the first structural model for the activated state of the transducin-PDE6 complex during visual excitation, enhancing our understanding of the molecular etiology of inherited retinal diseases.

Allosteric Regulation of Rod Photoreceptor Phosphodiesterase 6 (PDE6) Elucidated by Chemical Cross-Linking and Quantitative Mass Spectrometry

Photoreceptor phosphodiesterase (PDE6) is the central effector enzyme in the visual excitation pathway in rod and cone photoreceptors. Its tight regulation is essential for the speed, sensitivity, recovery, and adaptation of visual signaling. The rod PDE6 holoenzyme (Pαβγ₂) is composed of a catalytic heterodimer (Pαβ) that binds two inhibitory γ subunits. Each of the two catalytic subunits (Pα and Pβ) contains a catalytic domain responsible for cGMP hydrolysis and two tandem GAF domains, one of which binds cGMP noncatalytically. Unlike related GAF-containing PDEs where cGMP binding allosterically activates catalysis, the physiological significance of cGMP binding to the GAF domains of PDE6 is unknown. To elucidate the structural determinants of PDE6 allosteric regulators, we biochemically characterized PDE6 complexes in various allosteric states (Pαβ, Pαβ-cGMP, Pαβγ₂, and Pαβγ₂-cGMP) with a quantitative cross-linking/mass spectrometry approach. We employed a normalization strategy to dissect the cross-linking reactivity of individual residues in order to assess the spatial cross-linking propensity of detected pairs. In addition to identifying cross-linked pairs that undergo conformational changes upon ligand binding, we observed an asymmetric binding of the inhibitory γ-subunit and the noncatalytic cGMP to the GAFa domains of rod PDE6, as well as a stable open conformation of Pαβ catalytic dimer in different allosteric states. These results advance our understanding of the exquisite regulatory control of the lifetime of rod PDE6 activation/deactivation during visual signaling, as well as providing a structural basis for interpreting how mutations in rod PDE6 subunits can lead to retinal diseases.

The N termini of the inhibitory γ-subunits of phosphodiesterase-6 (PDE6) from rod and cone photoreceptors differentially regulate transducin-mediated PDE6 activation

Phosphodiesterase-6 (PDE6) plays a central role in both rod and cone phototransduction pathways. In the dark, PDE6 activity is suppressed by its inhibitory γ-subunit (Pγ). Rhodopsin-catalyzed activation of the G protein transducin relieves this inhibition and enhances PDE6 catalysis. We hypothesized that amino acid sequence differences between rod- and cone-specific Pγs underlie transducin's ability to more effectively activate cone-specific PDE6 than rod PDE6. To test this, we analyzed rod and cone Pγ sequences from all major vertebrate and cyclostome lineages and found that rod Pγ loci are far more conserved than cone Pγ sequences and that most of the sequence differences are located in the N-terminal region. Next we reconstituted rod PDE6 catalytic dimer (Pαβ) with various rod or cone Pγ variants and analyzed PDE6 activation upon addition of the activated transducin α-subunit (Gt_α*-GTPγS). This analysis revealed a rod-specific Pγ motif (amino acids 9-18) that reduces the ability of Gt_α*-GTPγS to activate the reconstituted PDE6. In cone Pγ, Asn-13 and Gln-14 significantly enhanced Gt_α*-GTPγS activation of cone Pγ truncation variants. Moreover, we observed that the first four amino acids of either rod or cone Pγ contribute to Gt_α*-GTPγS-mediated activation of PDE6. We conclude that physiological differences between rod and cone photoreceptor light responsiveness can be partially ascribed to ancient, highly conserved amino acid differences in the N-terminal regions of Pγ isoforms, demonstrating for the first time a functional role for this region of Pγ in the differential activation of rod and cone PDE6 by transducin.

Molecular architecture of photoreceptor phosphodiesterase elucidated by chemical cross-linking and integrative modeling

Photoreceptor phosphodiesterase (PDE6) is the central effector enzyme in visual excitation pathway in rod and cone photoreceptors. Its tight regulation is essential for the speed, sensitivity, recovery and adaptation of visual detection. Although major steps in the PDE6 activation/deactivation pathway have been identified, mechanistic understanding of PDE6 regulation is limited by the lack of knowledge about the molecular organization of the PDE6 holoenzyme (αβγγ). Here, we characterize the PDE6 holoenzyme by integrative structural determination of the PDE6 catalytic dimer (αβ), based primarily on chemical cross-linking and mass spectrometric analysis. Our models built from high-density cross-linking data elucidate a parallel organization of the two catalytic subunits, with juxtaposed α-helical segments within the tandem regulatory GAF domains to provide multiple sites for dimerization. The two catalytic domains exist in an open configuration when compared to the structure of PDE2 in the apo state. Detailed structural elements for differential binding of the γ-subunit to the GAFa domains of the α- and β-subunits are revealed, providing insight into the regulation of the PDE6 activation/deactivation cycle.

Identification of amino acid residues responsible for the selectivity of tadalafil binding to two closely related phosphodiesterases, PDE5 and PDE6

The 11 families of the Class I cyclic nucleotide phosphodiesterases (PDEs) are critical for regulation of cyclic nucleotide signaling. PDE5 (important in regulating vascular smooth muscle contraction) and PDE6 (responsible for regulating visual transduction in vertebrate photoreceptors) are structurally similar but have several functional differences whose structural basis is poorly understood. Using evolutionary trace analysis and structural homology modeling in conjunction with site-directed mutagenesis, we have tested the hypothesis that class-specific differences between PDE5 and PDE6 account for the biochemical and pharmacological differences in the two enzyme families. Replacing human PDE5 residues in the M-loop region of the binding site for the PDE5-selective inhibitor tadalafil (Cialis®) with the corresponding class-specific cone PDE6 residues (P773E, I778V, E780L, F787W, E796V, D803P, L804M, N806D, I813L, S815K) reduces tadalafil binding affinity to levels characteristic of PDE6. These mutations fail to alter vardenafil (Levitra®) affinity for the active site. Class-specific differences in PDE5 versus cone PDE6 that contribute to the accelerated catalytic efficiency of PDE6 were identified but required heterologous expression of full-length PDE5 constructs. Introduction of PDE6 residues into the background of the PDE5 protein sequence often led to loss of catalytic activity and reduced protein solubility, supporting the idea that multiple structural elements of PDE6 are highly susceptible to misfolding during heterologous expression. This work validates the use of PDE5 as a template to identify functional differences between PDE5 and PDE6 that will accelerate efforts to develop the next generation of PDE5-selective inhibitors with fewer adverse side effects resulting from PDE6 inhibition.

