Cyclic GMP-phosphodiesterase of rd retina: biosynthesis and content

Kinase activity profiling identifies putative downstream targets of cGMP/PKG signaling in inherited retinal neurodegeneration

Inherited retinal diseases (IRDs) are a group of neurodegenerative disorders that lead to photoreceptor cell death and eventually blindness. IRDs are characterised by a high genetic heterogeneity, making it imperative to design mutation-independent therapies. Mutations in a number of IRD disease genes have been associated with a rise of cyclic 3’,5’-guanosine monophosphate (cGMP) levels in photoreceptors. Accordingly, the cGMP-dependent protein kinase (PKG) has emerged as a new potential target for the mutation-independent treatment of IRDs. However, the substrates of PKG and the downstream degenerative pathways triggered by its activity have yet to be determined. Here, we performed kinome activity profiling of different murine organotypic retinal explant cultures (diseased rd1 and wild-type controls) using multiplex peptide microarrays to identify proteins whose phosphorylation was significantly altered by PKG activity. In addition, we tested the downstream effect of a known PKG inhibitor CN03 in these organotypic retina cultures. Among the PKG substrates were potassium channels belonging to the K_v1 family (KCNA3, KCNA6), cyclic AMP-responsive element-binding protein 1 (CREB1), DNA topoisomerase 2-α (TOP2A), 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 3 (F263), and the glutamate ionotropic receptor kainate 2 (GRIK2). The retinal expression of these PKG targets was further confirmed by immunofluorescence and could be assigned to various neuronal cell types, including photoreceptors, horizontal cells, and ganglion cells. Taken together, this study confirmed the key role of PKG in photoreceptor cell death and identified new downstream targets of cGMP/PKG signalling that will improve the understanding of the degenerative mechanisms underlying IRDs.

Newly Identified Chemicals Preserve Mitochondrial Capacity and Decelerate Loss of Photoreceptor Cells in Murine Retinal Degeneration Models

Purpose: Metabolic stress and associated mitochondrial dysfunction are implicated in retinal degeneration irrespective of the underlying cause. We identified seven unique chemicals from a Chembridge DiverSET screen and tested their protection against photoreceptor cell death in cell- and animal-based approaches. Methods: Calcium overload (A23187) was triggered in 661W murine photoreceptor-derived cells, and changes in redox potential and real-time changes in cellular metabolism were assessed using the MTT and Seahorse Biosciences XF assay, respectively. Cheminformatics to compare structures, and biodistribution in the living pig eye aided in selection of the lead compound. In-situ, retinal organ cultures of rd1 mouse and S334ter-line-3 rat were tested, in-vivo the light-induced retinal degeneration in albino Balb/c mice was used, assessing photoreceptor cell numbers histologically. Results: Of the seven chemicals, six were protective against A23187- and IBMX-induced loss of mitochondrial capacity, as measured by viability and respirometry in 661W cells. Cheminformatic analyses identified a unique pharmacophore with 6 physico-chemical features based on two compounds (CB11 and CB12). The protective efficacy of CB11 was further shown by reducing photoreceptor cell loss in retinal explants from two retinitis pigmentosa rodent models. Using eye drops, CB11 targeting to the pig retina was confirmed. The same eye drops decreased photoreceptor cell loss in light-stressed Balb/c mice. Conclusions: New chemicals were identified that protect from mitochondrial damage and lead to improved mitochondrial function. Using ex-vivo and in-vivo models, CB11 decreased the loss of photoreceptor cells in murine models of retinal degeneration and may be effective as treatment for different retinal dystrophies.

The role of cGMP-signalling and calcium-signalling in photoreceptor cell death: perspectives for therapy development

The second messengers, cGMP and Ca²⁺, have both been implicated in retinal degeneration; however, it is still unclear which of the two is most relevant for photoreceptor cell death. This problem is exacerbated by the close connections and crosstalk between cGMP-signalling and calcium (Ca²⁺)-signalling in photoreceptors. In this review, we summarize key aspects of cGMP-signalling and Ca²⁺-signalling relevant for hereditary photoreceptor degeneration. The topics covered include cGMP-signalling targets, the role of Ca²⁺ permeable channels, relation to energy metabolism, calpain-type proteases, and how the related metabolic processes may trigger and execute photoreceptor cell death. A focus is then put on cGMP-dependent mechanisms and how exceedingly high photoreceptor cGMP levels set in motion cascades of Ca²⁺-dependent and independent processes that eventually bring about photoreceptor cell death. Finally, an outlook is given into mutation-independent therapeutic approaches that exploit specific features of cGMP-signalling. Such approaches might be combined with suitable drug delivery systems for translation into clinical applications.

Suppression of cGMP-Dependent Photoreceptor Cytotoxicity With Mycophenolate Is Neuroprotective in Murine Models of Retinitis Pigmentosa

Purpose

To determine the effect of mycophenolate mofetil (MMF) on retinal degeneration on two mouse models of retinitis pigmentosa.

Methods

Intraperitoneal injections of MMF were administered daily in rd10 and c57 mice starting at postoperative day 12 (P12) and rd1 mice starting at P8. The effect of MMF was assessed with optical coherence tomography, immunohistochemistry, electroretinography, and OptoMotry. Whole retinal cyclic guanosine monophosphate (cGMP) and mycophenolic acid levels were quantified with mass spectrometry. Photoreceptor cGMP cytotoxicity was evaluated with cell counts of cGMP immunostaining.

