Mutation R238E in transducin-alpha yields a GTPase and effector-deficient, but not dominant-negative, G-protein alpha-subunit

PURPOSE

Certain forms of inherited and light-induced retinal degenerations are believed to involve excessive phototransduction signaling. A dominant-negative mutant of the visual G-protein, transducin, would represent a major tool in designing potential therapeutical strategies for this group of visual diseases. We thought to further investigate a novel mutant of the transducin-alpha subunit, R238E, that was recently reported to be a dominant-negative inhibitor of the rhodopsin/transducin/PDE visual system.

METHODS

The R238E substitution was introduced into a tranducin-like chimeric Gtalpha*-subunit. The nucleotide-bound state of the Gtalpha*R238E mutant was assessed using the trypsin-protection assay. The ability of the Gtalpha*R238E mutant to interact with Gtbetagamma, couple to photoexcited rhodopsin (R*), and undergo R*-stimulated guanine nucleotide exchange was examined by a GTPgammaS binding assay. The GTPase activity of the mutant Gtalpha* and its interaction with RGS proteins was characterized in the steady-state and single turnover measurements of GTP hydrolysis. A binding assay utilizing the fluorescently-labeled gamma-subunit of PDE6 (Pgamma) was employed to monitor the effector function of Gtalpha*R238E.

RESULTS

The Gtalpha*R238E mutant bound GDP and was capable of the AlF4--induced activational conformational change. The capacity of Gtalpha*R238E to couple to R* in the presence of Gtbetagamma was similar to that of Gtalpha*. However, the mutant GTPase activity was markedly impaired. This defect was further exacerbated by the diminished interactions of Gtalpha*R238E with the GAP proteins, RGS9 and RGS16. Another consequence of the mutation was the reduction in Gtalpha*R238E's affinity for Pgamma.

CONCLUSIONS

Transducin mutant Gtalpha*R238E exists in a nucleotide-bound state and is fully capable of activational coupling to R*. This mutation results in a significant impairment of Gtalpha*'s ability to hydrolyze GTP and interact with the inhibitory subunit of PDE6. This phenotype is entirely inconsistent with that of a dominant-negative inhibitor as recently reported.

The vertebrate phototransduction cascade: amplification and termination mechanisms

The biochemical cascade which transduces light into a neuronal signal in retinal photoreceptors is a heterotrimeric GTP-binding protein (G protein) signaling pathway called phototransduction. Works from psychophysicists, electrophysiologists, biochemists, and geneticists over several decades have come together to shape our understanding of how photon absorption leads to photoreceptor membrane hyperpolarization. The insights of phototransduction provide the foundation for a mechanistic account of signaling from many other G protein-coupled receptors (GPCR) found throughout nature. The application of reverse genetic techniques has strengthened many historic findings and helped to describe this pathway at greater molecular details. However, many important questions remain to be answered.

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.

A novel role for a Drosophila homologue of cGMP-specific phosphodiesterase in the active transport of cGMP

cGMP was first discovered in urine, demonstrating that kidney cells extrude this cyclic nucleotide. Drosophila Malpighian tubules provide a model renal system in which a homologue of mammalian PDE (phosphodiesterase) 6 is expressed. In humans, this cG-PDE (cGMP-specific PDE) is specifically expressed in the retinal system, where it controls visual signal transduction. In order to gain insight into the functional role of DmPDE6 (Drosophila PDE6-like enzyme) in epithelial function, we generated transgenic animals with targeted expression of DmPDE6 to tubule Type I (principal) cells. This revealed localization of DmPDE6 primarily at the apical membranes. As expected, overexpression of DmPDE6 resulted in elevated cG-PDE activity and decreased tubule cGMP content. However, such targeted overexpression of DmPDE6 creates a novel phenotype that manifests itself in inhibition of the active transport and efflux of cGMP by tubules. This effect is specific to DmPDE6 action, as no effect on cGMP transport is observed in tubules from a bovine PDE5 transgenic line which display reduced rates of fluid secretion, an effect not seen in DmPDE6 transgenic animals. Specific ablation of DmPDE6 in tubule principal cells, via expression of a targeted DmPDE6 RNAi (RNA interference) transgene, conferred increased active transport of cGMP, confirming a direct role for DmPDE6 in regulating cGMP transport in tubule principal cells. Pharmacological inhibition of DmPDE6 in wild-type tubules using the cG-PDE inhibitor, zaprinast, similarly results in stimulated cGMP transport. We provide the first demonstration of a novel role for a cG-PDE in modulating cGMP transport and efflux.

