The retinal cGMP phosphodiesterase gamma-subunit - a chameleon

Photoreceptor Phosphodiesterase (PDE6): Structure, Regulatory Mechanisms, and Implications for Treatment of Retinal Diseases

The photoreceptor phosphodiesterase (PDE6) is a member of large family of Class I phosphodiesterases responsible for hydrolyzing the second messengers cAMP and cGMP. PDE6 consists of two catalytic subunits and two inhibitory subunits that form a tetrameric protein. PDE6 is a peripheral membrane protein that is localized to the signal-transducing compartment of rod and cone photoreceptors. As the central effector enzyme of the G-protein coupled visual transduction pathway, activation of PDE6 catalysis causes a rapid decrease in cGMP levels that results in closure of cGMP-gated ion channels in the photoreceptor plasma membrane. Because of its importance in the phototransduction pathway, mutations in PDE6 genes result in various retinal diseases that currently lack therapeutic treatment strategies due to inadequate knowledge of the structure, function, and regulation of this enzyme. This review focuses on recent progress in understanding the structure of the regulatory and catalytic domains of the PDE6 holoenzyme, the central role of the multi-functional inhibitory γ-subunit, the mechanism of activation by the heterotrimeric G protein, transducin, and future directions for pharmacological interventions to treat retinal degenerative diseases arising from mutations in the PDE6 genes.

Photoreceptor phosphodiesterase (PDE6): activation and inactivation mechanisms during visual transduction in rods and cones

Rod and cone photoreceptors of the vertebrate retina utilize cGMP as the primary intracellular messenger for the visual signaling pathway that converts a light stimulus into an electrical response. cGMP metabolism in the signal-transducing photoreceptor outer segment reflects the balance of cGMP synthesis (catalyzed by guanylyl cyclase) and degradation (catalyzed by the photoreceptor phosphodiesterase, PDE6). Upon light stimulation, rapid activation of PDE6 by the heterotrimeric G-protein (transducin) triggers a dramatic drop in cGMP levels that lead to cell hyperpolarization. Following cessation of the light stimulus, the lifetime of activated PDE6 is also precisely regulated by additional processes. This review summarizes recent advances in the structural characterization of the rod and cone PDE6 catalytic and regulatory subunits in the context of previous biochemical studies of the enzymological properties and allosteric regulation of PDE6. Emphasis is given to recent advances in understanding the structural and conformational changes underlying the mechanism by which the activated transducin α-subunit binds to-and relieves inhibition of-PDE6 catalysis that is controlled by its intrinsically disordered, inhibitory γ-subunit. The role of the regulator of G-protein signaling 9-1 (RGS9-1) in regulating the lifetime of the transducin-PDE6 is also briefly covered. The therapeutic potential of pharmacological compounds acting as inhibitors or activators targeting PDE6 is discussed in the context of inherited retinal diseases resulting from mutations in rod and cone PDE6 genes as well as other inherited defects that arise from excessive cGMP accumulation in retinal photoreceptor cells.

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.

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.

Expression of phosphodiesterase 6 (PDE6) in human breast cancer cells

Considerable epidemiological evidence demonstrates a positive association between artificial light at night (LAN) levels and incidence rates of breast cancer, suggesting that exposure to LAN is a risk factor for breast cancer. There is a 30-50% higher risk of breast cancer in the highest LAN exposed countries compared to the lowest LAN countries, and studies showing higher incidence of breast cancer among shift workers exposed to more LAN have led the International Agency for Research on Cancer to classify shift work as a probable human carcinogen. Nevertheless, the means by which light can affect breast cancer is still unknown. In this study we examined established human breast cancer cell lines and patients’ primary breast cancer tissues for expression of genetic components of phosphodiesterase 6 (PDE6), a cGMP-specific PDE involved in transduction of the light signal, and previously thought to be selectively expressed in photoreceptors. By microarray analysis we find highly significant expression of mRNA for the PDE6B, PDE6C, and PDE6D genes in both the cell lines and patients’ tissues, minimal expression of PDE6A and PDE6G and no expression of PDE6H. Using antibody specific for PDE6β, we find expression of PDE6B protein in a wide range of patients’ tissues by immunohistochemistry, and in MCF-7 breast cancer cells by immunofluorescence and Western blot analysis. Considerable expression of key circadian genes, PERIOD 2, CLOCK, TIMELESS, CRYPTOCHROME 1, and CRYPTOCHROME 2 was also seen in all breast cancer cell lines and all patients’ breast cancer tissues. These studies indicate that genes for PDE6 and control of circadian rhythm are expressed in human breast cancer cells and tissues and may play a role in transducing the effects of light on breast cancer.

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.