Results

MMF treatment significantly delays the onset of retinal degeneration and cGMP-dependent photoreceptor cytotoxicity in rd10 and rd1 mice, albeit a more modest effect in the latter. In rd10 mice, treatment with MMF showed robust preservation of the photoreceptors up to P22 with associated suppression of cGMP immunostaining and microglial activation; The neuroprotective effect diminished after P22, but outer retinal thickness was still significantly thicker by P35 and OptoMotry response was significantly better up to P60. Whereas cGMP immunostaining of the photoreceptors were present in rd10 and rd1 mice, hyperphysiological whole retinal cGMP levels were observed only in rd1 mice.

Conclusions

Early treatment with MMF confers potent neuroprotection in two animal models of RP by suppressing the cGMP-dependent common pathway for photoreceptor cell death. The neuroprotective effect of MMF on cGMP-dependent cytotoxicity occurs independently of the presence of hyperphysiological whole retinal cGMP levels. Thus our data suggest that MMF may be an important new class of neuroprotective agent that could be useful in the treatment of patients with RP.

Techniques to Quantify cGMP Dysregulation as a Common Pathway Associated with Photoreceptor Cell Death in Retinitis Pigmentosa

The targeted development of neuroprotective therapies for retinitis pigmentosa (RP) depends upon a better understanding of the mechanisms of photoreceptor cell death. Nucleotide metabolite-associated photoreceptor cell death is an emerging area of research that is important in multiple models of RP, yet the exact pathophysiology remains to be elucidated. One common pathway of photoreceptor cell death in RP is cGMP dysregulation, which is underscored by its potential to be relevant in up to 30% of patients with RP. Optimizing tools for detecting and quantifying nucleotide metabolites in the retina is vital to expanding this area of research. Immunohistochemistry is useful for localizing abnormally high levels of cGMP in a cell-specific manner, while enzyme-linked immunosorbent assay and liquid chromatography-mass spectrometry are quantitative and more sensitive. These techniques can form the basis for more sophisticated experiments to elucidate upstream events in photoreceptor cell death, which will hopefully lead to the development of novel therapies for patients with RP.

Multipotent stem cells isolated from the adult mouse retina are capable of producing functional photoreceptor cells

Various stem cell types have been tested for their potential application in treating photoreceptor degenerative diseases, such as retinitis pigmentosa (RP) and age-related macular degeneration (AMD). Only embryonic stem cells (ESCs) have so far been shown to generate functional photoreceptor cells restoring light response of photoreceptor-deficient mice, but there is still some concern of tumor formation. In this study, we have successfully cultured Nestin(+)Sox2(+)Pax6(+) multipotent retinal stem cells (RSCs) from the adult mouse retina, which are capable of producing functional photoreceptor cells that restore the light response of photoreceptor-deficient rd1 mutant mice following transplantation. After they have been expanded for over 35 passages in the presence of FGF and EGF, the cultured RSCs still maintain stable proliferation and differentiation potential. Under proper differentiation conditions, they can differentiate into all the major retinal cell types found in the adult retina. More importantly, they can efficiently differentiate into photoreceptor cells under optimized differentiation conditions. Following transplantation into the subretinal space of slowly degenerating rd7 mutant eyes, RSC-derived photoreceptor cells integrate into the retina, morphologically resembling endogenous photoreceptors and forming synapases with resident retinal neurons. When transplanted into eyes of photoreceptor-deficient rd1 mutant mice, a RP model, RSC-derived photoreceptors can partially restore light response, indicating that those RSC-derived photoreceptors are functional. Finally, there is no evidence for tumor formation in the photoreceptor-transplanted eyes. Therefore, this study has demonstrated that RSCs isolated from the adult retina have the potential of producing functional photoreceptor cells that can potentially restore lost vision caused by loss of photoreceptor cells in RP and AMD.

Analysis of the RPE sheet in the rd10 retinal degeneration model

BACKGROUND

The normal RPE sheet in the C57BL/6J mouse is subclassified into two major tiling patterns: a regular generally hexagonal array covering most of the surface and a “soft network” near the ciliary body made of irregularly shaped cells. Physics models predict these two patterns based on contractility and elasticity of the RPE cell, and strength of cellular adhesion between cells.

HYPOTHESIS

We hypothesized and identified major changes in RPE regular hexagonal tiling pattern in rd10 compared to C57BL/6J mice.

RESULTS

In rd10 mice, RPE sheet damage was extensive but occurred later than expected, after most retinal degeneration was complete. RPE sheet changes occur in zones with a bullseye pattern. In the posterior zone, around the optic nerve, RPE cells take on larger irregular and varied shapes to maintain an intact monolayer. In mid periphery, RPE cells have a compressed or convoluted morphology that progress into ingrown layers of RPE under the retina. Cells in the periphery maintain their shape and size until the late stages of the RPE reorganization. The number of neighboring cells varies widely depending on zone and progression. RPE morphology continues to deteriorate after the photoreceptors have degenerated.

CONCLUSIONS

The RPE cells are bystanders to photoreceptor degeneration in the rd10 model, and the collateral damage to the RPE results in changes in morphology as early as 30 days old. Quantitative measures of the tiling patterns and histopathology detected here were scripted in a pipeline written in Perl and Cell Profiler (an open source MatLab plugin) and are directly applicable to RPE sheet images from noninvasive fundus autofluorescence (FAF), adaptive optics confocal scanning laser ophthalmoscope (AO-cSLO), and spectral domain optical coherence tomography (SD-OCT) of patients with early stage AMD or RP.

