Gene expression profiles of light-induced apoptosis in arrestin/rhodopsin kinase-deficient mouse retinas

Current Advancements in Mouse Models of Retinal Disease

The field of retinal degenerative (RDs) disease study has been in a state of exponential growth from discovering the underlying genetic components of such diseases as age-related macular degeneration (AMD) and retinitis pigmentosa (RP) to the first gene therapy developed and approved for human Leber congenital amaurosis. However, a source for high-fidelity animal models of these complex, multifactorial, and/or polygenic diseases is a need that has yet to be fulfilled. While models for AMD and RP do exist, they often require aging the animals for a year or more, feeding special diets, or introduction of external modulators such as exposure to cigarette smoke. Currently, work is being done to uncover high-fidelity naturally occurring models of these retinal diseases with the hope and intent of providing the vision community the tools it needs to better understand, treat, and, one day, cure the patients suffering from these devastating afflictions.

Complement Components Showed a Time-Dependent Local Expression Pattern in Constant and Acute White Light-Induced Photoreceptor Damage

Background: Photoreceptor cell death due to extensive light exposure and induced oxidative-stress are associated with retinal degeneration. A correlated dysregulation of the complement system amplifies the damaging effects, but the local and time-dependent progression of this mechanism is not thoroughly understood.

Methods: Light-induced photoreceptor damage (LD) was induced in Balb/c mice with white light illumination either for 24 h with 1000 lux (constant model) or 0.5 h with 5000 lux (acute model). Complement protein and mRNA expression levels were compared at 1 and 3 days post-LD for C1s, complement factor B (CFB), mannose binding lectin A, mannose-binding protein-associated serine protease 1 (MASP-1), C3, C4, C9, and complement factor P in retina and RPE/choroid. Histological analyses visualized apoptosis, microglia/macrophage migration, gliosis and deposition of the complement activation marker C3d. Systemic anaphylatoxin serum concentrations were determined using an ELISA.

Results: Apoptosis, gliosis and microglia/macrophage migration into the outer nuclear layer showed similar patterns in both models. Local complement factor expression revealed an early upregulation of complement factor mRNA in the acute and constant light regimen at 1 day post-treatment for c1s, cfb, masp-1, c3, c4 and c9 in the RPE/choroid. However, intraretinal complement mRNA expression for c1s, cfb, c3 and c4 was increased at 1 day in the constant and at 3 days in the acute model. A corresponding regulation on protein level in the retina following both LD models was observed for C3, which was upregulated at 1 day and correlated with increased C3d staining in the ganglion cell layer and at the RPE. In the RPE/choroid C1s-complex protein detection was increased at 3 days after LD irrespectively of the light intensities used.

Conclusion: LD in mouse eyes is correlated with local complement activity. The time-dependent local progression of complement regulation on mRNA and protein levels were equivalent in the acute and constant LD model, except for the intraretinal, time-dependent mRNA expression. Knowing the relative time courses of local complement expression and cellular activity can help to elucidate novel therapeutic options in retinal degeneration indicating at which time point of disease complement has to be rebalanced.

Integrative analysis of kinase networks in TRAIL-induced apoptosis provides a source of potential targets for combination therapy

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is an endogenous secreted peptide and, in preclinical studies, preferentially induces apoptosis in tumor cells rather than in normal cells. The acquisition of resistance in cells exposed to TRAIL or its mimics limits their clinical efficacy. Because kinases are intimately involved in the regulation of apoptosis, we systematically characterized kinases involved in TRAIL signaling. Using RNA interference (RNAi) loss-of-function and cDNA overexpression screens, we identified 169 protein kinases that influenced the dynamics of TRAIL-induced apoptosis in the colon adenocarcinoma cell line DLD-1. We classified the kinases as sensitizers or resistors or modulators, depending on the effect that knockdown and overexpression had on TRAIL-induced apoptosis. Two of these kinases that were classified as resistors were PX domain-containing serine/threonine kinase (PXK) and AP2-associated kinase 1 (AAK1), which promote receptor endocytosis and may enable cells to resist TRAIL-induced apoptosis by enhancing endocytosis of the TRAIL receptors. We assembled protein interaction maps using mass spectrometry-based protein interaction analysis and quantitative phosphoproteomics. With these protein interaction maps, we modeled information flow through the networks and identified apoptosis-modifying kinases that are highly connected to regulated substrates downstream of TRAIL. The results of this analysis provide a resource of potential targets for the development of TRAIL combination therapies to selectively kill cancer cells.

