Electrophysiology in Pseudohistoplasmosis and Chronic Idiopathic Retinal Vasculitis

We used computerized electroretinography (E.R.G.) flash to study 12 patients: five pseudohistoplasmosis (P.S.H.) and seven chronic idiopathic retinal vasculitis (C.I.R.V.), compared with 12 healthy controls. We found a moderate correlation in controls between white computer averaged b-wave amplitude and b-wave obtained with other wavelengths (over 600 nm red, over 550 nm orange, and under 500 nm blue), in scotopic (white/orange r = 0.41, P < 0.05; white/red r = 0.45, P < 0.05; white/blue r = 0.48, P < 0.02) and photopic conditions (white/orange r = 0.6, P < 0.01). For P.S.H. there was a highly significant correlation between white and all wavelengths in photopic (white/orange r = 1, white/red r = 0.96, P < 0.0001) and scotopic conditions (white/orange r = 0.97, P < 0.0001; white/red r = 0.96, P < 0.0001; white/blue r = 0.96). In vasculitis, as in controls, we found no white/red correlation and only in vasculitis we could not find a scotopic white/blue correlation. We conclude that these situations are electrophysiologically different. The closer correlation in P.S.H. suggests more efficient phototransduction and in C.I.R.V. there may also be a rod-selective dysfunction.

Intraperitoneal Injection ofGinkgo bilobaExtract Enhances Antioxidation Ability of Retina and Protects Photoreceptors After Light-Induced Retinal Damage in Rats

PURPOSE

To investigate the effect of Ginkgo biloba extract EGb 761, a free-radical scavenger, on the antioxidation capability of retina after light-induced retinal damage in rats in an attempt to understand the mechanism by which EGb 761 protects the photoreceptors after light-induced retinal damage.

METHODS

Seventy-two female Sprague-Dawley (SD) rats were evenly randomized into normal control group (NC group), light-induced retinal damage model group (M group), model + normal saline group (MN group), and model + EGb 761 group (ME group). Light-induced retinal damage model was induced via exposure to white light at 2740 +/- 120 lux for 6 hr. Rats in MN group and ME group were intraperitoneally injected daily with normal saline and 0.35% EGb 761 (100 mg/kg), respectively, 1 week before and 2 weeks after light exposure. The levels of malondialdehyde (MDA), total superoxide dismutase (T-SOD), glutathione peroxidase (GSH-Px), and catalase (CAT) in the retinal tissues were determined 24 hrs after light exposure; photoreceptor apoptosis was detected 4 days after light exposure. One and 2 weeks after light exposure, histopathologic examination was carried out, and the outer nuclear layer (ONL) thickness (number of nuclei) in the superior and inferior retina was counted.

RESULTS

Twenty-four hours after exposure, the MDA levels in the other three groups were significantly higher than that in the NC group (p < 0.05); those in the M and MN groups were similar to each other (p > 0.05); and that of the ME group was significantly lower than those in the M and MN group (p < 0.05). The activities of T-SOD, GSH-Px, and CAT were similar in the M and MN groups (p > 0.05); the activities in the M and MN groups were significantly lower than those in the NC and ME groups (p < 0.05); and the activities in the ME group were significantly higher than those in the M and MN groups (p < 0.05). Four days after exposure, the apoptotic photoreceptors within the ONL in the ME group were obviously fewer than those in the M and MN groups. One week and 2 weeks after exposure, the ONL thickness (number of nuclei) in the ME group was more than that in the M and MN groups but less than that in the NC group.

CONCLUSIONS

Intraperitoneal injection of EGb 761 can enhance the antioxidation ability of retina and partially inhibit the apoptosis of photoreceptors, thus exert a protective effect on photoreceptors.

Full-field electroretinograms in infants with hereditary tapetoretinal degeneration

Full-field electroretinograms were recorded from four infants (under the age of 12 months), with different types of tapetoretinal degeneration. Some of these patients, appear to have normal fundi. This article stresses the clinical usefulness of evaluation with electroretinography in young children.

Central corneal thickness and its relationship to intraocular pressure in mucopolysaccararidoses-1 following bone marrow transplantation

PURPOSE

To examine the ocular findings in mucopolysaccaridoses-1 (MPS-1) and the relationship between intraocular pressure (IOP) and central corneal thickness following bone marrow transplantation.

SETTING

clinical practice.

SUBJECTS

23 subjects with MPS-1 following bone marrow transplantation were examined.

OBSERVATION PROCEDURES

age, sex, visual acuity, presence of strabismus, refractive error, fundus examination, intraocular pressure, and central corneal thickness were assessed for each individual.

MAIN OUTCOME MEASURE

correlation of central corneal thickness with intraocular pressure. Only the right eye was used in correlations.

RESULTS

Forty-six eyes of 23 subjects were examined. All subjects had been treated with successful bone marrow transplantation with mean follow-up of 8.5 years. Ages ranged from 17 months to 19 years (SD 5.03). Of the subjects, 60.8% were female; 85.5% were hyperopic; 8.6% had best-corrected visual acuity of >/=6/12; 34.5% had best-corrected visual acuity of <6/12 and >6/36 with 56.9% <6/36; 30% had strabismus. IOP ranged from 13 to 41 mm Hg with four children on antiglaucoma therapy. Central corneal thickness ranged from 484 microm to 705 microm and was moderately correlated with measured IOP in the right eye (r = 0.56). Corneal opacification moderately correlated with central corneal thickness (r = 0.57). Seventy percent had a normal disk evaluation; 30% had abnormal disk cupping.

CONCLUSION

This is the first large case series of MPS-1 subjects demonstrating a correlation between measured IOP and central corneal thickness. All subjects had corneal opacification moderately correlating with corneal pachymetry.

Alterations in photoreceptor-bipolar cell signaling following ischemia/reperfusion in the rat retina

Studies of retinal ischemia/reperfusion indicate a disparity between the anatomical and functional results; while a large number of rod bipolar cells remain postischemia, there is a significant reduction in the amplitude of the scotopic b-wave of the electroretinogram (ERG). We investigated the alterations in photoreceptor-bipolar cell signaling following ischemia/reperfusion and suggest a mechanism for the decrease in b-wave amplitude. A cation channel probe (agmatine, 1-amino-4-guanidobutane, AGB) was used to assess cellular ion channel activity in neurochemically identified cells secondary to endogenous glutamate release or pharmacological manipulations. By applying the "neurochemical truth point" principle (Sun et al. [2007a] J Comp Neurol, this issue), we have been able to confirm the loss of specific subpopulations of neurons. ERG was used to assess gross retinal function, with parameters of the ERG model providing insight into changes in the phototransduction cascade and sensitivity of postreceptoral glutamate receptors. Following ischemia/reperfusion, rod bipolar cells maintained 2-amino-4-phosphonobutyric acid-responsive metabotropic glutamate receptors and displayed no change in sensitivity to flashes of light as assessed by ERG. Therefore, the loss in b-wave amplitude is likely due to alterations in photoreceptoral glutamate release detected as a change in postsynaptic AGB permeation into rod bipolar cells. Bipolar cell to amacrine cell signaling was also altered. The robust AGB entry into cholinergic amacrine cells was virtually absent in retinas that had undergone ischemia/reperfusion but remained in the AII amacrine cells. Such results suggest a loss of glutamate receptors and/or a change in receptor subunit expression in subpopulations of inner retinal neurons. Although many cells retain their characteristic neurochemical labeling following ischemia/reperfusion, caution should be used when assuming cells participate in functional retinal circuits based solely on the persistence of neurochemical labeling.

