cGMP phosphodiesterase of retinal rods is regulated by two inhibitory subunits

The ubiquitin-like modifier FAT10 inhibits retinal PDE6 activity and mediates its proteasomal degradation

The retina-specific chaperone aryl hydrocarbon interacting protein-like 1 (AIPL1) is essential for the correct assembly of phosphodiesterase 6 (PDE6), which is a pivotal effector enzyme for phototransduction and vision because it hydrolyzes cGMP. AIPL1 interacts with the cytokine-inducible ubiquitin-like modifier FAT10, which gets covalently conjugated to hundreds of proteins and targets its conjugation substrates for proteasomal degradation, but whether FAT10 affects PDE6 function or turnover is unknown. Here, we show that FAT10 mRNA is expressed in human retina and identify rod PDE6 as a retina-specific substrate of FAT10 conjugation. We found that AIPL1 stabilizes the FAT10 monomer and the PDE6-FAT10 conjugate. Additionally, we elucidated the functional consequences of PDE6 FAT10ylation. On the one hand, we demonstrate that FAT10 targets PDE6 for proteasomal degradation by formation of a covalent isopeptide linkage. On the other hand, FAT10 inhibits PDE6 cGMP hydrolyzing activity by noncovalently interacting with the PDE6 GAFa and catalytic domains. Therefore, FAT10 may contribute to loss of PDE6 and, as a consequence, degeneration of retinal cells in eye diseases linked to inflammation and inherited blindness-causing mutations in AIPL1.

Deep Genome Resequencing Reveals Artificial and Natural Selection for Visual Deterioration, Plateau Adaptability and High Prolificacy in Chinese Domestic Sheep

Sheep were one of the earliest domesticated animals. Both artificial and natural selection during domestication has resulted in remarkable changes in behavioral, physiological, and morphological phenotypes; however, the genetic mechanisms underpinning these changes remain unclear, particularly for indigenous Chinese sheep. In the present study, we performed pooled whole-genome resequencing of 338 sheep from five breeds representative of indigenous Chinese breeds and compared them to the wild ancestors of domestic sheep (Asian mouflon, Ovis orientalis) for detection of genome-wide selective sweeps. Comparative genomic analysis between domestic sheep and Asian mouflon showed that selected regions were enriched for genes involved in bone morphogenesis, growth regulation, and embryonic and neural development in domestic sheep. Moreover, we identified several vision-associated genes with funtional mutations, such as PDE6B (c.G2994C/p.A982P and c.C2284A/p.L762M mutations), PANK2, and FOXC1/GMSD in all five Chinese native breeds. Breed-specific selected regions were determined including genes such as CYP17 for hypoxia adaptability in Tibetan sheep and DNAJB5 for heat tolerance in Duolang sheep. Our findings provide insights into the genetic mechanisms underlying important phenotypic changes that have occurred during sheep domestication and subsequent selection.

Cryo-EM structure of phosphodiesterase 6 reveals insights into the allosteric regulation of type I phosphodiesterases

Cyclic nucleotide phosphodiesterases (PDEs) work in conjunction with adenylate/guanylate cyclases to regulate the key second messengers of G protein-coupled receptor signaling. Previous attempts to determine the full-length structure of PDE family members at high-resolution have been hindered by structural flexibility, especially in their linker regions and N- and C-terminal ends. Therefore, most structure-activity relationship studies have so far focused on truncated and conserved catalytic domains rather than the regulatory domains that allosterically govern the activity of most PDEs. Here, we used single-particle cryo-electron microscopy to determine the structure of the full-length PDE6αβ2γ complex. The final density map resolved at 3.4 Å reveals several previously unseen structural features, including a coiled N-terminal domain and the interface of PDE6γ subunits with the PDE6αβ heterodimer. Comparison of the PDE6αβ2γ complex with the closed state of PDE2A sheds light on the conformational changes associated with the allosteric activation of type I PDEs.

Gene Therapy Successfully Delays Degeneration in a Mouse Model of PDE6A-Linked Retinitis Pigmentosa (RP43)

Retinitis pigmentosa type 43 (RP43) is a blinding disease caused by mutations in the gene for rod phosphodiesterase 6 alpha (PDE6A). The disease process begins with a dysfunction of rod photoreceptors, subsequently followed by a currently untreatable progressive degeneration of the entire outer retina. Aiming at a curative approach via PDE6A gene supplementation, a novel adeno-associated viral (AAV) vector was developed for expression of the human PDE6A cDNA under control of the human rhodopsin promotor (rAAV8.PDE6A). This study assessed the therapeutic efficacy of rAAV8.PDE6A in the Pde6a^{nmf363/nmf363}-mutant mouse model of RP43. All mice included in this study were treated with sub-retinal injections of the vector at 2 weeks after birth. The therapeutic effect was monitored at 1 month and 6 months post injection. Biological function of the transgene was assessed in vivo by means of electroretinography. The degree of morphological rescue was investigated both in vivo using optical coherence tomography and ex vivo by immunohistological staining. It was found that the novel rAAV8.PDE6A vector resulted in a stable and efficient expression of PDE6A protein in rod photoreceptors of Pde6a^{nmf363/nmf363} mice following treatment at both the short- and long-term time points. The treatment led to a substantial morphological preservation of outer nuclear layer thickness, rod outer segment structure, and prolonged survival of cone photoreceptors for at least 6 months. Additionally, the ERG analysis confirmed a restoration of retinal function in a group of treated mice. Taken together, this study provides successful proof-of-concept for the cross-species efficacy of the rAAV8.PDE6A vector developed for use in human patients. Importantly, the data show stable expression and rescue effects for a prolonged period of time, raising hope for future translational studies based on this approach.

Unique structural features of the AIPL1-FKBP domain that support prenyl lipid binding and underlie protein malfunction in blindness

FKBP-domain proteins (FKBPs) are pivotal modulators of cellular signaling, protein folding, and gene transcription. Aryl hydrocarbon receptor-interacting protein-like 1 (AIPL1) is a distinctive member of the FKBP superfamily in terms of its biochemical properties, and it plays an important biological role as a chaperone of phosphodiesterase 6 (PDE6), an effector enzyme of the visual transduction cascade. Malfunction of mutant AIPL1 proteins triggers a severe form of Leber congenital amaurosis and leads to blindness. The mechanism underlying the chaperone activity of AIPL1 is largely unknown, but involves the binding of isoprenyl groups on PDE6 to the FKBP domain of AIPL1. We solved the crystal structures of the AIPL1-FKBP domain and its pathogenic mutant V71F, both in the apo form and in complex with isoprenyl moieties. These structures reveal a module for lipid binding that is unparalleled within the FKBP superfamily. The prenyl binding is enabled by a unique "loop-out" conformation of the β4-α1 loop and a conformational "flip-out" switch of the key W72 residue. A second major conformation of apo AIPL1-FKBP was identified by NMR studies. This conformation, wherein W72 flips into the ligand-binding pocket and renders the protein incapable of prenyl binding, is supported by molecular dynamics simulations and appears to underlie the pathogenicity of the V71F mutant. Our findings offer critical insights into the mechanisms that underlie AIPL1 function in health and disease, and highlight the structural and functional diversity of the FKBPs.