Internalization and recycling of the human prostacyclin receptor is modulated through its isoprenylation-dependent interaction with the delta subunit of cGMP phosphodiesterase 6

Prostacyclin, the major cyclooxygenase-derived product of arachidonic acid formed in the vasculature, mediates its potent anti-thrombotic and anti-proliferative effects through its G protein-coupled receptor (GPCR) termed the IP. Unlike many GPCRs, agonist-induced internalization of the IP occurs in an arrestin/GPCR kinase-independent manner. However, deletion of the IP COOH-terminal region prevented internalization suggesting that protein interactions at this region are involved in IP regulation. Using the COOH-terminal region of IP as bait we identified the delta subunit of cGMP phosphodiesterase 6 (PDE6delta) as a novel hIP-interacting protein in two independent yeast two-hybrid screens. Interaction of IP and PDE6delta was confirmed by co-immunoprecipitation in HEK293 cells, and in HEPG2 cells, which endogenously express neither IP nor PDE6delta. IP isoprenylation was critical for this interaction, as PDE6delta was unable to associate with an isoprenylation-deficient mutant IP (IPSSLC). PDE6delta overexpression altered the temporal pattern of agonist-induced internalization of IP, but not IPSSLC, in HEPG2 cells, increasing initial internalization but facilitating the return of IP to the cell surface despite the continued presence of agonist. Depletion of PDE6delta using short interfering RNA abolished cicaprost-induced IP internalization in human aortic smooth muscle cells. Recycling of IP, but not IPSSLC, upon agonist removal was facilitated by overexpression of PDE6delta. Thus PDE6delta interacts specifically with IP to modulate receptor trafficking.

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

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

Effects of the rd1 mutation and host strain on hippocampal learning in mice

Many of the inbred mouse strains commonly used in biomedical research are homozygous for the rd1 mutation of the Pde6b gene, which causes retinal degeneration. To dissociate the behavioural effects of rd1 homozygosity from those of the genetic background of the host strain in the most widely used paradigms for evaluating the cognitive abilities of mice, two rd1 homozygous strains (C3H/HeJ and CBA/J) were compared with two Pde6b wild-type strains, each possessing a genetic background identical (C3A.BLiA-Pde6b+/J) or very similar (CBA/CaJ) to that of its rd1 homozygous relative. In the fear conditioning procedure, the presence of the rd1 mutation had no effect on performance at any stage, as the superior contextual learning of the CBA/J and CBA/CaJ strains could be explained by genetic background effects alone. In the Morris water maze, only the Pde6b wild-type C3A.BLiA-Pde6b+/J and CBA/CaJ strains were able to demonstrate spatial learning. The study thus demonstrates how retinal degeneration and genetic background have different effects in these two tests of hippocampus-dependent learning and memory.

The N terminus of GTP gamma S-activated transducin alpha-subunit interacts with the C terminus of the cGMP phosphodiesterase gamma-subunit

Dynamic regulation of G-protein signaling in the phototransduction cascade ensures the high temporal resolution of vision. In a key step, the activated alpha-subunit of transducin (Galphat-GTP) activates the cGMP phosphodiesterase (PDE) by binding the inhibitory gamma-subunit (PDEgamma). Significant progress in understanding the interaction between Galphat and PDEgamma was achieved by solving the crystal structure of the PDEgamma C-terminal peptide bound to Galphat in the transition state for GTP hydrolysis (Slep, K. C., Kercher, M. A., He, W., Cowan, C. W., Wensel, T. G., and Sigler, P. B. (2001) Nature 409, 1071-1077). However, some of the structural elements of each molecule were absent in the crystal structure. We have probed the binding surface between the PDEgamma C terminus and activated Galphat bound to guanosine 5'-O-(3-thio)-triphosphate (GTPgammaS) using a series of full-length PDEgamma photoprobes generated by intein-mediated expressed protein ligation. For each of seven PDEgamma photoprobe species, expressed protein ligation allowed one benzoyl-L-phenylalaine substitution at selected hydrophobic C-terminal positions, and the addition of a biotin affinity tag at the extreme C terminus. We have detected photocross-linking from several PDEgamma C-terminal positions to the Galphat-GTPgammaS N terminus, particularly from PDEgamma residue 73. The overall percentage of cross-linking to the Galphat-GTPgammaSN terminus was analyzed using a far Western method for examining Galphat-GTPgammaS proteolytic digestion patterns. Furthermore, mass spectrometric analysis of cross-links to Galphat from a benzoyl-phenylalanine replacement at PDEgamma position 86 localized the region of photoinsertion to Galphat N-terminal residues Galphat-(22-26). This novel Galphat/PDEgamma interaction suggests that the transducin N terminus plays an active role in signal transduction.

