Photic injury promotes cleavage of p75NTR by TACE and nuclear trafficking of the p75 intracellular domain.

The p75 neurotrophin receptor (p75NTR) is a member of the tumor necrosis factor receptor superfamily that paradoxically mediates neuronal survival and differentiation or apoptotic cell death. Cleavage of p75NTR by a constitutively active metalloprotease could result in shedding of its extracellular domain (p75ECD) and generation of a pro-apoptotic intracellular domain (p75ICD). In this study, we established that exposure of a transgenic mouse photoreceptor cell line to intense light upregulated the expression of p75NTR and of the disintegrin metalloprotease tumor necrosis factor-converting enzyme (TACE) and resulted in apoptotic cell death. Light damage promoted TACE cleavage of p75NTR resulting in shedding of the soluble p75ECD and nuclear translocation of the p75ICD. Overexpression of TACE and p75NTR-induced p75NTR cleavage and secretion of p75ECD, but not nuclear transport of p75ICD. Light-induced cleavage of p75NTR, nuclear localization of p75ICD, and apoptosis were inhibited by IC-3, a metalloprotease inhibitor. Increased levels of p75NTR and TACE were observed in photoreceptor cells of animals with photic injury. Our findings support a role for TACE in the proteolytic cleavage of p75NTR and light-induced apoptosis.

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.

Therapeutic range of repetitive nanosecond laser exposures in selective RPE photocoagulation.

BACKGROUND: The aim of this study was to investigate whether selective damage the RPE while sparing the adjacent photoreceptors is possible with repetitive 200-ns pulses of Nd:YAG laser (532 nm) and what potential side effects can be expected with higher pulse energies. METHODS: We irradiated the retinas of 19 eyes of 10 chinchilla rabbits with 500 pulses from a Nd:YAG laser, each 200 ns in duration, at a repetition rate of 500 Hz (158 microns, 0-120 microJ). Threshold curves for different effects were established. Representative lesions were investigated by light and transmission electron microscopy. RESULTS: It was possible to produce lesions, which were only visible by fluorescein angiography. The ED50 threshold energy per pulse for visibility by fluorescein angiography was 2.1 microJ per pulse, for visibility by ophthalmoscopy 8.6 microJ. Bubble formation, an uncommon phenomenon in retinal photocoagulation, occurred at energies of 15-25 microJ. Hemorrhage occurred at surprisingly high energy levels of more than 100 microJ. Histology performed on lesions visible only by angiography showed damage primarily to the RPE and outer segments, with very little damage to some inner segments dependent on the energy used. CONCLUSIONS: Selective RPE damage is possible with repetitive 200-ns laser pulses and appropriate energy; however, the collateral damage to the adjacent retina is more pronounced than with repetitive microsecond laser pulses. There is no risk of hemorrhage of retinal photocoagulation with the repetitive 200-ns laser pulses at low energy levels which would be used clinically.

Basic fibroblast growth factor and local injury protect photoreceptors from light damage in the rat.

Injection of basic fibroblast growth factor (bFGF) into the eye, intravitreally or subretinally, delays photoreceptor degeneration in inherited retinal dystrophy in the rat, as does local injury to the retina (Faktorovich et al., 1990). To determine whether this heparin-binding peptide or local injury is effective in any other form of photoreceptor degeneration, we examined their protective roles in light damage. Albino rats of the F344 strain were exposed to 1 or 2 weeks of constant fluorescent light (115-200 footcandles), either with or without 1 microliter of bFGF solution (1150 ng/microliters in PBS) injected intravitreally or subretinally 2 d before the start of light exposure. Uninjected and intravitreally PBS-injected controls showed the loss of a majority of photoreceptor nuclei and the loss of most inner and outer segments after 1 week of light exposure, while intravitreal injection of bFGF resulted in significant photoreceptor rescue. The outer nuclear layer in bFGF-injected eyes was two to three times thicker than in controls, and the inner and outer segments showed a much greater degree of integrity. Following recovery in cyclic light for 10 d after 1 week of constant light exposure, bFGF-injected eyes showed much greater regeneration of photoreceptor inner and outer segments than did the controls. bFGF also increased the incidence of presumptive macrophages, located predominantly in the inner retina, but the evidence suggests they are not directly involved in photoreceptor rescue. Subretinal injection of bFGF resulted in photoreceptor rescue throughout most of the superior hemisphere in which the injection was made, with rescue extending into the inferior hemisphere in many of the eyes. Remarkably, the insertion of a dry needle or injection of PBS into the subretinal space also resulted in widespread photoreceptor rescue, extending through 70% or more of the superior hemisphere, and sometimes into the inferior hemispheres. This implicates the release and widespread diffusion of some endogenous survival-promoting factor from the site of injury in the retina. Our findings indicate that the photoreceptor rescue activity of bFGF is not restricted to inherited retinal dystrophy in the rat, and that light damage is an excellent model for studying the cellular site(s), kinetics, and molecular mechanisms of both the normal function of bFGF and its survival-promoting activity. Moreover, the injury-related rescue suggests that survival-promoting factors are readily available to provide a protective role in case of injury to the retina, presumably comparable to those that mediate the "conditioning lesion" effect in other neuronal systems.

Histopathology of commotio retinae.