Crb1 is a determinant of retinal apical Müller glia cell features

Mutations in the human Crumbs homologue-1 (CRB1) gene cause retinal blinding diseases, such as Leber congenital amaurosis and retinitis pigmentosa. In the previous studies we have shown that Crb1 resides in retinal Müller glia cells and that loss of Crb1 results in retinal degeneration (particularly in the inferior temporal quadrant of the mouse eye). Degeneration is increased by exposure to white light. Here, we studied the role of light and aging to gain a better understanding of the factors involved in the progress of retinal disease. Our data reveal that light is neither sufficient nor required to induce retinal disorganization and degeneration in young Crb1(-/-) mutant mice, suggesting that it rather modulates the retinal phenotype. Gene expression profiling showed that expression of five genes is altered in light-exposed Crb1(-/-) mutant retinas. Three of the five genes are involved in chromosome stabilization (Pituitary tumor transforming gene 1 or Pttg1, Establishment of cohesion 1 homolog 1 or Esco1, and a gene similar to histone H2B). In aged retinas, degeneration of photoreceptors, inner retinal neurons, and retinal pigment epithelium was practically limited to the inferior temporal quadrant. Loss of Crb1 in Müller glia cells resulted in an irregular number and size of their apical villi. We propose that Crb1 is required to regulate number and size of these Müller glia cell villi. The subsequent loss of retinal integrity resulted in neovascularization, in which blood vessels of the choroid protruded into the neural retina.

Identification of photoreceptor genes affected by PRPF31 mutations associated with autosomal dominant retinitis pigmentosa

Several ubiquitously expressed genes encoding pre-mRNA splicing factors have been associated with autosomal dominant retinitis pigmentosa (adRP), including PRPF31, PRPF3 and PRPF8. Molecular mechanisms by which defects in pre-mRNA splicing factors cause photoreceptor degeneration are not clear. To investigate the role of pre-mRNA splicing in photoreceptor gene expression and function, we have begun to search for photoreceptor genes whose pre-mRNA splicing is affected by mutations in PRPF31. Using an immunoprecipitation-coupled-microarray method, we identified a number of transcripts associated with PRPF31-containing complexes, including peripherin/RDS, FSCN2 and other photoreceptor-expressed genes. We constructed minigenes to study the effects of PRPF31 mutations on the pre-mRNA splicing of these photoreceptor specific genes. Our experiments demonstrated that mutant PRPF31 significantly inhibited pre-mRNA splicing of RDS and FSCN2. These observations suggest a functional link between ubiquitously expressed and retina-specifically expressed adRP genes. Our results indicate that PRPF31 mutations lead to defective pre-mRNA splicing of photoreceptor-specific genes and that the ubiquitously expressed adRP gene, PRPF31, is critical for pre-mRNA splicing of a subset of photoreceptor genes. Our results provide an explanation for the photoreceptor-specific phenotype of PRPF31 mutations.

Glutathione depletion induces differential apoptosis in cells of mouse retina, in vivo

Oxidative stress affects numerous intracellular macromolecules, and may result in cell death unless precisely regulated. Unregulated oxidative stress can be controlled by various cellular defense mechanisms such as glutathione (GSH) which can critically counteract the damaging effects of oxidative stress in mammalian cells. We determined the effects of unregulated oxidative stress induced by GSH depletion on cells in mouse retina. Mice were intraperitoneally injected with buthionine sulphoximine (BSO) at 1.5 g/kg. After 0, 1, 4, and 7 days of BSO administration, retinas were excised and sections were subjected to GSH assay and terminal uridine deoxynucleotidyl nick end labeling (TUNEL) analysis. After 4 days of BSO administration, the number of TUNEL positive cells was significantly increased. However, after 7 days, TUNEL positive cells returned to the basal level. The retinal region most affected by the BSO treatment appeared to be the outer nuclear layer where the photoreceptor cells reside. Different from cells in other regions, retinal cells in the inner nuclear layer increased in their apoptosis even after the first day of BSO injection, and the increase was further potentiated after 4 days. Taken together, our studies suggested that GSH depletion may cause unregulated oxidative stress to the cells in the retina and indeed increased cell death in the retina. The cells in the inner nuclear layer seemed to be affected earlier than the cells in other layers of the retina. The GSH level in the retina may be a crucial therapeutic target in preventing blindness.

[Clinical and electroretinographic profile of commotio retinae]

PURPOSE

To investigate clinical, anatomic and electroretinographic changes in eyes that suffered blunt ocular trauma with commotio retinae.

METHODS

Patients who presented commotio retinae after unilateral blunt ocular trauma less then 72 hours before were submitted to visual acuity testing, biomicroscopy, binocular indirect ophthalmoscopy, fluorescein angiography, optical coherence tomography and full-field electroretinography. Full-field ERG was repeated after 15 and 30 days.

RESULTS

Sixteen patients were included in the study. On the first examination there was a statistically significant difference between affected and fellow eye in all response amplitudes, without b/a ratio alteration, and a delay in single-flash cone response and 30-Hz flicker implicit time. On the second examination, the difference between the eyes remained for oscillatory potentials, but disappeared on the last examination. In fluorescein angiography, all patients presented mottled hyperfluorescence and hypofluorescent areas, due to alterations in the pigment barrier. On optical coherence tomography, we found optically empty spaces at the site of the lesion.

CONCLUSION

Found changes suggested photoreceptor and ganglion cells, but not Müller cell functional alterations, as well as pigment mobilization. These changes disappeared 30 days after the trauma.

Hyperoxia, hyperglycemia, and photoreceptor sensitivity in normal and diabetic subjects

The aim of this study was to investigate the effects of an increase in the saturation of blood oxygen (SaO2) and/or serum glucose on photoreceptor sensitivity in normal subjects and in patients with diabetes mellitus. We monitored cone and rod sensitivity by recording dark-adaptation curves to both green and red test stimuli while inhaling either air (20% O2 + 80% N2) or 100% oxygen in 12 normal subjects and 12 diabetic patients with no (10) or mild (2) retinopathy. We also repeated the experiment in 10 of the normal subjects under hyperglycemia (mean serum glucose: 161 mg/dl). Results show that in normal subjects the dark-adapted cone sensitivity is improved by an increase in SaO2 or by hyperglycemia. Final rod sensitivity is unchanged during hyperoxia and during hyperglycemia when measured with a green test spot. However the kinetics of dark adaptation are altered during hyperglycemia, and an increase in final sensitivity is observed when measured with the red test spot. Inhalation of oxygen during hyperglycemia in normal subjects reduces cone sensitivity compared to that found during hyperglycemia alone (Pasteur effect). In diabetic subjects the dark-adapted cone threshold is comparable to that found in normal subjects, and sensitivity also increases with an increase in SaO2. The final rod threshold, however, is impaired compared to that of the control group, and rod sensitivity is improved by increasing the SaO2. The results suggest that the metabolism of rods and cones may differ in normal subjects: in cones, the rate of metabolism can be augmented by increasing the available oxygen or glucose, whereas rods appear more insensitive to increased blood oxygen saturation and hyperglycemia. In diabetic subjects, both cone and rod metabolism can be increased by supplemental oxygen, indicative of an early rod deficit. The study lends weight to the hypothesis that dark-adapted rods in diabetics are hypoxic before the onset of retinopathy.

Altered Visual Sensitivity in Axial High Myopia: A Local Postreceptoral Phenomenon?

PURPOSE

The present study investigated retinal integrity in high myopia using spatial psychophysical tasks.