Functional mapping of interacting regions of the photoreceptor phosphodiesterase (PDE6) γ-subunit with PDE6 catalytic dimer, transducin, and regulator of G-protein signaling9-1 (RGS9-1)

The cGMP phosphodiesterase (PDE6) involved in visual transduction in photoreceptor cells contains two inhibitory γ-subunits (Pγ) which bind to the catalytic core (Pαβ) to inhibit catalysis and stimulate cGMP binding to the GAF domains of Pαβ. During visual excitation, interaction of activated transducin with Pγ relieves inhibition. Pγ also participates in a complex with RGS9-1 and other proteins to accelerate the GTPase activity of activated transducin. We studied the structural determinants for these important functions of Pγ. First, we identified two important sites in the middle region of Pγ (amino acids 27-38 and 52-54) that significantly stabilize the overall binding affinity of Pγ with Pαβ. The ability of Pγ to stimulate noncatalytic cGMP binding to the GAF domains of PDE6 has been localized to amino acids 27-30 of Pγ. Transducin activation of PDE6 catalysis critically depends on the presence of Ile54 in the glycine-rich region of Pγ in order to relieve inhibition of catalysis. The central glycine-rich region of Pγ is also required for transducin to increase cGMP exchange at the GAF domains. Finally, Thr-65 and/or Val-66 of Pγ are critical residues for Pγ to stimulate GTPase activity of transducin in a complex with RGS9-1. We propose that the glycine-rich region of Pγ is a primary docking site for PDE6-interacting proteins involved in the activation/inactivation pathways of visual transduction. This functional mapping of Pγ with its binding partners demonstrates the remarkable versatility of this multifunctional protein and its central role in regulating the activation and lifetime of visual transduction.

Characterization of conformational changes and protein-protein interactions of rod photoreceptor phosphodiesterase (PDE6)

As the central effector of visual transduction, the regulation of photoreceptor phosphodiesterase (PDE6) is controlled by both allosteric mechanisms and extrinsic binding partners. However, the conformational changes and interactions of PDE6 with known interacting proteins are poorly understood. Using a fluorescence detection system for the analytical ultracentrifuge, we examined allosteric changes in PDE6 structure and protein-protein interactions with its inhibitory γ-subunit, the prenyl-binding protein (PrBP/δ), and activated transducin. In solution, the PDE6 catalytic dimer (Pαβ) exhibits a more asymmetric shape (axial ratio of 6.6) than reported previously. The inhibitory Pγ subunit behaves as an intrinsically disordered protein in solution but binds with high affinity to the catalytic dimer to reconstitute the holoenzyme without a detectable change in shape. Whereas the closely related PDE5 homodimer undergoes a significant change in its sedimentation properties upon cGMP binding to its regulatory cGMP binding site, no such change was detected upon ligand binding to the PDE6 catalytic dimer. However, when Pαβ was reconstituted with Pγ truncation mutants lacking the C-terminal inhibitory region, cGMP-dependent allosteric changes were observed. PrBP/δ bound to the PDE6 holoenzyme with high affinity (K(D) = 6.2 nm) and induced elongation of the protein complex. Binding of activated transducin to PDE6 holoenzyme resulted in a concentration-dependent increase in the sedimentation coefficient, reflecting a dynamic equilibrium between transducin and PDE6. We conclude that allosteric regulation of PDE6 is more complex than for PDE5 and is dependent on interactions of regions of Pγ with the catalytic dimer.

Phosphodiesterase 6D, cGMP-specific rod delta

Network Map States Transitions Functions Protein Classes Sequence Interactions Pathways Domains & Motifs Protein Structure Orthologs Sequence Interactions Pathways Domains & Motifs Protein Structure Orthologs Blast Data.

Structural requirements of the photoreceptor phosphodiesterase gamma-subunit for inhibition of rod PDE6 holoenzyme and for its activation by transducin

The central enzyme of the visual transduction cascade, cGMP phosphodiesterase (PDE6), is regulated by its gamma-subunit (Pgamma), whose inhibitory constraint is released upon binding of activated transducin. It is generally believed that the last four or five C-terminal amino acid residues of Pgamma are responsible for blocking catalysis. In this paper, we showed that the last 10 C-terminal residues (Pgamma78-87) are the minimum required to completely block catalysis. The kinetic mechanism of inhibition by the Pgamma C terminus depends on which substrate is undergoing catalysis. We also discovered a second mechanism of Pgamma inhibition that does not require this C-terminal region and that is capable of inhibiting up to 80% of the maximal cGMP hydrolytic rate. Furthermore, amino acids 63-70 and/or the intact alpha2 helix of Pgamma stabilize binding of C-terminal Pgamma peptides by 100-fold. When PDE6 catalytic subunits were reconstituted with portions of the Pgamma molecule and tested for activation by transducin, we found that the C-terminal region (Pgamma63-87) by itself could not be displaced but that transducin could relieve inhibition of certain Pgamma truncation mutants. Our results are consistent with two distinct mechanisms of Pgamma inhibition of PDE6. One involves direct interaction of the C-terminal residues with the catalytic site. A second regulatory mechanism may involve binding of other regions of Pgamma to the catalytic domain, thereby allosterically reducing the catalytic rate. Transducin activation of PDE6 appears to require interaction with both the C terminus and other regions of Pgamma to effectively relieve its inhibitory constraint.

Probing the catalytic sites and activation mechanism of photoreceptor phosphodiesterase using radiolabeled phosphodiesterase inhibitors

Retinal photoreceptor phosphodiesterase (PDE6) is unique among the phosphodiesterase enzyme family not only for its catalytic heterodimer but also for its regulatory gamma-subunits (Pgamma) whose inhibitory action is released upon binding to the G-protein transducin. It is generally assumed that during visual excitation both catalytic sites are relieved of Pgamma inhibition upon binding of two activated transducin molecules. Because PDE6 shares structural and pharmacological similarities with PDE5, we utilized radiolabeled PDE5 inhibitors to probe the catalytic sites of PDE6. The membrane filtration assay we used to quantify [(3)H]vardenafil binding to PDE6 required histone II-AS to stabilize drug binding to the active site. Under these conditions, [(3)H]vardenafil binds stoichiometrically to both the alpha- and beta-subunits of the activated PDE6 heterodimer. [(3)H]vardenafil fails to bind to either the PDE6 holoenzyme or the PDE6 catalytic dimer reconstituted with Pgamma, consistent with Pgamma blocking access to the drug-binding sites. Following transducin activation of membrane-associated PDE6 holoenzyme, [(3)H]vardenafil binding increases in proportion to the extent of PDE6 activation. Both [(3)H]vardenafil binding and hydrolytic activity of transducin-activated PDE6 fail to exceed 50% of the value for the PDE6 catalytic dimer. However, adding a 1000-fold excess of activated transducin can stimulate the hydrolytic activity of PDE6 to its maximum extent. These results demonstrate that both subunits of the PDE6 heterodimer are able to bind ligands to the enzyme active site. Furthermore, transducin relieves Pgamma inhibition of PDE6 in a biphasic manner, with only one-half of the maximum PDE6 activity efficiently attained during visual excitation.

