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

Inhibition of mitochondrial pyruvate transport by zaprinast causes massive accumulation of aspartate at the expense of glutamate in the retina

Transport of pyruvate into mitochondria by the mitochondrial pyruvate carrier is crucial for complete oxidation of glucose and for biosynthesis of amino acids and lipids. Zaprinast is a well known phosphodiesterase inhibitor and lead compound for sildenafil. We found Zaprinast alters the metabolomic profile of mitochondrial intermediates and amino acids in retina and brain. This metabolic effect of Zaprinast does not depend on inhibition of phosphodiesterase activity. By providing (13)C-labeled glucose and glutamine as fuels, we found that the metabolic profile of the Zaprinast effect is nearly identical to that of inhibitors of the mitochondrial pyruvate carrier. Both stimulate oxidation of glutamate and massive accumulation of aspartate. Moreover, Zaprinast inhibits pyruvate-driven O2 consumption in brain mitochondria and blocks mitochondrial pyruvate carrier in liver mitochondria. Inactivation of the aspartate glutamate carrier in retina does not attenuate the metabolic effect of Zaprinast. Our results show that Zaprinast is a potent inhibitor of mitochondrial pyruvate carrier activity, and this action causes aspartate to accumulate at the expense of glutamate. Our findings show that Zaprinast is a specific mitochondrial pyruvate carrier (MPC) inhibitor and may help to elucidate the roles of MPC in amino acid metabolism and hypoglycemia.

Phosphodiesterase inhibition induces retinal degeneration, oxidative stress and inflammation in cone-enriched cultures of porcine retina

Inherited retinal degenerations affecting both rod and cone photoreceptors constitute one of the causes of incurable blindness in the developed world. Cyclic guanosine monophosphate (cGMP) is crucial in the phototransduction and, mutations in genes related to its metabolism are responsible for different retinal dystrophies. cGMP-degrading phosphodiesterase 6 (PDE6) mutations cause around 4-5% of the retinitis pigmentosa, a rare form of retinal degeneration. The aim of this study was to evaluate whether pharmacological PDE6 inhibition induced retinal degeneration in cone-enriched cultures of porcine retina similar to that found in murine models. PDE6 inhibition was induced in cone-enriched retinal explants from pigs by Zaprinast. PDE6 inhibition induced cGMP accumulation and triggered retinal degeneration, as determined by TUNEL assay. Western blot analysis and immunostaining indicated that degeneration was accompanied by caspase-3, calpain-2 activation and poly (ADP-ribose) accumulation. Oxidative stress markers, total antioxidant capacity, thiobarbituric acid reactive substances (TBARS) and nitric oxide measurements revealed the presence of oxidative damage. Elevated TNF-alpha and IL-6, as determined by enzyme immunoassay, were also found in cone-enriched retinal explants treated with Zaprinast. Our study suggests that this ex vivo model of retinal degeneration in porcine retina could be an alternative model for therapeutic research into the mechanisms of photoreceptor death in cone-related diseases, thus replacing or reducing animal experiments.

Retinitis pigmentosa: rapid neurodegeneration is governed by slow cell death mechanisms

For most neurodegenerative diseases the precise duration of an individual cell's death is unknown, which is an obstacle when counteractive measures are being considered. To address this, we used the rd1 mouse model for retinal neurodegeneration, characterized by phosphodiesterase-6 (PDE6) dysfunction and photoreceptor death triggered by high cyclic guanosine-mono-phosphate (cGMP) levels. Using cellular data on cGMP accumulation, cell death, and survival, we created mathematical models to simulate the temporal development of the degeneration. We validated model predictions using organotypic retinal explant cultures derived from wild-type animals and exposed to the selective PDE6 inhibitor zaprinast. Together, photoreceptor data and modeling for the first time delineated three major cell death phases in a complex neuronal tissue: (1) initiation, taking up to 36 h, (2) execution, lasting another 40 h, and finally (3) clearance, lasting about 7 h. Surprisingly, photoreceptor neurodegeneration was noticeably slower than necrosis or apoptosis, suggesting a different mechanism of death for these neurons.