METHODS

Ten axial high myopes (-8.5 to -11.5 D) and 10 age-matched control subjects (+/-1.0 D) were recruited. All participants underwent clinical examination and ocular biometry and demonstrated no visible macular disease with visual acuities better than 6/12. Foveal summation thresholds were determined for white and S-cone-isolating spots of various diameters up to 5.4 degrees and spatial contrast sensitivity to luminance sine wave gratings (0.5-9.7 cyc/deg). Data were analyzed after correction for the magnification induced by eye size and correcting lens power.

RESULTS

Spatial summation for both white and S-cone-isolating spots showed a generalized loss of sensitivity at all spot sizes in myopes relative to control subjects (P = 0.01). Critical areas at maximum summation were significantly larger in myopes, for S-cone isolating spots only, after image size correction (P = 0.048). Sensitivity at maximum summation correlated negatively with vitreous chamber depth for both targets (P = 0.005). Sensitivities for S-cone and luminance spots also correlated (P < 0.001), indicating widespread dysfunction. Myopes displayed contrast sensitivity losses at high spatial frequencies (P </= 0.006) with a normal peak contrast sensitivity.

CONCLUSIONS

These data can be interpreted to indicate that highly myopic eyes have either (1) a reduction in the number of receptors and/or a reduction in their sensitivity or, (2) a reduction in the sensitivity of postreceptoral processes. The presence of normal contrast sensitivity at low spatial frequencies indicates dysfunction at a postreceptoral level in high myopes.

The genomic response to retinal disease and injury: evidence for endothelin signaling from photoreceptors to glia

Regardless of proximal cause, photoreceptor injury or disease almost invariably leads to the activation of Muller cells, the principal glial cells in the retina. This observation implies the existence of signaling systems that inform Muller cells of the health status of photoreceptors. It further suggests that diverse types of photoreceptor damage elicit a limited range of biochemical responses. Using the mouse retina, we show by microarray, RNA blot, and in situ hybridization that the genomic responses to both light damage and inherited photoreceptor degeneration involve a relatively small number of genes and that the genes activated by these two insults overlap substantially with one another and with the genes activated by retinal detachment. Among the induced transcripts, those coding for endothelin2 (Edn2) are unusual in that they are localized to photoreceptors and are also highly induced in all of the tested models of photoreceptor disease or injury. Acute light damage also leads to a >10-fold increase in endothelin receptor B (Ednrb) in Muller cells 24 h after injury. These observations suggest that photoreceptor-derived EDN2 functions as a general stress signal, that EDN2 signals to Muller cells by binding to EDNRB, and that Muller cells can increase their sensitivity to EDN2 as part of the injury response.

The effects of indocyanine green and endoillumination on rabbit retina: an electroretinographic and histological study

AIM

To evaluate the functional and morphological retinal toxicity associated with intravitreal injection of indocyanine green (ICG) dye in rabbit eyes during vitrectomy with endoillumination.

METHODS

20 eyes of 10 New Zealand pigmented rabbits were used in the study. All eyes underwent pars plana vitrectomy and removal of posterior vitreous cortex under endoillumination. In one eye of each rabbit, intravitreal injection of 0.1 ml of 2.5 mg/ml ICG was applied for 30 seconds followed by 10 minutes of endoillumination. The control eye had endoillumination only without ICG injection. Dark adapted and light adapted electroretinograms (ERGs) were performed before the surgery and 1 week after surgery for serial comparisons. Rabbits were killed 1 week after surgery and eyes were enucleated for histological examination.

RESULTS

Serial ERG comparisons showed significant reduction in the light adapted a-wave amplitude (p = 0.037) and significant delays in the dark adapted and light adapted b-wave latencies (p = 0.020 and p = 0.038, respectively) in the ICG treated eyes. Histological examinations demonstrated loss of photoreceptor outer segments with focal absence of photoreceptors in some areas in the ICG injected eyes.

CONCLUSIONS

Vitrectomy followed by intravitreal injection of 2.5 mg/ml ICG for 30 seconds with endoillumination may result in retinal toxicity causing functional and morphological retinal damages in rabbit eyes. The lowest concentration of ICG should be used if necessary for intraocular use to prevent potential retinal toxicity.

Brain-Derived Neurotrophic Factor Gene Delivery to Müller Glia Preserves Structure and Function of Light-Damaged Photoreceptors

PURPOSE

To test the hypothesis that adenovirus (Ad)-mediated gene delivery of brain-derived neurotrophic factor (BDNF) to Müller cells can protect photoreceptors from light-induced retinal degeneration.

METHODS

Adult Sprague-Dawley rats received an intraocular injection of Ad.BDNF, control Ad containing the green fluorescent protein (GFP) gene, or BDNF recombinant protein. Animals were then exposed to 5, 10, or 16 days of constant light. The effect of Ad.BDNF on photoreceptor survival was examined histologically, by measuring the outer nuclear layer (ONL) thickness, and functionally, by measuring the electroretinographic (ERG) response.

RESULTS

Ad.BDNF mediated sustained expression of bioactive neurotrophin by Müller cells that lasted for at least 30 days after viral vector administration. BDNF gene delivery to Müller glia markedly increased the survival and structural integrity of light-damaged photoreceptors. For example, after 10 days of exposure to light, the average percentage of ONL preservation in the superior central retina of eyes that received Ad.BDNF was 71%, compared with 46% in eyes that received a control Ad.GFP or 15% in contralateral eyes. Of importance, retinas exposed to Ad.BDNF had more photoreceptor nuclei than retinas that received a single intraocular injection of BDNF recombinant protein. The neuroprotective effect of Ad.BDNF was accompanied by preservation of the ERG response of the treated eyes.

CONCLUSIONS

These data provide proof of the concept that BDNF gene transfer into Müller cells is an effective strategy for preserving structure and function of photoreceptors in retinal degeneration.

Photoreceptor synapses degenerate early in experimental choroidal neovascularization

Severe visual loss in patients with age-related macular degeneration is associated with the development of choroidal neovascularization (CNV). The pathogenic mechanisms for CNV formation have been extensively investigated, but remarkably little research has addressed the mechanisms for dysfunction of the retina in CNV. Using laser-induced CNV in mice, we evaluated the mechanisms of retinal dysfunction. At 3 days, 1 week, 2 weeks, and 4 weeks after laser application, retinas under experimental CNV were characterized physiologically (ERG recordings, synaptic uptake of the exocytotic marker FM1-43, and light-induced translocation of transducin), histologically, and immunohistochemically. ERG amplitudes were reduced by 20% at 1 week after CNV. Depolarization-induced FM1-43 uptake in photoreceptor synapses was selectively reduced by 45% at 1 week after CNV. Although photoreceptor outer segments were shortened by 36%, light adaptation as measured by transducin translocation was mostly preserved. Early in CNV (3 days to 1 week), Muller cells demonstrated induction of c-fos and pERK expression. Also, the density of macrophage-like, F4/80 immunoreactive cells increased approximately 3-fold. Minimal photoreceptor death occurred during the first week, and was variable thereafter. At later times in CNV formation (> or =2 weeks), expression of photoreceptor synaptic markers was reduced in the outer plexiform layer, indicating loss of photoreceptor synaptic terminals. ERG amplitudes, synaptic uptake of FM1-43, and the induction of c-fos and pERK in Muller cells were altered within 1 week of experimental CNV, suggesting that during CNV formation, deficits in retinal function, in particular photoreceptor synaptic function, precede degeneration of photoreceptor terminals and photoreceptor cell death.