Direct allosteric regulation between the GAF domain and catalytic domain of photoreceptor phosphodiesterase PDE6

Photoreceptor cGMP phosphodiesterase (PDE6) is the central enzyme in the visual transduction cascade. The PDE6 catalytic subunit contains a catalytic domain and regulatory GAF domains. Unlike most GAF domain-containing cyclic nucleotide phosphodiesterases, little is known about direct allosteric communication of PDE6. In this study, we demonstrate for the first time direct, inter-domain allosteric communication between the GAF and catalytic domains in PDE6. The binding affinity of PDE6 for pharmacological inhibitors or for the C-terminal region of the inhibitory gamma subunit (Pgamma), known to directly inhibit PDE6 catalysis, was increased approximately 2-fold by ligands binding to the GAF domain. Binding of the N-terminal half of Pgamma to the GAF domains suffices to induce this allosteric effect. Allosteric communication between GAF and catalytic domains is reciprocal, in that drug binding to the catalytic domain slowed cGMP dissociation from the GAF domain. Although cGMP hydrolysis was not affected by binding of Pgamma1-60, Pgamma lacking its last seven amino acids decreased the Michaelis constant of PDE6 by 2.5-fold. Pgamma1-60 binding to the GAF domain increased vardenafil but not cGMP affinity, indicating that substrate- and inhibitor-binding sites do not totally overlap. In addition, prolonged incubation of PDE6 with vardenafil or sildenafil (but not 3-isobutyl-1-methylxanthine and zaprinast) induced a distinct conformational change in the catalytic domain without affecting the binding properties of the GAF domains. We conclude that although Pgamma-mediated regulation plays the dominant role in visual excitation, the direct, inter-domain allosteric regulation described in this study may play a feedback role in light adaptational processes during phototransduction.

Assay and functional properties of PrBP(PDEdelta), a prenyl-binding protein interacting with multiple partners

A 17-kDa prenyl-binding protein, PrBP(PDEdelta), is highly conserved among various species from human to Caenorhabditis elegans. First identified as a putative regulatory delta subunit of the cyclic nucleotide phosphodiesterase (PDE6) purified from mammalian photoreceptor cells, PrBP(PDEdelta) has been hypothesized to reduce activation of PDE6 by the heterotrimeric G-protein, transducin, thereby desensitizing the photoresponse. However, recent work shows that PrBP(PDEdelta) interacts with numerous prenylated proteins at their farnesylated or geranylgeranylated C-termini, as well as with non-prenylated proteins. These polypeptides include small GTPases such as Rab13, Ras, Rap, and Rho6, as well as components involved in phototransduction (e.g., rod and cone PDE6, rod and cone opsin kinases). Expression of PrBP(PDEdelta) in tissues and organisms not expressing PDE6, the demonstration of multiple interacting partners with PrBP(PDEdelta), and its low abundance in rod outer segments all argue against it being a regulatory PDE6 subunit. This raises intriguing questions as to its physiological functions. In this chapter, we review the current status of PrBP(PDEdelta) and describe some of the assays used to determine these interactions in detail. In mammalian photoreceptors, the results are consistent with a role of PrBP(PDEdelta) in the transport of prenylated proteins from their site of synthesis in the inner segment to the outer segment where phototransduction occurs.

The glutamic acid-rich protein-2 (GARP2) is a high affinity rod photoreceptor phosphodiesterase (PDE6)-binding protein that modulates its catalytic properties

The glutamic acid-rich protein-2 (GARP2) is a splice variant of the beta-subunit of the cGMP-gated ion channel of rod photoreceptors. GARP2 is believed to interact with several membrane-associated phototransduction proteins in rod photoreceptors. In this study, we demonstrated that GARP2 is a high affinity PDE6-binding protein and that PDE6 co-purifies with GARP2 during several stages of chromatographic purification. We found that hydrophobic interaction chromatography succeeds in quantitatively separating GARP2 from the PDE6 holoenzyme. Furthermore, the 17-kDa prenyl-binding protein, abundant in retinal cells, selectively released PDE6 (but not GARP2) from rod outer segment membranes, demonstrating the specificity of the interaction between GARP2 and PDE6. Purified GARP2 was able to suppress 80% of the basal activity of the nonactivated, membrane-bound PDE6 holoenzyme at concentrations equivalent to its endogenous concentration in rod outer segment membranes. However, GARP2 was unable to reverse the transducin activation of PDE6 (in contrast to a previous study) nor did it significantly alter catalysis of the fully activated PDE6 catalytic dimer. The high binding affinity of GARP2 for PDE6 and its ability to regulate PDE6 activity in its dark-adapted state suggest a novel role for GARP2 as a regulator of spontaneous activation of rod PDE6, thereby serving to lower rod photoreceptor "dark noise" and allowing these sensory cells to operate at the single photon detection limit.

Efficacy and selectivity of phosphodiesterase-targeted drugs in inhibiting photoreceptor phosphodiesterase (PDE6) in retinal photoreceptors

PURPOSE

Phosphodiesterase (PDE) inhibitors are important therapeutic agents, but their effects on photoreceptor PDE (PDE6) and photoreceptor cells are poorly understood. The potency and selectivity of various classes of PDE inhibitors on purified rod and cone PDE6 and on intact rod outer segments (ROS) were characterized.

METHODS

The inhibition constant (K(i)) of isozyme-selective PDE inhibitors was determined for purified rod and cone PDE6. Perturbations of cGMP levels in isolated ROS suspensions by PDE inhibitors were quantitated by a cGMP enzyme-linked immunoassay.

RESULTS

Most PDE5-selective inhibitors were excellent PDE6 inhibitors. Vardenafil, a potent PDE5 inhibitor (K(i) = 0.2 nM), was the most potent PDE6 inhibitor tested (K(i) = 0.7 nM). Zaprinast was the only drug that inhibited PDE6 more potently than did PDE5. PDE1-selective inhibitors were equally effective in inhibiting PDE6. In intact ROS, PDE inhibitors elevated cGMP levels, but none fully inhibited PDE6. Their potency for elevating cGMP levels in ROS was much lower than their ability to inhibit the purified enzyme. Competition between PDE5/6-selective drugs and the inhibitory gamma-subunit for the active site of PDE6 is proposed to reduce the effectiveness of drugs at the enzyme-active site.

CONCLUSIONS

Several classes of PDE inhibitors inhibit PDE6 equally as well as the PDE family to which they are targeted. In intact ROS, high PDE6 concentrations, binding of the gamma-subunit to the active site, and calcium feedback mechanisms attenuate the effectiveness of PDE inhibitors to inhibit PDE6 and disrupt the cGMP signaling pathway during visual transduction.

Cyclic guanosine 5'-monophosphate binding to regulatory GAF domains of photoreceptor phosphodiesterase

Of the 11 families of mammalian cyclic nucleotide phosphodiesterases (PDEs), 5 contain regulatory domains capable of binding cyclic guanosine 5'-monophosphate (cGMP). The best understood of the GAF-containing PDEs is the family of rod (PDE6R) and cone (PDE6C) photo receptor PDEs. Binding of cGMP to the rod PDE6 catalytic dimer (alphabeta) allosterically regulates the affinity of the inhibitory subunits of PDE6 (gamma) for the enzyme. Two nonidentical, high- affinity cGMP-binding sites exist on the nonactivated mammalian PDE6R holoenzyme (alphabetagammagamma). One of the sites does not readily exchange with free cGMP when the catalytic dimer is complexed with Pgamma. On dissociation of gamma from the catalytic dimer, one of the two cGMP-binding sites undergoes a transition from high to low affinity. This chapter describes techniques to quantify cGMP binding to PDE6 in order to study the regulatory significance of the GAF domains. For high-affinity cGMP binding sites on PDE6, membrane filtration is the method of choice because of its speed, simplicity, and sensitivity. However, lower-affinity cGMP-binding sites require a method that does not perturb the equilibrium between bound and free ligand. The use of ammonium sulfate solutions during filtration extends to lower-binding affinities the useful range of membrane filtration. However, a centrifugal separation technique that minimizes perturbation of the cGMP-binding equilibrium is also presented for measuring lower-affinity cGMP-binding sites. These methods are applicable to understanding the regulatory mechanisms regulating other GAF-containing PDEs as well.