Next-generation sequencing targeted disease panel in rod-cone retinal dystrophies in Māori and Polynesian reveals novel changes and a common founder mutation

IMPORTANCE

This study identifies unique genetic variation observed in a cohort of Māori and Polynesian patients with rod-cone retinal dystrophies using a targeted next-generation sequencing retinal disease gene panel.

BACKGROUND

With over 250 retinal disease genes identified, genetic diagnosis is still only possible in 60-70% of individuals and even less within unique ethnic groups.

DESIGN

Prospective genetic testing in patients with rod-cone retinal dystrophies identified from the New Zealand Inherited Retinal Disease Database, PARTICIPANTS: Sixteen patients of Māori and Polynesian ancestry.

METHODS

Next-generation sequencing of a targeted retinal gene panel. Sanger sequencing for a novel PDE6B mutation in subsequent Māori patients.

MAIN OUTCOME MEASURES

Genetic diagnosis, genotype-phenotype correlation.

RESULTS

Thirteen unique pathogenic variants were identified in 9 of 16 (56.25%) patients in 10 different genes. A definitive genetic diagnosis was made in 7/16 patients (43.7%). Six changes were novel and not in public databases of human variation. In four patients, a homozygous, novel pathogenic variant (c.2197G > C, p.(Ala 733Pro)) in PDE6B was identified and also present in a further five similarly affected Māori patients.

CONCLUSIONS AND RELEVANCE

Over half of the Māori and Polynesian patients with inherited rod-cone diseases have no pathogenic variant(s) detected with a targeted retinal next-generation sequencing strategy, which is supportive of novel genetic mechanisms in this population. A novel PDE6B founder variant is likely to account for 16% of recessive inherited retinal dystrophy in Māori. Careful characterization of the clinical presentation permits identification of further Māori patients with a similar phenotype and simplifies the diagnostic algorithm.

Mechanisms of mutant PDE6 proteins underlying retinal diseases

Mutations in PDE6 genes encoding the effector enzymes in rods and cones underlie severe retinal diseases including retinitis pigmentosa (RP), autosomal dominant congenital stationary night blindness (adCSNB), and achromatopsia (ACHM). Here we examined a spectrum of pathogenic missense mutations in PDE6 using the system based on co-expression of cone PDE6C with its specialized chaperone AIPL1 and the regulatory Pγ subunit as a potent co-chaperone. We uncovered two mechanisms of PDE6C mutations underlying ACHM: (a) folding defects leading to expression of catalytically inactive proteins and (b) markedly diminished ability of Pγ to co-chaperone mutant PDE6C proteins thereby dramatically reducing the levels of functional enzyme. The mechanism of the Rambusch adCSNB associated with the H258N substitution in PDE6B was probed through the analysis of the model mutant PDE6C-H262N. We identified two interrelated deficits of PDE6C-H262N: disruption of the inhibitory interaction of Pγ with mutant PDE6C that markedly reduced the ability of Pγ to augment the enzyme folding. Thus, we conclude that the Rambusch adCSNB is triggered by low levels of the constitutively active PDE6. Finally, we examined PDE6C-L858V, which models PDE6B-L854V, an RP-linked mutation that alters the protein isoprenyl modification. This analysis suggests that the type of prenyl modifications does not impact the folding of PDE6, but it modulates the enzyme affinity for its trafficking partner PDE6D. Hence, the pathogenicity of PDE6B-L854V likely arises from its trafficking deficiency. Taken together, our results demonstrate the effectiveness of the PDE6C expression system to evaluate pathogenicity and elucidate the mechanisms of PDE6 mutations in retinal diseases.

Domain organization and conformational plasticity of the G protein effector, PDE6

The cGMP phosphodiesterase of rod photoreceptor cells, PDE6, is the key effector enzyme in phototransduction. Two large catalytic subunits, PDE6α and -β, each contain one catalytic domain and two non-catalytic GAF domains, whereas two small inhibitory PDE6γ subunits allow tight regulation by the G protein transducin. The structure of holo-PDE6 in complex with the ROS-1 antibody Fab fragment was determined by cryo-electron microscopy. The ∼11 Å map revealed previously unseen features of PDE6, and each domain was readily fit with high resolution structures. A structure of PDE6 in complex with prenyl-binding protein (PrBP/δ) indicated the location of the PDE6 C-terminal prenylations. Reconstructions of complexes with Fab fragments bound to N or C termini of PDE6γ revealed that PDE6γ stretches from the catalytic domain at one end of the holoenzyme to the GAF-A domain at the other. Removal of PDE6γ caused dramatic structural rearrangements, which were reversed upon its restoration.

Membrane protein transport in photoreceptors: the function of PDEδ: the Proctor lecture

This lecture details the elucidation of cGMP phosphodiesterase (PDEδ), discovered 25 years ago by Joe Beavo at the University of Washington. PDEδ, once identified as a fourth PDE6 subunit, is now regarded as a promiscuous prenyl-binding protein and important chaperone of prenylated small G proteins of the Ras superfamily and prenylated proteins of phototransduction. Alfred Wittinghofer's group in Germany showed that PDEδ forms an immunoglobulin-like β-sandwich fold that is closely related in structure to other lipid-binding proteins, for example, Uncoordinated 119 (UNC119) and RhoGDI. His group cocrystallized PDEδ with ARL (Arf-like) 2(GTP), and later with farnesylated Rheb (ras homolog expressed in brain). PDEδ specifically accommodates farnesyl and geranylgeranyl moieties in the absence of bound protein. Germline deletion of the Pde6d gene encoding PDEδ impeded transport of rhodopsin kinase (GRK1) and PDE6 to outer segments, causing slowly progressing, recessive retinitis pigmentosa. A rare PDE6D null allele in human patients, discovered by Tania Attié-Bitach in France, specifically impeded trafficking of farnesylated phosphatidylinositol 3,4,5-trisphosphate (PIP3) 5-phosphatase (INPP5E) to cilia, causing severe syndromic ciliopathy (Joubert syndrome). Binding of cargo to PDEδ is controlled by Arf-like proteins, ARL2 and ARL3, charged with guanosine-5'-triphosphate (GTP). Arf-like proteins 2 and 3 are unprenylated small GTPases that serve as cargo displacement factors. The lifetime of ARL3(GTP) is controlled by its GTPase-activating protein, retinitis pigmentosa protein 2 (RP2), which accelerates GTPase activity up to 90,000-fold. RP2 null alleles in human patients are associated with severe X-linked retinitis pigmentosa (XLRP). Germline deletion of RP2 in mouse, however, causes only a mild form of XLRP. Absence of RP2 prolongs the activity of ARL3(GTP) that, in turn, impedes PDE6δ-cargo interactions and trafficking of prenylated protein to the outer segments. Hyperactive ARL3(GTP), acting as a hyperactive cargo displacement factor, is predicted to be key in the pathobiology of RP2-XLRP.