Sustained elevation of intracellular cGMP causes oxidative stress triggering calpain-mediated apoptosis in photoreceptor degeneration

Sustained elevation in cGMP and a concomitant increase in intracellular Ca(2+) levels in the rd1 photoreceptors are followed by a rapid loss of photoreceptors. In a murine-derived photoreceptor cell line, 661W, treated with the phosphodiesterase inhibitor IBMX or the cyclic GMP-gated channel agonist 8-bromo-cGMP, it was previously found that the induced cell death was mediated by calpain and caspase-3. Because oxidative stress is a common product of ionic imbalance or elevated Ca(2+), we tested the role of oxidative stress in cGMP-induced photoreceptor cell death. In the rd1 mouse retina, oxidative stress was found to precede calpain and caspase-3 activation. In 661W cells, the increase in intracellular cGMP and Ca(2+) resulted in the generation of reactive oxygen species (ROS), the activation of oxidative stress enzymes, and the activation of calpain, followed by apoptosis mediated by the effector caspase-3. All these events, including calpain activation, were ameliorated by docosahexanoic acid (DHA). The cell-permeable inhibitor of calpain, SJA6017, while inhibiting cell death, had no effect on the generation of oxidative stress. These results establish a central role for oxidative stress in cGMP-induced cell death and suggest a ROS-mediated sequential activation of signal transduction events, which provide targets for future treatment strategies.

Up-regulation and increased phosphorylation of protein kinase C (PKC) delta, mu and theta in the degenerating rd1 mouse retina

The rd1 mouse serves as a model for inherited photoreceptor degeneration: retinitis pigmentosa. Microarray techniques were employed to compare the transcriptomes of rd1 and congenic wild-type retinas at postnatal day 11, when degenerative processes have started but most photoreceptors are still present. Of the several genes that were differentially expressed, focus was put on those associated with the protein kinase C (PKC) signaling pathway, in particular PKCdelta, mu and theta. Microarray identified these as being up-regulated in the rd1 retina, which was confirmed by QRT-PCR. Western blotting and immunostaining, using antibodies against either total or phosphorylated variants of the PKC isoforms, revealed increased expression and phosphorylation of PKCdelta, mu and theta in the rd1 retina at the protein level as well. Our results suggest that these PKC isoforms are involved in rd1 degeneration.

Calpain is activated in degenerating photoreceptors in the rd1 mouse

The retinal degeneration (rd)1 mouse displays an inherited retinal degeneration and therefore allows studies of the molecular mechanisms behind the blinding disease retinitis pigmentosa. Activation of the calcium-dependent protease calpain has been suggested to play an important role in cell death in various tissues, but little is known about the expression and activity of calpain during inherited retinal degeneration. Using microarray techniques, transcript levels of cyclic AMP response element-binding protein (CREB)-1, calpastatin and of various calpain genes were analysed in the rd1 mouse compared with its wild-type control. Expression of distinct calpain isoforms and calpastatin was investigated using immunofluorescence and immunoblotting. Gene transcription and protein expression levels were compared with calpain activity using an enzymatic assay that allowed monitoring of calpain activity at the cellular level. We found that CREB-1 and calpastatin expression was reduced in rd1 retinas, whereas calpain activity was substantially increased in rd1 photoreceptors. Calpain activity peaked at postnatal day 13, together with rd1 photoreceptor cell death. Calpain-specific inhibitors decreased calpain activity in situ. These results indicate that activation of calpains correlates with rd1 photoreceptor cell death, which raises the possibility of using calpain inhibitors to prevent or delay photoreceptor degeneration.

Differential modification of phosducin protein in degenerating rd1 retina is associated with constitutively active Ca2+/calmodulin kinase II in rod outer segments

Retinitis pigmentosa comprises a heterogeneous group of incurable progressive blinding diseases with unknown pathogenic mechanisms. The retinal degeneration 1 (rd1) mouse is a retinitis pigmentosa model that carries a mutation in a rod photoreceptor-specific phosphodiesterase gene, leading to rapid degeneration of these cells. Elucidation of the molecular differences between rd1 and healthy retinae is crucial for explaining this degeneration and could assist in suggesting novel therapies. Here we used high resolution proteomics to compare the proteomes of the rd1 mouse retina and its congenic, wild-type counterpart at postnatal day 11 when photoreceptor death is profound. Over 3000 protein spots were consistently resolved by two-dimensional gel electrophoresis and subjected to a rigorous filtering procedure involving computer-based spot analyses. Five proteins were accepted as being differentially expressed in the rd1 model and subsequently identified by mass spectrometry. The difference in one such protein, phosducin, related to an altered modification pattern in the rd1 retina rather than to changed expression levels. Additional experiments showed phosducin in healthy retinae to be highly phosphorylated in the dark- but not in the light-adapted phase. In contrast, rd1 phosducin was highly phosphorylated irrespective of light status, indicating a dysfunctional rd1 light/dark response. The increased rd1 phosducin phosphorylation coincided with increased activation of calcium/calmodulin-activated protein kinase II, which is known to utilize phosducin as a substrate. Given the increased rod calcium levels present in the rd1 mutation, calcium-evoked overactivation of this kinase may be an early and long sought for step in events leading to photoreceptor degeneration in the rd1 mouse.