Purification of PDE6 isozymes from mammalian retina

The photoreceptor phosphodiesterase (PDE6) is the central effector of visual transduction in vertebrate retinal photoreceptors. Distinct isozymes of PDE6 exist in rods and cones. Mammalian retina serves as an abundant source of tissue for PDE6 purification. Methods are described for the isolation and purification of membrane-associated PDE6 from rod outer segment membranes. Purification of cone PDE6 from the soluble fraction of retinal extracts is also described. Several procedures that can purify the rod and cone isozymes to homogeneity, including anion exchange, hydrophobic interaction, gel filtration, hydroxyapatite, and immunoaffinity chromatography, are presented. A method to activate PDE6 by limited proteolysis of its inhibitory gamma-subunit is also provided.

Rod and cone opsin families differ in spectral tuning domains but not signal transducing domains as judged by saturated evolutionary trace analysis

The visual receptor of rods and cones is a covalent complex of the apoprotein, opsin, and the light-sensitive chromophore, 11-cis-retinal. This pigment must fulfill many functions including photoactivation, spectral tuning, signal transmission, inactivation, and chromophore regeneration. Rod and cone photoreceptors employ distinct families of opsins. Although it is well known that these opsin families provide unique ranges in spectral sensitivity, it is unclear whether the families have additional functional differences. In this study, we use evolutionary trace (ET) analysis of 188 vertebrate opsin sequences to identify functionally important sites in each opsin family. We demonstrate the following results. (1) The available vertebrate opsin sequences produce a definitive description of all five vertebrate opsin families. This is the first demonstration of sequence saturation prior to ET analysis, which we term saturated ET (SET). (2) The cone opsin classes have class-specific sites compared to the rod opsin class. These sites reside in the transmembrane region and tune the spectral sensitivity of each opsin class to its characteristic wavelength range. (3) The cytoplasmic loops, primarily responsible for signal transmission and inactivation, are essentially invariant in rod versus cone opsins. This indicates that the electrophysiological differences between rod and cone photoreceptors cannot be ascribed to differences in the protein interaction regions of the opsins. SET shows that chromophore binding and regeneration are the only aspects of opsin structure likely to have functionally significant differences between rods and cones, whereas excitatory and adaptational properties of the opsin families appear to be functionally invariant.

cGMP signaling in vertebrate retinal photoreceptor cells

The visual transduction pathway in vertebrate photoreceptors transforms a light stimulus entering the photoreceptor outer segments into an electrical response at the synapses of rod and cone photoreceptor cells. This process is mediated by complex biochemical pathways that precisely regulate cGMP levels, thereby controlling the extent, duration, and adaptation of the photoreceptor to the light stimulus. This review first summarizes the major mechanisms of regulating cytoplasmic cGMP levels (synthesis, degradation, buffering, and efflux) as well as the primary targets of action of cGMP (cyclic nucleotide-gated ion channels, cGMP-dependent protein kinase, and cGMPregulated phosphodiesterases). This information is applied to our current understanding of how these processes operate in the signal-transducing outer segment of rod and cone photoreceptors to carry out visual excitation, recovery, and adaptation in response to light stimulation.

Evaluation of the 17-kDa prenyl-binding protein as a regulatory protein for phototransduction in retinal photoreceptors

The mammalian rod photoreceptor phosphodiesterase (PDE6) holoenzyme is isolated in both a membrane-associated and a soluble form. Membrane binding is a consequence of prenylation of PDE6 catalytic subunits, whereas soluble PDE6 is purified with a 17-kDa prenyl-binding protein (PDEdelta) tightly bound. This protein, here termed PrBP/delta, has been hypothesized to reduce activation of PDE6 by transducin, thereby desensitizing the photoresponse. To test the potential role of PrBP/delta in regulating phototransduction, we examined the abundance, localization, and potential binding partners of PrBP/delta in retina and in purified rod outer segment (ROS) suspensions whose physiological and biochemical properties are well characterized. The amphibian homologue of PrBP/delta was cloned and sequenced and found to have 82% amino acid sequence identity with mammalian PrBP/delta. In contrast to bovine ROS, all of the PDE6 in purified frog ROS is membrane-associated. However, addition of recombinant frog PrBP/delta can solubilize PDE6 and prevent its activation by transducin. PrBP/delta also binds other prenylated photoreceptor proteins in vitro, including opsin kinase (GRK1/GRK7) and rab8. Quantitative immunoblot analysis of the PrBP/delta content of purified ROS reveals insufficient amounts of PrBP/delta (<0.1 PrBP/delta per PDE6) to serve as a subunit of PDE6 in either mammalian or amphibian photoreceptors. The immunolocalization of PrBP/delta in frog and bovine retina shows greatest PrBP/delta immunolabeling outside the photoreceptor cell layer. Within photoreceptors, only the inner segments of frog double cones are strongly labeled, whereas bovine photoreceptors reveal more PrBP/delta labeling near the junction of the inner and outer segments (connecting cilium) of photoreceptors. Together, these results rule out PrBP/delta as a PDE6 subunit and implicate PrBP/delta in the transport and membrane targeting of prenylated proteins (including PDE6) from their site of synthesis in the inner segment to their final destination in the outer segment of rods and cones.

Characteristics of photoreceptor PDE (PDE6): similarities and differences to PDE5

Phosphodiesterase 6 (PDE6) is highly concentrated in the retina. It is most abundant in the internal membranes of retinal photoreceptors, where it reduces cytoplasmic levels of cyclic guanosine monophosphate (cGMP) in rod and cone outer segments in response to light. The rod PDE6 holoenzyme comprises alpha and beta catalytic subunits and two identical inhibitory gamma subunits. Each catalytic subunit contains three distinct globular domains corresponding to the catalytic domain and two GAF domains (responsible for allosteric cGMP binding). The PDE6 catalytic subunits resemble PDE5 in amino-acid sequence as well as in three-dimensional structure of the catalytic dimer; preference for cGMP over cyclic adenosine monophosphate (cAMP) as a substrate; and the ability to bind cGMP at the regulatory GAF domains. Most PDE5 inhibitors inhibit PDE6 with similar potency, and electroretinogram studies show modest effects of PDE5 inhibitors on visual function-an observation potentially important in designing PDE5-specific therapeutic agents.