Temporal and spatial regulation of cAMP signaling in disease: role of cyclic nucleotide phosphodiesterases

Since its discovery, cAMP has been proposed as one of the most versatile second messengers. The remarkable feature of cAMP to tightly control highly diverse physiological processes, including metabolism, homeostasis, secretion, muscle contraction, cell proliferation and migration, immune response, and gene transcription, is reflected by millions of different articles worldwide. Compartmentalization of cAMP in space and time, maintained by mainly phosphodiesterases, contributes to the maintenance of equilibrium inside the cell where one signal can trigger many different events. Novel cAMP sensors seem to carry out certain unexpected signaling properties of cAMP and thereby to permit delicate adaptations of biologic responses. Measuring space and time events with biosensors will increase our current knowledge on the pathophysiology of diseases, such as chronic obstructive pulmonary disease, asthma, cognitive impairment, cancer, and renal and heart failure. Further insights into the cAMP dynamics will help to optimize the pharmacological treatment for these diseases.

A truncated form of rod photoreceptor PDE6 β-subunit causes autosomal dominant congenital stationary night blindness by interfering with the inhibitory activity of the γ-subunit

Autosomal dominant congenital stationary night blindness (adCSNB) is caused by mutations in three genes of the rod phototransduction cascade, rhodopsin (RHO), transducin α-subunit (GNAT1), and cGMP phosphodiesterase type 6 β-subunit (PDE6B). In most cases, the constitutive activation of the phototransduction cascade is a prerequisite to cause adCSNB. The unique adCSNB-associated PDE6B mutation found in the Rambusch pedigree, the substitution p.His258Asn, leads to rod photoreceptors desensitization. Here, we report a three-generation French family with adCSNB harboring a novel PDE6B mutation, the duplication, c.928-9_940dup resulting in a tyrosine to cysteine substitution at codon 314, a frameshift, and a premature termination (p.Tyr314Cysfs*50). To understand the mechanism of the PDE6β1-314fs*50 mutant, we examined the properties of its PDE6-specific portion, PDE6β1-313. We found that PDE6β1-313 maintains the ability to bind noncatalytic cGMP and the inhibitory γ-subunit (Pγ), and interferes with the inhibition of normal PDE6αβ catalytic subunits by Pγ. Moreover, both truncated forms of the PDE6β protein, PDE6β1-313 and PDE6β1-314fs*50 expressed in rods of transgenic X. laevis are targeted to the phototransduction compartment. We hypothesize that in affected family members the p.Tyr314Cysfs*50 change results in the production of the truncated protein, which binds Pγ and causes constitutive activation of the phototransduction thus leading to the absence of rod adaptation.

General pathophysiology in retinal degeneration

Retinal degeneration, including that seen in age-related macular degeneration and retinitis pigmentosa (RP), is the most common form of neural degenerative disease in the world. There is great genetic and allelic heterogeneity of the various retinal dystrophies. Classifications of these diseases can be ambiguous, as there are similar clinical presentations in retinal degenerations arising from different genetic mechanisms. As would be expected, alterations in the activity of the phototransduction cascade, such as changes affecting the renewal and shedding of the photoreceptor OS, visual transduction, and/or retinol metabolism have a great impact on the health of the retina. Mutations within any of the molecules responsible for these visual processes cause several types of retinal and retinal pigment epithelium degenerative diseases. Apoptosis has been implicated in the rod cell loss seen in a mouse model of RP, but the precise mechanisms that connect the activation of these pathways to the loss of phosphodiesterase (PDE6β) function has yet to be defined. Additionally, the activation of apoptosis by CCAAT/-enhancer-binding protein homologous protein (CHOP), after activation of the unfolded protein response pathway, may be responsible for cell death, although the mechanism remains unknown. However, the mechanisms of cell death after loss of function of PDE6, which is a commonly studied mammalian model in research, may be generalizable to loss of function of different key proteins involved in the phototransduction cascade.

Detergents stabilize the conformation of phosphodiesterase 6

Membrane-bound phosphodiesterase 6 (PDE6) plays an important role in visual signal transduction by regulating cGMP levels in rod photoreceptor cells. Our understanding of PDE6 catalysis and structure suffers from inadequate characterization of the α and β subunit catalytic core, interactions of the core with two intrinsically disordered, proteolysis-prone inhibitory PDEγ (Pγ) subunits, and binding of two types of isoprenyl-binding protein δ, called PrBP/δ, to the isoprenylated C-termini of the catalytic core. Structural studies of native PDE6 have been also been hampered by the lack of a heterologous expression system for the holoenzyme. In this work, we purified PDE6 in the presence of PrBP/δ and screened for additives and detergents that selectively suppress PDE6 basal activity while sparing that of the trypsin-activated enzyme. Some detergents removed PrBP/δ from the PDE complex, separating it from the holoenzyme after PDE6 purification. Additionally, selected detergents also significantly reduced the level of dissociation of PDE6 subunits, increasing their homogeneity and stabilizing the holoenzyme by substituting for its native membrane environment.

Binding of cGMP to the transducin-activated cGMP phosphodiesterase, PDE6, initiates a large conformational change involved in its deactivation

Retinal photoreceptor phosphodiesterase (PDE6), a key enzyme for phototransduction, consists of a catalytic subunit complex (Pαβ) and two inhibitory subunits (Pγs). Pαβ has two noncatalytic cGMP-binding sites. Here, using bovine PDE preparations, we show the role of these cGMP-binding sites in PDE regulation. Pαβγγ and its transducin-activated form, Pαβγ, contain two and one cGMP, respectively. Only Pαβγ shows [(3)H]cGMP binding with a K(d) ∼ 50 nM and Pγ inhibits the [(3)H]cGMP binding. Binding of cGMP to Pαβγ is suppressed during its formation, implying that cGMP binding is not involved in Pαβγγ activation. Once bound to Pαβγ, [(3)H]cGMP is not dissociated even in the presence of a 1000-fold excess of unlabeled cGMP, binding of cGMP changes the apparent Stokes' radius of Pαβγ, and the amount of [(3)H]cGMP-bound Pαβγ trapped by a filter is spontaneously increased during its incubation. These results suggest that Pαβγ slowly changes its conformation after cGMP binding, i.e. after formation of Pαβγ containing two cGMPs. Binding of Pγ greatly shortens the time to detect the increase in the filter-trapped level of [(3)H]cGMP-bound Pαβγ, but alters neither the level nor its Stokes' radius. These results suggest that Pγ accelerates the conformational change, but does not add another change. These observations are consistent with the view that Pαβγ changes its conformation during its deactivation and that the binding of cGMP and Pγ is crucial for this change. These observations also imply that Pαβγγ changes its conformation during its activation and that release of Pγ and cGMP is essential for this change.