Early alterations in mitochondrial reserve capacity; a means to predict subsequent photoreceptor cell death

Although genetic and environmental factors contribute to neurodegenerative disease, the underlying etiology common to many diseases might be based on metabolic demand. Mitochondria are the main producer of ATP, but are also the major source of reactive oxygen species. Under normal conditions, these oxidants are neutralized; however, under environmental insult or genetic susceptibility conditions, oxidative stress may exceed cellular antioxidant capacities, leading to degeneration. We tested the hypothesis that loss in mitochondrial reserve capacity plays a causative role in neuronal degeneration and chose a cone photoreceptor cell line as our model. 661W cells were exposed to agents that mimic oxidant stress or calcium overload. Real-time changes in cellular metabolism were assessed using the multi-well Seahorse Biosciences XF24 analyzer that measures oxygen consumption (OCR) and extracellular acidification rates (ECAR). Cellular stress resulted in an early loss of mitochondrial reserve capacity, without affecting basal respiration; and ECAR was increased, representing a compensatory shift of ATP productions toward glycolysis. The degree of change in energy metabolism was correlated with the amount of subsequent cell death 24-hours post-treatment, the concentration-dependent loss in mitochondrial reserve capacity correlated with the number of live cells. Our data suggested first, that loss in mitochondrial reserve capacity is a major contributor in disease pathogenesis; and second, that the XF24 assay might represent a useful surrogate assay amenable to the screening of agents that protect against loss of mitochondrial reserve capacity. In future experiments, we will explore these concepts for the development of neuroprotective agents.

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.

PDE5 Inhibitors as Potential Tools in the Treatment of Cystic Fibrosis

Despite great advances in the understanding of the genetics and pathophysiology of cystic fibrosis (CF), there is still no cure for the disease. Using phosphodiesterase type 5 (PDE5) inhibitors, we and others have provided evidence of rescued F508del-CFTR trafficking and corrected deficient chloride transport activity. Studies using PDE5 inhibitors in mice homozygous for the clinically relevant F508del mutation have been conducted with the aim of restoring F508del-CFTR protein function. We demonstrated, by measuring transepithelial nasal potential difference in F508del mice following intraperitoneal injection of sildenafil, vardenafil, or taladafil at clinical doses are able to restore the decreased CFTR-dependent chloride transport across the nasal mucosa. Moreover, vardenafil, but not sildenafil, stimulates chloride transport through the normal CFTR protein. We developed a specific nebulizer setup for mice, with which we demonstrated, through a single inhalation of PDE5 inhibitors, local activation of CFTR protein in CF. Significant potential advantages of inhalation drug therapy over oral or intravenous routes include rapid onset of pharmacological action, reduced systemic secondary effects, and reduced effective drug doses compared to the drug delivered orally; this underlines the relevance and impact of our work for translational science. More recently, we analyzed the bronchoalveolar lavage of CF and wild-type mice for cell infiltrates and expression of pro-inflammatory cytokines and chemokines; we found that the CFTR activating effect of vardenafil, selected as a representative long-lasting PDE5 inhibitor, breaks the vicious circle of lung inflammation which plays a major role in morbi-mortality in CF. Our data highlight the potential use of PDE5 inhibitors in CF. Therapeutic approaches using clinically approved PDE5 inhibitors to address F508del-CFTR defects could speed up the development of new therapies for CF.

Light-driven calcium signals in mouse cone photoreceptors

Calcium mediates various neuronal functions. The complexity of neuronal Ca²⁺ signaling is well exemplified by retinal cone photoreceptors, which, with their distinct compartmentalization, offer unique possibilities for studying the diversity of Ca²⁺ functions in a single cell. Measuring subcellular Ca²⁺ signals in cones under physiological conditions is not only fundamental for understanding cone function, it also bears important insights into pathophysiological processes governing retinal neurodegeneration. However, due to the proximity of light-sensitive outer segments to other cellular compartments, optical measurements of light-evoked Ca²⁺ responses in cones are challenging. We addressed this problem by generating a transgenic mouse (HR2.1:TN-XL) in which both short- and middle-wavelength-sensitive cones selectively express the genetically encoded ratiometric Ca²⁺ biosensor TN-XL. We show that HR2.1:TN-XL allows recording of light-evoked Ca²⁺ responses using two-photon imaging in individual cone photoreceptor terminals and to probe phototransduction and its diverse regulatory mechanisms with pharmacology at subcellular resolution. To further test this system, we asked whether the classical, nitric oxide (NO)-soluble guanylyl-cyclase (sGC)-cGMP pathway could modulate Ca²⁺ in cone terminals. Surprisingly, NO reduced Ca²⁺ resting levels in mouse cones, without evidence for direct sGC involvement. In conclusion, HR2.1:TN-XL mice offer unprecedented opportunities to elucidate light-driven Ca²⁺ dynamics and their (dys)regulation in cone photoreceptors.