Phosphodiesterase 5 (PDE 5) inhibitors for the treatment of male erectile disorder: Attaining selectivity versus PDE6

The role of phosphodiesterase type 5 (PDE5) in the mechanism of male erection has been well understood, and several drugs inhibiting this enzyme are being used for the treatment of erectile dysfunction (ED). Discovery of inhibitors with improved selectivity versus other PDE isozymes could lead to drugs with improved safety profile. Achievement of selectivity versus PDE6, co-inhibition of which results in disturbances of color perception, remains the most challenging aspect of current drug discovery programs. The present review describes several case studies, where significant (>100 fold) selectivity versus PDE6 has been attained via investigation of structure-activity relationships (SAR). Special attention is given to the chemical routes leading to novel chemotypes and allowing efficient exploration of their SAR's. Strategies for attaining inhibitor selectivity discussed below may be applicable for other drug discovery programs.

The regulatory subunit of PDE6 interacts with PACSIN in photoreceptors

PURPOSE

In photoreceptors, phosphodiesterase 6 (PDE6) is regulated in response to light, due to the shuttling of a regulatory subunit (PDE6gamma) between the catalytic subunits of PDE6 and the activated form of transducin. We showed previously that PDE6gamma is able to interact with the Src-homology type 3 (SH3) domain of formin-binding protein 17 (FBP17), a protein involved in membrane receptor endocytosis. FBP17 was not detected in rat retina. Therefore, we looked for other SH3 domain-containing proteins that might interact with PDE6gamma in rat photoreceptors.

METHODS

Several SH3 domains highly homologous to this domain of FBP17 were found by structural alignment. Yeast two-hybrid system and GST pull-downs were used to test interaction of PDE6gamma with these putative partners. Expression patterns in rat retina of the SH3 containing candidates were also determined by immunohistochemistry and western blotting. GST pull-downs and co-immunopreciptations were then used to test in vivo interaction with PDE6gamma in rat retina extracts. Colocalization and light translocation of PDE6gamma and one of its partner were studied by confocal microscopy.

RESULTS

PDE6gamma interacts in vitro with a number of SH3 domains. These interactions involve a polyprolin motif located between amino acids 20 and 28 of PDE6gamma. Several of the putative partners of PDE6gamma are expressed in photoreceptor cells and might therefore interact in vivo with PDE6gamma. Our results show that only PACSIN, a protein implicated in endocytosis, was found to interact with PDE6gamma in rat retina extracts. The colocalization of the two proteins occurs in photoreceptor inner segments and synapses and is greatly enhanced upon illumination of the retina.

CONCLUSIONS

PDE6gamma function is mostly documented in the regulation of phototransduction. Our results provide evidence that in vitro PDE6gamma has a broad pattern of SH3 containing partners expressed in photoreceptors. PDE6gamma interaction with PACSIN points to a possible role of PDE6gamma in endocytosis. Further studies will be needed to understand the exact role of PDE6gamma-PACSIN interactions in photoreceptors. The description of this new function of PDE6gamma might help to understand the molecular mechanism of the severe retinal degeneration observed in PDE6gamma knock-out mice.

Ribozyme knockdown of the gamma-subunit of rod cGMP phosphodiesterase alters the ERG and retinal morphology in wild-type mice

PURPOSE

To generate an animal model of retinal degeneration by using AAV-mediated ribozyme knockdown of the gamma-subunit of the rod cGMP phosphodiesterase (PDEgamma) mRNA in the retina of wild-type mice.