Phosphodiesterase 6 subunits are expressed and altered in idiopathic pulmonary fibrosis

Background

Idiopathic Pulmonary Fibrosis (IPF) is an unresolved clinical issue. Phosphodiesterases (PDEs) are known therapeutic targets for various proliferative lung diseases. Lung PDE6 expression and function has received little or no attention. The present study aimed to characterize (i) PDE6 subunits expression in human lung, (ii) PDE6 subunits expression and alteration in IPF and (iii) functionality of the specific PDE6D subunit in alveolar epithelial cells (AECs).

Methodology/Principal Findings

PDE6 subunits expression in transplant donor (n = 6) and IPF (n = 6) lungs was demonstrated by real-time quantitative (q)RT-PCR and immunoblotting analysis. PDE6D mRNA and protein levels and PDE6G/H protein levels were significantly down-regulated in the IPF lungs. Immunohistochemical analysis showed alveolar epithelial localization of the PDE6 subunits. This was confirmed by qRT-PCR from human primary alveolar type (AT)II cells, demonstrating the down-regulation pattern of PDE6D in IPF-derived ATII cells. In vitro, PDE6D protein depletion was provoked by transforming growth factor (TGF)-β1 in A549 AECs. PDE6D siRNA-mediated knockdown and an ectopic expression of PDE6D modified the proliferation rate of A549 AECs. These effects were mediated by increased intracellular cGMP levels and decreased ERK phosphorylation.

Conclusions/Significance

Collectively, we report previously unrecognized PDE6 expression in human lungs, significant alterations of the PDE6D and PDE6G/H subunits in IPF lungs and characterize the functional role of PDE6D in AEC proliferation.

Structural characterization of the rod cGMP phosphodiesterase 6

Rod cGMP phosphodiesterase 6 (PDE6) is a key enzyme of the phototransduction cascade, consisting of PDE6alpha, PDE6beta, and two regulatory PDE6gamma subunits. PDE6 is membrane associated through isoprenyl membrane anchors attached to the C-termini of PDE6alpha and PDE6beta and can form a complex with prenyl-binding protein delta (PrBP/delta), an isoprenyl-binding protein that is highly expressed in photoreceptors. The stoichiometry of PDE6-PrBP/delta binding and the mechanism by which the PDE6-PrBP/delta complex assembles have not been fully characterized, and the location of regulatory PDE6gamma subunits within the protein assembly has not been elucidated. To clarify these questions, we have developed a rapid purification method for PDE6-PrBP/delta from bovine rod outer segments utilizing recombinant PrBP/delta. Transmission electron microscopy of negatively stained samples revealed the location of PrBP/delta and, thus, where the carboxyl-termini of PDE6alpha and PDE6beta must be located. The three-dimensional structure of the PDE6alphabetagamma complex was determined up to 18 A resolution from single-particle projections and was interpreted by model building to identify the probable location of isoprenylation, PDE6gamma subunits, and catalytic sites.

Mechanism for the regulation of mammalian cGMP phosphodiesterase6. 2: isolation and characterization of the transducin-activated form

Rod photoreceptor cGMP phosphodiesterase (PDE6) consists of a catalytic subunit complex (Palphabeta) and two inhibitory subunits (Pgamma). In the accompanying article, using bovine photoreceptor outer segment homogenates, we show that Pgamma as a complex with the GTP-bound transducin alpha subunit (GTP-Talpha) dissociates from Palphabetagammagamma on membranes, and the Palphabetagammagamma becomes Pgamma-depleted. Here, we identify and characterize the Pgamma-depleted PDE. After incubation with or without guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS), Palphabeta complexes are extracted. When a hypotonic buffer is used, Palphabetagammagamma, Palphabetagamma, and a negligible amount of a Palphabeta complex containing Pgamma are isolated with GTPgammaS, and only Palphabetagammagamma is obtained without GTPgammaS. When an isotonic buffer containing Pdelta, a prenyl-binding protein, is used, Palphabetagammagammadelta, Palphabetagammadeltadelta, and a negligible amount of a Palphabeta complex containing Pgamma and Pdelta are isolated with GTPgammaS, and Palphabetagammagammadelta is obtained without GTPgammaS. Neither Palphabeta nor Palphabetagammagamma complexed with GTPgammaS-Talpha is found under any condition we examined. Palphabetagamma has approximately 12 times higher PDE activity and approximately 30 times higher Pgamma sensitivity than those of Palphabetagammagamma. These results indicate that the Pgamma-depleted PDE is Palphabetagamma. Isolation of Palphabetagammagammadelta and Palphabetagammadeltadelta suggests that one C-terminus of Palphabeta is involved in the Palphabetagammagamma interaction with membranes, and that Pgamma dissociation opens another C-terminus for Pdelta binding, which may lead to the expression of high PDE activity. Cone PDE behaves similarly to rod PDE in the anion exchange column chromatography. We conclude that the mechanisms for PDE activation are similar in mammalian and amphibian photoreceptors as well as in rods and cones.

Mechanism for the regulation of mammalian cGMP phosphodiesterase6. 1: identification of its inhibitory subunit complexes and their roles

Cyclic GMP phosphodiesterase (PDE) in bovine rod photoreceptor outer segments (OS) comprises a catalytic subunit complex (Palphabeta) and two inhibitory subunits (Pgamma) and is regulated by the alpha subunit of transducin (Talpha). Here, we show an overall mechanism for PDE regulation by identifying Pgamma complexes in OS homogenates prepared with an isotonic buffer. Before Talpha activation, three Pgamma complexes exist in the soluble fraction. Complex a, a minor complex, contains Palphabeta, Talpha, and a protein named Pdelta. Complex b, Palphabetagammagamma( b ), has a PDE activity similar to that of membranous Palphabetagammagamma, Palphabetagammagamma( M ), and its level, although its large portion is Pdelta-free, is estimated to be 20-30% of the total Palphabetagammagamma. Complex c, (Pgamma.GDP-Talpha) (2) ( c ) , appears to be a dimer of Pgamma.GDP-Talpha. Upon Talpha activation, (1) complex a stays unchanged, (2) Palphabetagammagamma( b ) binds to membranes, (3) the level of (Pgamma.GDP-Talpha) (2) ( c ) is reduced as its GTP-form is produced, (4) complex d, Pgamma.GTP-Talpha( d ), is formed on membranes and its substantial amount is released to the soluble fraction, and (5) membranous Palphabetagammagamma, Palphabetagammagamma( M ) and/or Palphabetagammagamma( b ), becomes Pgamma-depleted. These observations indicate that Pgamma as a complex with GTP-Talpha dissociates from Palphabetagammagamma on membranes and is released to the soluble fraction and that Pgamma-depleted PDE is the GTP-Talpha-activated PDE. After GTP hydrolysis, both (Pgamma.GDP-Talpha) (2) ( c ) and Pgamma.GDP-Talpha( d ), without liberating Pgamma, deactivate Pgamma-depleted PDE. The preferential order to be used for the deactivation is membranous Pgamma.GDP-Talpha( d ), solubilized Pgamma.GDP-Talpha( d ) and (Pgamma.GDP-Talpha) (2) ( c ) . Release of Pgamma.GTP-Talpha complexes to the soluble fraction is relevant to light adaptation.