Retinal light damage: mechanisms and protection

By its action on rhodopsin, light triggers the well-known visual transduction cascade, but can also induce cell damage and death through phototoxic mechanisms - a comprehensive understanding of which is still elusive despite more than 40 years of research. Herein, we integrate recent experimental findings to address several hypotheses of retinal light damage, premised in part on the close anatomical and metabolic relationships between the photoreceptors and the retinal pigment epithelium. We begin by reviewing the salient features of light damage, recently joined by evidence for retinal remodeling which has implications for the prognosis of recovery of function in retinal degenerations. We then consider select factors that influence the progression of the damage process and the extent of visual cell loss. Traditional, genetically modified, and emerging animal models are discussed, with particular emphasis on cone visual cells. Exogenous and endogenous retinal protective factors are explored, with implications for light damage mechanisms and some suggested avenues for future research. Synergies are known to exist between our long term light environment and photoreceptor cell death in retinal disease. Understanding the molecular mechanisms of light damage in a variety of animal models can provide valuable insights into the effects of light in clinical disorders and may form the basis of future therapies to prevent or delay visual cell loss.

Effect of g protein-coupled receptor kinase 1 (Grk1) overexpression on rod photoreceptor cell viability

PURPOSE

Photoreceptor rhodopsin kinase (Rk, G protein-dependent receptor kinase 1 [Grk1]) phosphorylates light-activated opsins and channels them into an inactive complex with visual arrestins. Grk1 deficiency leads to human retinopathy and heightened susceptibility to light-induced photoreceptor cell death in the mouse. The goal of this study was to determine whether excess Grk1 activity is protective against photoreceptor cell death.

METHODS

Grk1-overexpressing transgenic mice (Grk1(+)) were generated by using a bacterial artificial chromosome (BAC) construct containing mouse Grk1, along with its flanking sequences. Quantitative reverse transcription-PCR, immunoblot analysis, immunostaining, and activity assays were combined with electrophysiology and morphometric analysis, to evaluate Grk1 overexpression and its effect on physiologic and morphologic retinal integrity. Morphometry and nucleosome release assays measured differences in resistance to photoreceptor cell loss between control and transgenic mice exposed to intense light.

RESULTS

Compared with control animals, the Grk1(+) transgenic line had approximately a threefold increase in Grk1 transcript and immunoreactive protein. Phosphorylated opsin immunochemical staining and in vitro phosphorylation assays confirmed proportionately higher Grk1 enzyme activity. Grk1(+) mice retained normal rod function, normal retinal appearance, and lacked evidence of spontaneous apoptosis when reared in cyclic light. In intense light, Grk1(+) mice showed photoreceptor damage, and their susceptibility was more pronounced than that of control mice with prolonged exposure times.

CONCLUSIONS

Enhancing visual pigment deactivation does not appear to protect against apoptosis; however, excess flow of opsin into the deactivation pathway may actually increase susceptibility to stress-induced cell death similar to some forms of retinal degeneration.

A novel mutation in GRK1 causes Oguchi disease in a consanguineous Pakistani family

PURPOSE

The purpose of this study was to identify the underlying molecular genetic defect in a large consanguineous Pakistani family with Oguchi disease who had been given a diagnosis of autosomal recessive retinitis pigmentosa.

METHODS

The family was genotyped with the Affymetrix 10K single nucleotide polymorphism array. Fine-mapping of a common homozygous region on chromosome 13q was performed using fluorescent microsatellite markers. Mutation analysis was done by direct sequencing of the candidate gene GRK1 located in the region. The segregation of a novel mutation in the family and the frequency of the identified mutation in the Pakistani population were determined by StuI RFLP analysis.

RESULTS

Genetic mapping supported the diagnosis of typical Oguchi disease in a Pakistani family and also resulted in the identification of a novel nonsense mutation (c.614C>A; p.S205X) in exon 1 of GRK1. This mutation is predicted to result in premature termination of the protein product, thereby affecting the phototransduction cascade. A clinical reappraisal of the family revealed that all patients homozygous for this variant had Oguchi disease.