Glial cell-mediated spread of retinal degeneration during detachment: a hypothesis based upon studies in rabbits

In human subjects with peripheral retinal detachments, visual deficits are not restricted to the detached retina but are also present in the non-detached tissue. Based upon studies on a rabbit model of rhegmatogenous retinal detachment, we propose a glial cell-mediated mechanism of spread of retinal degeneration into non-detached retinal areas which may also have importance for the understanding of alterations in the human retina. Both detached and attached portions of the rabbit retina display photoreceptor cell degeneration and cystic degeneration of the innermost layers. An inverse mode of photoreceptor cell degeneration in the attached tissue suggests a disturbed support of the photoreceptor cells by Müller cells which show various indications of gliosis (increased expression of intermediate filaments, cell hypertrophy, decreased plasma membrane K(+) conductance, increased Ca(2+) responsiveness to purinergic stimulation) in both detached and attached tissues. We propose that gliotic alterations of Müller cells contribute to the degeneration of the attached retina, via disturbance of glial homeostasis mechanisms. A down-regulation of the K(+) conductance of Müller cells may prevent effective retinal K(+) and water clearance, and may favor photoreceptor cell degeneration and edema development.

Photoreceptor differentiation and integration of retinal progenitor cells transplanted into transgenic rats

Previous studies evaluating neural stem cells transplanted into the mature retina have demonstrated limited levels of graft-host integration and photoreceptor differentiation. The purpose of this investigation is to enhance photoreceptor cell differentiation and integration of retinal progenitor cells (RPC) following subretinal transplantation into retinal degenerate rats by optimization of isolation, expansion, and transplantation procedures. RPCs were isolated from human placental alkaline phosphatase (hPAP)-positive embryonic day 17 (E17) rat retina and expanded in serum-free defined media. RPCs at passage 2 underwent in vitro induction with all trans retinoic acid or were transplanted into the subretinal space of post-natal day (P) 17 S334ter-3 and S334ter-5 transgenic rats. Animals were examined post-operatively by ophthalmoscopy and optical coherence tomography (OCT) at weeks 1 and 4. Differentiation profiles of RPCs, both in vitro and in vivo were analysed microscopically by immunohistochemistry for various retinal cell specific markers. Our results demonstrated that the majority of passage 2 RPCs differentiated into retina-specific neurons expressing rhodopsin after in vitro induction. Following subretinal transplantation, grafted cells formed a multi-layer cellular sheet in the subretinal space in both S334ter-3 and S334ter-5 rats. Prominent retina-specific neuronal differentiation was observed in both rat lines as evidenced by recoverin or rhodopsin staining in 80% of grafted cells. Less than 5% of the grafted cells expressed glial fibrillary acidic protein. Synapsin-1 (label for nerve terminals) positive neural processes were present at the graft-host interface. Expression profiles of the grafted RPCs were similar to those of RPCs induced to differentiate in vitro using all-trans retinoic acid. In contrast to our previous study, grafted RPCs can demonstrate extensive rhodopsin expression, organize into layers, and show some features of apparent integration with the host retina following subretinal transplantation in slow and fast retinal degenerate rats. The similarity of the in vitro and in vivo RPC differentiation profiles suggests that intrinsic signals may have a significant contribution to RPC cell fate determination.

Absorption of the eye lens and macular pigment derived from the reflectance of cone photoreceptors

We measured the amplitude of the directional component of the bleached fundus reflectance, the so-called optical Stiles-Crawford effect, as a function of wavelength. The directional reflectance originates from within the outer segments of the photoreceptors. Thus only two anterior absorbers are of importance: macular pigment and the crystalline lens. Analysis of spectra obtained in pseudophakes established that the cone photoreceptors act as spectrally neutral reflectors. The reflectance spectra, expressed in density units, resembled the macular pigment density spectrum. Studying age effects in the lens of normal subjects resulted in a description of the optical density of the lens in terms of a "young" and an "aged" template. The young template represents the pigment O-beta-glucoside of 3-hydroxykynurenine, which dominates the light absorption in young eyes and decreases with age. The aged template represents the pigments accumulating in the lens with age. The total optical density increased with age, but it was lower in the wavelength region 500-650 nm than was previously assumed on the basis of psychophysical studies. Analysis of the spectra also provided precise individual estimates of the optical density of macular pigment. Finally, we observed a decrease in the photoreceptor reflectivity with age, possibly reflecting a degradation of the photoreceptors.

Light deprivation slows but does not prevent the loss of photoreceptors in taurine transporter knockout mice

Taurine transporter knockout mice show severe retinal degeneration at an early age. The study was designed to determine whether degeneration also takes place in the absence of light. Mice were maintained up to 6 weeks of age in cyclic lighting or in total darkness. Degeneration took place in both groups, but was more rapid in animals exposed to standard cyclic illumination. At the ultrastructural level the retinas showed features characteristic of apoptosis but not of necrosis.

CONCLUSIONS

Cell differentiation is not seriously affected by the lack of a functional taurine transporter but mature photoreceptor cells do not survive without an intact transporter, even in the dark.

The nature of dominant mutations of rhodopsin and implications for gene therapy

Mutations in the rhodopsin gene are the most common cause of retinitis pigmentosa (RP) among human patients. The nature of the rhodopsin mutations has critical implications for the design of strategies for gene therapy. Nearly all rhodopsin mutations are dominant. Although dominance does not arise because of haploinsufficiency, it is unclear whether it is caused by gain-of-function or dominant-negative mutations. Current strategies for gene therapy have been devised to deal with toxic, gain-of-function mutations. However, analysis of results of transgenic and targeted expression of various rhodopsin genes in mice suggests that dominance may arise as a result of dominant-negative mutations. This has important consequences for gene therapy. The effects of dominant-negative mutations can be alleviated, in principle, by supplementation with additional wild-type rhodopsin. If added wild-type rhodopsin could slow retinal degeneration in human patients, as it does in mice, it would represent a valuable new strategy for gene therapy of RP caused by dominant rhodopsin mutations.

Abnormal dark-adapted ERG in cats heterozygous for a recessively inherited rod-cone degeneration

PURPOSE

To study retinal function in cats homozygous and heterozygous for a recessively inherited rod-cone degeneration.

METHODS

Dark-adapted electroretinograms (ERGs) were performed on early affected, heterozygous (ophthalmoscopically normal), and clinically normal, nonrelated cats. Responses to blue stimuli over a 3.9-log unit range were recorded.

RESULTS

Lower b-wave amplitudes than normal were observed in heterozygotes and early affected cats. The amplitudes of the heterozygotes took an intermediate position between normal and early affected cats. Normalized amplitude/intensity data suggest a normal dynamic range in carriers. B-wave implicit times in carriers were comparable to those of normal cats.

CONCLUSIONS

These results show that heterozygotes have an altered retinal function, although they are ophthalmoscopically normal. It is difficult to electrophysiologically differentiate heterozygotes from affected cats with the very early stage of retinal degeneration.

Temporal visual filtering in diabetes mellitus

The background modulation method was used to investigate the temporal response of the magnocellular pathway in diabetic patients and controls. The luminance threshold for detecting a moving, 2 degrees, achromatic target was measured as a function of background flicker frequency from 5 to 45 Hz. A model of photoreceptor kinetics integrated with difference of Gaussian receptive fields [Vis. Neurosci. 13 (1996) 173] was used to analyse the data. Diabetic patients with significant maculopathy showed raised thresholds at 8.75, 12.5, 15 and 17.5 Hz. Estimates of photoreceptor summation time were the same in both groups, but receptive field centre-to-surround delay showed an increasing trend in the diabetic patients.

