Faculty Bibliography

Death of retinal photoreceptors by apoptosis is observed under many physiological and pathological conditions such as histogenesis, retinal dystrophies and light-induced photoreceptor degeneration. To date, little is known about regulatory mechanisms for apoptosis in the retina. The tumor suppressor gene p53 is a regulator of apoptosis in a number of systems, however, p53-independent apoptosis has also been described. We have therefore investigated whether the lack of p53 influences the dark-adapted ERG in C57BL/6 p53-/- mice compared to p53+/+ control littermates under physiological (regular light-dark cycle) conditions. We also recorded ERGs at 12 to 14 h in darkness following diffuse bright light exposure to 8,000 or 15,000 lux for 2 h. ERG analysis over a range of 6 logarithmic units of light intensity revealed normal and virtually identical a-, b-, c-waves and oscillatory potentials in dark-adapted p53+/+ and p53-/- mice. After exposure to diffuse white fluorescent light strong decreases of all ERG components were found to be very similar in both genotypes. These data support the notion that the p53 protein is neither essential for normal retinal function nor for processes involved in light-induced depression of the ERG in mice.

Apoptosis is a genetically regulated form of cell death. Individual cells show condensed nuclear chromatin and cytoplasm, and biochemical analysis reveals fragmentation of the DNA. Ensuing cellular components, apoptotic bodies, are removed by macrophages or neighboring cells. Genes involved in the regulation of apoptosis as well as stimuli and signal transduction systems, are only beginning to be understood in the retina. Therefore, we developed a new in vivo model system for the investigation of events leading to apoptosis in the retina and the pigment epithelium. We induced apoptosis in retinal photoreceptors and the pigment epithelium of albino rats by exposure to 3000 lux of diffuse, cool white fluorescent light for short time periods of up to 120 minutes. Animals were killed at different time intervals during and after light exposure. The eyes were enucleated and the lower central retina was processed for light- and electron microscopy. DNA fragmentation was analysed in situ by TdT-mediated dUTP nick-end labeling (TUNEL) or by gel electrophoresis of total retinal DNA. We observed that the timing of apoptosis in the photoreceptors and pigment epithelium was remarkably different, the pigment epithelium showing a distinct delay of several hours before the onset of apoptosis. In photoreceptors, apoptosis was induced within 90 minutes of light exposure, with the morphological appearance of apoptosis preceding the fragmentation of DNA. In the pigment epithelium, the morphological appearance of apoptosis and DNA fragmentation were coincident. Different regulative mechanisms may lead to apoptotic cell death in the retinal photoreceptors and pigment epithelium. This in vivo model system will allow measurement of dose-responses, a potential spectral dependence and the molecular background of apoptotic mechanisms in the retina.

Apoptotic cell death in the retina was recently demonstrated in animal models of the hereditary human retinal dystrophy known as retinitis pigmentosa. Although recent evidence indicates that the proto-oncogene c-fos is a mediator of apoptosis, its precise role is unclear. In fact, under some conditions, c-fos may even protect against apoptotic cell death. In the retina, c-fos is physiologically expressed in a diurnal manner and is inducible by light. We previously observed a light-elicited, dose-dependent apoptotic response in rat photoreceptors. To determine whether c-fos is involved in the light-induced apoptotic pathway we have used control mice and mice lacking c-fos. We found that following dark adaptation and two hours of light exposure both groups of animals exhibited only a few apoptotic cells. However, at 12 and 24 additional hours after light exposure, apoptosis increased dramatically in controls but was virtually absent in those mice lacking c-fos. Therefore, c-fos is essential for light-induced apoptosis of photoreceptors. Notably, c-fos is continuously upregulated concomitant with apoptotic photoreceptor death in our system and in animal models of retinitis pigmentosa (Agarwal, N. et al., Invest. Ophthalmol. Vis.Sci. Suppl. 36, S638 and Rich, K.A. et al., Invest. Ophthalmol. Vis. Sci. Suppl. 35, 1833). Inhibition of c-fos expression might therefore represent a novel therapeutic strategy to retard the time course of retinal dystrophies and light-induced retinal degeneration.

Are observations on ultraviolet (UV)- and visible light-induced ocular changes in animals relevant for human pathology? Different conclusions are drawn by different groups, depending on their perspective: while in the epidemiologist's view the evidence for those lesions is mostly limited or insufficient, laboratory scientists continually extend observations on radiation damage in animals. Consequently, there are diverging views on the necessity and specifications for eye protection. In this review, problems of epidemiological surveys and observations in humans and animal studies are discussed, and natural and artificial protection of the eye is outlined. The human and animal eye has an inherent potential for photochemical lesions due to chromophores including the visual pigments that are present at birth. Lifelong light exposure gives rise to additional absorbing molecules. With decreasing wavelengths of the electromagnetic spectrum the number of absorbing molecules rises; therefore, the likelihood of a photochemical reaction grows. As the spectral energy is augmented, more damage will occur. In our view, the knowledge gained from laboratory studies is a significant component of the total evidence from different fields-epidemiology, clinical observations, model studies and theoretical calculations-that UV radiation and short-wavelength visible light can cause acute and chronic changes in ocular structures. Such changes may comprise irreversible damage. Following recently issued recommendations of the major visual health organizations in the United States, protection against UV and blue light should be incorporated into the spectrum of safety considerations for sunglasses.

beta-Receptor blockers may exert a spectrum of metabolic and humoral effects, which might differ depending on the specific adrenoreceptor characteristics of the individual agents. We investigated the influence of celiprolol, a beta 1-blocker with beta 2-agonistic and, possibly, additional weak alpha-receptor antagonistic properties, on insulin sensitivity (SI), glucose homeostasis, and lipid profile in 20 young, healthy, normotensive individuals. SI, fasting plasma glucose and insulin, serum total triglycerides (TG), lipoprotein cholesterol (C) fractions, lipoprotein a [Lp(a)], and plasma atrial natriuretic factor (ANF) levels were determined before and after acute glucose loading under placebo conditions and after 3 weeks of celiprolol administration. The participants were instructed to follow a 3-day standard diet (2,500 kcal/day, 45% carbohydrates, 40% fat, and 15% protein) and an overnight fast before measurements were recorded. As compared with control values, SI, fasting plasma glucose and insulin, the areas under the glucose and insulin curves, the k value of glucose disappearance after glucose load, and serum cholesterol fractions, TG, and Lp(a) were unchanged during celiprolol administration. However, celiprolol significantly reduced plasma ANF levels (p < 0.02). The latter increased in response to acute hyperglycemia/hyperinsulinemia with placebo (p < 0.05) but not with celiprolol. Although diastolic blood pressure (DBP) decreased slightly during the first and second week of celiprolol administration, BP and heart rate (HR) did not differ significantly after 3 weeks on celiprolol treatment as compared with placebo conditions. Our findings demonstrate that in healthy lean humans beta-receptor modulation with celiprolol is neutral with regard to SI and lipoprotein metabolism. Moreover, glucose loading stimulates whereas celiprolol decreases plasma ANF levels.(ABSTRACT TRUNCATED AT 250 WORDS)