Regulation of photoreceptor phosphodiesterase (PDE6) by phosphorylation of its inhibitory gamma subunit re-evaluated

Phosphorylation of the inhibitory gamma subunit (Pgamma) of rod cGMP phosphodiesterase (PDE6) has been reported to turn off visual excitation without the requirement for inactivation of the photoreceptor G-protein transducin. We evaluated the significance of Pgamma phosphorylation for PDE6 regulation by preparing Pgamma stoichiometrically phosphorylated at Thr(22) or at Thr(35). Phosphorylation of Pgamma at either residue caused a minor decrease--not the previously reported increase--in the ability of Pgamma to inhibit catalysis at the active site of purified PDE6 catalytic dimers. Likewise, Pgamma phosphorylation had little effect on its potency to inhibit transducin-activated PDE6 depleted of its endogenous Pgamma subunits. The strength of Pgamma interaction with the regulatory GAF domain of PDE6 was reduced severalfold upon Pgamma phosphorylation at Thr(22) (but not Thr(35)), as judged by allosteric changes in cGMP binding to these noncatalytic sites on the enzyme (Mou, H., and Cote, R. H. (2001) J. Biol. Chem. 276, 27527-27534). In contrast, the effects of Pgamma phosphorylation on its interactions with activated transducin were much more pronounced. Phosphorylation of Pgamma at either Thr(22) or Thr(35) greatly diminished its ability to bind activated transducin, consistent with earlier work. In situ phosphorylation of Pgamma by endogenous rod outer segment kinases was enhanced severalfold upon light activation, but only approximately 10% of the endogenous Pgamma was phosphorylated. This is attributed to Pgamma being a poor substrate for protein kinases when associated with the PDE6 holoenzyme. We conclude that, contrary to previous reports, Pgamma phosphorylation at either Thr(22) or Thr(35) modestly weakens its direct interactions with PDE6. However, Pgamma phosphorylation subsequent to its dissociation from PDE6 is likely to abolish its binding to activated transducin and may serve to make phosphorylated Pgamma available to regulate other signal transduction pathways (e.g. mitogen-activated protein kinase; Wan, K. F., Sambi, B. S., Frame, M., Tate, R., and Pyne, N. J. (2001) J. Biol. Chem. 276, 37802-37808) in photoreceptor cells.

Molecular organization of bovine rod cGMP-phosphodiesterase 6

Phosphodiesterase 6 (PDE6), a multisubunit (alphabetagamma(2)delta) enzyme, plays a major role in visual function by hydrolysing cGMP in response to a light stimulus. Solubilized bovine rod PDE6 molecules depleted of their gamma subunits were purified to homogeneity from bovine retinal rods and their molecular organization was investigated by electron microscopy. Image analysis of single particles revealed the three-dimensional dimeric arrangement of the purified alphabetadelta complex, and the internal organization of each catalytic subunit into three distinct domains at a resolution of 2.8 nm. The relative volume of each domain is consistent with sequence analysis and functional data, which suggest that these domains correspond to the catalytic and two GAF domains. This hypothesis was confirmed by immunolabelling experiments, which located the N-terminal part of the catalytic subunit where the major interaction between the two alphabeta subunits was found to occur. The 3D molecular organization of human platelet PDE5 appears highly homologous to that of bovine rod PDE6, as predicted by similarities in their primary sequences. These observations describe the quaternary organization of the catalytic PDE6 alphabeta complex, and place the catalytic and regulatory domains on a structural model.

The Catalytic and GAF Domains of the Rod cGMP Phosphodiesterase (PDE6) Heterodimer Are Regulated by Distinct Regions of Its Inhibitory γ Subunit

The central effector of visual transduction in retinal rod photoreceptors, cGMP phosphodiesterase (PDE6), is a catalytic heterodimer (alphabeta) to which low molecular weight inhibitory gamma subunits bind to form the nonactivated PDE holoenzyme (alphabetagamma(2)). Although it is known that gamma binds tightly to alphabeta, the binding affinity for each gamma subunit to alphabeta, the domains on gamma that interact with alphabeta, and the allosteric interactions between gamma and the regulatory and catalytic regions on alphabeta are not well understood. We show here that the gamma subunit binds to two distinct sites on the catalytic alphabeta dimer (K(D)(1) < 1 pm, K(D)(2) = 3 pm) when the regulatory GAF domains of bovine rod PDE6 are occupied by cGMP. Binding heterogeneity of gamma to alphabeta is absent when cAMP occupies the noncatalytic sites. Two major domains on gamma can interact independently with alphabeta with the N-terminal half of gamma binding with 50-fold greater affinity than its C-terminal, inhibitory region. The N-terminal half of gamma is responsible for the positive cooperativity between gamma and cGMP binding sites on alphabeta but has no effect on catalytic activity. Using synthetic peptides, we identified regions of the amino acid sequence of gamma that bind to alphabeta, restore high affinity cGMP binding to low affinity noncatalytic sites, and retard cGMP exchange with both noncatalytic sites. Subunit heterogeneity, multiple sites of gamma interaction with alphabeta, and positive cooperativity of gamma with the GAF domains are all likely to contribute to precisely controlling the activation and inactivation kinetics of PDE6 during visual transduction in rod photoreceptors.

Mechanism of transducin activation of frog rod photoreceptor phosphodiesterase. Allosteric interactiona between the inhibitory gamma subunit and the noncatalytic cGMP-binding sites

The rod photoreceptor phosphodiesterase (PDE) is unique among all known vertebrate PDE families for several reasons. It is a catalytic heterodimer (alphabeta); it is directly activated by a G-protein, transducin; and its active sites are regulated by inhibitory gamma subunits. Rod PDE binds cGMP at two noncatalytic sites on the alphabeta dimer, but their function is unclear. We show that transducin activation of frog rod PDE introduces functional heterogeneity to both the noncatalytic and catalytic sites. Upon PDE activation, one noncatalytic site is converted from a high affinity to low affinity state, whereas the second binding site undergoes modest decreases in binding. Addition of gamma to transducin-activated PDE can restore high affinity binding as well as reducing cGMP exchange kinetics at both sites. A strong correlation exists between cGMP binding and gamma binding to activated PDE; dissociation of bound cGMP accompanies gamma dissociation from PDE, whereas addition of either cGMP or gamma to alphabeta dimers can restore high affinity binding of the other molecule. At the active site, transducin can activate PDE to about one-half the turnover number for catalytic alphabeta dimers completely lacking bound gamma subunit. These results suggest a mechanism in which transducin interacts primarily with one PDE catalytic subunit, releasing its full catalytic activity as well as inducing rapid cGMP dissociation from one noncatalytic site. The state of occupancy of the noncatalytic sites on PDE determines whether gamma remains bound to activated PDE or dissociates from the holoenzyme, and may be relevant to light adaptation in photoreceptor cells.

cGMP binding to noncatalytic sites on mammalian rod photoreceptor phosphodiesterase is regulated by binding of its gamma and delta subunits