Inhibitory modulation of photoreceptor melatonin synthesis via a nitric oxide-mediated mechanism

Nitric oxide (NO) has been suggested to have many physiological functions in the vertebrate retina, including a role in light-adaptive processes. The aim of this study was to examine the influence of the NO-donor sodium nitroprusside (SNP) on the activity of arylalkylamine-N-acetyltransferase (AA-NAT; EC. 2.3.1.87), the activity of which responds to light and reflects the changes in retinal melatonin synthesis--a key feature of light-adaptive responses in photoreceptors. Incubation of dark-adapted and dark-maintained retinas with SNP lead to the NO-specific suppression of AA-NAT activity, with NO suppressing AA-NAT activity to a level similar to that seen in the presence of dopaminergic agonists or light. Increased levels of cGMP appeared to be causally involved in the suppression of AA-NAT activity by SNP, as non-hydrolysable analogues of cGMP and the cGMP-specific phosphodiesterase (PDE) inhibitor zaprinast also significantly suppressed AA-NAT activity, while an inhibitor of soluble guanylate cyclase blocked the effect of SNP. While this chain of events may not be part of the normal physiology of the retina, it could be important in pathological circumstances that are associated with marked increase in levels of cGMP, as is found to be the case in certain forms photoreceptor degeneration, which are produced by defects in cGMP phosphodiesterase activity.

Does constitutive phosphorylation protect against photoreceptor degeneration in Rpe65-/- mice?

Despite the presence of this virgin opsin, Rpe65-/- rods are behaving like dark-adapted rods. These results argue that opsin which has not been exposed to 11-cis retinal and is constitutively phosphorylated, does not generate the activity generally associated with the bleached apoprotein. However, increased light-independent activation of transducin (due to bleached opsin) could be demonstrated after the addition of exogenous 11-cis retinal. We hypothesize that free opsin in the Rpe65-/- rods does not cause degeneration of rods by constitutive activation of the phototransduction cascade; but rather rods may die due to other causes such as the impairment of RPE function due to excess unprocessed retinyl-esters in the RPE.

Calcium-induced calpain mediates apoptosis via caspase-3 in a mouse photoreceptor cell line

The rd mouse, an accepted animal model for photoreceptor degeneration in retinitis pigmentosa, has a recessive mutation for the gene encoding the beta-subunit of the cGMP phosphodiesterase. This mutation results in high levels of cGMP, which leaves an increased number of the cGMP-gated channels in the open state, thus allowing intracellular calcium (Ca(2+)) to rise to toxic levels, and rapid photoreceptor degeneration follows. To delineate the events in rd photoreceptor degeneration, we demonstrated an increase in calpain and caspase-3 activity, hypothesizing that Ca(2+)-mediated apoptosis in photoreceptors is mediated by calpain, involving mitochondrial depolarization and caspase-3 activation. To examine this hypothesis further, a murine photoreceptor-derived cell line (661W) was treated with the Ca(2+) ionophore A23187, cGMP-gated channel agonist 8-bromo-cGMP, or phosphodiesterase inhibitor isobutylmethylxanthine to mimic the increased Ca(2+) influx seen in the rd photoreceptors. Ca(2+)-induced cell death in 661W cells was found to be mediated by calpain and caspase-3 and could be completely inhibited by the calpain inhibitor SJA6017, implicating both calpain and caspases in the apoptotic process. The apoptotic events correlated in an SJA6017-inhibitable manner with bid cleavage, mitochondrial depolarization, cytochrome c release, and caspase-3 and -9 activation. We concluded that Ca(2+) influx in the rd model of photoreceptor degeneration leads to the activation of the cysteine protease calpain, which executes apoptosis via modulation of caspase-3 activity.

Differential expression of rod photoreceptor cGMP-phosphodiesterase alpha and beta subunits: mRNA and protein levels

The catalytic core of photoreceptor-specific cGMP-phosphodiesterase (PDE) consists of two subunits, PDEalpha and PDEbeta, that are homologous and have similar domain organization but are encoded by different genes. We have examined the PDEalpha and PDEbeta mRNA steady-state and protein levels as well as the biosynthesis rate of these proteins in developing and fully differentiated retinas. We have also determined the translational efficiency of PDE subunits and the role of their mRNA structures in regulating protein synthesis. In mature retinas, PDEalpha and PDEbeta are represented by approximately 1.5 x 108 and 7.5 x 108 copies/microg retinal mRNA, respectively. The levels of these transcripts in developing photoreceptors (P10) are approximately 75% of those at P30. Quantification of protein concentration indicated that PDEalpha and PDEbeta are equally expressed in developing and fully differentiated photoreceptors. Furthermore, the PDEalpha/PDEbeta ratios obtained throughout a 2-h pulsechase period revealed a similar turnover rate for both subunits. The observed discordance between the mRNA and protein levels of PDEalpha and PDEbeta suggested post-transcriptional regulation of their expression. We found that PDEalpha mRNA is translated more efficiently than either of the two PDEbeta transcripts expressed in retina. Therefore, the lower level of PDEalpha mRNA is compensated by its more efficient translation to achieve equimolar expression with PDEbeta. We also analyzed the effect of PDEalpha and PDEbeta mRNA 5'- and 3'-untranslated regions as well as that of their coding regions on protein synthesis. We determined that the PDE-coding regions play a critical role in the differential translation of these subunits.

