Relationship between mitochondrial DNA damage and photoreceptor death in developing and adult retina, assessed in normal and degenerative rat strains

Phenotypic characterization of P23H and S334ter rhodopsin transgenic rat models of inherited retinal degeneration

We produced 8 lines of transgenic (Tg) rats expressing one of two different rhodopsin mutations in albino Sprague-Dawley (SD) rats. Three lines were generated with a proline to histidine substitution at codon 23 (P23H), the most common autosomal dominant form of retinitis pigmentosa in the United States. Five lines were generated with a termination codon at position 334 (S334ter), resulting in a C-terminal truncated opsin protein lacking the last 15 amino acid residues and containing all of the phosphorylation sites involved in rhodopsin deactivation, as well as the terminal QVAPA residues important for rhodopsin deactivation and trafficking. The rates of photoreceptor (PR) degeneration in these models vary in proportion to the ratio of mutant to wild-type rhodopsin. The models have been widely studied, but many aspects of their phenotypes have not been described. Here we present a comprehensive study of the 8 Tg lines, including the time course of PR degeneration from the onset to one year of age, retinal structure by light and electron microscopy (EM), hemispheric asymmetry and gradients of rod and cone degeneration, rhodopsin content, gene dosage effect, rapid activation and invasion of the outer retina by presumptive microglia, rod outer segment disc shedding and phagocytosis by the retinal pigmented epithelium (RPE), and retinal function by the electroretinogram (ERG). The biphasic nature of PR cell death was noted, as was the lack of an injury-induced protective response in the rat models. EM analysis revealed the accumulation of submicron vesicular structures in the interphotoreceptor space during the peak period of PR outer segment degeneration in the S334ter lines. This is likely due to the elimination of the trafficking consensus domain as seen before as with other rhodopsin mutants lacking the C-terminal QVAPA. The 8 rhodopsin Tg lines have been, and will continue to be, extremely useful models for the experimental study of inherited retinal degenerations.

Neuroprotective effect of Myo/Nog cells in the stressed retina

Myo/Nog cells are essential for eye development in the chick embryo and respond to injury in adult tissues. These cells express mRNA for the skeletal muscle specific transcription factor MyoD, the bone morphogenetic protein (BMP) inhibitor Noggin and the cell surface protein recognized by the G8 monoclonal antibody (mAb). In this study, we determined that Myo/Nog cells are present in low numbers in the retina of the mouse eye. G8-positive Myo/Nog cells were distinguished from neuronal, Müller and microglial cells that were identified with antibodies to calretinin, Chx10, glial fibrillary acidic protein and ionized calcium binding adaptor molecule 1, respectively. In the neonatal retina, the number of Myo/Nog cells increased in parallel with cell death induced by transient exposure to hyperoxia. In this model of retinopathy of prematurity, depletion of Myo/Nog cells by intravitreal injection of the G8 mAb and complement increased cell death. These findings demonstrate that Myo/Nog cells are a distinct population of cells, not previously described in the retina, which increases in response to retinal damage and mitigate hypoxia-induced cell death.

Analysis of fine structure and biochemical changes of retina during aging of Wistar albino rats

BACKGROUND

Aging is a biological phenomenon that involves an increase of oxidative stress associated with gradual degradation of the structure and function of the retina. Gender differences and subsequent deterioration of retinal cell layers is an interesting topic, especially because there is no published work concerning it.

METHODS

One hundred and twenty male and female Wistar albino rats ages 1, 6, 18, 30 and 42 months (n = 20 equal for male and female) were used. At the time interval, retinae were investigated by light and transmission electron microscopy, assessments of neurotransmitters, anti-oxidant enzymes (catalase, superoxide dismustase and glutathione S transferase), caspase-3 and -7, malonadialdhyde, and DNA fragmentation.

RESULTS

Light and transmission electron microscopy observations of the older specimens (30 and 42 months) revealed apparent deterioration of retinal cell layers, especially ganglion and nerve fibres, nuclear, pigmented epithelium and stacked membranes of the photoreceptor's outer segments. Males were highly susceptible to aging processes. Retinal DNA fragmentation was remarked parallel with increase of apoptic markers caspase 3 and 7. Concomitantly, there was a marked reduction of neurotransmitters and anti-oxidant enzymes, and an increase of lipid peroxidation.

CONCLUSIONS

Aging contributed to an increase of oxidative stress resulting from damage of mitochondria in retinal cells, a decrease of the anti-oxidant enzyme system and an increase of markers of retinal cell death.