Retinal remodeling triggered by photoreceptor degenerations

Many photoreceptor degenerations initially affect rods, secondarily leading to cone death. It has long been assumed that the surviving neural retina is largely resistant to this sensory deafferentation. New evidence from fast retinal degenerations reveals that subtle plasticities in neuronal form and connectivity emerge early in disease. By screening mature natural, transgenic, and knockout retinal degeneration models with computational molecular phenotyping, we have found an extended late phase of negative remodeling that radically changes retinal structure. Three major transformations emerge: 1) Müller cell hypertrophy and elaboration of a distal glial seal between retina and the choroid/retinal pigmented epithelium; 2) apparent neuronal migration along glial surfaces to ectopic sites; and 3) rewiring through evolution of complex neurite fascicles, new synaptic foci in the remnant inner nuclear layer, and new connections throughout the retina. Although some neurons die, survivors express molecular signatures characteristic of normal bipolar, amacrine, and ganglion cells. Remodeling in human and rodent retinas is independent of the initial molecular targets of retinal degenerations, including defects in the retinal pigmented epithelium, rhodopsin, or downstream phototransduction elements. Although remodeling may constrain therapeutic intervals for molecular, cellular, or bionic rescue, it suggests that the neural retina may be more plastic than previously believed.

Entrainment of circadian rhythm to a photoperiod reversal shows retinal dystrophy in RPE65(-/-) mice

Light entrainment of circadian rhythms is mediated by classical "visual" photoreceptors (rods and cones) as well as "nonvisual" photoreceptive elements (light-detecting cells that do not contribute to classical "vision"). This paper aimed to assess whether light entrainment of locomotor circadian rhythms in mice with impaired rods and cones differs from normal controls and whether this technique, alongside existing techniques, could be used to assess visual function. The study was primarily interested in differences between the entrainment of circadian rhythms of normal-sighted C57Bl/6J mouse and the C57Bl/RPE65 knockout mouse (RPE65(-/-)), although C3H/HeJ (rd/rd) mice were included as a preexisting model of retinal degeneration. Circadian rhythms of motor activity before and after a 12-h light reversal were assessed in custom-built cages that continuously monitored movement. The controls showed a significantly higher mesor and amplitude when compared to the RPE65(-/-) and rd/rd mice. Despite the loss of rods and cones, the RPE65(-/-) and rd/rd maintained a 24-h circadian rhythm entrained to light similar to controls and were capable of circadian reentrainment to a 12-h light reversal. Importantly, this light reentrainment of the circadian phase occurred at a significantly slower rate in the retinal degenerate models than in the controls. The RPE65(-/-) model demonstrates a retinal degenerate reentrainment phenotype when compared to the rd/rd model. It is suggested that these retinal degenerate mice retain the ability to detect light for the purposes of circadian rhythm entrainment. However, alterations of specific parameters of the circadian rhythm with loss of rods and cones may provide measures of loss of visual function (sight).

The retinal anatomy and function of the myelin mutant taiep rat

PURPOSE

To study the histology and the physiological function of the retina in the neurological myelin mutant, taiep rats during the postnatal developmental period (P20-P360).

METHODS

Electroretinography (ERG) was applied to evaluate intensity dependence and spectral sensitivity of the responses to light. Retinal histology, morphometry, and immunocytochemistry were used to characterize the structure of the retina, with particular emphasis on the Müller (glial) cells.

RESULTS

In the taiep rats of all ages studied, the scotopic ERG showed normal a- and b-wave amplitudes and latencies; likewise, the scotopic spectral sensitivity function was the same for control and taiep animals, with a maximal sensitivity (lambda(max)) at 500 nm. However, in adult taiep rats (P90 to P360) a secondary cornea-positive wave ('b(2)') was observed in response to high stimulus intensities, which never occurred in controls. This correlated with the observation that in the photopic ERG responses of the taiep rats, the b-wave was reduced in amplitude, and was followed by a rapid cornea-negative after-potential. After 1 year of life, in taiep rats the outer plexiform layer (OPL) became slightly thinner and the inner plexiform/ganglion cell layers (IPL/GCL) appeared to be swollen, and increased in thickness; in addition, the number of retinal neurons (particularly, of photoreceptor cells) slightly decreased. Increased GFAP immunoreactivity revealed a hypertrophy and reactivity of the Müller cells in 1-year-old taiep rats.

CONCLUSIONS

The present results suggest the occurrence of a relatively mild and slowly progressing neural retinal alteration in taiep rats, which becomes histologically and functionally evident at the end of the first year of life, and mainly affects the circuit(s) of the photopic ON-response. It is speculated that this alteration is due to missing/altered signals from demyelinated optic nerve.

Experimental Retinal Reattachment: A New Perspective

In the feline model, retinal detachment initiates a cascade of changes that include photoreceptor- cell "deconstruction," apoptotic death of some photoreceptors, neurite outgrowth from second- and third-order neurons, remodeling of photoreceptor synaptic terminals, and Müller-cell gliosis. We have previously shown that reattachment within 24 h halts or reverses many of these presumed detrimental changes. However, in patients with retinal detachments, reattachment cannot always be performed within this 24-h window. Moreover, recovery of vision following successful reattachment surgery in the macula is often imperfect. Here, we examine the ability of relatively long-term reattachment (28 d) to stop or reverse several cellular events that occur at 3 d of detachment. In contrast to earlier studies of reattachment, which focused on the regeneration of outer segments, we focus our attention here on other cellular events such as neuronal remodeling and gliosis. Some of these changes are reversed by reattachment, but reattachment itself appears to stimulate other changes that are not associated with detachment. The implications of these events for the return of vision are unknown, but they do indicate that simply reattaching the retina does not return the retina to its pre-detachment state within 28 d.

Morphologic characteristics of retinal degeneration induced by sodium iodate in mice

PURPOSE

Retinal degeneration induced by sodium iodate (NaIO( 3)) in mice was evaluated morphologically.

METHODS

Male and female ICR and C57BL mice were intraperitoneally administered 100 mg/kg NaIO(3) at 7 weeks of age, and were killed 6, 12, 24 hrs, and 3, 7 and 28 days after the treatment. Retinas were examined histologically, ultrastructurally, immunohistochemically, and by the TUNEL method.

RESULTS

Retinal degeneration was evoked in all NaIO(3)-treated mice. The primary site of damage appeared in the retinal pigment epithelial (RPE) cells followed by photoreceptor cell degeneration. Initially, the RPE cells showed necrosis starting 6 hrs post-NaIO(3), followed by photoreceptor outer segment disruption and photoreceptor cell apoptosis at 24 hrs; photoreceptor cell apoptosis peaked at day 3 and was completed by day 7. At day 3, Müller cell proliferation, macrophage migration within the retina, and regeneration of damaged RPE cells occurred. Finally at day 7 and day 28, the retina showed a mosaic pattern of relatively normal retina and areas lacking RPE cells and photoreceptor cells.

CONCLUSIONS

RPE cell necrosis followed by photoreceptor cell apoptosis and the resulting mosaic pattern of the retina phenotypically resembles gyrate atrophy of the choroid and retina.