Molecular aspects of retinal degenerative diseases

Retinal degeneration, either acquired or inherited, is a major cause of visual impairment and blindness in humans. Inherited retinal degeneration comprises a large group of diseases that result in the loss of photoreceptor cells. To date, 131 retinal disease loci have been identified, and 76 of the genes at these loci have been isolated (RetNet Web site). Several of these genes were first considered candidates because of their chromosomal localization or homology to genes involved in retinal degeneration in other organisms. In this review, I will discuss recent advances in the identification of genes that cause retinal degeneration, and I will describe the mechanisms of photoreceptor death and potential treatments for retinal degenerative diseases.

Regulation of cAMP by light and dopamine receptors is dysfunctional in photoreceptors of dystrophic retinal degeneration slow(rds) mice

cAMP levels in dark and light were studied in dystrophic retinal degeneration slow(rds) mice, which carry a mutation in the rds/peripherin gene. cAMP levels were measured in vivo, in freshly isolated retinas, and in vitro in the presence of glutamate, which confines light modulation to photoreceptors. Experiments were conducted on young animals, when significant numbers of viable photoreceptor cells are present. In vivo levels of cAMP are higher in illuminated rds/rds retinas than levels measured in normal BALB/c retinas. Light-evoked down-regulation of cAMP levels was observed in vitro in normal photoreceptors. These measurements were made in the presence of the cyclic nucleotide phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine; therefore, they reflect an inhibition of cAMP formation. In contrast, light had no effect on cAMP formation in photoreceptors of mutant mice, measured under identical conditions. Thus, elevated levels of cAMP in rds/rds retinas in vivo result from abnormalities in cAMP synthesis in the mutant photoreceptor cells. In addition to regulation by light, cAMP formation in photoreceptor cells is regulated by dopamine, acting through dopamine D4 receptors. A dopamine D2/D4 receptor agonist, quinpirole, reduced cAMP levels in dark-adapted normal retinas in vitro, but not in rds/rds retinas. Our data indicate that alterations in a signal transduction pathway that leads to inhibition of adenylyl cyclase might underlie the abnormalities in cAMP levels in mutant rds/rds retinas. Heterozygous rds/+ photoreceptors demonstrated a normal pattern of light-evoked and quinpirole-mediated down-regulation of cAMP. Thus, partial expression of the normal phenotype is sufficient to render normal characteristics of cAMP regulation to the photoreceptors of the heterozygous mouse. The data obtained in the present study might be relevant to the understanding of photoreceptor pathology of patients with peripherin/rds mutations.

Expression of Bcl-2 protects against photoreceptor degeneration in retinal degeneration slow (rds) mice

The retinal degeneration slow or rds gene encodes rds/peripherin, an integral membrane glycoprotein in the outer segments of rod and cone photoreceptors. Mice homozygous for a null mutation in rds fail to develop outer segments and undergo subsequent degeneration of photoreceptors by the apoptotic pathway. Mutations in the human RDS gene are responsible for several forms of inherited blindness including autosomal-dominant retinitis pigmentosa and macular degeneration. Here, we examined the effects of ectopic Bcl-2 expression in transgenic photoreceptors on the rate of retinal degeneration in rds mutant mice. We observed an approximately twofold preservation of photoreceptors compared with nontransgenic rds mutant mice at 3 months. Immunoblot analysis showed similar levels of Bcl-2 in 2-, 3-, and 4-week-old transgenic mice. Expression of Bcl-2 in the rds mouse did not lead to outer segment formation and did not induce cell death. These results suggest that Bcl-2 expression may be an effective therapeutic strategy in humans with mutations in RDS or other genes that affect the integrity of photoreceptor outer segments.

Animal models of human retinal dystrophies

Naturally occurring retinal dystrophies in laboratory and companion animals represent a wealth of different conditions, some of which are important from a comparative point of view, and all of which offer opportunities to further the understanding of retinal function and reaction in health and disease. The study of animal models of retinal dystrophies has provided candidate genes for investigation in conditions of man such as retinitis pigmentosa and has also led to the identification of new genes and even new families of genes. Mutations in the gene for the beta subunit of cyclic GMP phosphodiesterase cause retinal dystrophies in man, mice and dog, and mutations in the gene for the structural protein peripherin/RDS result in a retinal dystrophy in the mouse and a spectrum of differing retinal dystrophies in man. Animals with homologous retinal dystrophies to man may make useful models for investigation of treatment either by drugs or by gene therapy. Furthermore the use of transgenics and gene targeting in laboratory mice offers the opportunity to create new models of human retinal dystrophies and also to investigate the effect of gene dysfunction.

Retinal projections in mice with inherited retinal degeneration: implications for circadian photoentrainment

The availability of naturally occurring and transgenic retinal mutants has made the mouse an attractive experimental model to address questions regarding photoentrainment of circadian rhythms. However, very little is known about the retinal cells and the retinal projections to the nuclei of the murine circadian timing system. Furthermore, the effect of inherited retinal degeneration on these projections is not understood. In this report, we have used pseudorabies virus as a neuroanatomical tract tracer in mice to address a series of questions: Which retinal cells mediate circadian responses to light? What is the nature of the retinohypothalamic projection? What is the impact of the inherited retinal disorder, retinal degenerate (rd/rd), on the structures of the photoentrainment pathway? Our results show that a class ofretinal ganglion cell, morphologically similar to the type III ganglion cells of the rat, appears to project to central circadian structures of the mouse. They are few in number and sparsely distributed throughout the retina. The low number and broad distribution of these specialized retinal ganglion cells may be an adaptive mechanism to integrate environmental irradiance without compromising the spatial resolution required for vision. In addition, viral infection of conelike and rodlike photoreceptors and amacrinelike cells suggest that these cells may mediate or contribute to circadian responses to light. Inherited retinal degeneration has no obvious effect on the anatomy of the retinal cells or their projections to the circadian axis. These anatomical findings are consistent with our previous findings showing that aged rd/rd mice are capable of regulating their circadian rhythms by light with unattenuated sensitivity.