AIPL1, a protein associated with childhood blindness, interacts with alpha-subunit of rod phosphodiesterase (PDE6) and is essential for its proper assembly

Mutations in the gene coding for AIPL1 cause Leber congenital amaurosis (LCA), a severe form of childhood blindness. The severity in disease is reflected in the complete loss of vision and rapid photoreceptor degeneration in the retinas of mice deficient in AIPL1. Our previous observations suggest that rod photoreceptor degeneration in retinas lacking AIPL1 is due to the massive reduction in levels of rod cGMP phosphodiesterase (PDE6) subunits (alpha, beta, and gamma). To date, the crucial link between AIPL1 and the stability of PDE6 subunits is not known. In this study using ex vivo pulse label analysis, we demonstrate that AIPL1 is not involved in the synthesis of PDE6 subunits. However, ex vivo pulse-chase analysis clearly shows that in the absence of AIPL1, rod PDE6 subunits are rapidly degraded by proteasomes. We further demonstrate that this rapid degradation of PDE6 is due to the essential role of AIPL1 in the proper assembly of synthesized individual PDE6 subunits. In addition, using a novel monoclonal antibody generated against AIPL1, we show that the catalytic subunit (alpha) of PDE6 associates with AIPL1 in retinal extracts. Our studies establish that AIPL1 interacts with the catalytic subunit (alpha) of PDE6 and is needed for the proper assembly of functional rod PDE6 subunits.

Up-regulation of 3'5'-cyclic guanosine monophosphate-specific phosphodiesterase in the porcine cumulus-oocyte complex affects steroidogenesis during in vitro maturation

The 3'5'-cyclic GMP (cGMP) pathway is known to influence ovarian functions, including steroidogenesis, ovulation, and granulosa cell proliferation. We show here that cGMP-phosphodiesterase (PDE) activity increased in a gonadotropin-dependent manner more than 3-fold in the cumulus-oocyte complex (COC) after 24 h in vitro maturation (IVM) and up to 5-fold after 48 h. Further characterization of this increase demonstrated that the activity was located primarily in cumulus cells, and was sensitive to sildenafil and zaprinast, two inhibitors specific to both type 5 and 6 PDEs. RT-PCR experiments showed that the mRNAs for cGMP-degrading PDEs 5A and 6C are present in the COC before and after 30 h IVM. Western blotting confirmed the presence of PDE 5A in the COC. Western blotting of PDE 6C revealed a significant up-regulation in the COC during IVM. Isolation and analysis of detergent-resistant membranes suggested that PDE 6C protein, along with half of the total sildenafil-sensitive cGMP-degradation activity, is associated with detergent-resistant membrane in the COC after 30 h IVM. Treatment of porcine COC with sildenafil during IVM caused a significant decrease in gonadotropin-stimulated progesterone secretion. Together, these results constitute the first report exploring the contribution of cGMP-PDE activity in mammalian COC, supporting a functional clustering of the enzyme, and providing the first evidence of its role in steroidogenesis.

Signal transducing membrane complexes of photoreceptor outer segments

Signal transduction in outer segments of vertebrate photoreceptors is mediated by a series of reactions among multiple polypeptides that form protein-protein complexes within or on the surface of the disk and plasma membranes. The individual components in the activation reactions include the photon receptor rhodopsin and the products of its absorption of light, the three subunits of the G protein, transducin, the four subunits of the cGMP phosphodiesterase, PDE6 and the four subunits of the cGMP-gated cation channel. Recovery involves membrane complexes with additional polypeptides including the Na(+)/Ca(2+), K(+) exchanger, NCKX2, rhodopsin kinases RK1 and RK7, arrestin, guanylate cyclases, guanylate cyclase activating proteins, GCAP1 and GCAP2, and the GTPase accelerating complex of RGS9-1, G(beta5L), and membrane anchor R9AP. Modes of membrane binding by these polypeptides include transmembrane helices, fatty acyl or isoprenyl modifications, polar interactions with lipid head groups, non-polar interactions of hydrophobic side chains with lipid hydrocarbon phase, and both polar and non-polar protein-protein interactions. In the course of signal transduction, complexes among these polypeptides form and dissociate, and undergo structural rearrangements that are coupled to their interactions with and catalysis of reactions by small molecules and ions, including guanine nucleotides, ATP, Ca(2+), Mg(2+), and lipids. The substantial progress that has been made in understanding the composition and function of these complexes is reviewed, along with the more preliminary state of our understanding of the structures of these complexes and the challenges and opportunities that present themselves for deepening our understanding of these complexes, and how they work together to convert a light signal into an electrical signal.

The G protein cascade of visual transduction: kinetics and regulation

In retinal rods photoexcited rhodopsin (R*) catalyses the activation of transducin (T) by GTP, which in turn activates the cGMP phosphodiesterase (PDE). The ensuing decrease in cGMP concentration reduces the cell membrane's channel conductance. To account for the kinetics of the response to light, all underlying biochemical reactions must reach maximum speed and be turned off within a second. Kinetic analysis of transducin activation suggests that because of the fast lateral diffusion of T, the rate-limiting step is not the collision between R* and T but the entry of GTP after the release fo GDP from the R*-bound T alpha. T alpha-GTP dissociates from both R* and T beta gamma and diffuses through the cytoplasm to activate PDE. In suspensions of bovine rod outer segments, time-resolved microcalorimetry yields rates of approximately 1-2 s-1 for the GTPase of T alpha and the correlated deactivation of PDE. But for isolated T alpha-GTP the single turnover GTPase rate measured by a stopped-flow technique is only 0.05 s-1. To activate PDE, T alpha-GTP binds tightly to the PDE gamma subunit. In vitro the soluble T alpha-GTP.PDE gamma complex dissociates from activated PDE alpha beta. Thus PDE gamma might be the GTPase activator of T alpha, but no GTPase acceleration was observed in isolated T alpha-GTP.PDE gamma. The GTPase activation must depend on the interaction of T alpha-GTP.PDE gamma with membrane-bound PDE alpha beta.