The binding of cGMP to the noncatalytic sites on two isoforms of the phosphodiesterase (PDE) from mammalian rod outer segments has been characterized to evaluate their role in regulating PDE during phototransduction. Nonactivated, membrane-associated PDE (PDE-M, alpha beta gamma2) has one exchangeable site for cGMP binding; endogenous cGMP remains nonexchangeable at the second site. Non-activated, soluble PDE (PDE-S, alpha beta gamma2 delta) can release and bind cGMP at both noncatalytic sites; the delta subunit is likely responsible for this difference in cGMP exchange rates. Removal of the delta and/or gamma subunits yields a catalytic alphabeta dimer with identical catalytic and binding properties for both PDE-M and PDE-S as follows: high affinity cGMP binding is abolished at one site (KD >1 microM); cGMP binding affinity at the second site (KD approximately 60 nM) is reduced 3-4-fold compared with the nonactivated enzyme; the kinetics of cGMP exchange to activated PDE-M and PDE-S are accelerated to similar extents. The properties of nonactivated PDE can be restored upon addition of gamma subunit. Occupancy of the noncatalytic sites by cGMP may modulate the interaction of the gamma subunit with the alphabeta dimer and thereby regulate cytoplasmic cGMP concentration and the lifetime of activated PDE during visual transduction in photoreceptor cells.

Regulation of photoreceptor phosphodiesterase catalysis by its non-catalytic cGMP-binding sites

The photoreceptor 3':5'-cyclic nucleotide phosphodiesterase (PDE) is the central enzyme of visual excitation in rod photoreceptors. The hydrolytic activity of PDE is precisely regulated by its inhibitory gamma subunit (Pgamma), which binds directly to the catalytic site. We examined the inhibition of frog rod outer segment PDE by endogenous Pgamma, as well as by synthetic peptides corresponding to its central and C-terminal domains, to determine whether the non-catalytic cGMP-binding sites on the catalytic alphabeta dimer of PDE allosterically regulate PDE activity. We found that the apparent binding affinity of Pgamma for PDE was 28 pM when cGMP occupied the non-catalytic sites, whereas Pgamma had an apparent affinity only 1/16 of this when the sites were empty. The elevated basal activity of PDE with empty non-catalytic sites can be decreased by the addition of nanomolar levels of cGMP, demonstrating that the high-affinity non-catalytic sites on the PDE catalytic dimer mediate this effect. No evidence for a direct allosteric effect of the non-catalytic sites on catalysis could be detected for the activated enzyme lacking bound Pgamma. The intrinsic affinity of a synthetic C-terminal (residues 63-87) Pgamma peptide to bind and to inhibit the hydrolytic activity of activated PDE was enhanced 300-fold in the presence of cGMP compared with cAMP. We conclude that the binding of cGMP to the non-catalytic sites of PDE induces an allosteric change in the structure of the catalytic domain that greatly enhances the interaction of the C-terminus of Pgamma with the catalytic domain.

Potency and mechanism of action of E4021, a type 5 phosphodiesterase isozyme-selective inhibitor, on the photoreceptor phosphodiesterase depend on the state of activation of the enzyme

The ability of inhibitors selective for the type 5 phosphodiesterase isozyme (PDE5) to act on the photoreceptor PDE isozyme (PDE6, the central effector enzyme for visual transduction) is poorly understood. Because PDE5 inhibitors are currently used as therapeutic agents, it is important to assess the potency and mechanism of action of this class of PDE inhibitor on PDE6. We show that E4021 (sodium 1-[6-chloro-4-(3, 4-methylenedioxybenzyl)-aminoquinazolin-2-yl]piperidine-4-ca rboxylate sesquihydrate) inhibits activated PDE6 (KI = 1.7 nM) as potently as PDE5. This makes E4021 the most potent inhibitor of PDE6 discovered to date. The effectiveness of E4021 to inhibit nonactivated PDE6 (with bound inhibitory gamma subunits) is reduced 40-fold compared with the activated enzyme. Furthermore, at intermediate E4021 concentrations and high cGMP concentrations, nonactivated PDE undergoes activation of cGMP hydrolysis rather than inhibition. We demonstrate direct competition of E4021 and the gamma subunits for binding to the catalytic site. Measurements of cGMP binding to noncatalytic regulatory sites on the catalytic subunits of PDE6 rule out an allosteric effect of E4021 by direct binding to these noncatalytic sites. We conclude that E4021 is a competitive inhibitor of cGMP hydrolysis and that the gamma subunit also competes with both E4021 and substrate for catalytic site binding. An understanding of the effects of PDE5-targeted drugs on retinal PDE6 requires a knowledge of the complex interactions among substrate, drug, and inhibitory gamma subunit at the catalytic site of both nonactivated and activated forms of PDE6.

Structural features of the noncatalytic cGMP binding sites of frog photoreceptor phosphodiesterase using cGMP analogs

The cGMP-specific phosphodiesterase (PDE) of retinal photoreceptors is a central regulatory enzyme in the visual transduction pathway of vertebrate vision. Although the mechanism of activation of PDE by transducin is well understood, the role of the noncatalytic cGMP binding sites located on the catalytic subunits of PDE remains obscure. We report here for the first time the molecular basis of the noncovalent interactions between cGMP and the high affinity, noncatalytic cGMP binding sites of frog photoreceptor PDE. None of the tested cGMP analogs were able to bind with greater affinity than cGMP itself, and the noncatalytic sites were unable to bind cAMP. The major determinant for discrimination of cGMP over cAMP is in the N-1/C-6 region of the purine ring of cGMP where hydrogen bonding probably stabilizes the selective binding of cGMP. Substitutions at the C-2 position demonstrate that this region of the molecule plays a secondary but significant role in stabilizing cGMP binding to PDE through hydrogen bond interactions. The unaltered hydrogen at the C-8 position is also important for high affinity binding. A significant interaction between the binding pocket and the ribose ring of cGMP occurs at the 2'-hydroxyl position. Steric constraints were greatest in the C-8 and possibly the C-6/N-1 regions, whereas the C-2/N-3 and C-2' regions tolerated bulky substituents better. Several lines of evidence indicate that the noncatalytic site binds cGMP in the anti-conformation. The numerous noncovalent interactions between cGMP and the noncatalytic binding pocket of the photoreceptor PDE described in this study account for both the high affinity for cGMP and the high level of discrimination of cGMP from other cyclic nucleotides at the noncatalytic site.