Inhibition of cyclic nucleotide phosphodiesterases by methylxanthines and related compounds

Naturally occurring methylxanthines were the first inhibitors of cyclic nucleotide (cN) phosphodiesterases (PDEs) to be discovered. To improve potency and specificity for inhibition of various PDEs in research and for treatment of diseases, thousands of compounds with related structures have now been synthesized. All known PDE inhibitors contain one or more rings that mimic the purine in the cN substrate and directly compete with cN for access to the catalytic site; this review focuses on inhibitors that contain a nucleus that is closely related to the xanthine ring of theophylline and caffeine and the purine ring of cNs. The specificity and potency of these compounds for blocking PDE action have been improved by appending groups at positions on the rings as well as by modification of the number and distribution of nitrogens and carbons in those rings. Several of these inhibitors are highly selective for particular PDEs; potent and largely selective PDE5 inhibitors are used clinically for treatment of erectile dysfunction [sildenafil (Viagra™), tadalafil (Cialis™) and vardenafil (Levitra™)] and pulmonary hypertension [sildenafil (Revatio™) and tadalafil (Adenocirca)]. Related compounds target other PDEs and show therapeutic promise for a number of maladies.

Phosphodiesterase 6C, cGMP-specific cone alpha'

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.

Phosphodiesterase inhibitors: factors that influence potency, selectivity, and action

Cyclic nucleotide phosphodiesterases (PDEs) are promising targets for pharmacological intervention. The presence of multiple PDE genes, diversity of the isoforms produced from each gene, selective tissue and cellular expression of the isoforms, compartmentation within cells, and an array of conformations of PDE proteins are some of the properties that challenge the development of drugs that target these enzymes. Nevertheless, many of the characteristics of PDEs are also viewed as unique opportunities to increase specificity and selectivity when designing novel compounds for certain therapeutic indications. This chapter provides a summary of the major concepts related to the design and use of PDE inhibitors. The overall structure and properties of the catalytic domain and conformations of PDEs are summarized in light of the most recent X-ray crystal structures. The distinctive properties of catalytic domains of different families as well as the technical challenges associated with probing PDE properties and their interactions with small molecules are discussed. The effect of posttranslational modifications and protein-protein interactions are additional factors to be considered when designing PDE inhibitors. PDE inhibitor interaction with other proteins needs to be taken into account and is also discussed.

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 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.

Structural basis of phosphodiesterase 6 inhibition by the C-terminal region of the gamma-subunit

The inhibitory interaction of phosphodiesterase-6 (PDE6) with its gamma-subunit (Pgamma) is pivotal in vertebrate phototransduction. Here, crystal structures of a chimaeric PDE5/PDE6 catalytic domain (PDE5/6cd) complexed with sildenafil or 3-isobutyl-1-methylxanthine and the Pgamma-inhibitory peptide Pgamma(70-87) have been determined at 2.9 and 3.0 A, respectively. These structures show the determinants and the mechanism of the PDE6 inhibition by Pgamma and suggest the conformational change of Pgamma on transducin activation. Two variable H- and M-loops of PDE5/6cd form a distinct interface that contributes to the Pgamma-binding site. This allows the Pgamma C-terminus to fit into the opening of the catalytic pocket, blocking cGMP access to the active site. Our analysis suggests that disruption of the H-M loop interface and Pgamma-binding site is a molecular cause of retinal degeneration in atrd3 mice. Comparison of the two PDE5/6cd structures shows an overlap between the sildenafil and Pgamma(70-87)-binding sites, thereby providing critical insights into the side effects of PDE5 inhibitors on vision.

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.

Phosphodiesterase 5 inhibition improves synaptic function, memory, and amyloid-beta load in an Alzheimer's disease mouse model

Memory loss, synaptic dysfunction, and accumulation of amyloid beta-peptides (A beta) are major hallmarks of Alzheimer's disease (AD). Downregulation of the nitric oxide/cGMP/cGMP-dependent protein kinase/c-AMP responsive element-binding protein (CREB) cascade has been linked to the synaptic deficits after A beta elevation. Here, we report that the phosphodiesterase 5 inhibitor (PDE5) sildenafil (Viagra), a molecule that enhances phosphorylation of CREB, a molecule involved in memory, through elevation of cGMP levels, is beneficial against the AD phenotype in a mouse model of amyloid deposition. We demonstrate that the inhibitor produces an immediate and long-lasting amelioration of synaptic function, CREB phosphorylation, and memory. This effect is also associated with a long-lasting reduction of A beta levels. Given that side effects of PDE5 inhibitors are widely known and do not preclude their administration to a senile population, these drugs have potential for the treatment of AD and other diseases associated with elevated A beta levels.

