The gamma-subunit of the rod photoreceptor cGMP-binding cGMP-specific PDE is expressed in mouse lung

Autosomal-recessive early-onset retinitis pigmentosa caused by a mutation in PDE6G, the gene encoding the gamma subunit of rod cGMP phosphodiesterase

Retinitis pigmentosa (RP) is the most common form of hereditary retinal degeneration, with a worldwide prevalence of 1 in 4000. Over 30 genes and loci have been implicated in nonsyndromic autosomal-recessive (ar) RP. Genome-wide homozygosity mapping was conducted in two sibships from an extended consanguineous Muslim Arab Israeli family segregating ar severe early-onset RP. A shared homozygous region on chromosome 17q25.3 was identified in both sibships, with an overlap of 4.7 Mb. One of the genes located in this interval is PDE6G, encoding for the inhibitory gamma subunit of rod photoreceptor cyclic GMP-phosphodiesterase. Mutations in the genes encoding for the catalytic subunits of this holoenzyme, PDE6A and PDE6B, cause arRP. Sequencing of all coding exons, including exon-intron boundaries, revealed a homozygous single base change (c.187+1G>T) located in the conserved intron 3 donor splice site of PDE6G. This mutation cosegregated with the disease in the extended family. We used an in vitro splicing assay to demonstrate that this mutation leads to incorrect splicing. Affected individuals had markedly constricted visual fields. Both scotopic and photopic electroretinograms were severely reduced or completely extinct. Funduscopy showed typical bone spicule-type pigment deposits spread mainly at the midperiphery, as well as pallor of the optic disk. Macular involvement was indicated by the lack of foveal reflex and typical cystoid macular edema, proved by optical coherence tomography. These findings demonstrate the positive role of the gamma subunit in maintaining phosphodiesterase activity and confirm the contribution of PDE6G to the etiology of RP in humans.

Cyclic nucleotide phosphodiesterase (PDE) superfamily: a new target for the development of specific therapeutic agents

Cyclic nucleotide phosphodiesterases (PDEs), which are ubiquitously distributed in mammalian tissues, play a major role in cell signaling by hydrolyzing cAMP and cGMP. Due to their diversity, which allows specific distribution at cellular and subcellular levels, PDEs can selectively regulate various cellular functions. Their critical role in intracellular signaling has recently designated them as new therapeutic targets for inflammation. The PDE superfamily represents 11 gene families (PDE1 to PDE11). Each family encompasses 1 to 4 distinct genes, to give more than 20 genes in mammals encoding the more than 50 different PDE proteins probably produced in mammalian cells. Although PDE1 to PDE6 were the first well-characterized isoforms because of their predominance in various tissues and cells, their specific contribution to tissue function and their regulation in pathophysiology remain open research fields. This concerns particularly the newly discovered families, PDE7 to PDE11, for which roles are not yet established. In many pathologies, such as inflammation, neurodegeneration, and cancer, alterations in intracellular signaling related to PDE deregulation may explain the difficulties observed in the prevention and treatment of these pathologies. By inhibiting specifically the up-regulated PDE isozyme(s) with newly synthesized potent and isozyme-selective PDE inhibitors, it may be potentially possible to restore normal intracellular signaling selectively, providing therapy with reduced adverse effects.

Roles of the intracellular regions of angiotensin II receptor AT2 in mediating reduction of intracellular cGMP levels

We have shown previously that the angiotensin II (Ang II) receptor AT2 reduces the intracellular levels of cGMP in Xenopus oocytes when activated by ligand binding, and the C-terminal cytoplasmic tail of the AT2 acts as a negative regulator of this function. Here we report the effects of mutations in the 2nd and 3rd intracellular loops of AT2 on AT2-mediated cGMP reduction. Mutating the highly conserved DRY motif (D141G-R142G-Y143A) of the 2nd ICL implicated in activating G(alpha) subunit of trimeric G-proteins did not affect AT2-mediated cGMP reduction. Moreover, anti-Gialpha antibody or phosphodiesterase inhibitor IBMX did not inhibit AT2-mediated cGMP reduction, suggesting that Gialpha activation and subsequent phosphodiesterase activation are not involved in this function. In contrast, mutations T250R-R251N and L255F-K256R located in the C-terminus of the 3rd ICL of AT2 retained ligand-binding properties of the wild-type AT2, and its ability to interact with the ErbB3 in yeast two-hybrid assay, but abolished AT2-mediated cGMP reduction. Similarities in the roles of ICLs of AT2 in AT2-mediated cGMP reduction in oocytes, and AT2-mediated SHP1 activation in COS-7 cells, (need of 3rd ICL for both functions and lack of involvement of DRY motif), suggest that the cascade of events in these two signaling mechanisms could be similar, and that an oocyte-specific SHP1-like protein may be involved in AT2-mediated cGMP reduction in these cells.

