Gene expression and protein localization of calmodulin-dependent phosphodiesterase in adult rat retina

The Sleep Quality- and Myopia-Linked PDE11A-Y727C Variant Impacts Neural Physiology by Reducing Catalytic Activity and Altering Subcellular Compartmentalization of the Enzyme

Recently, a Y727C variant in the dual-specific 3',5'-cyclic nucleotide phosphodiesterase 11A (PDE11A-Y727C) was linked to increased sleep quality and reduced myopia risk in humans. Given the well-established role that the PDE11 substrates cAMP and cGMP play in eye physiology and sleep, we determined if (1) PDE11A protein is expressed in the retina or other eye segments in mice, (2) PDE11A-Y7272C affects catalytic activity and/or subcellular compartmentalization more so than the nearby suicide-associated PDE11A-M878V variant, and (3) Pde11a deletion alters eye growth or sleep quality in male and female mice. Western blots show distinct protein expression of PDE11A4, but not PDE11A1-3, in eyes of Pde11a WT, but not KO mice, that vary by eye segment and age. In HT22 and COS-1 cells, PDE11A4-Y727C reduces PDE11A4 catalytic activity far more than PDE11A4-M878V, with both variants reducing PDE11A4-cAMP more so than PDE11A4-cGMP activity. Despite this, Pde11a deletion does not alter age-related changes in retinal or lens thickness or axial length, nor vitreous or anterior chamber depth. Further, Pde11a deletion only minimally changes refractive error and sleep quality. That said, both variants also dramatically alter the subcellular compartmentalization of human and mouse PDE11A4, an effect occurring independently of dephosphorylating PDE11A4-S117/S124 or phosphorylating PDE11A4-S162. Rather, re-compartmentalization of PDE11A4-Y727C is due to the loss of the tyrosine changing how PDE11A4 is packaged/repackaged via the trans-Golgi network. Therefore, the protective impact of the Y727C variant may reflect a gain-of-function (e.g., PDE11A4 displacing another PDE) that warrants further investigation in the context of reversing/preventing sleep disturbances or myopia.

The sleep quality- and myopia-linked PDE11A-Y727C variant impacts neural physiology by reducing catalytic activity and altering subcellular compartmentalization of the enzyme

Recently, a Y727C variant in the dual-specific 3',5'-cyclic nucleotide phosphodiesterase 11A (PDE11A-Y727C) was linked to increased sleep quality and reduced myopia risk in humans. Given the well-established role that the PDE11 substrates cAMP and cGMP play in eye physiology and sleep, we determined if 1) PDE11A protein is expressed in the retina or other eye segments in mouse, 2) PDE11A-Y7272C affects catalytic activity and/or subcellular compartmentalization more so than the nearby suicide-associated PDE11A-M878V variant, and 3) Pde11a deletion alters eye growth or sleep quality in male and female mice. Western blots show distinct protein expression of PDE11A4, but not PDE11A1-3, in eyes of Pde11a WT-but not KO mice-that vary by eye segment and age. In HT22 and COS-1 cells, PDE11A4-Y727C reduces PDE11A4 catalytic activity far more than PDE11A4-M878V, with both variants reducing PDE11A4-cAMP more so than PDE11A4-cGMP activity. Despite this, Pde11a deletion does not alter age-related changes in retinal or lens thickness, axial length, nor vitreous or anterior chamber depth. Further, Pde11a deletion only minimally changes refractive error and sleep quality. That said, both variants also dramatically alter the subcellular compartmentalization of human and mouse PDE11A4, an effect occurring independently of dephosphorylating PDE11A4-S117/S124 or phosphorylating PDE11A4-S162. Rather, re-compartmentalization of PDE11A4-Y727C is due to the loss of the tyrosine changing how PDE11A4 is packaged/repackaged via the trans-Golgi network. Therefore, the protective impact of the Y727C variant may reflect a gain-of-function (e.g., PDE11A4 displacing another PDE) that warrants further investigation in the context of reversing/preventing sleep disturbances or myopia.