N-terminal half of the cGMP phosphodiesterase gamma-subunit contributes to stabilization of the GTPase-accelerating protein complex

In the visual signal terminating transition state, the cyclic GMP phosphodiesterase (PDE6) inhibitory γ-subunit (PDEγ) stimulates GTPase activity of the α-subunit of transducin (αt) by enhancing the interaction between αt and its regulator of G protein signaling (RGS9), which is constitutively bound to the type 5 G protein β-subunit (β5). Although it is known from a crystal structure of partial molecules that the PDEγ C terminus contacts with both αt and RGS9, contributions from the intrinsically disordered PDEγ N-terminal half remain unclear. In this study, we were able to investigate this issue using a photolabel transfer strategy that allows for mapping the interface of full-length proteins. We observed label transfer from PDEγ N-terminal positions 50, 30, and 16 to RGS9·β5 in the GTPase-accelerating protein (GAP) complex composed of PDEγ·αt·RGS9·β5. In support of a direct PDEγ N-terminal interaction with RGS9·β5, the PDEγ N-terminal peptide PDEγ(1-61) abolished label transfer to RGS9·β5, and another N-terminal peptide, PDEγ(10-30), disassembled the GAP complex in label transfer and pulldown experiments. Furthermore, we determined that the PDEγ C-terminal interaction with αt was enhanced whereas the N-terminal interaction was weakened upon changing the αt conformation from the signaling state to the transition state. This "rearrangement" of PDEγ domain interactions with αt appears to facilitate the interaction of the PDEγ N-terminal half with RGS9·β5 and hence its contribution to optimal stabilization of the GAP complex.

In vitro interaction of tubulin with the photoreceptor cGMP phosphodiesterase gamma-subunit

The alpha and beta tubulins compose the microtubule cytoskeleton which is involved in many cellular processes such as vesicular transport. The photoreceptor cells in the retina are neurons specialized for phototransduction. Here we report a novel interaction between tubulin and the photoreceptor cGMP phosphodiesterase (PDE6) gamma subunit (PDE gamma). The specificity and molecular details of the PDE gamma:tubulin interaction were analyzed through the experiments of pull down, microtubule co-sedimentation, and NMR spectroscopy. The tubulin-interacting site was identified to be in the PDE gamma C-terminal I67-G85 region, and the interaction interface appeared to be distinct from those with the other PDE gamma targets in phototransduction. We also observed that PDE gamma interacted with tubulin in a GTP-dependent manner. Our findings offer implications for non-phototransduction role(s) of PDE gamma in the photoreceptor neurons.

Complementary interactions of the rod PDE6 inhibitory subunit with the catalytic subunits and transducin

Activation of the cyclic GMP phosphodiesterase (PDE6) by transducin is the central event of visual signal transduction. How the PDE6 inhibitory gamma-subunit (Pgamma) interacts with the catalytic subunits (Palphabeta) and the transducin alpha-subunit (alpha(t)) in this process is not entirely clear. Here we have investigated this issue, taking advantage of site-specific label transfer from throughout the full-length Pgamma molecule to both alpha(t) and Palphabeta. The interaction profiling and pull-down experiments revealed that the Pgamma C- terminal domain accounted for the major interaction with alpha(t) bound with guanosine 5'-3-O-(thio)triphosphate (alpha(t)GTPgammaS) in comparison with the central region, whereas an opposite pattern was observed for the Pgamma-Palphabeta interaction. This complementary feature was further exhibited when both alpha(t)GTPgammaS and Palphabeta were present and competing for Pgamma interaction, with the Pgamma C-terminal domain favoring alpha(t), whereas the central region demonstrated a preference for Palphabeta. Furthermore, alpha(t)GTPgammaS co-immunoprecipitated with PDE6 and vice versa in a Pgamma-dependent manner. Either Palphabeta or alpha(t)GTPgammaS could be pulled down by the Btn-Pgamma molecules on streptavidin beads that were saturated by the other partner, indicating simultaneous binding of these two partners to Pgamma. These data together indicate that complementary Pgamma interactions with its two targets facilitate the alpha(t).PDE6 "transducisome" formation. Thus, our study provides new insights into the molecular mechanisms of PDE6 activation.

cGMP-phosphodiesterase 6, transducin and Wnt5a/Frizzled-2-signaling control cGMP and Ca(2+) homeostasis in melanoma cells

Malignant melanoma is one of the most aggressive human neoplasms which develop from the malignant transformation of normal epithelial melanocytes and share the lineage with retinal cells. cGMP-phosphodiesterase 6 (PDE6) is one of the cancer-retina antigens newly identified in melanoma cells. Normally, PDE6 hydrolyzes the photoreceptor second messenger cGMP allowing the visual signal transduction in photoreceptor cells. cGMP also play an important signaling role in stimulating melanogenesis in human melanocytes. Here, we present evidence that PDE6 is a key enzyme regulating the cGMP metabolism in melanoma cells. Decrease in intracellular cGMP leads to calcium accumulation in melanoma cells. In these cells, cGMP-phosphodiesterase 6 can be activated by another cancer-retina antigen, transducin, through Wnt5a-Frizzled-2 cascade, which leads to a lowering of cGMP and an increase in intracellular calcium mobilization. Thus, the aberrant expression of PDE6 may control cGMP metabolism and calcium homeostasis in melanoma cells.

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.