The inhibitory gamma subunit of the type 6 retinal cGMP phosphodiesterase functions to link c-Src and G-protein-coupled receptor kinase 2 in a signaling unit that regulates p42/p44 mitogen-activated protein kinase by epidermal growth factor

The inhibitory gamma subunit of the retinal photoreceptor type 6 cGMP phosphodiesterase (PDEgamma) is phosphorylated by G-protein-coupled receptor kinase 2 on threonine 62 and regulates the epidermal growth factor- dependent stimulation of p42/p44 mitogen-activated protein kinase in human embryonic kidney 293 cells. We report here that PDEgamma is in a pre-formed complex with c-Src and that stimulation of cells with epidermal growth factor promotes the association of GRK2 with this complex. c-Src has a critical role in the stimulation of the p42/p44 mitogen-activated protein kinase cascade by epidermal growth factor, because c-Src inhibitors block the activation of this kinase by the growth factor. Mutation of Thr-62 (to Ala) in PDEgamma produced a GRK2 phosphorylation-resistant mutant that was less effective in associating with GRK2 in response to epidermal growth factor and did not potentiate the stimulation of p42/p44 mitogen-activated protein kinase by this growth factor. The transcript for a short splice variant version of PDEgamma lacking the Thr-62 phosphorylation site is also expressed in certain mammalian cells and, in common with the Thr-62 mutant, failed to potentiate the stimulatory effect of epidermal growth factor on p42/p44 mitogen-activated protein kinase. The mutation of Thr-22 (to Ala) in PDEgamma, which is a site for phosphorylation by p42/p44 mitogen-activated protein kinase, resulted in a prolonged activation of p42/p44 mitogen-activated protein kinase by epidermal growth factor, suggesting a role for this phosphorylation event in the negative feedback control of PDEgamma.

An assessment of the role of the inhibitory gamma subunit of the retinal cyclic GMP phosphodiesterase and its effect on the p42/p44 mitogen-activated protein kinase pathway in animal and cellular models of pulmonary hypertension

1. We have previously reported that the inhibitory gamma subunit of the rod photoreceptor type 6 cyclic GMP phosphodiesterase (PDEgamma) is expressed in nonretinal tissues and is involved in the stimulation of the p42/p44 mitogen-activated protein kinase (MAPK) pathway by growth factors and G-protein-coupled receptor agonists. We have now investigated whether PDEgamma plays a role in modulating chronic hypoxic-dependent mitogenic signalling pathways in pulmonary smooth muscle from rats with pulmonary hypertension (PHT). 2. We show for the first time that PDEgamma is expressed in rat main, first, intrapulmonary and resistance pulmonary arteries. Moreover, its expression is increased in all the arteries to varying extents by chronic hypoxia. The extent of the increased expression of PDEgamma is correlated with an enhanced activation of p42/p44 MAPK in these vessels. 3. We also report that PDEgamma translation from mRNA transcript is increased in cultured human pulmonary artery smooth muscle cells subjected to chronic hypoxia for 14 days. This was correlated with hypoxic-dependent increase in p42/p44 MAPK activation. 4. In conclusion, our studies identify for the first time a major chronic hypoxic-dependent change in the phenotypic expression of an intermediate protein regulating mitogenic signalling in pulmonary arteries. This may have a significant effect on arterial remodelling in PHT. Future studies will focus on strategies designed to knockout rod PDEgamma to assess whether this rescues rats from chronic hypoxic-dependent changes in arterial remodelling and PHT.

The identification of the inhibitory gamma-subunits of the type 6 retinal cyclic guanosine monophosphate phosphodiesterase in non-retinal tissues: differential processing of mRNA transcripts

Here, we report that mouse lung expresses gamma-subunit (PDEgamma) transcripts of the rod and cone photoreceptor cGMP phosphodiesterase genes (Pde6g and Pde6h, respectively). Moreover, a major 14-kDa protein (p14) in lung membranes was immunostained with antibodies that react with both rod and cone PDEgamma. We show that p14 is, in fact, a mixture of rod and cone PDEgamma, based on three additional lines of evidence. First, p14 was also immunostained with antibodies specific for the cone PDEgamma isoform. Second, the expression of p14 immunostained with antibodies recognizing both rod and cone PDEgamma was substantially reduced in lung membranes from Pde6g-/- mice. In contrast, the fraction of p14 stained with cone PDEgamma-specific antibodies was not altered in the Pde6g-/- mice. Third, the absence of the Pde6g transcript was correlated with reduced levels of p14 in Pde6g-/- mice. We have also found that mouse lung contains a small Pde6h transcript that has a 41-bp deletion resulting in a frame change, derived by differential mRNA processing of exon 3 of Pde6h. BLAST searches also revealed a rat ovary EST that has the same 41-bp deletion causing the same frame change. However, the premature in-frame stop codon seen in the short Pde6h transcript is absent and the regular stop codon is out of frame leading to a predicted ORF extension into the 3' UTR. These findings show that rod and cone PDEgamma isoforms are expressed in lung and seem to have a critical role in regulating p42/p44 mitogen-activated protein kinase signaling.

