Light-dependent phosphorylation of the gamma subunit of cGMP-phophodiesterase (PDE6gamma) at residue threonine 22 in intact photoreceptor neurons

Cone cell dysfunction attenuates retinal neovascularization in oxygen-induced retinopathy mouse model

Aberrant neovascularization is the most common feature in retinopathy of prematurity (ROP), which leads to the retinal detachment and visual defects in neonates with a low gestational age eventually. Understanding the regulation of inappropriate angiogenic signaling benefits individuals at-risk. Recently, neural activity originating from the specific neural activity has been considered to contribute to retinal angiogenesis. Here, we explored the impact of cone cell dysfunction on oxygen-induced retinopathy (OIR), a mouse model commonly employed to understand retinal diseases associated with abnormal blood vessel growth, using the Gnat2cpfl3 (cone photoreceptor function loss-3) strain of mice (regardless of the sex), which is known for its inherent cone cell dysfunction. We found that the retinal avascular area, hypoxic area, and neovascular area were significantly attenuated in Gnat2cpfl3 OIR mice compared to those in C57BL/6 OIR mice. Moreover, the HIF-1α/VEGF axis was also reduced in Gnat2cpfl3 OIR mice. Collectively, our results indicated that cone cell dysfunction, as observed in Gnat2cpfl3 OIR mice, leads to attenuated retinal neovascularization. This finding suggests that retinal neural activity may precede and potentially influence the onset of pathological neovascularization.

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

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

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

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

PKM2 ablation enhanced retinal function and survival in a preclinical model of retinitis pigmentosa

Retinitis pigmentosa (RP) is a neurodegenerative disorder that causes irreversible vision loss in over 1.5 million individuals world-wide. The genetic heterogeneity of RP necessitates a broad therapy that is able to provide treatment in a gene- and mutation- non-specific manner. In this study, we identify the therapeutic benefits of metabolic reprogramming by targeting pyruvate kinase M2 (PKM2) in a Pde6β preclinical model of RP. The genetic contributions of PKM2 inhibition in retinal degeneration were evaluated through histology and electroretinogram (ERG) followed by a statistical analysis using a linear regression model. Notably, PKM2 ablation resulted in thicker retinal layers in Pde6β-mutated mice as compared to the controls, suggesting greater photoreceptor survival. Consistent with these anatomical findings, ERG analyses revealed that the maximum b-wave is on average greater in Pkm2 knockout mice than in mice with intact Pkm2, indicating enhanced photoreceptor function. These rescue phenotypes from Pkm2 ablation in a preclinical model of RP indicate that a metabolome reprogramming may be useful in treating RP.

A novel Ca2+-feedback mechanism extends the operating range of mammalian rods to brighter light

A previously unidentified calcium-dependent mechanism contributes to light adaptation in mammalian rods.

Sensory cells adjust their sensitivity to incoming signals, such as odor or light, in response to changes in background stimulation, thereby extending the range over which they operate. For instance, rod photoreceptors are extremely sensitive in darkness, so that they are able to detect individual photons, but remain responsive to visual stimuli under conditions of bright ambient light, which would be expected to saturate their response given the high gain of the rod transduction cascade in darkness. These photoreceptors regulate their sensitivity to light rapidly and reversibly in response to changes in ambient illumination, thereby avoiding saturation. Calcium ions (Ca²⁺) play a major role in mediating the rapid, subsecond adaptation to light, and the Ca²⁺-binding proteins GCAP1 and GCAP2 (or guanylyl cyclase–activating proteins [GCAPs]) have been identified as important mediators of the photoreceptor response to changes in intracellular Ca²⁺. However, mouse rods lacking both GCAP1 and GCAP2 (GCAP^−/−) still show substantial light adaptation. Here, we determined the Ca²⁺ dependency of this residual light adaptation and, by combining pharmacological, genetic, and electrophysiological tools, showed that an unknown Ca²⁺-dependent mechanism contributes to light adaptation in GCAP^−/− mouse rods. We found that mimicking the light-induced decrease in intracellular [Ca²⁺] accelerated recovery of the response to visual stimuli and caused a fourfold decrease of sensitivity in GCAP^−/− rods. About half of this Ca²⁺-dependent regulation of sensitivity could be attributed to the recoverin-mediated pathway, whereas half of it was caused by the unknown mechanism. Furthermore, our data demonstrate that the feedback mechanisms regulating the sensitivity of mammalian rods on the second and subsecond time scales are all Ca²⁺ dependent and that, unlike salamander rods, Ca²⁺-independent background-induced acceleration of flash response kinetics is rather weak in mouse rods.

Silencing of tuberin enhances photoreceptor survival and function in a preclinical model of retinitis pigmentosa (an american ophthalmological society thesis)

PURPOSE

To assess the functional consequences of silencing of tuberin, an inhibitor of the mTOR signaling pathway, in a preclinical model of retinitis pigmentosa (RP) in order to test the hypothesis that insufficient induction of the protein kinase B (PKB)-regulated tuberin/mTOR self-survival pathway initiates apoptosis.