Ubiquitinylation and ubiquitin-dependent proteolysis in vertebrate photoreceptors (rod outer segments). Evidence for ubiquitinylation of Gt and rhodopsin

In corroboration of the hypothesized regulation of phototransduction proteins by the ubiquitin-dependent pathway, we identified free ubiquitin (8 kDa) and ubiquitin-protein conjugates (50 to >200 kDa; pI 5.3-6.8 by two-dimensional electrophoresis) in bovine rod outer segments (ROS). A 38-kDa ubiquitinylated protein and transducin (Gt) were eluted together from light-adapted ROS membranes with GTP. When ROS were dark-adapted, this 38-kDa ubiquitinylated species and Gt were readily solubilized in buffer lacking GTP. These data are consistent with ubiquitinylation of Gt and corroborate previous cell-free experiments identifying Gt as a substrate for ubiquitin-dependent proteolysis (Obin, M. S., Nowell, T., and Taylor, A. (1994) Biochem. Biophys. Res. Commun. 200, 1169-1176). Evidence for ubiquitinylation of rhodopsin (36 kDa), the (photo)receptor coupled to Gt, included (i) the presence in ROS membranes "stripped" of peripheral membrane proteins of numerous ubiquitin-protein conjugates, including two whose masses (44 and 50 kDa) are consistent with mono- and diubiquitinylated rhodopsin; (ii) catalysis by permeabilized ROS of 125I-labeled ubiquitin-protein conjugates whose masses (42, 50, and 58 kDa) suggest a "ladder" of mono-, di-, and triubiquitinylated rhodopsin; (iii) parallel mobility shifts on SDS-polyacrylamide gels of rhodopsin and these 125I-labeled ubiquitin-protein conjugates; and (iv) generation of enhanced levels of 125I-labeled ubiquitin-protein conjugates when stripped, detergent-solubilized ROS membranes (95% rhodopsin) were incubated with reticulocyte lysate. A functional ubiquitin-dependent pathway in ROS is demonstrated by the presence of (i) the ubiquitin-activating enzyme (E1); (ii) four ubiquitin carrier proteins (E214K, E220K, E225K, and E235K) and pronounced activity of E214K, an enzyme required for "N-end rule" proteolysis; (iii) ATP-dependent 26 S proteasome activity that rapidly degrades high mass 125I-labeled ubiquitin-ROS protein conjugates; and (iv) distinct ubiquitin C-terminal isopeptidase/hydrolase activities, including potent ubiquitin-aldehyde-insensitive activity directed at high mass ubiquitinylated moieties. Considered together, the data support a novel role for the ubiquitin-dependent pathway in the regulation of mammalian phototransduction protein levels and/or activities and provide the first identification of a non-calpain proteolytic system in photoreceptors.

Developmental expression of a synaptic ribbon antigen (B16) in mouse retina

We investigated the developmental expression of the synaptic antigen B16 in the outer and inner plexiform layers in the retina. We compared the expression of this protein with published accounts of retinal ribbon synaptogenesis. The time course of B16 expression was found to be similar, if not identical, to the time course of synaptogenesis in the photoreceptor terminal, supporting our idea of the synaptic nature of the B16 protein. This study provides further evidence which suggests that the B16 antigen is a ribbon associated protein. The morphology, location, and developmental expression of this antigen is coincident with the structure and development of the ribbon synaptic elements located in both the photoreceptor and bipolar terminals. The emergence of the B16 protein was also examined in retina from mice homozygous for the retinal degeneration (rd) mutation which affects photoreceptor synaptogenesis. The B16 expression in the rd retina parallels that in the wild-type retina at early stages of development. However, significant changes in the morphology and staining pattern of the B16 antigen in the rd retina occurring at later postnatal stages are consistent with the degeneration observed in the photoreceptor terminals.

Appearance of cGMP-phosphodiesterase immunoreactivity parallels the morphological differentiation of photoreceptor outer segments in the rat retina

We have investigated by immunofluorescence the appearance of immunoreactive guanosine 3'-5' cyclic monophosphate phosphodiesterase (cGMP-PDE) during the postnatal development of the retina of the pigmented rat. We show that a sudden increase in immunoreactivity takes place during postnatal day five (P5), when rod outer segments begin to form; immunoreactivity develops rapidly in the following days. Labeling is restricted to the developing photoreceptor outer segments, sparing other retinal cells, as confirmed by electron microscopy immunocytochemistry. In addition, cGMP-PDE immunoreactivity follows a center-to-periphery gradient paralleling photoreceptor differentiation. It appears that cGMP-PDE is expressed when the photoreceptor subcellular compartments are already formed, and represents a specific marker of late photoreceptor differentiation. The appearance of cGMP-PDE during development is temporally correlated with the appearance of other proteins of the phototransduction machinery.