Photoreceptor phosphodiesterase: interaction of inhibitory gamma subunit and cyclic GMP with specific binding sites on catalytic subunits

The photoreceptor phosphodiesterase (PDE6) is the central effector enzyme in the phototransduction cascade of photoreceptor cells. It is the only known PDE isoform the activity of which is regulated by interaction with a heterotrimeric G protein. The rod PDE6 holoenzyme is a tetrameric protein consisting of two large catalytic alpha and beta subunits and two small gamma subunits, which serve as potent inhibitors of PDE6. In dark-adapted photoreceptors, the gamma subunits maintain PDE6 activity at a low level. When exposed to light the visual pigment rhodopsin activates the retinal G protein, transducin, leading to release of the inhibitory action of the gamma subunits. In addition to the active sites where cGMP is hydrolyzed, the alpha and beta catalytic subunits have high-affinity, noncatalytic cGMP binding sites. These noncatalytic sites do not directly regulate cGMP catalysis at the active site, but rather can modulate the affinity with which the gamma subunits bind to the catalytic subunits. This article describes a number of experimental approaches that have recently been developed for studying the interactions between catalytic and inhibitory subunits of PDE6, as well as the dynamics of cGMP binding to and dissociation from the PDE6 noncatalytic sites.

cGMP binding sites on photoreceptor phosphodiesterase: role in feedback regulation of visual transduction

A central step in vertebrate visual transduction is the rapid drop in cGMP levels that causes cGMP-gated ion channels in the photoreceptor cell membrane to close. It has long been a puzzle that the cGMP phosphodiesterase (PDE) whose activation causes this decrease contains not only catalytic sites for cGMP hydrolysis but also noncatalytic cGMP binding sites. Recent work has shown that occupancy of these noncatalytic sites slows the rate of PDE inactivation. We report here that PDE activation induced by activated transduction lowers the cGMP binding affinity for noncatalytic sites on PDE and accelerates the dissociation of cGMP from these sites. These sites can exist in three states: high affinity (Kd = 60 nM) for the nonactivated PDE, intermediate affinity (Kd approximately 180 nM) when the enzyme is activated in a complex with transducin, and low affinity (Kd > 1 microM) when transducin physically removes the inhibitory subunits of PDE from the PDE catalytic subunits. Activation of PDE by transducin causes a 10-fold increase in the rate of cGMP dissociation from one of the two noncatalytic sites; physical removal of the inhibitory subunits from the PDE catalytic subunits further accelerates the cGMP dissociation rate from both sites > 50-fold. Because PDE molecules lacking bound cGMP inactivate more rapidly, this suggests that a prolonged cGMP decrease may act as a negative feedback regulator to generate the faster, smaller photoresponses characteristic of light-adapted photoreceptors.

Regulation of intracellular cyclic GMP concentration by light and calcium in electropermeabilized rod photoreceptors

This study examines the regulation of cGMP by illumination and by calcium during signal transduction in vertebrate retinal photoreceptor cells. We employed an electropermeabilized rod outer segment (EP-ROS) preparation which permits perfusion of low molecular weight compounds into the cytosol while retaining many of the features of physiologically competent, intact rod outer segments (ROS). When nucleotide-depleted EP-ROS were incubated with MgGTP, time- and dose-dependent increases in intracellular cGMP levels were observed. The steady state cGMP concentration in EP-ROS (0.007 mol cGMP per mol rhodopsin) approached the cGMP concentration in intact ROS. Flash illumination of EP-ROS in a 250-nM free calcium medium resulted in a transient decrease in cGMP levels; this occurred in the absence of changes in calcium concentration. The kinetics of the cGMP response to flash illumination of EP-ROS were similar to that of intact ROS. To further examine the effects of calcium on cGMP metabolism, dark-adapted EP-ROS were incubated with MgGTP containing various concentrations of calcium. We observed a twofold increase in cGMP steady state levels as the free calcium was lowered from 1 microM to 20 nM; this increase was comparable to the behavior of intact ROS. Measurements of guanylate cyclase activity in EP-ROS showed a 3.5-fold increase in activity over this range of calcium concentrations, indicating a retention of calcium regulation of guanylate cyclase in EP-ROS preparations. Flash illumination of EP-ROS in either a 50- or 250-nM free calcium medium revealed a slowing of the recovery time course at the lower calcium concentration. This observation conflicts with any hypothesis whereby a reduction in free calcium concentration hastens the recovery of cytoplasmic cGMP levels, either by stimulating guanylate cyclase activity or by inhibiting phosphodiesterase activity. We conclude that changes in the intracellular calcium concentration during visual transduction may have more complex effects on the recovery of the photoresponse than can be accounted for solely by guanylate cyclase activation.

Intracellular cGMP concentration in rod photoreceptors is regulated by binding to high and moderate affinity cGMP binding sites

cGMP is the second messenger for visual excitation in vertebrate rod photoreceptors. However, no direct correlation has been observed between the measured total cGMP concentration in the rod outer segment and the electrical response of these cells to photic stimulation. To address this discrepancy, we have quantitated the number and affinities of cGMP binding sites in the rod outer segment to determine the cytoplasmic free cGMP concentration that is involved in visual transduction. We identified two distinct classes of cGMP binding sites in amphibian rod outer segments: 1) high affinity binding sites with a KD1 = 60 nM and a site density of 30 microM, and 2) moderate affinity binding sites with a KD2 = 6.6 microM and a site density of 78 microM. These two classes of binding sites are calculated to bind 94% of the total cellular cGMP, thereby lowering the cytoplasmic cGMP concentration to 3.5 microM in dark-adapted rod outer segments. This value is consistent with predictions of the cytoplasmic cGMP concentration based on activation of the cGMP-gated ion channel of rod photoreceptors. The kinetics of cGMP dissociation from high affinity binding sites indicate that this class of sites would dissociate its bound cGMP too slowly to participate in visual excitation and recovery to flash illumination. This binding of cGMP to intracellular binding sites provides a non-enzymatic mechanism by which photoreceptor cells regulate the concentration and restrict the diffusion of this second messenger during visual transduction.

Mechanisms of amplification, deactivation, and noise reduction in invertebrate photoreceptors

In this review we have discussed the problem of deactivation at both the rhodopsin and G protein levels. Of particular interest is the novel observation that rhodopsin deactivation can be modulated by light. This modulation is likely to play an important role in light adaptation by reducing the gain of transduction. One interesting possibility is that this modulation involves the phosphorylation of an arrestin-like molecule, but this remains to be tested. One of the experimental advantages of Limulus photoreceptors is the large size of the single photon responses and the fact that even single G proteins produce a detectable response. This made possible the observation that nonhydrolyzable GTP analogues produce discrete transient events rather than the step-like events that would be predicted by previous models. This observation led us to a new view of how enzyme deactivation is coupled to GTP hydrolysis on G protein. According to this view, enzymes are activated by G protein, but can be deactivated by processes that are not dependent on G protein or the hydrolysis of GTP. We have conducted several types of experiments, including some on the vertebrate rod system, that strongly support this hypothesis. A second major theme of this review is transduction noise. The available biochemical evidence suggests that both G protein and G protein-activated enzymes are likely to become spontaneously active and generate undesirable noise. Our measurements indicate, however, that this noise is orders of magnitude smaller than would be predicted by simple models, suggesting that special mechanisms must exist for suppressing this noise. We have proposed a specific mechanism by which enzymes regulated allosterically by multiple subunits could act as coincidence detectors to reduce transduction noise. Finally, there is the fundamental question of which second messengers have a direct role in invertebrate phototransduction. After Fesenko et al. (1985) showed that the light-dependent conductance in vertebrate rods was modulated by cGMP and not by Ca2+, there was rapid progress in understanding the vertebrate photoreceptor transduction mechanism. Now that it has been established that invertebrate light-dependent channels are regulated by cGMP and not by Ca2+, we can expect rapid progress in understanding invertebrate phototransduction. A key question that needs to be answered is whether the InsP3-Ca2+ pathway somehow triggers changes in cGMP or whether there is an altogether different pathway by which cGMP metabolizing enzymes are affected by light.