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

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

The inhibitory gamma subunit of the type 6 retinal cyclic guanosine monophosphate phosphodiesterase is a novel intermediate regulating p42/p44 mitogen-activated protein kinase signaling in human embryonic kidney 293 cells

The inhibitory gamma subunits of the retinal rod and cone photoreceptor type 6 retinal cyclic guanosine monophosphate phosphodiesterase (PDEgamma) are expressed in non-retinal tissues. Here, we show that PDEgamma interacts with the G-protein-coupled receptor kinase 2 signaling system to regulate the epidermal growth factor- and thrombin-dependent stimulation of p42/p44 mitogen-activated protein kinase in human embryonic kidney 293 cells. This is based upon several lines of evidence. First, the transfection of cells with an antisense rod PDEgamma plasmid construct, which reduced endogenous rod PDEgamma expression, ablated the epidermal growth factor- and thrombin-dependent stimulation of p42/p44 mitogen-activated protein kinase. Second, the transfection of cells with recombinant rod or cone PDEgamma and/or G-protein-coupled receptor kinase 2 increased the stimulation of p42/p44 mitogen-activated protein kinase by epidermal growth factor or thrombin. In contrast, a G-protein-coupled receptor kinase 2 phosphorylation-resistant rod PDEgamma mutant failed to increase the epidermal growth factor- or thrombin-dependent stimulation of p42/p44 mitogen-activated protein kinase and, in fact, functioned as a dominant negative. Thrombin also stimulated the association of endogenous rod PDEgamma with dynamin II, which was increased in cells transfected with rod PDEgamma or G-protein-coupled receptor kinase 2. Dynamin II plays a critical role in regulating endocytosis of receptor signal complexes required for activation of p42/p44 mitogen-activated protein kinase. Therefore, PDEgamma may have an important role in promoting endocytosis of receptor signal complexes leading to the activation of p42/p44 mitogen-activated protein kinase. We conclude that PDEgamma is an entirely novel intermediate regulating mitogenic signaling from both receptor tyrosine kinase and G-protein-coupled receptors in human embryonic kidney 293 cells.

The gamma subunit of the rod photoreceptor cGMP phosphodiesterase can modulate the proteolysis of two cGMP binding cGMP-specific phosphodiesterases (PDE6 and PDE5) by caspase-3

We have investigated whether the proteolysis of members of the cGMP binding phosphodiesterases (PDE6, PDE5A1, and PDE10A2) by caspase-3 is modulated by the gamma inhibitor subunit of PDE6. We show here that purified caspase-3 proteolyses PDE6, an enzyme composed of two nonidentical catalytic subunits (termed alpha and beta) with molecular mass of 88 and 84 kDa. The proteolysis of PDE6 produced a single fragment with a molecular mass of 78 kDa. This corresponds to the possible cleavage of the caspase-3 consensus DFVD site (amino acids: 164-168) in the alpha subunit and leads to a 50% decrease in the cGMP hydrolysing activity of the enzyme. The addition of rod PDEgamma to the incubation completely blocked the cleavage of PDE6 by caspase-3. In contrast, rod PDEgamma converted PDE5A1 (molecular mass of 98 kDa) to a better substrate for caspase-3. This resulted in the formation of four major fragments with molecular mass of 82-83, 67, 43, and 34 kDa. In addition, caspase-3 induced an approximately 80% reduction in the activity of a partially purified preparation of PDE5A1 in the presence of rod PDEgamma. Caspase-3 also cleaved PDE10A2 (molecular mass of 95 kDa) to a single 48-kDa fragment. This was consistent with cleavage of the DLFD site (amino acids: 312-315) in PDE10A2. In contrast with both PDE6 and PDE5A1, rod PDEgamma was without effect on this enzyme. These data show that rod PDEgamma interacts with at least two members of the cGMP binding PDE family (PDE5A1 and PDE6) and can exert differential effects on the cleavage of these enzymes by caspase-3.

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.

cAMP-dependent induction of PDE5 expression in murine neuroblastoma cell differentiation

The present study demonstrates, in both hybrid NG108-15 and mouse neuroblastoma N18TG2 cells, the presence and regulation of PDE5 mRNA during cell differentiation. PDE5 cDNA probes in Northern blot analysis recognize a approximately 9 kb transcript in bovine lung as well as in mouse neuroblastoma cells. Hybridization on total RNA extracted from dibutyryl-cAMP-treated NG108-15 cells shows a 5-fold increase of PDE5 9 kb mRNA: such an increase is not observed in N18TG2 although we observed a similar increase in the enzymatic activity of both cell lines. Our data demonstrate that PDE5 gene expression can be regulated by cAMP and suggest the existence of a complex regulatory system for PDE5 activity.