METHODS

Two hammerhead ribozymes, HRz35 and HRz42, were designed to target the PDEgamma gene in wild-type C57BL/6 mice. The efficiency and specificity of the ribozyme cleavage was tested in vitro against three different types of target: short synthetic RNA oligomers, longer targets transcribed from clones, and full-length mRNA from total retinal RNA extracts. After in vitro validation, the ribozymes were cloned and packaged in a recombinant adenoassociated virus (rAAV) containing a proximal 472-bp murine rod opsin promoter (MOPS) to drive ribozyme synthesis. Three-week-old wild-type C57BL/6 mice were injected subretinally with the vectors. For treated versus partner control retinas, responses to light were measured by full-field electroretinography (ERG), and retinal tissues were examined by light microscopy. Messenger RNA and protein levels of PDEgamma gene were monitored by reverse transcription-polymerase chain reaction (RT-PCR) and Western immunoblot assay.

RESULTS

The ribozymes had comparable in vitro kinetic properties in multiple turnover kinetic analyses. Ribozyme HRz35 exhibited a K(cat) of 0.48 minute(-1) and a K(m) of 980 nM, and HRz42 showed a K(cat) of 0.17 minute(-1) and a K(m) of 971 nM. Both ribozymes cleaved at accessible sites in the RNA, as they digested long transcripts transcribed from clones and full-length mRNA from total retinal RNA extracts in vitro. At increasing intervals after subretinal injection with either AAV ribozyme, a 30% to 90% reduction in a- and b-wave amplitudes was observed compared with those in contralateral control eyes that were not injected. Retinal tissue analysis showed that loss of the photoreceptor cells and PDEgamma mRNA and protein paralleled the ERG results.

CONCLUSIONS

Ribozyme-mediated somatic knockdown of wild-type PDEgamma mRNA in vivo can efficiently reduce the target RNA leading to a loss in rod photoreceptors and in rod-mediated ERG amplitudes, thus generating an animal model of retinal degeneration resembling human RP in an essentially normal adult retina. This vector ribozyme technique should be applicable to other genes associated with RP and perhaps also to mRNAs of phototransduction genes not yet associated with RP. Application of this approach may be age and species independent.

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

PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Genotype-phenotype correlation of mouse pde6b mutations

PURPOSE

To identify the underlying molecular defects causing retinal degeneration in seven N-ethyl-N-nitrosourea (ENU) induced mutant alleles of the Pde6b gene and to analyze the timescale of retinal degeneration in these new models of retinitis pigmentosa.

METHODS

Conformation sensitive capillary electrophoresis and DNA sequencing were used to identify the mutations in the Pde6b gene. Visual acuity testing was performed with a visual-tracking drum at ages ranging from postnatal day 25 to week 10. Retinal examination was performed with an indirect ophthalmoscope. Animals were killed and eyes were prepared for histologic analysis.

RESULTS

Point mutations in the seven new alleles of Pde6b were identified: Three generated premature stop codons, two were missense mutations, and two were splice mutations. The three stop codon mutants and one of the splice mutants had phenotypes indistinguishable from the Pde6b(rd1) mouse in rapidity of onset of retinal degeneration, suggesting that they are null alleles. However, the remaining alleles showed slower onset of retinal degeneration, as determined by visual acuity testing, fundus examination, and histology, indicating that they are hypomorphic alleles.

CONCLUSIONS

These data demonstrate a correlation between genotype and phenotype. Four of the mutants with severe genetic lesions have rapid onset of retinal degeneration, as determined by fundus examination. These mice were indistinguishable from Pde6b(rd1) mice, which are effectively blind by 3 weeks of age. In contrast, the milder genetic lesions show a slower progression of the disease and provide the community with models that more closely mimic human retinitis pigmentosa.

Analysis of PDE6 function using chimeric PDE5/6 catalytic domains

cGMP-phosphodiesterases of the PDE6 family are expressed in retinal photoreceptor cells, where they mediate the phototransduction cascade. A system for expression of PDE6 in vitro is lacking, thus straining progress in understanding the structure-function relationships of the photoreceptor enzyme. Here, we report generation and characterization of bacterially expressed chimeric PDE5/6 catalytic domains which are highly soluble, catalytically active, and sensitive to inhibition by the PDE6 Pgamma subunit. Two flexible PDE6 loops, H and M, impart chimeric PDE5/6 catalytic domains with PDE6-like properties. The replacement of the PDE6 H-loop into the PDE5 catalytic domain increases the catalytic rate and the K(m) value for cGMP hydrolysis, whereas the substitution of the M-loop produces catalytic PDE domains responsive to Pgamma. Multiple PDE6 segments preventing functional expression of the catalytic domain are identified, supporting the requirement for specialized photoreceptor chaperones to assist PDE6 folding in vivo.