Translational regulation of the rod photoreceptor cGMP-phosphodiesterase: the role of the 5'- and 3'-untranslated regions

We have established earlier that rod photoreceptor cGMP-phosphodiesterase (PDE6) alpha and beta subunits are equally represented in the retina at the protein level and have similar turnover rates. mRNA quantification revealed five PDE6beta messages for every PDE6alpha transcript pointing at post-transcriptional regulation of PDE6alpha and PDE6beta expression. Indeed, the wild-type PDE6alpha mRNA was translated 5-fold more efficiently than that of PDE6beta. The coding regions of these subunits had a major contribution in this process. Here, we extend our study of translational regulation of PDE6 subunits and present a detailed analysis of the role of PDE6alpha and PDE6beta 5'- and 3'-UTRs (untranslated regions) in this process. We showed that both the short and long PDE6beta 5'-UTRs lead to more efficient protein synthesis than the PDE6alpha 5'-UTR. The 3'-UTRs of PDE6alpha and PDE6beta stimulated translation by approximately 2- and 3-fold, respectively. However, the positive effect of the PDE6alpha or PDE6beta 3'-UTRs was not observed when these regions were placed in constructs containing the 5'-UTR of the corresponding PDE6 subunit. Furthermore, it appears that PDE6alpha 5'- and 3'-UTRs may be involved in a base pairing interaction that reduces the efficiency of protein synthesis. Finally, using progressive deletion analysis of the PDE6alpha 5'-UTR, we have identified several regions that have significant contribution in regulation of protein synthesis. Based on these and earlier published data, it can be stated that an equimolar level of PDE6alpha and PDE6beta synthesized from different amounts of mRNA (ratio of PDE6alpha to PDE6beta mRNA in the retina is 1:5) is achieved as a result of combinatorial effects of 5'-UTRs and coding regions of PDE6alpha and PDE6beta mRNAs on translational regulation.

The discovery of signal transduction by G proteins: a personal account and an overview of the initial findings and contributions that led to our present understanding

The realization that there existed a G-protein coupled signal transduction mechanism developed gradually and was initially the result of an ill fated quest for uncovering the mechanism of action of insulin, followed by a refocused research in many laboratories, including mine, on how GTP acted to increase hormonal stimulation of adenylyl cyclase. Independent research into how light-activated rhodopsin triggers a response in photoreceptor cells of the retina and the attendant biochemical studies joined midway and, without the left hand knowing well what the right hand was doing, preceded classical G protein research in identifying the molecular players responsible for signal transduction by G proteins.

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.

Structural determinants of the PDE6 GAF A domain for binding the inhibitory gamma-subunit and noncatalytic cGMP

Photoreceptor cGMP phosphodiesterases (PDE6 family) are modular enzymes with each catalytic subunit containing two N-terminal regulatory GAF domains, GAF A and GAF B. The GAF A domains contribute to dimerization of the PDE6 catalytic subunits and to binding of the inhibitory Pgamma subunits, and represent candidate sites for noncatalytic binding of cGMP. We performed a mutational analysis of selected residues from the GAF A domain of cone PDEalpha' to identify the cGMP-binding pocket and delineate the Pgamma-binding surface. Results of this analysis establish the noncatalytic cGMP-binding site within the PDE6 GAF A domain and suggest that occupation of the pocket by cGMP is required for high-affinity binding of Pgamma to the proximate contact surface.

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.

AIPL1, the protein that is defective in Leber congenital amaurosis, is essential for the biosynthesis of retinal rod cGMP phosphodiesterase

Aryl hydrocarbon receptor-interacting protein-like 1 (AIPL1) is a member of the FK-506-binding protein family expressed specifically in retinal photoreceptors. Mutations in AIPL1 cause Leber congenital amaurosis, a severe early-onset retinopathy that leads to visual impairment in infants. Here we show that knockdown of AIPL1 expression in mice also produces a retinopathy but over a more extended time course. Before any noticeable pathology, there was a reduction in the level of rod cGMP phosphodiesterase (PDE) proportional to the decrease in AIPL1 expression, whereas other photoreceptor proteins were unaffected. Consistent with less PDE in rods, flash responses had a delayed onset, a reduced gain, and a slower recovery of flash responses. We suggest that AIPL1 is a specialized chaperone required for rod PDE biosynthesis. Thus loss of AIPL1 would result in a condition that phenocopies retinal degenerations in the rd mouse and in a subgroup of human patients.

Differential inhibitor sensitivity between human recombinant and native photoreceptor cGMP-phosphodiesterases (PDE6s)

Human photoreceptor cGMP-phosphodiesterases (PDE6s) are important reagents in PDE inhibitor discovery. However, recombinant human PDE6s have not been expressed, and isolation of native human PDE6s is highly difficult. In this study, the catalytic subunit(s) of human rod and cone PDE6s (PDE6alphabeta and PDE6alpha', respectively) were co-expressed or expressed separately as catalytically active enzymes. Sildenafil inhibited both the recombinant PDE6s in a dose-dependent manner with Ki values of 94 and 98 nM, respectively. These Ki values were four-fold higher than that (25 nM) of a human native PDE6 preparation. Similarly, 3-isobutyl-1-methylxanthine (IBMX)'s Ki values for the recombinant PDE6s were five- to eight-fold higher than that of the native enzyme. However, E4021 and zaprinast exhibited much (30-80-fold) lower potencies for the recombinant PDE6s than for the native enzyme. Additional PDE5 inhibitors representing other structural classes and possessing different selectivity against native PDE6 also showed different potencies against the recombinant and native PDE6s. In particular, one class of xanthine analogues exhibited significantly (5-15-fold) higher potencies for the recombinant PDE6s than for the native enzyme. Our data demonstrates that the recombinant and native PDE6s exhibit differential sensitivity to inhibitors, and cautions the use of recombinant catalytic subunits of PDE6 in drug discovery or in structural/functional studies.

Phosphodiesterases in the vascular system

Cyclic adenosine 3',5'-monophosphate (cAMP) and cyclic guanosine 3',5'-monophosphate (cGMP) are second messengers involved in the intracellular signal transduction of a variety of extracellular stimuli in several tissues. In the vascular system, these nucleotides play important roles in the regulation of vascular tone and in the maintenance of the mature contractile phenotype in smooth muscle cells. Given that cyclic nucleotide signaling regulates a wide variety of cellular functions, it is not surprising that cyclic nucleotide phosphodiesterases (PDEs). In paticular, the accumulating data showing that there are a large number of different PDE isozymes have triggered an equally large increase in interest about these enzymes. At least 11 different gene families of PDEs are currently known to exist in mammalian tissues. Most families contain several distinct genes, and many of these genes are expressed in different tissues as functionally unique alternative splice variants. This article reviews many of the important aspects about the structure, cellular localization, and regulation of each family of PDEs. Particular emphasis is placed on new information obtained in the last few years about vascular disease. The development of novel methods to deliver more potent and selective PDE inhibitors to individual cell types and subcellular locations will lead to new therapeutic uses for this class of drugs in diseases of the vascular system.

Photoreceptor cGMP phosphodiesterase delta subunit (PDEdelta) functions as a prenyl-binding protein

Bovine PDEdelta was originally copurified with rod cGMP phosphodiesterase (PDE) and shown to interact with prenylated, carboxymethylated C-terminal Cys residues. Other studies showed that PDEdelta can interact with several small GTPases including Rab13, Ras, Rap, and Rho6, all of which are prenylated, as well as the N-terminal portion of retinitis pigmentosa GTPase regulator and Arl2/Arl3, which are not prenylated. We show by immunocytochemistry with a PDEdelta-specific antibody that PDEdelta is present in rods and cones. We find by yeast two-hybrid screening with a PDEdelta bait that it can interact with farnesylated rhodopsin kinase (GRK1) and that prenylation is essential for this interaction. In vitro binding assays indicate that both recombinant farnesylated GRK1 and geranylgeranylated GRK7 co-precipitate with a glutathione S-transferase-PDEdelta fusion protein. Using fluorescence resonance energy transfer techniques exploiting the intrinsic tryptophan fluorescence of PDEdelta and dansylated prenyl cysteines as fluorescent ligands, we show that PDEdelta specifically binds geranylgeranyl and farnesyl moieties with a Kd of 19.06 and 0.70 microm, respectively. Our experiments establish that PDEdelta functions as a prenyl-binding protein interacting with multiple prenylated proteins.