Identification of a nonsense mutation in the rod photoreceptor cGMP phosphodiesterase beta-subunit gene of the rd mouse

Retinal degeneration in the mouse mutant, rd, was previously shown to be a disorder of cyclic nucleotide metabolism involving a deficiency in the activity of the rod photoreceptor cGMP phosphodiesterase (PDE). We have characterized the normal and rd PDE beta-subunit gene, and their respective transcripts, by PCR and direct sequence analysis. We show that the gene consists of at least 22 exons ranging in size from 48 base pairs to several hundred base pairs, covering greater than 25 kilobases. Within a 67-base-pair exon of the rd PDE beta-subunit gene, we identified a nonsense ochre mutation (a C----A transversion in codon 347) that truncates the normal gene product, eliminating more than one-half of the peptide chain, including the putative catalytic domain. The consequences of the truncation are consistent with the observed phenotypes in rd mice heterozygous and homozygous for the disorder. The nonsense mutation was also found in another related and in six unrelated strains displaying the rd phenotype, indicating that the rd allele arose from a single genetic event. The results strongly argue for the nonsense mutation being responsible for retinal degeneration in the rd mouse.

Acylation and glycosylation of rhodopsin in the rd mouse

Retinas of 9-10-day-old rd and control mice were incubated for 2 hr with [14C]leucine along with either tritiated palmitic acid or galactose to investigate the acylation or glycosylation, respectively, of rhodopsin. Although other laboratories have reported that phosphorylation of rhodopsin is not detectable in rd retinas, the two post-translational modifications of rhodopsin investigated in the present work are detectable. The rod outer segments (ROS) were separated from the retinal debris containing the rough endoplasmic reticulum (RER) of photoreceptor cells by vortexing and then by linear sucrose gradients. The rhodopsin from the RER was purified by affinity chromatography and gel electrophoresis. In the acylation studies, the mean ratio of palmitate to leucine in the rd mouse was nearly twice that of controls (11.73 +/- 2.84 v. 6.81 +/- 1.04). Possible explanations for the disparity between the two groups could include: (1) a diminished internal pool size of the fatty acid; or (2) acylation of amino acids such as serine or threonine which normally are not acylated in rhodopsin. Treatment of purified rhodopsin with 1 M hydroxylamine released similar amounts of palmitate from the rd mice and controls. Hence, the higher ratio of palmitate to leucine in rd mice is apparently due to a diminished internal pool size. In the glycosylation studies, the ratio of galactose to leucine was very similar between rd mice and controls, 1.7 +/- 0.43 v. 2.47 +/- 0.74. Protein content and specific activity were determined for the crude ROS preparations and for the remaining retinal debris. Although the amount of ROS protein differed significantly between the two groups, the specific activities did not.(ABSTRACT TRUNCATED AT 250 WORDS)

Retinal degeneration in the rd mouse is caused by a defect in the beta subunit of rod cGMP-phosphodiesterase

Mice homozygous for the rd mutation display hereditary retinal degeneration and the classic rd lines serve as a model for human retinitis pigmentosa. In affected animals the retinal rod photoreceptor cells begin degenerating at about postnatal day 8, and by four weeks no photoreceptors are left. Degeneration is preceded by accumulation of cyclic GMP in the retina and is correlated with deficient activity of the rod photoreceptor cGMP-phosphodiesterase. We have recently isolated a candidate complementary DNA for the rd gene from a mouse retinal library and completed the characterization of cDNAs encoding all subunits of bovine photoreceptor phosphodiesterase. The candidate cDNA shows strong homology with a cDNA encoding the bovine phosphodiesterase beta subunit. Here we present evidence that the candidate cDNA is the murine homologue of bovine phosphodiesterase beta cDNA. We conclude that the mouse rd locus encodes the rod photoreceptor cGMP-phosphodiesterase beta subunit.

Evidence for reduced binding of cyclic GMP to cyclic GMP phosphodiesterase in photoreceptors of mice heterozygous for the rd gene

The binding of radiolabelled cGMP to rod outer segment proteins has been investigated in mice, heterozygous for the recessive rd gene that leads to rod dysplasia. Two binding sites were detected, by Scatchard analysis, in a crude cGMP phosphodiesterase fraction, extracted with an EDTA wash from outer segments. Affinities were normal but the capacity of both was reduced 25-35%. Photoaffinity labelling with 3H-cGMP, followed by SDS PAGE and fluorography, suggested that cGMP PDE was the principal binding component in the extracts. If the finding reflects cGMP binding in situ, it might explain the 30-40% lower than normal level of cGMP found in the +/rd retina. Visual pigment has been regenerated in isolated normal and heterozygotic retinas by the application of active isomers of cis-retinal, and the time course of cGMP recovery to 'dark-adapted' levels monitored. The increase in the concentration of cGMP was significantly delayed as compared to that of rhodopsin. No differences in time course or kinetics of recovery were discerned between the two genotypes.

Reconstitution of a light-stimulated adenylate cyclase from retina and Neurospora crassa preparations. Characterization of the heterologous systems using normal and degenerative retinas

Adenylate cyclase catalytic subunits from Neurospora crassa membranes may interact with regulatory factors from membranes of bovine retinal rod outer segments (pretreated with N-ethylmaleimide), reconstituting a heterologous system which, in the presence of light, is catalytically active in assay mixtures containing MgATP. Maximal activation was observed at 550 nm. Transducin-depleted retinal membranes were not capable of reconstituting the heterologous light-stimulated adenylate cyclase system. Addition of a transducin preparation to depleted membranes restored the reconstitution capacity of these membranes. A similar heterologous adenylate cyclase system was reconstituted with Neurospora and mouse retinal whole membranes (pretreated with N-ethylmaleimide). Membranes from mice suffering photoreceptor degeneration (rd homozygotes) did not reconstitute an heterologous adenylate cyclase system.