Preservation of visual responsiveness in the superior colliculus of RCS rats after retinal pigment epithelium cell transplantation

The dystrophic RCS rat undergoes progressive photoreceptor degeneration due to a primary defect in retinal pigment epithelial (RPE) cells. This has a major impact on central visual responsiveness. Here we have examined how functional deterioration is contained by subretinal transplantation of immortalized human RPE cells. Transplantation was done at three to four weeks of age prior to significant photoreceptor loss and recipients were kept on cyclosporin. At six months of age, sensitivity maps and multi-unit response properties were obtained across the visual field by recording at 76 equidistant sites encompassing the whole superior colliculus.A significant degree of functional protection, both in terms of area of responsive retina and response characteristics was observed following RPE transplantation. At best, the sensitivity, latency of onset, and response rise time were all maintained within normal ranges and this was achieved with no more than half of the normal complement of photoreceptors. Although partial, the degree of anatomical preservation (both in terms of outer nuclear layer thickness and area of rescue) correlated well with the level of preserved visual sensitivities. Sham injections also resulted in rescue, though the area of preservation was strictly confined to the needle injury site and the response properties were significantly worse than with RPE injections. This study shows that central physiological responsiveness and correlated retinal morphology can be preserved in an animal model of retinal disease by implantation of an immortalized cell line. The use of retinal sensitivity measurements provides a background for assessing higher visual functions in these animals and a direct comparison for human perimetry measures.

Retinal degeneration and RPE transplantation in Rpe65(-/-) mice

PURPOSE

To determine whether transplanting normal retinal pigment epithelium (RPE) into the subretinal space influences photoreceptor function and degeneration in Rpe65(-/-) mice.

METHODS

RPE cells were isolated from eyes of normal mice and transplanted to the subretinal space of one eye of Rpe65(-/-) mice. The other eye received a subretinal injection of saline or was not touched. Corneal electroretinograms (ERGs) from both eyes were monitored before and after surgery to follow progression of the degeneration. The width of the outer nuclear layer was measured in the area of transplantation and compared with a similar area in control retinas.

RESULTS

Transplantation of RPE increased ERG amplitude maximally at 3.7 weeks after surgery. This rescue effect slowly diminished with time. Sham surgery had little effect on the ERG. The width of the outer nuclear layer in the area receiving RPE transplants was slightly greater than in control subjects. Evidence of the presence of RPE transplants in the subretinal space decreased with time after transplantation without signs of inflammation.

CONCLUSIONS

Retinal degeneration in the Rpe65(-/-) mice is slowly progressive. Photoreceptor function can be transiently increased for several months and anatomic degeneration slightly reduced in Rpe65(-/-) mice by RPE cell transplantation. Loss of the rescue effect may be due to degeneration of the transplanted RPE.

Encapsulated cell-based delivery of CNTF reduces photoreceptor degeneration in animal models of retinitis pigmentosa

PURPOSE

The objective of the present study was to evaluate the therapeutic efficacy of ciliary neurotrophic factor (CNTF) delivered through encapsulated cells directly into the vitreous of the eye in an rcd1 canine model of retinitis pigmentosa. The dose-range effect of the treatment was also investigated.

METHODS

Polymer membrane capsules (1.0 cm in length and 1.0 mm in diameter) were loaded with mammalian cells that were genetically engineered to secrete CNTF. The cell-containing capsules were then surgically implanted into the vitreous of one eye of rcd1 dogs at 7 weeks of age, when retinal degeneration is in progress but not complete. The contralateral eyes were not treated. The capsules remained in the eyes for 7 weeks. At the end of the studies, the capsules were explanted, and CNTF output and cell viability were evaluated. The eyes were processed for histologic evaluation.

RESULTS

In each animal, the number of rows of photoreceptor nuclei in the outer nuclear layer (ONL) was significantly higher in the eye that received a CNTF-secreting implant than in the untreated contralateral eye. No adverse effects were observed on the retina in the treated eyes. The explanted capsules produced a low level of CNTF. The cells in the capsules remained viable and densely distributed throughout.

CONCLUSIONS

CNTF delivered through encapsulated cells directly into the vitreous of the eye protects photoreceptors in the PDE6B-deficient rcd1 canine model. Furthermore, sparing of photoreceptors appeared dose-dependent with minimum protection observed at CNTF doses of 0.2 to 1.0 ng/d. Incrementally greater protection was achieved at higher doses. The surgically implanted, cell-containing capsules were well tolerated, and the cells within the capsule remained viable for the 7-week implantation interval. These results suggest that encapsulated cell therapy may provide a safe and effective strategy for treating retinal disorders in humans.

Developmental expression of nerve growth factor in the eye of rats affected by inherited retinopathy: correlative aspects with retinal structural degeneration

We have previously reported that exogenous administration of nerve growth factor (NGF) in C3H/HeJ mouse strain affected by retinitis pigmentosa (RP) delayed retinal degeneration, suggesting that NGF may be implicated in retinal development. Whether NGF is present in the developing eye was not investigated. To address this question we have used Royal College of Surgeons (RCS) rats, characterised by photoreceptor loss during postnatal life. The results of these studies showed that while the thickness of the outer nuclear layer (ONL) of RCS is comparable to controls, while the amount of NGF expressed in the eye of this mutant rat is significant lower, as compared to control eye. This observation suggests that the lower presence of NGF in the eye of RCS rats during early postnatal life might be one critical key factor implicated in RP. The results of these studies will be presented and discussed.

Critical role of photoreceptor apoptosis in functional damage after retinal detachment

PURPOSE

Although apoptosis is assumed to play a pivotal role in retinal function loss, its mechanism and real influence on retinal function are still unclear. To investigate the relation between retinal function and apoptosis, we studied photoreceptor apoptosis in experimental retinal detachment (RD).

METHODS

We induced RD by subretinal injection of sodium hyaluronate in Brown Norway rats. Apoptotic photoreceptors were detected by TdT-dUTP Terminal Nick-End Labeling (TUNEL). To evaluate the function of the detached retina, electroretinograms (ERGs) were taken on day 1, 3 with corneal electrodes and full-field stimulation.

RESULTS

Apoptotic DNA fragmentation appeared 12 hours after RD, was most prominent on day 3, and decreased thereafter. The ERGs showed that the amplitudes of dark-adapted a-waves and light adapted 2 Hz b-waves decreased immediately after RD and continued to decrease over time. The administration of Fas/Fc chimera recombinant protein or a caspase inhibitor, Z-VAD.fmk, failed to prevent either photoreceptor apoptosis or retinal functional damage. In contrast, brain derived neurotrophic factor (BDNF) and basic fibroblast growth factor (bFGF) significantly impeded both apoptosis and dysfunction. The ERGs recognized the functional changes sensitively, and these ERG changes correlated well to the amount of photoreceptor apoptosis. Immunohistochemical study showed that apoptosis-inducing factor (AIF), a novel caspase-independent apoptotic factor, was relocalized from mitochondria to the nucleus in this process.

CONCLUSIONS

The present results showed that apoptosis was a key phenomenon in the retinal dysfunction in RD and that this process was transmitted mainly by mitochondria-dependent pathways rather than Fas/Fas-L or downstream caspase dependent pathways.

"On" response dysfunction in multifocal posterior pigment epitheliopathy

PURPOSE

To determine the electroretinographic properties of one patient with multifocal posterior pigment epitheliopathy.

METHODS

Rod and cone electroretinograms (ERGs) and photopic ERGs elicited by long-duration stimuli were studied in a patient with multifocal posterior pigment epitheliopathy.

RESULTS

The amplitudes of both the rod and cone ERGs were significantly reduced. Photopic ERGs elicited by long-duration stimuli demonstrated that the b-wave ("on" response) was abolished but the d-wave ("off" response) was reduced by only amplitude.

CONCLUSION

The ERG findings in multifocal posterior pigment epitheliopathy indicate that there is dysfunction not only of the photoreceptors but also in the signal transmission specific for the "on" pathway.

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.