CONCLUSIONS

This is the first report to describe a mutation causing typical Oguchi disease in a large consanguineous Pakistani family. This mutation segregated in eight affected members.

Depleting Rac1 in mouse rod photoreceptors protects them from photo-oxidative stress without affecting their structure or function

In nonphagocytic cells, Rac1 is a component of NADPH oxidase that produces reactive oxygen species [Ushio-Fukai M (2006) Sci STKE 2006:re8]. Rac1 is expressed abundantly in mammalian retinal photoreceptors, where it is activated in response to light stimuli [Balasubramanian N, Slepak VZ (2003) Curr Biol 13:1306-1310]. We used Cre-LoxP conditional gene targeting to knock down Rac1 expression in mouse rod photoreceptors and found protection against light-induced photoreceptor death compared with WT litter-mates. We also found a similar protective effect on rods using apocynin, which inhibits NADPH oxidase activity. These results implicate both neuronal Rac1 and NADPH oxidase in cell death in this model of CNS degeneration. Studies in which dominant-mutants of Rac1 were expressed in transgenic Drosophila species demonstrated that Rac1 is a key regulator of photoreceptor morphogenesis and polarity [Chang HY, Ready DF (2000) Science 290:1978-1980]. However, we found that diminished Rac1 expression in mouse rods had no effect on retinal structure or function examined by light microscopy, electron microscopy, rhodopsin measurement, electroretinogram activity, and visual acuity, indicating rod outer segment morphogenesis proceeded normally in Rac1 conditional knockout mice. The lack of structural or functional effect of Rac1 depletion on photoreceptors, but protection under conditions of stress, indicate that the Rac1 pathway warrants exploration as a target for therapy in retinal neurodegenerative diseases.

Gene expression profiling of light-induced retinal degeneration in phototransduction gene knockout mice

Exposure to light can induce photoreceptor cell death and exacerbate retinal degeneration. In this study, mice with genetic knockout of several genes, including rhodopsin kinase (Rhok-/-), arrestin (Sag-/-), transducin (Gnat1-/-), c-Fos (c-Fos-/-) and arrestin/transducin (Sag-/-/Gnat1-/-), were examined. We measured the expression levels of thousands of genes in order to investigate their roles in phototransduction signaling in light-induced retinal degeneration using DNA microarray technology and then further explored the gene network using pathway analysis tools. Several cascades of gene components were induced or inhibited as a result of corresponding gene knockout under specific light conditions. Transducin deletion blocked the apoptotic signaling induced by exposure to low light conditions, and it did not require c-Fos/AP-1. Deletion of c-Fos blocked the apoptotic signaling induced by exposure to high intensity light. In the present study, we identified many gene transcripts that are essential for the initiation of light-induced rod degeneration and proposed several important networks that are involved in pro- and anti-apoptotic signaling. We also demonstrated the different cascades of gene components that participate in apoptotic signaling under specific light conditions.

Characterization of phototransduction gene knockouts revealed important signaling networks in the light-induced retinal degeneration

Understanding the molecular pathways mediating neuronal function in retinas can be greatly facilitated by the identification of genes regulated in the retinas of different mutants under various light conditions. We attempted to conduct a gene chip analysis study on the genes regulated during rhodopsin kinase (Rhok^−/−) and arrestin (Sag^−/−) knockout and double knockouts in mice retina. Hence, mice were exposed to constant illumination of 450 lux or 6,000 lux on dilated pupils for indicated periods. The retinas were removed after the exposure and processed for microarray analysis. Double knockout was associated with immense changes in gene expression regulating a number of apoptosis inducing transcription factors. Subsequently, network analysis revealed that during early exposure the transcription factors, p53, c-MYC, c-FOS, JUN, and, in late phase, NFκB, appeared to be essential for the initiation of light-induced retinal rod loss, and some other classical pro- and antipoptotic genes appeared to be significantly important as well.