Interactions between catalytic and inhibitory subunits of PDE6

Rod and cone photoreceptor guanosine 3'5'-cyclic-monophosphate phosphodiesterases (PDEs) are classified into the PDE6 family of cyclic nucleotide PDEs. A unique feature of the PDE6 enzymes is the presence of inhibitory gamma-subunits (Pgamma). The inhibitory interaction between Pgamma and the rod PDE6alphabeta catalytic subunits is critically important for understanding the mechanism of phototransduction. Recent insights into the molecular interface between Pgamma and PDE6alphabeta have been achieved using mutagenesis of Pgamma, fluorescence labeling, and crosslinking approaches.

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

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

Properties of the interaction of Arf-like protein 2 with PDEdelta

Arf-like proteins (Arl) share certain characteristic features with the Arf subfamily of Ras superfamily proteins, but their function is unknown. Here, we show by a variety of spectroscopic techniques that Arl2, unlike most other Ras-related proteins, has micromolar rather than picomolar affinity for nucleotides. As a consequence of low affinity, nucleotide dissociation rates are rather fast, arguing that it is not regulated by guanine nucleotide exchange factors. Arl2 is isolated as prey in a yeast double hybrid screen using phosphodiesterase 6delta (PDEdelta) as bait. This interaction is dependent on GTP, and the binding of PDEdelta substantially stabilizes GTP binding, increasing affinity and decreasing dissociation rates by a similar factor. Among all Arl proteins tested, PDEdelta only interacted with the closely related proteins Arl2 and Arl3, strongly suggesting that Arl2/3 are specific regulators of PDEdelta.

Distinct gene expression profiles and reduced JNK signaling in retinitis pigmentosa caused by RP1 mutations

To understand the mechanisms underlying autosomal dominant progressive retinitis pigmentosa (RP) caused by the mutations of the RP1 gene and to identify molecules that play roles in the early disease process, we used Affymetrix U74Av2 microarrays to compare the gene expression profiles of retinas from Rp1-/- and Rp1+/+ mice at postnatal days (P) 7, 10, 14, 18 and 21. These profiles were independently verified by comparison with results of retinal serial analysis of gene expression, U74Av2 array studies of mouse retinas, real-time PCR and in situ hybridization. We found that the disruption of Rp1 significantly affected the expression of multiple clusters of genes whose products were involved in diverse biological pathways. The molecular responses to the disruption of Rp1 changed dramatically during development and were distinct from responses to the disruption of photoreceptor transcription factors (Crx-/- or Nrl-/-) and a phototransduction molecule (Pde6brd1). We found specific alterations of gene expression in the c-Jun N-terminal kinase (JNK) signaling cascades. Western analysis confirmed that the phosphorylation of key members in the JNK signaling cascades (i.e. JNK1, JNK2, MAP2, MKK4 and c-Jun) is reduced, whereas phospho-ERK and phospho-p38 are unchanged, in Rp1-/- retinas at P18-21. Immunostaining demonstrated that, like Rp1, phospho-JNKs and phospho-MAP2 are present in outer segments of photoreceptors. Our studies reveal unique molecular phenotypes in multiple biological pathways and the specific reduction of JNK signaling cascades in RP1 diseases, and suggest that RP1, a doublecortin-containing microtubule associated protein, and JNK signaling cascades play integral roles in photoreceptor development and maintenance. Our studies further suggest JNK-related therapeutic strategies for RP1 diseases.

NIH Swiss and Black Swiss mice have retinal degeneration and performance deficits in cognitive tests

Swiss mice are among the most commonly used outbred strains in biomedical research. Because prior knowledge of the baseline phenotypes of mouse strains will allow informed selection of strains for particular experiments, we sought to characterize the behavior of two previously untested outbred Swiss strains--NIH Swiss and Black Swiss--in the two most widely used paradigms for evaluating the cognitive abilities of mice. Unlike the C57BL/6J and C57BL/6J-Tyr(c-2J) controls, animals of both outbred Swiss strains were unable to demonstrate learning in the Morris water maze and contextual fear conditioning paradigms. A polymerase chain reaction assay revealed that all of the NIH Swiss and Black Swiss mice tested were homozygous for the recessive retinal degeneration 1 mutation of the Pde6b gene. Histological examination of NIH Swiss and Black Swiss mouse eyes confirmed the presence of retinal degeneration, which causes visual image blindness. These findings indicate that NIH Swiss and Black Swiss mice are visually im paired and thus may be unsuitable for use in some experiments.