METHODS

In an unbiased genome-scale approach, kinase peptide substrate arrays were used to analyze self-survival pathways at the onset of photoreceptor degeneration. The mutant Pde6b(H620Q)/Pde6b(H620Q) at P14 and P18 photoreceptor outer segment (OS) lysates were labeled with P-ATP and hybridized to an array of 1,164 different synthetic peptide substrates. At this stage, OS of Pde6b(H620Q)/Pde6b(H620Q) rods are morphologically normal. In vitro kinase assays and immunohistochemistry were used to validate phosphorylation. Short hairpin RNA (shRNA) gene silencing was used to validate tuberin's role in regulating survival.

RESULTS

At the onset of degeneration, 162 peptides were differentially phosphorylated. Protein kinases A, G, C (AGC kinases), and B exhibited increased activity in both peptide array and in vitro kinase assays. Immunohistochemical data confirmed altered phosphorylation patterns for phosphoinositide-dependent kinase-1 (PDK1), ribosomal protein S6 (RPS6), and tuberin. Tuberin gene silencing rescued photoreceptors from degeneration.

CONCLUSIONS

Phosphorylation of tuberin and RPS6 is due to the upregulated activity of PKB. PKB/tuberin cell growth/survival signaling is activated before the onset of degeneration. Substrates of the AGC kinases in the PKB/tuberin pathway are phosphorylated to promote cell survival. Knockdown of tuberin, the inhibitor of the mTOR pathway, increased photoreceptor survival and function in a preclinical model of RP.

cAMP controls rod photoreceptor sensitivity via multiple targets in the phototransduction cascade

In early studies, both cyclic AMP (cAMP) and cGMP were considered as potential secondary messengers regulating the conductivity of the vertebrate photoreceptor plasma membrane. Later discovery of the cGMP specificity of cyclic nucleotide–gated channels has shifted attention to cGMP as the only secondary messenger in the phototransduction cascade, and cAMP is not considered in modern schemes of phototransduction. Here, we report evidence that cAMP may also be involved in regulation of the phototransduction cascade. Using a suction pipette technique, we recorded light responses of isolated solitary rods from the frog retina in normal solution and in the medium containing 2 µM of adenylate cyclase activator forskolin. Under forskolin action, flash sensitivity rose more than twofold because of a retarded photoresponse turn-off. The same concentration of forskolin lead to a 2.5-fold increase in the rod outer segment cAMP, which is close to earlier reported natural day/night cAMP variations. Detailed analysis of cAMP action on the phototransduction cascade suggests that several targets are affected by cAMP increase: (a) basal dark phosphodiesterase (PDE) activity decreases; (b) at the same intensity of light background, steady background-induced PDE activity increases; (c) at light backgrounds, guanylate cyclase activity at a given fraction of open channels is reduced; and (d) the magnitude of the Ca²⁺ exchanger current rises 1.6-fold, which would correspond to a 1.6-fold elevation of [Ca²⁺]_in. Analysis by a complete model of rod phototransduction suggests that an increase of [Ca²⁺]_in might also explain effects (b) and (c). The mechanism(s) by which cAMP could regulate [Ca²⁺]_in and PDE basal activity is unclear. We suggest that these regulations may have adaptive significance and improve the performance of the visual system when it switches between day and night light conditions.

Function of the asparagine 74 residue of the inhibitory γ-subunit of retinal rod cGMP-phophodiesterase (PDE) in vivo

The inhibitory subunit of rod cyclic guanosine monophosphate (cGMP) phosphodiesterase, PDE6γ, is a major component of rod transduction and is required to support photoreceptor integrity. The N74A allele of PDE6γ has previously been shown in experiments carried out in vitro to reduce the regulatory inhibition on the PDE6 catalytic core subunits, PDE6αβ. This should, in intact rods, lead to an increase in basal (dark) PDE6 activity producing a state equivalent to light adaptation in the rods and we have examined this possibility using ERG and suction-electrode measurements. The murine opsin promoter was used to drive the expression of a mutant N74A and a wild-type PDE6γ control transgene in the photoreceptors of +/Pde6g(tm1) mice. This transgenic line was crossed with Pde6g(tm1)/Pde6g(tm1) mice to generate animals able to synthesize only the transgenic mutant PDE6γ. We find that the N74A mutation did not produce a significant decrease in circulating current, a decrease in sensitivity or affect the kinetics of the light response, all hallmarks of the light-adapted state. In an in vitro assay of the PDE purified from the N74A transgenic mice and control mice we could find no increase in basal activity of the mutant PDE6. Both the results from the physiology and the biochemistry experiments are consistent with the interpretation that the mutation causes a much milder phenotype in vivo than was predicted from observations made using a cell-free assay system. The in vivo regulation of PDE6γ on PDE6αβ may be more dynamic and context-dependent than was replicated in vitro.

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.