Defective development of photoreceptor membranes in a mouse model of recessive retinal degeneration

Retinal neurodegeneration occurs in several inherited diseases. Some of the most severe disease alleles involve mutations at the C-terminus of rhodopsin, but in no case is the pathogenic mechanism leading to cell death well understood. We have examined a mouse model of recessive retinal degeneration caused by a knock-in of a human rhodopsin-EGFP fusion gene (hrhoG/hrhoG) at the rhodopsin locus. Whereas heterozygous mutant mice were indistinguishable from control mice, homozygous mutant mice had retinal degeneration. We hypothesized that degeneration might be due to aberrant rhodopsin signaling; however, inhibiting signaling by rearing mice in total darkness had no effect on the rate of degeneration. Using confocal and electron microscopy, we identified the fundamental defect as failed biogenesis of disk membranes, which is observed at the earliest stages of outer segment development. These results reveal that in addition to its role in transport and sorting of rhodopsin to disk membranes, rhodopsin is also essential for formation of disks.

Signalling of the M3-muscarinic receptor to the anti-apoptotic pathway

The process of programmed cell death (or apoptosis) occurs widely in tissue maintenance and embryonic development, and is under tight regulatory control. It is now clear that one of the important regulators of apoptosis are G-protein-coupled receptors. In the present study, we investigate the regulatory mechanism employed by the Gq/11-coupled M3-muscarinic receptor in mediating an anti-apoptotic response. Using a CHO (Chinese-hamster ovary) cell model, we demonstrate that the M3-muscarinic receptor anti-apoptotic response is independent of calcium/phospholipase C signalling. This response can, however, be inhibited by the transcriptional inhibitor actinomycin D at a concentration that inhibits the rapid increase in gene transcription mediated by M3-muscarinic receptor stimulation. Furthermore, apoptosis in CHO cells induced by the DNA-damaging agent, etoposide, is associated with a fall in the levels of the anti-apoptotic Bcl-2 protein. This fall in Bcl-2 protein concentration can be attenuated by M3-muscarinic receptor stimulation. We conclude, therefore, that the M3-muscarinic receptor signals to the anti-apoptotic pathway via a mechanism that is independent of calcium/phospholipase C signalling, but in a manner that involves both gene transcription and the up-regulation of Bcl-2 protein.

Current issues for DNA microarrays: platform comparison, double linear amplification, and universal RNA reference

DNA microarray technology has been widely used to simultaneously determine the expression levels of thousands of genes. A variety of approaches have been used, both in the implementation of this technology and in the analysis of the large amount of expression data. However, several practical issues still have not been resolved in a satisfactory manner, and among the most critical is the lack of agreement in the results obtained in different array platforms. In this study, we present a comparison of several microarray platforms [Affymetrix oligonucleotide arrays, custom complementary DNA (cDNA) arrays, and custom oligo arrays printed with oligonucleotides from three different sources] as well as analysis of various methods used for microarray target preparation and the reference design. The results indicate that the pairwise correlations of expression levels between platforms are relative low overall but that the log ratios of the highly expressed genes are strongly correlated, especially between Affymetrix and cDNA arrays. The microarray measurements were compared with quantitative real-time-polymerase chain reaction (QRT-PCR) results for 23 genes, and the varying degrees of agreement for each platform were characterized. We have also developed and tested a double amplification method which allows the use of smaller amounts of starting material. The added round of amplification produced reproducible results as compared to the arrays hybridized with single round amplified targets. Finally, the reliability of using a universal RNA reference for two-channel microarrays was tested and the results suggest that comparisons of multiple experimental conditions using the same control can be accurate.

A bioinformatics analysis of memory consolidation reveals involvement of the transcription factor c-rel

Consolidation of long-term memory (LTM) is a complex process requiring synthesis of new mRNAs and proteins. Many studies have characterized the requirement for de novo mRNA and protein synthesis; however, few studies have comprehensively identified genes regulated during LTM consolidation. We show that consolidation of long-term contextual memory in the hippocampus triggers altered expression of numerous genes encompassing many aspects of neuronal function. Like contextual memory formation, this altered gene expression required NMDA receptor activation and was specific for situations in which the animal formed an association between a physical context and a sensory stimulus. Using a bioinformatics approach, we found that regulatory elements for several transcription factors are over-represented in the upstream region of genes regulated during consolidation of LTM. Using a knock-out mouse, we found that c-rel, one of the transcription factors identified in our bioinformatics study, is necessary for hippocampus-dependent long-term memory formation.