Differential expression of rod photoreceptor cGMP-phosphodiesterase alpha and beta subunits: mRNA and protein levels

The catalytic core of photoreceptor-specific cGMP-phosphodiesterase (PDE) consists of two subunits, PDEalpha and PDEbeta, that are homologous and have similar domain organization but are encoded by different genes. We have examined the PDEalpha and PDEbeta mRNA steady-state and protein levels as well as the biosynthesis rate of these proteins in developing and fully differentiated retinas. We have also determined the translational efficiency of PDE subunits and the role of their mRNA structures in regulating protein synthesis. In mature retinas, PDEalpha and PDEbeta are represented by approximately 1.5 x 108 and 7.5 x 108 copies/microg retinal mRNA, respectively. The levels of these transcripts in developing photoreceptors (P10) are approximately 75% of those at P30. Quantification of protein concentration indicated that PDEalpha and PDEbeta are equally expressed in developing and fully differentiated photoreceptors. Furthermore, the PDEalpha/PDEbeta ratios obtained throughout a 2-h pulsechase period revealed a similar turnover rate for both subunits. The observed discordance between the mRNA and protein levels of PDEalpha and PDEbeta suggested post-transcriptional regulation of their expression. We found that PDEalpha mRNA is translated more efficiently than either of the two PDEbeta transcripts expressed in retina. Therefore, the lower level of PDEalpha mRNA is compensated by its more efficient translation to achieve equimolar expression with PDEbeta. We also analyzed the effect of PDEalpha and PDEbeta mRNA 5'- and 3'-untranslated regions as well as that of their coding regions on protein synthesis. We determined that the PDE-coding regions play a critical role in the differential translation of these subunits.

Phosphorylation by cyclin-dependent protein kinase 5 of the regulatory subunit (Pgamma) of retinal cgmp phosphodiesterase (PDE6): its implications in phototransduction

Cyclic GMP phosphodiesterase (PDE6) is a key enzyme in vertebrate retinal phototransduction. After GTP/GDP exchange on the a subunit of transducin (Talpha) by illuminated rhodopsin, the GTP-bound form Talpha (GTP/Talpha) interacts with the regulatory subunit (Pgamma) of PDE6 to activate cGMP hydrolytic activity. The regulatory mechanism of PDE6 has been believed to be a typical G protein-mediated signal transduction process. We found that cyclin-dependent protein kinase 5 (Cdk5) phosphorylates Pgamma complexed with GTP/Talpha in vitro and in vivo. Phosphorylated Py dissociates from GTP/Talpha without GTP hydrolysis and interacts effectively with catalytic subunits of PDE6 to inhibit the enzyme activity. These observations provide new twists to the current model of retinal phototransduction. In this article, in addition to the details of Py phosphorylation by Cdk5, we review previous studies implying the Pgamma phosphorylation and the turnoff of PDE6 without GTP hydrolysis and indicate the direction for future studies of Py phosphorylation, including the possible involvement of Ca2+/Ca2+-binding proteins.

Signal transduction in the visual cascade involves specific lipid-protein interactions

In retinal rod photoreceptor cells, transducin (Gt) and cyclic GMP phosphodiesterase (PDE) are peripherally anchored to the cytoplasmic surface of the disk saccules. We have examined the role of specific phospholipids in the interaction of these proteins with native osmotically intact disk vesicles, employing spin-labeled phospholipid analogues (2% of total phospholipids) and bovine serum albumin back-exchange assay. Inactive GDP-bound transducin exclusively reduced the extraction of negatively charged phosphatidylserine. The effect disappeared upon activation of the G-protein with guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS). PDE affected the extraction of the zwitterionic phosphatidylcholine and, to a smaller extent, of phosphatidylethanolamine. When active GtGTPgammaS interacted with the PDE to form the active effector, the interaction with phosphatidylcholine was specifically enhanced. Each copy of the G-protein bound 3 +/- 1 molecules of phosphatidylserine, whereas the PDE bound a much larger amount (70 +/- 10) of a mixture of phosphatidylcholine and ethanolamine. The results are interpreted as a head group-specific and state-dependent interaction of the signaling proteins with the phospholipids of the photoreceptor membrane.

Molecular evolution of proteins involved in vertebrate phototransduction

Vision is one of the most important senses for vertebrates. As a result, vertebrates have evolved a highly organized system of retinal photoreceptors. Light triggers an enzymatic cascade, called the phototransduction cascade, that leads to the hyperpolarization of photoreceptors. It is expected that a systematic comparison of phototransduction cascades of various vertebrates can provide insights into the diversity of vertebrate photoreceptors and into the evolution of vertebrate vision. However, only a few attempts have been made to compare each phototransduction protein participating in this cascade. Here, we determine phylogenetic trees of the vertebrate phototransduction proteins and compare them. It is demonstrated that vertebrate opsin sequences fall into five fundamental subfamilies. It is speculated that this is crucial for the diversity of the spectral sensitivity observed in vertebrate photoreceptors and provides the vertebrates with the molecular tools to discriminate the color of incident light. Other phototransduction proteins can be classified into only a few subfamilies. Cones generally share isoforms of phototransduction proteins that are different from those found in rods. The difference in sensitivity to light between rods and cones is likely due to the difference in the molecular properties of these isoforms. The phototransduction proteins seem to have co-evolved as a system. Switching the expression of these isoforms may characterize individual vertebrate photoreceptors.

Binding of cGMP to GAF domains in amphibian rod photoreceptor cGMP phosphodiesterase (PDE). Identification of GAF domains in PDE alphabeta subunits and distinct domains in the PDE gamma subunit involved in stimulation of cGMP binding to GAF domains

Retinal cGMP phosphodiesterase (PDE6) is a key enzyme in vertebrate phototransduction. Rod PDE contains two homologous catalytic subunits (Palphabeta) and two identical regulatory subunits (Pgamma). Biochemical studies have shown that amphibian Palphabeta has high affinity, cGMP-specific, non-catalytic binding sites and that Pgamma stimulates cGMP binding to these sites. Here we show by molecular cloning that each catalytic subunit in amphibian PDE, as in its mammalian counterpart, contains two homologous tandem GAF domains in its N-terminal region. In Pgamma-depleted membrane-bound PDE (20-40% Pgamma still present), a single type of cGMP-binding site with a relatively low affinity (K(d) approximately 100 nm) was observed, and addition of Pgamma increased both the affinity for cGMP and the level of cGMP binding. We also show that mutations of amino acid residues in four different sites in Pgamma reduced its ability to stimulate cGMP binding. Among these, the site involved in Pgamma phosphorylation by Cdk5 (positions 20-23) had the largest effect on cGMP binding. However, except for the C terminus, these sites were not involved in Pgamma inhibition of the cGMP hydrolytic activity of Palphabeta. In addition, the Pgamma concentration required for 50% stimulation of cGMP binding was much greater than that required for 50% inhibition of cGMP hydrolysis. These results suggest that the Palphabeta heterodimer contains two spatially and functionally distinct types of Pgamma-binding sites: one for inhibition of cGMP hydrolytic activity and the second for activation of cGMP binding to GAF domains. We propose a model for the Palphabeta-Pgamma interaction in which Pgamma, by binding to one of the two sites in Palphabeta, may preferentially act either as an inhibitor of catalytic activity or as an activator of cGMP binding to GAF domains in frog PDE.