Multiple Roles of cAMP in Vertebrate Retina

cAMP is a key regulatory molecule that controls many important processes in the retina, including phototransduction, cell development and death, growth of neural processes, intercellular contacts, retinomotor effects, and so forth. The total content of cAMP changes in the retina in a circadian manner following the natural light cycle, but it also shows local and even divergent changes in faster time scales in response to local and transient changes in the light environment. Changes in cAMP might also manifest or cause various pathological processes in virtually all cellular components of the retina. Here we review the current state of knowledge and understanding of the regulatory mechanisms by which cAMP influences the physiological processes that occur in various retinal cells.

The photoreceptor protective cGMP-analog Rp-8-Br-PET-cGMPS interacts with cGMP-interactors PKGI, PDE1, PDE6, and PKAI in the degenerating mouse retina

The inherited eye disease retinitis pigmentosa (RP) causes the loss of photoreceptors by a still unknown cell death mechanism. During this degeneration, cyclic guanosine-3',5'-monophosphate (cGMP) levels become elevated, leading to over-activation of the cGMP-binding protein cGMP-dependent protein kinase (PKG). cGMP analogs selectively modified to have inhibitory actions on PKG have aided in impeding photoreceptor death, and one such cGMP analog is Rp-8-Br-PET-cGMPS. However, cGMP analogs have previously been shown to interact with numerous targets, so to better understand the therapeutic action of Rp-8-Br-PET-cGMPS, it is necessary to elucidate its target-selectivity and hence what potential cellular mechanism(s) it may affect within the photoreceptors. Here, we, therefore, applied affinity chromatography together with mass spectrometry to isolate and identify Rp-8-Br-PET-cGMPS interactors from retinas derived from three different murine RP models (i.e., rd1, rd2, and rd10 mice). Our findings revealed that Rp-8-Br-PET-cGMPS bound seven known cGMP-binding proteins, including PKG1β, PDE1β, PDE1c, PDE6α, and PKA1α. Furthermore, an additional 28 proteins were found to be associated with Rp-8-Br-PET-cGMPS. This latter group included MAPK1/3, which is known to connect with cGMP/PKG in other systems. However, in organotypic retinal cultures, Rp-8-Br-PET-cGMPS had no effect on photoreceptor MAPK1/3 expression or activity. To summarize, Rp-8-Br-PET-cGMPS is more target specific compared to regular cGMP.

Myopia in African Americans Is Significantly Linked to Chromosome 7p15.2-14.2

Purpose

The purpose of this study was to perform genetic linkage analysis and association analysis on exome genotyping from highly aggregated African American families with nonpathogenic myopia. African Americans are a particularly understudied population with respect to myopia.

Methods

One hundred six African American families from the Philadelphia area with a family history of myopia were genotyped using an Illumina ExomePlus array and merged with previous microsatellite data. Myopia was initially measured in mean spherical equivalent (MSE) and converted to a binary phenotype where individuals were identified as affected, unaffected, or unknown. Parametric linkage analysis was performed on both individual variants (single-nucleotide polymorphisms [SNPs] and microsatellites) as well as gene-based markers. Family-based association analysis and transmission disequilibrium test (TDT) analysis modified for rare variants was also performed.

Results

Genetic linkage analysis identified 2 genomewide significant variants at 7p15.2 and 7p14.2 (in the intergenic region between MIR148A and NFE2L3 and in the noncoding RNA LOC401324) and 2 genomewide significant genes (CRHR2 and AVL9) both at 7p14.3. No genomewide results were found in the association analyses.

Conclusions

This study identified a significant linkage peak in African American families for myopia at 7p15.2 to 7p14.2, the first potential risk locus for myopia in African Americans. Interesting candidate genes are located in the region, including PDE1C, which is highly expressed in the eyes, and known to be involved in retinal development. Further identification of the causal variants at this linkage peak will help elucidate the genetics of myopia in this understudied population.

cGMP-Phosphodiesterase Inhibition Prevents Hypoxia-Induced Cell Death Activation in Porcine Retinal Explants