Cyclic GMP levels and membrane current during onset, recovery, and light adaptation of the photoresponse of detached frog photoreceptors

We have used a preparation of isolated, intact rod photoreceptors to correlate the effects of flash illumination on the intracellular cyclic GMP content and the membrane current. We find that the recovery of cyclic GMP levels after brief flash illumination requires approximately twice as much time as the recovery of the membrane current. In contrast, the subsecond kinetics of the cyclic GMP response to light are faster than the kinetics of membrane current suppression. Both cyclic GMP and the membrane current show graded responses to a wide range of flash intensities; however, in a low Ca2+-Ringer's solution, dim flashes can trigger a decrease in cyclic GMP concentration with no corresponding decrease in membrane current. These results suggest that either other factors can regulate the membrane current, or that measurements of total cellular cyclic GMP do not accurately reflect dynamic changes in cyclic GMP concentration in the vicinity of the light-regulated channel. Changes in cyclic GMP concentration in the presence of background illumination exhibit adaptational behavior similar to that observed in a light-adapted photoresponse: acceleration in the response kinetics and a decrease in response amplitude. That this result is observed in rods depleted of internal Ca2+ suggests a Ca2+-independent mechanism by which background illumination can accelerate the cyclic GMP response.

Characterization of guanylate cyclase in squid photoreceptors

Light causes a rapid, 1.7-fold increase in cyclic GMP concentration in intact squid retinas (Johnson et al. (1986]. To determine whether light-induced changes in cyclic GMP concentration result from activation of guanylate cyclase, we have studied the regulation of guanylate cyclase activity in squid (Loligo pealei) photoreceptors. The enzyme is membrane-associated and activity is enhanced by the detergents Triton X-100 or digitonin. The enzyme requires divalent cations, Mn2+ being preferred over Mg2+. The dependence of enzyme activity on the MnGTP concentration deviates from simple Michaelis-Menten kinetics. Under conditions where a light-induced binding of GTP to the guanine nucleotide regulatory protein can be observed, no light-induced change in guanylate cyclase could be detected.

Changes in cGMP concentration correlate with some, but not all, aspects of the light-regulated conductance of frog rod photoreceptors

Cyclic GMP has been implicated in controlling the light-regulated conductance of rod photoreceptors of the vertebrate retina. However, there is little direct evidence correlating changes in cGMP concentration with the light-regulated permeability mechanism in living cells. A preparation of intact frog rod outer segments suspended in a Ringer's medium containing low Ca2+ has been used to demonstrate that initial changes in total cellular cGMP concentration parallel changes in the light-regulated membrane current over a wide range of light intensities. At light intensities bleaching from 160 to 5.6 X 10(6) rhodopsin molecules/rod/s, decreases in the response latency for the cGMP kinetics parallel decreases in the latent period of the electrical response. Further, changes in the rate of the cGMP decrease parallel the rate of membrane current suppression as the light intensity is varied. Up to 10(5) cGMP molecules are hydrolyzed per photolyzed rhodopsin, consistent with in vitro studies showing that each bleached rhodopsin can activate over 100 phosphodiesterase molecules. Addition of the Ca2+ ionophore, A23187, does not affect the initial kinetics of the cGMP decrease or of the electrical response, excluding a direct role for Ca2+ in the initial events of phototransduction. These results are consistent with cGMP being the intracellular messenger that links rhodopsin isomerization with changes in membrane permeability upon illumination. It is unlikely, however, that light-induced changes in total cGMP concentration are the sole regulators of membrane current. This is suggested by several observations: at bright light intensities, the subsecond light-induced cGMP decrease is essentially complete prior to complete suppression of membrane current; maximal light-induced decreases in cGMP concentration occur at all light intensities tested, whereas the extent of membrane current suppression varies over the same range of light intensities; changing the external Ca2+ concentration from 1 mM to 10 nM in the dark causes an increase in membrane current that is significantly more rapid than corresponding changes in cGMP concentration. Thus, light-induced changes in total cellular cGMP concentration correlate with some, but not all, aspects of the visual excitation process in vertebrate photoreceptors.

cGMP influences guanine nucleotide binding to frog photoreceptor G-protein

A rapid light-induced decrease in cGMP is thought to play a role in regulating the permeability or light sensitivity of photoreceptor membranes. Photo-excited rhodopsin activates a guanine nucleotide-binding protein (G-protein) by catalyzing the exchange of bound GDP for GTP. This G-protein X GTP complex activates the phosphodiesterase resulting in a decrease in cGMP concentration. We have observed two processes in vitro which may be relevant for the regulation of G-protein activation. First, we have found that free GDP binds to G-protein with an affinity similar to that of GTP. These two nucleotides appear to compete for a common site. Since G-protein X GDP does not activate phosphodiesterase, light-induced changes in the GTP/GDP ratio known to occur on illumination may serve to reduce G-protein activation and hence reduce phosphodiesterase activation. Second, addition of cGMP in the presence of equimolar GTP and GDP causes GTP binding to G-protein to be enhanced compared to GDP binding. This effect increases as the cGMP concentration is increased from 0.05 to 2 mM. Thus, light-induced decreases in cGMP concentration may also act as a feedback control in reducing G-protein activation. One or both of these processes may be involved in the desensitization (light adaptation) of rod photoreceptors.

Light-induced decreases in cGMP concentration precede changes in membrane permeability in frog rod photoreceptors

This study examines whether changes in cGMP concentration initiated by illumination of frog rod photoreceptors occur rapidly enough to implicate cGMP as an intermediate between rhodopsin activation in the disc membrane and permeability changes in the plasma membrane. Previous studies using whole retinas or isolated outer segments have provided conflicting evidence on the role of cGMP in the initial events of phototransduction. The rod photoreceptor preparation employed in this work consists of purified suspensions of outer segments still attached to the mitochondria-rich ellipsoid portion of the inner segment. These photoreceptors are known to retain normal electrophysiological responses to illumination and have cGMP levels comparable to those measured in the intact retina. When examined under several different conditions, changes in cGMP concentrations were found to occur as rapidly or more rapidly than the suppression of the membrane dark current. Subsecond changes in cGMP concentration were analyzed with a rapid quench apparatus and confirmed by comparison with a rapid freezing technique. In a 1 mM Ca2+ Ringer's solution, cGMP levels decrease to 65% of their final extent within 200 ms after bright illumination; changes in membrane dark current follow a similar time course. When the light intensity is decreased to 8000 rhodopsins bleached per rod per s, the light-induced cGMP decrease is completed within 50 ms, with 7 X 10(5) cGMP molecules hydrolyzed per rhodopsin bleached. During this time the dark current has not yet begun to change. Thus, under physiological conditions it is clear that changes in cGMP concentration precede permeability changes at the plasma membrane. The correlation of rapid changes in cGMP levels with changes in membrane current leave open the possibility that changes in cGMP concentration may be an obligatory step in the reaction sequence linking rhodopsin activation by light and the resultant decrease in sodium permeability of the plasma membrane.