Three-dimensional structure of non-activated cGMP phosphodiesterase 6 and comparison of its image with those of activated forms

Cyclic GMP phosphodiesterase (PDE6) in rod photoreceptors, a key enzyme in vertebrate phototransduction, consists of two homologous catalytic subunits (Palpha and Pbeta) and two identical regulatory subunits (Pgammas). Pgamma regulates the PDE activity through its direct interaction with transducin. Here, using electron microscopy and image analysis of single particles, we show the three-dimensional organization of the basic form of bovine PDE, Palphabetagammagamma, and compare its average image with those of Pgamma-released PDE. The structure of Palphabetagammagamma appears to be a flattened bell-shape, with dimensions of 150 x 108 x 60A, and with a handle-like protrusion attached to the top of the structure. Except for the protrusion, the organization consists of two homologous structures arranged side by side, with each structure having three distinct regions, showing pseudo twofold symmetry. These characteristics are consistent with a model in which the overall structure of Palphabetagammagamma is determined by hetero-dimerization of Palpha and Pbeta, with each subunit consisting of one catalytic and two GAF regions. A comparison of the average image of Palphabetagammagamma with those of Pgamma-released PDE suggests that Pgamma release does not affect the overall structure of Palphabeta, and that the Palphabeta C-terminus, but not Pgamma, is a determinant for the Palphabeta orientation on carbon-coated grids. These observations suggest that the basic structure of PDE does not change during its regulation, which implies that Palphabeta is regulated by its regional interaction with Pgamma.

The positive role of the carboxyl terminus of the gamma subunit of retinal cGMP-phosphodiesterase in maintaining phosphodiesterase activity in vivo

The inhibitory rod cyclic GMP-phosphodiesterase gamma subunit, PDEgamma, is a key component of the photoresponse and is required to support rod integrity. Pdeg(tm1)/Pdeg(tm1) mice that lack PDEgamma due to a targeted disruption of the gene encoding PDEgamma, (Pdeg) suffer from a very rapid and severe photoreceptor degeneration. Previously, deletions in the carboxyl-terminal domain of PDEgamma blocked its ability to inhibit trypsin-activated PDE activity, in vitro. In other words, these mutations eliminated PDEgamma's control on the catalytic activity of PDEalpha and PDEbeta. To study the in vivo effects resulting from the deletion of the last seven amino acids of the PDEgamma carboxyl terminal, this PDEgamma allele (Del7C) was introduced as a transgene Pdeg(tm1)/Pdeg(tm1) mice. These animals could only synthesize transgenic mutant PDEgamma. The mutant retinas were expected to display a higher basal level of PDE activity and lower cGMP levels in light and darkness than the PDEgamma knockout mice, which would allow the rescue of their photoreceptors. Instead, our results showed that the Del7C transgene could not complement the Pdeg(tm1)/Pdeg(tm1) mutant for photoreceptor survival. In fact, animals carrying the Del7C transgene have low PDE activity as well as reduced PDEalpha and PDEbeta content.

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.

Expression and role of phosphodiesterase 6 in the chicken pineal gland

The chicken pineal gland is directly photosensitive, with light causing an inhibition of melatonin synthesis. A possible role of phosphodiesterase 6 (PDE6, the primary effector of retinal phototransduction) in mediating this response was investigated. RT-PCR, DNA sequencing and northern blots revealed the presence of RNA encoding both catalytic and regulatory subunits of PDE6 in the chicken pineal gland. Both rod and cone forms of PDE6 subunits mRNA were detected. The concentration of the transcripts encoding PDE6 catalytic subunits peaked at night. Western blot analysis of chicken pineal proteins with an antibody directed against the catalytic subunits of bovine rod PDE6 identified a single immunoreactive protein of 97 kDa. Anion exchange chromatography of chicken pineal soluble proteins revealed a peak of PDE6 activity that accounted for about 30% of cyclic GMP-hydrolysis. In cultured chick pineal glands, arylalkylamine N-acetyltransferase (AA-NAT), the rate-limiting enzyme of melatonin synthesis, was protected from inhibition by light when selective PDE5/6 inhibitors (zaprinast, DMPPO) were added to the culture medium. PDE5/6 inhibitors did not affect AA-NAT activity in the dark. In contrast, a general PDE inhibitor (IBMX) increased AA-NAT in a light-independent manner. Together, the data indicate that rod and cone forms of PDE6 are expressed in chick pineal cells and that this enzyme plays a role in the inhibition of melatonin synthesis by light.

In vivo studies of the gamma subunit of retinal cGMP-phophodiesterase with a substitution of tyrosine-84

The inhibitory rod cGMP phosphodiesterase gamma subunit (PDEgamma) is a major component of the photoresponse and is required to support rod integrity. Pdeg(tm1)/Pdeg(tm1) mice (which lack PDEgamma owing to a targeted disruption of the Pdeg gene) suffer from a very rapid and severe photoreceptor degeneration. The Y84G (Tyr(84)-->Gly) allele of PDEgamma has previously been shown in experiments carried out in vitro to reduce the regulatory control of the PDE catalytic core (PDEalphabeta) exerted by the wild-type gamma subunit. To determine the effects of this mutation on in vivo function, the murine opsin promoter was used to direct expression to the photoreceptors of +/Pdeg(tm1) mice of a mutant Y84G and a wild-type PDEgamma control transgene. The transgenic mice were crossed with Pdeg(tm1)/Pdeg(tm1) mice to generate animals able to synthesize only the transgenic PDEgamma. Our results showed that wild-type PDEgamma and Y84G transgenes could complement the Pdeg(tm1)/Pdeg(tm1) mutant for photoreceptor survival. The mutation caused a significant biochemical defect in PDE activation by transducin. However, the Y84G mutation did not fully eliminate the control of PDEgamma on the PDE catalytic core in vivo; the expression of the mutant subunit was associated with only a 10-fold reduction in the amplitude of the a-wave and a 1.5-fold decrease in the b-wave of the corneal electroretinogram. Unexpectedly, the mutation caused a much 'milder' phenotype in vivo than was predicted from the biochemical assays in vitro.

Adaptation in vertebrate photoreceptors

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