[Vitamin A metabolism and retinal degeneration]

Vitamin A plays an important role in the pathogenesis and during the course of retinal degenerations. Following a brief overview of the metabolic pathway of Vitamin A from the entrance into the body to the arrival in retinal target structures, the most important retinal degenerations that are related to the Vitamin A-metabolism are presented. Ways of prevention or therapy of such diseases are discussed in the light of the still incomplete knowledge about basic mechanisms of retinol transport and metabolism.

[Necessary visual information for restoring reading with a retinal implant in a blind patients with massive retinal degeneration of photoreceptors]

DEFINITION OF THE PROBLEM

Our goal is to determine the minimum of information necessary for elementary reading, using a retinal implant. This concerns particularly the fragmentation (pixellisation) of the presented image and its position in the visual field. Fragmentation corresponds to the number of electrodes available, the position of the image in the visual field is equivalent to the site of the implant on the retina.

MATERIAL AND METHODS

10 degrees x 10 degrees windows, containing isolated words or letters, were presented to six healthy subjects on a computer screen. A coupling between the computer and an eye tracker stabilizes these images in an area of the visual field. This coupling constantly corrects the position of the image on the screen according to the direction of gaze.

RESULTS

1) A rapid decrease of the performance is observed at a certain threshold of pixellisation, dependent on the eccentricity of presentation of the images. 2) In central vision, about 400 pixels are sufficient to recognize 80% of the four-letters words. At 10 degrees of eccentricity, about 1225 pixels are needed. 3) An acceptable comprehension of a text (identification of four words out of five), is impossible at eccentricities higher than 10 degrees 4) About 50 pixels are sufficient for a satisfactory recognition of isolated letters, independently of their eccentricity.

CONCLUSION

These data validate the method of investigation and provide valuable indications regarding minimal visual requirements in prosthetic vision.

Quantitative relationship of the scotopic and photopic ERG to photoreceptor cell loss in light damaged rats

In order to use the ERG to track the effects of potential photoreceptor rescue treatments, we have compared retinal histology to the ERG in light damage. Male albino CD rats (40) were purchased at 7 weeks of age and reared in 50 lx cyclic light until 8 week old. They were exposed to a range of light intensities using white fluorescent light (1000, 1500, 2000, 2500 or 3000 lx) for 24 or 48 hr (n = 5 per group). Controls remained in dim cyclic light. Seven days after exposure, dark and light adapted ERGs were recorded from threshold up to 200 cd m-2 using 50 ms Ganzfeld white light stimuli. The STR, and scotopic and photopic b-wave thresholds and amplitudes were measured. After recording the ERG, the eyes were removed from the animals in each of the five 48 hr light exposed groups and control group for histological measurements. These included: (1) outer nuclear layer width in rod photoreceptor cell number (cell count) and micrometers, and (2) outer + inner segment layer width along the vertical meridian in the inferior retina. The product of cell count and outer + inner segment length was calculated. All histological measures showed a statistically significant linear relationship to light exposure intensity (P < 0.0001): r2 = 0.94 (cell count), 0.90 (outer nuclear layer width), 0.77 (outer + inner segment length). The log of the scotopic b-wave threshold and log amplitude showed a significant linear correlation to all histological parameters (P < 0.0001) and there was no significant difference between b-wave threshold and amplitude for any one of the histology measures used. However, overall, log b-wave threshold was significantly better correlated to histology P < 0.02. Only log b-wave amplitude showed a significant increase in variability in light damaged retinas (P < 0.02). The b-wave threshold intensity increased 0.33 log cd m-2 and the maximum amplitude decreased 0.23 log microV with each 10% decrease in cell number in the outer nuclear layer. The sensitivity of the scotopic threshold response, which originates from third order neurons, changed much more slowly with cell loss, than did the b-wave (P < 0.0005) and was well fit by a linear relationship to cell loss. The increase in photopic b-wave threshold was not significant for a cell loss of less than 70-80%. Neither the photopic or scotopic b-wave could be reliably recorded with more than 80% cell loss, but the scotopic threshold response remained. Both the scotopic and photopic ERG showed similar waveform changes near the threshold, including loss of the positive going b-wave and the predominance of a negative going response. Outer nuclear layer cell counts in this study showed the same relationship to log b-wave threshold elevation, as has been previously shown for whole retinal rhodopsin content in light damage, indicating that regional histology measurements can be good indicators of overall cell survival. Both the b-wave threshold and amplitude can be reliably used to track photoreceptor cell loss due to the damaging effects of constant light, but the scotopic threshold response may be more useful in severe damage.

Understanding the origin of visual percepts elicited by electrical stimulation of the human retina

BACKGROUND

The success of a retinal prosthesis for patients with outer retinal degeneration (ORD) depends on the ability to electrically stimulate retinal cells other than photoreceptors. Experiments were undertaken in human volunteers to ascertain whether electrical stimulation of cells other than photoreceptors will result in the perception of light.

METHODS

In two subjects, two areas of laser damage (argon green and krypton red) were created in an eye scheduled for exenteration due to recurrent cancer near the eye. In the operating room prior to exenteration, under local anesthesia, a hand-held stimulating device was inserted via the pars plana and positioned over the damaged areas and normal retina. Subjects' psychophysical responses to electrical stimulation were recorded.

RESULTS

In both subjects, electrical stimulation produced the following perceptions. Normal retina: dark oval (subject 1), dark half-moon (subject 2); krypton red laser-treated retina: small, white light (both subjects); argon green laser treated retina: thin thread (subject 1), thin hook (subject 2). Histologic evaluation of the krypton red-treated retina showed damage confined to the outer retinal layers, while the argon green-treated area evinced damage to both the outer and the inner nuclear layers.

CONCLUSION

The perception produced by electrical stimulation was dependent on the retinal cells present. Electrical stimulation of the krypton red-ablated area best simulated the electrically elicited visual perceptions of our blind, ORD patients, suggesting that the site of stimulation in blind patients is the inner retinal neurons.

Leber congenital amaurosis caused by a homozygous mutation (R90W) in the homeodomain of the retinal transcription factor CRX: direct evidence for the involvement of CRX in the development of photoreceptor function

The CRX (cone-rod homeobox) gene is specifically expressed in developing and mature photoreceptors and encodes an otd/Otx-like paired homeodomain protein. Mutant alleles of the CRX gene have recently been associated with autosomal dominant cone-rod dystrophy (CORD) as well as dominant Leber congenital amaurosis (LCA). Since LCA is more commonly inherited in an autosomal recessive manner, we examined a cohort of recessive LCA patients for CRX mutations. A homozygous substitution of arginine (R) at codon 90 by tryptophan (W) was identified in the CRX homeodomain of one of the probands who was nearly blind from birth. A group of 48 control individuals and 190 previously characterized CORD probands did not reveal this sequence change. The mutant CRXR90W homeodomain demonstrated decreased binding to the previously identified cis sequence elements in the rhodopsin promoter. In transient transfection experiments, the mutant protein showed significantly reduced ability to transactivate the rhodopsin promoter, as well as lower synergistic activation with the bZIP transcription factor NRL. Heterozygosity of the mutant CRX (R90W) allele was detected in both parents and in an older sibling. Ophthalmologic examination and electro-retinography revealed a subtle abnormality of cone function in both the parents. These data suggest that the R90W mutation results in a CRX protein with reduced DNA binding and transcriptional regulatory activity and that the subsequent changes in photoreceptor gene expression lead to the very early onset severe visual impairment in LCA.