Retinal hypoxia and oxidative stress are involved in several retinal degenerations including diabetic retinopathy, glaucoma, central retinal artery occlusion, or retinopathy of prematurity. The second messenger cyclic guanosine monophosphate (cGMP) has been reported to be protective for neuronal cells under several pathological conditions including ischemia/hypoxia. The purpose of this study was to evaluate whether the accumulation of cGMP through the pharmacological inhibition of phosphodiesterase (PDE) with Zaprinast prevented retinal degeneration induced by mild hypoxia in cultures of porcine retina. Exposure to mild hypoxia (5% O₂) for 24h reduced cGMP content and induced retinal degeneration by caspase dependent and independent (PARP activation) mechanisms. Hypoxia also produced a redox imbalance reducing antioxidant response (superoxide dismutase and catalase activities) and increasing superoxide free radical release. Zaprinast reduced mild hypoxia-induced cell death through inhibition of caspase-3 or PARP activation depending on the cell layer. PDE inhibition also ameliorated the effects of mild hypoxia on antioxidant response and the release of superoxide radical in the photoreceptor layer. The use of a PKG inhibitor, KT5823, suggested that cGMP-PKG pathway is involved in cell survival and antioxidant response. The inhibition of PDE, therefore, could be useful for reducing retinal degeneration under hypoxic/ischemic conditions.

Ca(2+)/calmodulin-activated phosphodiesterase 1A is highly expressed in rabbit cardiac sinoatrial nodal cells and regulates pacemaker function

Constitutive Ca(2+)/calmodulin (CaM)-activation of adenylyl cyclases (ACs) types 1 and 8 in sinoatrial nodal cells (SANC) generates cAMP within lipid-raft-rich microdomains to initiate cAMP-protein kinase A (PKA) signaling, that regulates basal state rhythmic action potential firing of these cells. Mounting evidence in other cell types points to a balance between Ca(2+)-activated counteracting enzymes, ACs and phosphodiesterases (PDEs) within these cells. We hypothesized that the expression and activity of Ca(2+)/CaM-activated PDE Type 1A is higher in SANC than in other cardiac cell types. We found that PDE1A protein expression was 5-fold higher in sinoatrial nodal tissue than in left ventricle, and its mRNA expression was 12-fold greater in the corresponding isolated cells. PDE1 activity (nimodipine-sensitive) accounted for 39% of the total PDE activity in SANC lysates, compared to only 4% in left ventricular cardiomyocytes (LVC). Additionally, total PDE activity in SANC lysates was lowest (10%) in lipid-raft-rich and highest (76%) in lipid-raft-poor fractions (equilibrium sedimentation on a sucrose density gradient). In intact cells PDE1A immunolabeling was not localized to the cell surface membrane (structured illumination microscopy imaging), but located approximately within about 150nm inside of immunolabeling of hyperpolarization-activated cyclic nucleotide-gated potassium channels (HCN4), which reside within lipid-raft-rich microenvironments. In permeabilized SANC, in which surface membrane ion channels are not functional, nimodipine increased spontaneous SR Ca(2+) cycling. PDE1A mRNA silencing in HL-1 cells increased the spontaneous beating rate, reduced the cAMP, and increased cGMP levels in response to IBMX, a broad spectrum PDE inhibitor (detected via fluorescence resonance energy transfer microscopy). We conclude that signaling via cAMP generated by Ca(2+)/CaM-activated AC in SANC lipid raft domains is limited by cAMP degradation by Ca(2+)/CaM-activated PDE1A in non-lipid raft domains. This suggests that local gradients of [Ca(2+)]-CaM or different AC and PDE1A affinity regulate both cAMP production and its degradation, and this balance determines the intensity of Ca(2+)-AC-cAMP-PKA signaling that drives SANC pacemaker function.

A role for TREK1 in generating the slow afterhyperpolarization in developing starburst amacrine cells

Slow afterhyperpolarizations (sAHPs) play an important role in establishing the firing pattern of neurons that in turn influence network activity. sAHPs are mediated by calcium-activated potassium channels. However, the molecular identity of these channels and the mechanism linking calcium entry to their activation are still unknown. Here we present several lines of evidence suggesting that the sAHPs in developing starburst amacrine cells (SACs) are mediated by two-pore potassium channels. First, we use whole cell and perforated patch voltage clamp recordings to characterize the sAHP conductance under different pharmacological conditions. We find that this conductance was calcium dependent, reversed at EK, blocked by barium, insensitive to apamin and TEA, and activated by arachidonic acid. In addition, pharmacological inhibition of calcium-activated phosphodiesterase reduced the sAHP. Second, we performed gene profiling on isolated SACs and found that they showed strong preferential expression of the two-pore channel gene kcnk2 that encodes TREK1. Third, we demonstrated that TREK1 knockout animals exhibited an altered frequency of retinal waves, a frequency that is set by the sAHPs in SACs. With these results, we propose a model in which depolarization-induced decreases in cAMP lead to disinhibition of the two-pore potassium channels and in which the kinetics of this biochemical pathway dictate the slow activation and deactivation of the sAHP conductance. Our model offers a novel pathway for the activation of a conductance that is physiologically important.