Cyclic GMP signaling is involved in the luteinizing hormone-dependent meiotic maturation of mouse oocytes

It is well established that cAMP signaling is an important regulator of the oocyte meiotic cell cycle. Conversely, the function of cGMP during oocyte maturation is less clear. Herein, we evaluated the expression of cGMP-hydrolyzing phosphodiesterases (PDEs) in the somatic and germ cell compartments of the mouse ovarian follicle and demonstrate that PDE5 is preferentially expressed in somatic cells. Cyclic GMP is a potent inhibitor of cAMP hydrolysis from oocyte extracts, with a 50% inhibitory concentration of 97 nM. Luteinizing hormone (LH) stimulation of cultured preovulatory follicles results in a marked decrease in cGMP content, and a nadir is reached in 1.5 h; similarly, oocyte cGMP levels decrease after gonadotropin stimulation in vivo. The LH-dependent decrease in cGMP requires activation of the epidermal growth factor network. Treatment of follicles with a PDE5 inhibitor increases cGMP in the follicle well above unstimulated levels. Although LH causes a decrease in cGMP in follicles preincubated with PDE5 inhibitors, the levels of this nucleotide remain above unstimulated levels. Under these conditions of elevated cGMP, LH stimulation does not cause oocyte maturation after 5 h of incubation. Microinjection of a cGMP-specific PDE into oocytes causes meiotic maturation of wild-type oocytes, suggesting that an intraoocyte pool of cGMP is involved in the maintenance of meiotic arrest. This effect is absent in PDE3A-deficient oocytes. Taken together, these findings provide evidence that cGMP and cAMP signaling cooperate in maintaining meiotic arrest via regulation of PDE3A and that a decrease in cGMP in the somatic compartment is one of the signals contributing to meiotic maturation.

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.

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.

Chronic sildenafil in men with diabetes and erectile dysfunction

Erectile dysfunction frequently represents a neurovascular complication of diabetes mellitus, and it has been calculated that almost 50% of diabetic men will have erectile dysfunction within 6 years after diagnosis. Penile endothelial and smooth muscle cell dysfunction are due to molecular pathway abnormalities (i.e., activation of PKC, increased oxidative stress and overproduction of advanced-glycosylation end products). The response rate to oral drug therapies, such as sildenafil, is lower than in most other groups. Because therapeutic alternatives (i.e., intracavernous injections with vasoactive agents) are not curative, clinical trials aimed to demonstrate rehabilitative effects with daily phosphodiesterase type-5 inhibitors are ongoing. If this approach proves successful, it will determine many advantages over the intracavernosal treatment and potentially induce sexual rehabilitation.

Structural determinants of phosphodiesterase 6 response on binding catalytic site inhibitors

To predict the response of retinal phosphodiesterase on binding catalytic site inhibitors, a homology model of the catalytic domain of subunit alpha of type 6 phosphodiesterase has been built by selecting an experimental structure of type 5 phosphodiesterase as template. Guanosine monophosphate and inhibitors (sildenafil, zaprinast) docked to the type 6 phosphodiesterase binding crevice similarly to the experimental conformations of guanosine monophosphate and sildenafil in the catalytic domain of type 5 phosphodiesterase. Inhibitors, but not guanosine monophosphate, interacted with Phe778 and Met759 (sildenafil) or Met759 (zaprinast), the key residues involved in the interaction between the catalytic binding domain and the inhibitory gamma subunit of type 6 phosphodiesterase. Agreeing with predictions obtained by modelling binding, both inhibitors (1 and 10muM) enhanced the amplitude of electric light responses of the isolated rat retina, however, the enhancement was smaller for the more efficacious inhibitor sildenafil. These paradoxical responses can be explained as a result of the enhancement of light activation of PDE6 through the competition between the catalytic site inhibitors and the gamma subunit residues for catalytic domain residues Phe778 and Met759.