The pathogenesis of commotio retinae in animal models has included extracellular edema, intracellular edema, and photoreceptor outer segment disruption. Histopathologic findings obtained within 24 hours of blunt trauma in a human eye with clinically-observed commotio retinae revealed photoreceptor outer segment disruption and damage to the retinal pigment epithelium. The major site of injury in commotio retinae seems to be at the level of the photoreceptor outer segment-retinal pigment epithelium junction.

Localized current field hyperthermia: effect on normal ocular tissue.

Using a 500 kHz radiofrequency electromagnetic heating system, the effects of localized current field hyperthermia in normal rabbit eyes were examined. A specially designed scleral plaque placed on normal rabbit eyes was heated to temperatures of 43 degrees C, 45 degrees C, and 47 degrees C for a period of 45 min. The effects of hyperthermia were monitored by clinical examination, fluorescein angiography, electroretinography and histopathology. A graded effect with increasing temperature was found at the lower temperature, and it was confined to the treatment field. At 47 degrees C the electroretinogram was extinguished due to diffuse photoreceptor damage outside the treatment field, as demonstrated by histopathology and electron microscopy. This study indicates that hyperthermia at 45 degrees C for 45 min is the maximum allowable temperature without causing diffuse retinal damage in the normal rabbit eye.

Antagonistic effects of adrenalectomy and ether/surgical stress on light-induced photoreceptor damage.

Light-induced damage to retinal photoreceptors in influenced by the endocrine status of the animal during the period of exposure. Experimental manipulation of the pituitary gland and of prolactin levels has been shown to affect retinal damage in rats exposed to visible light. When rats are experimentally stressed, prolactin secretion from the pituitary gland occurs as does secretion of adrenocorticotropic hormone (ACTH), which stimulates the release of adrenal cortical hormones. Since prolactin appears to influence retinal damage and since stressed animals have increased serum levels of prolactin, a comparison of photoreceptor damage in animals in which the adrenal glands were removed or which had been experimentally stressed was undertaken in this study. Adrenalectomized rats had thicker outer nuclear layer (ONL) measurements than those found in sham-operated animals. Stressed rats had severely damaged retinas with cystic degeneration and significantly reduced ONL thickness measurements as compared to retinas of unstressed and adrenalectomized rats. Therefore hormones of the pituitary-adrenal system appear to be involved in the damage to the retina by light, and this response may be related to an interaction or synergism between the adrenal gland, stress, and prolactin secretion.

Repair and late degeneration of the primate foveola after injury by argon laser.

Each foveola of nine rhesus monkey eyes was subjected to a single, mild, calibrated exposure from an argon laser (1 to 1.4 mW for 10 to 20 min). Observations from ophthalmoscopy, fundus photography, and fluorescein angiography were correlated with light and electron microscopic studies. The ophthalmoscopic changes consisted of initial whitening and subsequent but persistent depigmentation of the foveola. Fluorescein angiography showed a pattern consistent with "window defect" of the retinal pigment epithelium (RPE). In the first 6 months after injury, the pathologic changes consisted of initial vacuolar changes of the RPE, followed by persistent hypopigmentation and slow development of membranous bodies or lipoidal degeneration in these cells. Initial alteration and subsequent incomplete reformation of cone outer segments were also noted. In animals sacrificed 3 to 4 years after injury, separation of RPE from Bruch's membrane, with production of various abnormal basement membranes and intercellular cystoid changes in the overlying retina, was observed. The morphologic observations suggest that although the mildly injured epithelial cells may recover, they may develop functional incompetence at a later date, resulting in serous detachment of RPE and foveal (macular) edema.

Retinal damage produced by intraocular fiber optic light.

We exposed the maculas of owl monkey eyes to light from an intraocular fiber optic light source similar to that used for human pars plana vitrectomy. Retinal irradiance was calculated at 0.22 W/cm2. Eyes were exposed for time intervals ranging from 30 minutes to five minutes and were observed after light treatment by fundus photography and fluorescein angiography. Tissue was obtained for light and electron microscopy by animal killing at one hour, 24 hours, one week, and four weeks. Fundus lesions were seen ophthalmoscopically as early as five hours following 30 minutes of light exposure. Significant damage to the photoreceptor layer and less damage to the pigment epithelium was present by light and electron microscopy as early as one hour after 30 minutes of light exposure. By one month complete loss of photoreceptors with Müller cell junctions between inner retina and flattened abnormal retinal pigment epithelium cells was observed. Fluorescein angiography revealed significant staining of the pigment epithelium and outer retina 24 hours after 30 minutes of light exposure. No leakage from retinal vessels occurred. At one month following light treatment, transmission of choroidal fluorescein through window defects in the pigment epithelium was present with no retinal staining. The threshold for ophthalmoscopically visible fundus lesions in this study was 15 minutes of light exposure. Ten minutes of light treatment was the threshold for microscopic changes. Short light exposures damaged the outer retina and spared the pigment epithelium. Removing a substantial amount of the infrared light from our light source did not protect the retina from damage. Removal of light between 400 and 500 nm is probably more helpful in protecting the retina. Intermittent light exposure of the retina seemed as harmful as uninterrupted illumination for the same cumulative period of time. We speculate that the retinal damage caused by intraocular fiber optic light has primarily a photic mechanism. Damage to the retinal pigment epithelium may be secondary to outer retinal damage. The present levels of intraocular light used for human pars plana vitrectomy are probably safe in most instances. Lengthy preretinal membrane stripping procedures during vitrectomy, however, may pose a threat of light damage to the retina. This damage must be appreciated as continued efforts are made to produce brighter sources of intraocular light for human pars plana vitrectomy.