Calcium-dependent increases in protein kinase-A activity in mouse retinal ganglion cells are mediated by multiple adenylate cyclases

Neurons undergo long term, activity dependent changes that are mediated by activation of second messenger cascades. In particular, calcium-dependent activation of the cyclic-AMP/Protein kinase A signaling cascade has been implicated in several developmental processes including cell survival, axonal outgrowth, and axonal refinement. The biochemical link between calcium influx and the activation of the cAMP/PKA pathway is primarily mediated through adenylate cyclases. Here, dual imaging of intracellular calcium concentration and PKA activity was used to assay the role of different classes of calcium-dependent adenylate cyclases (ACs) in the activation of the cAMP/PKA pathway in retinal ganglion cells (RGCs). Surprisingly, depolarization-induced calcium-dependent PKA transients persist in barrelless mice lacking AC1, the predominant calcium-dependent adenylate cyclase in RGCs, as well as in double knockout mice lacking both AC1 and AC8. Furthermore, in a subset of RGCs, depolarization-induced PKA transients persist during the inhibition of all transmembrane adenylate cyclases. These results are consistent with the existence of a soluble adenylate cyclase that plays a role in calcium-dependent activation of the cAMP/PKA cascade in neurons.

Differential distribution of exchange proteins directly activated by cyclic AMP within the adult rat retina

The recently discovered exchange protein directly activated by cAMP (Epac), a guanine exchange factor for the G-protein RAP-1, is directly activated by cAMP independently of protein kinase A (PKA). While cAMP is known to be an important second messenger in the retina, the presence of Epac has not been investigated in this tissue. The goal of the present study was to determine if the Epac1 and Epac2 genes are present and to characterize their location within the retina. Western blot analysis revealed that Epac1 and Epac2 proteins are expressed within the retina, and the presence of mRNA was demonstrated with the aid of reverse transcriptase polymerase chain reaction (RT-PCR). Additionally, we used immunofluorescence and confocal microscopy to demonstrate that Epac1 and Epac2 have overlapping as well as unique distributions within the retina. Both are present within horizontal cells, rod and cone bipolar cells, cholinergic amacrine cells, retrograde labeled retinal ganglion cells, and Müller cells. Uniquely, Epac2 was expressed by cone photoreceptor inner and outer segments, cell bodies, and synaptic terminals. In contrast, Epac1 was expressed in vesicular glutamate transporter 1 (VGlut1) and C-terminal binding protein 2 (CtBP2) positive photoreceptor synaptic terminals. Together, these results provide evidence that Epac1 and Epac2 are differentially expressed within the retina and provide the framework for further functional studies of cAMP pathways within the retina.

The novel distribution of phosphodiesterase-4 subtypes within the rat retina

Phosphodiesterases (PDEs) are important regulators of signal transduction processes. While much is known about the function of cyclic GMP-specific PDEs in the retina, much less is known about the closely related, cyclic AMP-specific PDEs. The purpose of the present study is to characterize and localize PDE4 within the adult rat retina. We have used Western blotting, RT-PCR, and immunohistochemistry together with retrograde labeling to determine the presence and location of each PDE4 subtype. Western blot analysis revealed that multiple isoforms of PDE4A, B, and D subtypes are present within the retina, whereas the PDE4C subtype was absent. These data were confirmed by RT-PCR. Using immunohistochemistry we show that all three PDE4s are abundantly expressed within the retina where they all colocalize with retrograde-labeled retinal ganglion cells, as well as bipolar cells, horizontal cells, and cholinergic amacrine cells, whereas Müller cells lack PDE4 expression. Uniquely, PDE4B was expressed by the inner and outer segments of rod photoreceptors as well as their terminals within the outer plexiform layer. Collectively, our results demonstrate that PDE4s are abundantly expressed throughout the rodent retina and this study provides the framework for further functional studies.