Cloning and sequencing of the gamma-subunit of retinal cyclic-GMP phosphodiesterase from rd mouse

Molecular cloning, sequencing and Northern blot analysis have been performed on the gamma-subunit of cyclic-GMP phosphodiesterase (cGMP-PDE gamma) from the retina of mice affected with the rd mutation. The full length cDNA has a total of 926 bp which include 261 bp of coding region, 121 bp of 5'-untranslated and 544 bp of 3'-untranslated regions. The latter contains a poly A tail of 50 bp. The coding region has only one base changed from the normal mouse cGMP-PDE gamma cDNA, a C replaced by an A at position 105 (Tuteja and Farber, 1988), and 91% homology with the coding region of the bovine retinal enzyme (Ovchinnikov et al., 1986). The mRNA of cGMP-PDE gamma is 900 bp long in both normal and rd mouse retinas and codes for a protein containing 87 amino acids. The deduced amino acid sequence of cGMP-PDE gamma from rd retina has 100% homology with that of the enzyme from normal mouse retina and 97.7% homology with that of the bovine cGMP-PDE gamma suggesting that if this subunit of cGMP-PDE is properly transcribed and translated, it is not defective in the degenerative rd mouse retina.

Biochemical characterization of retinal protein and phospholipid synthesis in mice exposed transplacentally to N-methyl-N-nitrosourea

Transplacental exposure to the DNA alkylating agent N-methyl-N-nitrosourea on day 16 of gestation in CD-1 albino mice induces a degeneration of the retina, the severity of which depends upon the dosage level of the drug. A 1 mg kg-1 dose provokes a progressive retinal degeneration in the offspring which begins at about 4-6 weeks of age and is characterized by gradual thinning of the retinal layers. A 15 mg kg-1 dosage of MNU provokes severe retinal dysplasia characterized morphologically by rosettes in the outer nuclear layer and loss of rod outer segments (ROS). In the present biochemical experiments, retinal protein synthesis was examined in mice 2-, 4-, and 6 weeks of age exposed to 1 mg kg-1 MNU and 2- and 5 weeks of age exposed to 15 mg kg-1 MNU. Phospholipid synthesis was examined in mice 2-, 4-, 6- and 12 weeks of age exposed to 1 mg kg-1 MNU and at 2 weeks in mice exposed to 15 mg kg-1 MNU. Retinas were incubated for 2 hr at 37 degrees C in media supplemented with either [3H]leucine for protein synthesis studies or [3H]glycerol for phospholipid synthesis experiments. Aliquots of crude ROS and the retinal debris were taken for protein determination, scintillation counting, SDS-PAGE separation of labeled opsin, phosphorus determination and TLC separation of phospholipids. Results indicated that mice exposed to 1 mg kg-1 MNU did not differ significantly from age-matched controls in these measurements, whereas mice exposed to 15 mg kg-1 MNU were significantly different from controls. These results suggest that even as early as 2 weeks of age protein and lipid metabolism are adversely affected in mice exposed to the higher dose of the alkylating agent at a critical time in retinal development, but general protein and lipid synthesis is not affected in animals exposed to 1 mg kg-1 MNU at least up to 12 weeks of age. These studies suggest further investigation of more subtle derangement in the retinal function in animals exposed to low levels of MNU.

Opsin, G-protein and 48-kDa protein in normal and rd mouse retinas: developmental expression of mRNAs and proteins and light/dark cycling of mRNAs

Retinal degeneration in rd mice is manifested during the most rapid period of postnatal photoreceptor differentiation and is hypothesized to be caused by a lesion in cGMP metabolism. We have studied the sequence of developmental expression of three proteins involved in the cGMP cascade and the mRNAs from which they are translated, in rd and control mouse retinas. Slot blot analysis of retinal RNAs indicates that the mRNAs coding for opsin, the alpha, beta and gamma subunits of G-protein and 48-kDa protein each has the same time for onset of expression in normal and diseased retinas. G beta and 48-kDa protein mRNAs are already detectable at birth, opsin mRNA appears by postnatal day 5 (P5), G gamma mRNA at P6 and G alpha mRNA by P8. The levels of all these mRNAs decrease in the diseased retinas after P11-P12, correlating with the reduction in photoreceptor cell number that characterizes the rd disease. Immunocytochemistry indicates that the 48-kDa protein is present at birth, G gamma and opsin are detectable at P4 and G alpha at P7. After P7, opsin and G-protein immunoreactivity are localized throughout the photoreceptor cell in the rd retinas but they are found only in the outer segment in control retinas. The 48-kDa protein immunoreactivity, which is observed in the whole photoreceptor layer both in rd and control retinas throughout development, is the only one of all immunoreactivities analysed that remains at 2 months of age in the rd retina and is probably localized in cones. However, at 6 months of age, 48-kDa protein immunoreactive cells are no longer present in the rd retina. We have also investigated whether there is a daily rhythm for the levels of mRNA present at different times during the light/dark periods in developing rd/rd and rd/+ retinas and in adult normal (+/+) retinas. We find that the levels of each mRNA analysed appear to cycle in the +/+ adult retina, with the greatest amount of opsin and the three subunits of G-protein mRNAs occurring just before light onset and the greatest amount of 48-kDa protein mRNA occurring just before lights off. Cycling in the developing diseased or control retinas (P0-P12) is not observed and may be masked by the pronounced cell growth that occurs during this period.