Comparative analysis of transcriptional profiles between two apoptotic pathways of light-induced retinal degeneration

Light exposure can exacerbate the condition of a variety of human retinal diseases by increasing the rate of photoreceptor cell death. How light negatively affects photoreceptor cell survival is not yet fully understood. Previous studies involving light damage models have revealed two independent apoptotic pathways: low levels of light induce retinal degeneration in the arrestin -/- mouse via constitutive activation of the phototransduction cascade, whereas strong light exposure to the retina, such as in an albino eye, elicits photoreceptor cell death via activator protein (AP-1) induction. In order to better understand the initial gene expression changes underlying light damage, dark-reared arrestin -/- and albino BALB/c mice were exposed to constant white light (2000 lux), and their retinal morphology was assessed as a function of time. The expression profiles of retinal transcripts were then compared between dark-adapted and light-exposed arrestin -/-, pigmented wild-type and BALB/c mice at a time point when morphological changes were minimal. As expected, the dark-adapted samples showed little difference in expression pattern between the three genotypes. Among the genes differentially regulated by light in BALB/c, but not arrestin -/- retinas, were c-fos and other stress-induced early response genes. In both mouse models, a marked increase in expression of the bZIP family of transcription factors was observed. Our results show a select group of unique and overlapping sets of genes induced by light in the two mouse models. These expression changes may constitute the underlying initiating events leading to the two distinct mechanisms of light damage.

Piecing together the timetable for visual transduction with transgenic animals

Transgenic mice bearing null or functional mutations are being used to define the roles of specific elements in phototransduction and also to time the molecular interactions. Genetic manipulation of the collision frequency between rhodopsin and transducin molecules identified this parameter as rate-limiting for the photoresponse onset. Genetic interference with rhodopsin phosphorylation and arrestin binding, transducin shut-off and calcium feedback has revealed their respective roles in shaping the response waveform. The timetable for all of these molecular events determines the amplitude, kinetics and reproducibility of the photoresponse.

Age-related trends in gene expression in the chemosensory-nasal mucosae of senescence-accelerated mice

We have utilized high-density GeneChip oligonucleotide arrays to investigate the use of the senescence-accelerated mouse (SAM) as a biogerontological resource to identify patterns of gene expression in the chemosensory-nasal mucosa. Gene profiling in chronologically young and old mice of the senescence-resistant (SAMR) and senescence-prone (SAMP) strains revealed 133 known genes that were modulated by a three-fold or greater change either in one strain or the other or in both strains during aging. We also identified known genes in our study which based on their encoded proteins were identified as aging-related genes in the aging neocortex and cerebellum of mice as reported by Lee et al. (2000) [Nat. Genet. 25 (2000) 294]. Changes in gene profiles for chemosensory-related genes including olfactory and vomeronasal receptors, sensory transduction-associated proteins, and odor and pheromone transport molecules in the young SAMR and SAMP were compared with age-matched C57BL/6J mice. An analysis of known gene expression profiles suggests that changes in the expression of immune factor genes and genes associated with cell cycle progression and cell death were particularly prominent in the old SAM strains. A preliminary cellular validation study supported the dysregulation of cell cycle-related genes in the old SAM strains. The results of our initial study indicated that the use of the SAM models of aging could provide substantive information leading to a more fundamental understanding of the aging process in the chemosensory-nasal mucosa at the genomic, molecular, and cellular levels.

Naturally occurring rhodopsin mutation in the dog causes retinal dysfunction and degeneration mimicking human dominant retinitis pigmentosa

Rhodopsin is the G protein-coupled receptor that is activated by light and initiates the transduction cascade leading to night (rod) vision. Naturally occurring pathogenic rhodopsin (RHO) mutations have been previously identified only in humans and are a common cause of dominantly inherited blindness from retinal degeneration. We identified English Mastiff dogs with a naturally occurring dominant retinal degeneration and determined the cause to be a point mutation in the RHO gene (Thr4Arg). Dogs with this mutant allele manifest a retinal phenotype that closely mimics that in humans with RHO mutations. The phenotypic features shared by dog and man include a dramatically slowed time course of recovery of rod photoreceptor function after light exposure and a distinctive topographic pattern to the retinal degeneration. The canine disease offers opportunities to explore the basis of prolonged photoreceptor recovery after light in RHO mutations and determine whether there are links between the dysfunction and apoptotic retinal cell death. The RHO mutant dog also becomes the large animal needed for preclinical trials of therapies for a major subset of human retinopathies.