Normal retina releases a diffusible factor stimulating cone survival in the retinal degeneration mouse

The role of cellular interactions in the mechanism of secondary cone photoreceptor degeneration in inherited retinal degenerations in which the mutation specifically affects rod photoreceptors was studied. We developed an organ culture model of whole retinas from 5-week-old mice carrying the retinal degeneration mutation, which at this age contain few remaining rods and numerous surviving cones cocultured with primary cultures of mixed cells from postnatal day 8 normal-sighted mice (C57BL/6) retinas or retinal explants from normal (C57BL/6) or dystrophic (C3H/He) 5-week-old mice. After 7 days, the numbers of residual cone photoreceptors were quantified after specific peanut lectin or anti-arrestin antibody labeling by using an unbiased stereological approach. Examination of organ cultured retinas revealed significantly greater numbers of surviving cones (15-20%) if cultured in the presence of retinas containing normal rods as compared with controls or cocultures with rod-deprived retinas. These data indicate the existence of a diffusible trophic factor released from retinas containing rod cells and acting on retinas in which only cones are present. Because cones are responsible for high acuity and color vision, such data could have important implications not only for eventual therapeutic approaches to human retinal degenerations but also to define interactions between retinal photoreceptor types.

Rod and cone function in the Nougaret form of stationary night blindness

BACKGROUND

Recently, a mutation (Gly38Asp) was identified in the alpha subunit of rod transducin in members of the Nougaret pedigree affected with dominantly inherited stationary night blindness.

OBJECTIVE

To evaluate retinal function in patients with the Gly38Asp gene defect.

DESIGN

Ocular examinations, including specialized measures of rod and cone function.

SETTING

A clinical research facility in Boston, Mass.

PATIENTS

A father (aged 48 years) and son (aged 25 years) with the Gly38Asp mutation.

MAIN OUTCOME MEASURES

Psychophysical thresholds to white and narrowband lights and full-field electroretinographic (ERG) responses.

RESULTS

Both patients showed dark-adapted thresholds to white light that were elevated approximately 2 log-units across the retina. Spectral sensitivity testing revealed thresholds that seemed to be governed mostly by rods. Although both patients' dark-adapted ERG responses to a dim blue flash were nondetectable, their dark-adapted ERGs to a white flash showed an a-wave with cone and rod components and a b-wave amplitude larger than what could have been generated by cone function alone. Rod ERGs to bright blue flashes had subnormal, but detectable, amplitudes that seemed to result from a profound reduction in sensitivity. The patients also showed loss of a cone subcomponent in the dark-adapted response to a red flash. The abnormal dark-adapted ERG responses of the patients could be simulated in the ERG responses of normal subjects tested with blue, white, and red flashes presented in the presence of a mesopic background.

CONCLUSIONS

Although the Nougaret form of stationary night blindness has been cited as a prototype of absent rod function with normal cone function, our findings, based on the genealogically and genotypically documented descendants of Jean Nougaret, show that rod function is present, although subnormal, and that there is slight impairment of cone function. The data also suggest that these abnormalities can be simulated by light-adapting the normal retina, compatible with the proposal that the rod transducin encoded by the mutant gene is constitutively active and that the night blindness results from partial desensitization of rods caused by the constitutive activity.

Early onset photoreceptor abnormalities induced by targeted disruption of the interphotoreceptor retinoid-binding protein gene

Vision in all vertebrates is dependent on an exchange of retinoids between the retinal pigment epithelium and the visual photoreceptors. It has been proposed that the interphotoreceptor retinoid-binding protein (IRBP) is essential for this intercellular exchange, and that it serves to prevent the potentially cytotoxic effects of retinoids. Although its precise function in vivo has yet to be defined, the early expression of IRBP suggests that it may also be required for normal photoreceptor development. To further assess the biological role of IRBP, we generated transgenic mice with targeted disruption of the IRBP gene (IRBP-/- mice). Specifically, homologous recombination was used to replace the first exon and promoter region of the IRBP gene with a phosphoglycerate kinase-promoted neomycin-resistant gene. Immunocytochemical and Western blot analyses demonstrated the absence of IRBP expression in the IRBP-/- mice. As early as postnatal day 11, histological examination of the retinas of IRBP-/- mice revealed a loss of photoreceptor nuclei and changes in the structural integrity of the receptor outer segments. At 30 d of age, the photoreceptor abnormalities in IRBP-/- mice were more severe, and electroretinographic recordings revealed a marked loss in photic sensitivity. In contrast, no morphological or electrophysiological changes were detected in age-matched heterozygotes. These observations indicate that normal photoreceptor development and function are highly dependent on the early expression of IRBP, and that in the absence of IRBP there is a slowly progressive degeneration of retinal photoreceptors.

A case of localized retinal damage in thallium poisoning

PURPOSE

To determine the cause of visual impairment and to document the late eye disturbances in a case of thallium poisoning.

PATIENT

A 44-year-old woman presented with a history of repeated attacks of complete alopecia over a period of several months, diffuse pain in both legs, transient gastrointestinal disturbances, abasia with a progressive paraparesis, paresthesia in the fingertips, and polyneuropathy. She complained of slowly progressive visual deterioration in both eyes which began about six months after the first attack of alopecia. The optic discs showed distinct signs of temporal atrophy together with a deep temporal excavation. The Goldmann perimetry revealed an absolute central scotoma. Traces of thallium were found in the urine and in the serum. The district attorney later discovered that her husband had been trying to poison her with thallium.

METHODS

The clinical and electrophysiological examinations included visual evoked potentials (VEP) and electroretinography (flash ERG, multifocal ERG and pattern ERG).

RESULTS

The VEP showed a reduction in amplitude and a prolonged latency indicating a conduction block. The pattern ERG was initially normal. At a follow-up examination 6 years later, a slight amplitude reduction in the pattern ERG was found. The multifocal ERG showed a diminished amplitude in the center of the retina (up to +/- 10 degrees visual angle).

CONCLUSIONS

The electrophysiological investigations in our patient--who had an optic atrophy--indicated a conduction block of the retinal nerve fibers (VEP) and an additional lesion at or before the retinal bipolar cells (multifocal ERG), localized in the central +/- 10 degrees. These findings suggest that thallium poisoning can lead to a combined lesion of the retinal nerve fibers and the neural retina.

Light-induced cell death of retinal photoreceptors in the absence of p53

PURPOSE

Cell death by apoptosis is essential for normal development and tissue homeostasis, and it is involved also in a variety of pathologic processes. Apoptosis is the final common pathway of photoreceptor cell death in retinal dystrophies and degeneration. So far, little is known about genes regulating apoptosis in the retina. The tumor-suppressor gene product p53 is a potent regulator of apoptosis in numerous systems. However, p53-independent apoptotic pathways also have been described. In this study the authors investigated the role of p53 in the light-induced apoptosis of retinal photoreceptors using mice lacking p53.

METHODS

Free-moving p53-/- and p53+/+ mice were dark adapted and were exposed to 8,500 or 15,000 lux of diffuse, cool, white fluorescent light for 2 hours. Animals were killed before and immediately after light exposure or at 12 hours in darkness after light exposure. Eyes were enucleated and processed for light and electron microscopy and histochemistry (TdT-dUTP terminal nick-end labeling method). Isolated retinas were subjected to the extraction of total retinal DNA. Electroretinogram (ERG) recordings were performed at all time points.

RESULTS

Morphologic, biochemical, histochemical, and ERG analysis showed that the retinas of untreated p53-/- mice and wild-type control mice were structurally and functionally indistinguishable. After exposure to diffuse white fluorescent light, light-induced photoreceptor cell death was analyzed and was found to be the same in both groups of mice.

CONCLUSIONS

These data suggest that light-induced apoptosis of photoreceptors is independent of functional p53.