PKG activity causes photoreceptor cell death in two retinitis pigmentosa models

Photoreceptor degeneration in retinitis pigmentosa is one of the leading causes of hereditary blindness in the developed world. Although causative genetic mutations have been elucidated in many cases, the underlying neuronal degeneration mechanisms are still unknown. Here, we show that activation of cGMP-dependent protein kinase (PKG) hallmarks photoreceptor degeneration in rd1 and rd2 human homologous mouse models. When induced in wild-type retinae, PKG activity was both necessary and sufficient to trigger cGMP-mediated photoreceptor cell death. Target-specific, pharmacological inhibition of PKG activity in both rd1 and rd2 retinae strongly reduced photoreceptor cell death in organotypic retinal explants. Likewise, inhibition of PKG in vivo, using three different application paradigms, resulted in robust photoreceptor protection in the rd1 retina. These findings suggest a pivotal role for PKG activity in cGMP-mediated photoreceptor degeneration mechanisms and highlight the importance of PKG as a novel target for the pharmacological intervention in RP.

Probing neurochemical structure and function of retinal ON bipolar cells with a transgenic mouse

Retinal ON bipolar cells make up about 70% of all bipolar cells. Glutamate hyperpolarizes these cells by binding to the metabotropic glutamate receptor mGluR6, activating the G-protein G(o1), and closing an unidentified cation channel. To facilitate investigation of ON bipolar cells, we here report on the production of a transgenic mouse (Grm6-GFP) in which enhanced green fluorescent protein (EGFP), under control of mGluR6 promoter, was expressed in all and only ON bipolar cells. We used the mouse to determine density of ON bipolar cells, which in central retina was 29,600 cells/mm(2). We further sorted the fluorescent cells and created a pure ON bipolar cDNA library that was negative for photoreceptor unique genes. With this library, we determined expression of 27 genes of interest. We obtained positive transcripts for G(o) interactors: regulators of G-protein signaling (RGS), Ret-RGS1 (a variant of RGS20), RGS16, RGS7, purkinje cell protein 2 (PCP2, also called L7 or GPSM4), synembryn (RIC-8), LGN (GPSM2), RAP1GAP, and Gbeta5; cGMP modulators: guanylyl cyclase (GC) 1alpha1, GC1beta1, phosphodiesterase (PDE) 1C, and PDE9A; and channels: inwardly rectifying potassium channel Kir2.4, transient receptor potential TRPC2, and sperm-specific cation channels CatSper 2-4. The following transcripts were not found in our library: AGS3 (GPSM1), RGS10, RGS19 (GAIP), calbindin, GC1alpha2, GC1beta2, PDE5, PDE2A, amiloride-sensitive sodium channel ACCN4, and CatSper1. We then localized Kir2.4 to several cell types and showed that, in ON bipolar cells, the channel concentrates in their dendritic tips. The channels and modulators found in ON bipolar cells likely shape their light response. Additional uses of the Grm6-GFP mouse are also discussed.

Gene expression and protein localization of calmodulin-dependent phosphodiesterase during ontogenesis of chick retina

Calmodulin-dependent phosphodiesterase (PDE1) is a key enzyme in cyclic nucleotides metabolism. We studied its gene expression and protein localization during retinal development in chick embryos. Western blot and densitometric analysis demonstrated that the expression of the three isoforms changed during development. PDE1A was highly expressed at the early stages and decreased as development proceeded. PDE1B expression remained relatively low and constant over time. PDE1C showed a prominent increase (13-fold) between embryonic day (E) 7 and E13, followed by a moderate increase between E13 and postnatal day (P) 1. The presence of the enzyme in the different retinal locations was strongly modulated by development. PDE1A immunostaining was first detected at the ganglion cell level (E7), then in the outer retina (E15-E21). At P5, the immunostaining was confined in the optic fiber layer. Isoform C immunolocalization followed the same inner-outer pattern as isoform A. At 5 days posthatching (P5), the immunoreactivity was restricted, as well as for the isoform A, in the optic fiber layer. The isoform B immunolabelling was low and evenly distributed across the retina at all stages. The different developmental profiles of PDE1A, PDE1B, and PDE1C induced a temporal modulation in cyclic nucleotides concentration, suggesting specific roles of this enzyme in the morphofunctional development of retinal circuitry.