Stable transmission of targeted gene modification using single-stranded oligonucleotides with flanking LNAs

Targeted mutagenesis directed by oligonucleotides (ONs) is a promising method for manipulating the genome in higher eukaryotes. In this study, we have compared gene editing by different ONs on two new target sequences, the eBFP and the rd1 mutant photoreceptor βPDE cDNAs, which were integrated as single copy transgenes at the same genomic site in 293T cells. Interestingly, antisense ONs were superior to sense ONs for one target only, showing that target sequence can by itself impart strand-bias in gene editing. The most efficient ONs were short 25 nt ONs with flanking locked nucleic acids (LNAs), a chemistry that had only been tested for targeted nucleotide mutagenesis in yeast, and 25 nt ONs with phosphorothioate linkages. We showed that LNA-modified ONs mediate dose-dependent target modification and analyzed the importance of LNA position and content. Importantly, when using ONs with flanking LNAs, targeted gene modification was stably transmitted during cell division, which allowed reliable cloning of modified cells, a feature essential for further applications in functional genomics and gene therapy. Finally, we showed that ONs with flanking LNAs aimed at correcting the rd1 stop mutation could promote survival of photoreceptors in retinas of rd1 mutant mice, suggesting that they are also active in vivo.

New pyrazolo[1′,5′:1,6]pyrimido[4,5-d]pyridazin-4(3H)-ones as potent and selective PDE5 inhibitors

A series of potent PDE5 inhibitors with high selectivity versus PDE6 isoenzymes was identified.

Sulfhydryl-reactive, cleavable, and radioiodinatable benzophenone photoprobes for study of protein-protein interaction

The major task in proteomics is to understand how proteins interact with their partners. The photo-cross-linking technique enables direct probing of protein-protein interaction. Here we report the development of three novel sulfhydryl-reactive benzophenone photoprobes of short "arm" length, each with a substitution of either amino, iodo, or nitro at the para-position, rendering the benzophenone moiety directly radioiodinatable. Their potential for study of protein-protein interaction was assessed using the inhibitory subunit of rod cGMP phosphodiesterase (PDEgamma) and the activated transducin alphasubunit (G alpha t-GTPgammaS) as a model system. These photoprobes proved to be stable at neutral pH and dithiothreitol-cleavable in addition. The PDEgamma constructs derivatized at the C-terminal positions with these probes could be readily purified, had unaltered PDEgamma functional activity, and were shown to photo-cross-link to G alpha t-GTPgammaS with an efficiency as high as 40%. Additionally, the amino benzophenone probe was radioiodinated, facilitating sensitive detection of label transfer. The uniquely combined features of these benzophenone photoprobes promise robust and flexible methods for characterization of protein-protein interaction, either by mass spectrometry when a nonradioactive label is available or by autoradiography when using radioiodinated derivatives.

PDE6 is an effector for the Wnt/Ca2+/cGMP-signalling pathway in development

Wnt signalling in development operates via members of the Frizzleds, G-protein-coupled receptors that bind specific Wnt ligands and mediate signalling via distinct pathways. The Wnt/Ca(2+)/cGMP pathway mediated by Frizzled-2 was discovered recently. Activation of this pathway leads to increased intracellular concentrations of Ca(2+) and decreased intracellular concentrations of cGMP. The nature of the phosphodiesterase responsible for this Frizzled-2-mediated effect on cGMP levels was identified based on three separate criteria: (i) sensitivity to selective enzyme inhibitors, (ii) behaviour on chromatographic separation, and (ii) isolation by two-dimensional gels in tandem with direct mapping by MS of tryptic digests of the activity. On the basis of results from these three analyses, the cGMP-specific phosphodiesterase, PDE6, is demonstrated to be an effector for the Wnt/Ca(2+)/cGMP signalling pathway of development, which is mediated by Frizzled-2.