PRCD is concentrated at the base of photoreceptor outer segments and is involved in outer segment disc formation.

Mutations of the photoreceptor disc component (PRCD) gene are associated with rod-cone degeneration in both dogs and humans. Prcd is expressed in the mouse eye as early as embryonic day 14. In the adult mouse retina, PRCD is expressed in the outer segments of both rod and cone photoreceptors. Immunoelectron microscopy revealed that PRCD is located at the outer segment rim and that it is highly concentrated at the base of the outer segment. Prcd-knockout mice present with progressive retinal degeneration, starting at 20 weeks of age and onwards. This process is reflected by a significant and progressive reduction of both scotopic and photopic electroretinographic responses and by thinning of the retina, and specifically of the outer nuclear layer, indicating photoreceptor loss. Electron microscopy revealed severe damage to photoreceptor outer segments, which is associated with immigration of microglia cells to the Prcd-knockout retina and accumulation of vesicles in the inter-photoreceptor space. Phagocytosis of photoreceptor outer segment discs by the retinal pigmented epithelium is severely reduced. Our data show that Prcd-knockout mice serve as a good model for retinal degeneration caused by PRCD mutations in humans. Our findings in these mice support the involvement of PRCD in outer segment disc formation of both rod and cone photoreceptors. Furthermore, they suggest a feedback mechanism which coordinates the rate of photoreceptor outer segment disc formation, shedding and phagocytosis. This study has important implications for understanding the function of PRCD in the retina, as well as for future development of treatment modalities for PRCD deficiency in humans.

Safety of intravitreal clindamycin in albino rabbit eyes.

PURPOSE: To study the potential toxic effects of intravitreal clindamycin on the retina of albino rabbits, by assessing functional and morphological retinal changes. METHODS: Eight albino rabbits were included in the study. In each rabbit, 1 mg/0.1 ml clindamycin was injected into the vitreous of the right (experimental) eye, and 0.1 ml saline was injected into the vitreous of the left (control) eye. The electroretinogram (ERG) was recorded before injection, 3 days, 1, 2, and 4 weeks post-injection. The visual evoked potential (VEP) was recorded 4 weeks post-injection. Clinical examination was conducted at all time points. The eyes were enucleated at the termination of the follow-up period in order to prepare the retinas for histology in order to assess retinal structure. RESULTS: ERG and VEP responses that were recorded from the experimental eye at different times following intravitreal clindamycin injection were very similar to the corresponding responses that were recorded from the control eyes. Clinical examination was normal in all eyes, and no histological damage was observed. CONCLUSIONS: Intravitreal injection of 1 mg clindamycin does not cause functional or morphological signs of retinal toxicity in albino rabbits, during a period of 4 weeks post-injection. These findings support the clinical use of 1 mg intravitreal clindamycin.

The retinal toxicity profile towards assemblies of Amyloid-ß indicate the predominant pathophysiological activity of oligomeric species.

Amyloid-ß (Aß), reported as a significant constituent of drusen, was implicated in the pathophysiology of age-related macular degeneration (AMD), yet the identity of the major pathogenic Aß species in the retina has remained hitherto unclear. Here, we examined the in-vivo retinal impact of distinct supramolecular assemblies of Aß. Fibrillar (Aß40, Aß42) and oligomeric (Aß42) preparations showed clear biophysical hallmarks of amyloid assemblies. Measures of retinal structure and function were studied longitudinally following intravitreal administration of the various Aß assemblies in rats. Electroretinography (ERG) delineated differential retinal neurotoxicity of Aß species. Oligomeric Aß42 inflicted the major toxic effect, exerting diminished ERG responses through 30 days post injection. A lesser degree of retinal dysfunction was noted following treatment with fibrillar Aß42, whereas no retinal compromise was recorded in response to Aß40 fibrils. The toxic effect of Aß42 architectures was further reflected by retinal glial response. Fluorescence labelling of Aß42 species was used to detect their accumulation into the retinal tissue. These results provide conceptual evidence of the differential toxicity of particular Aß species in-vivo, and promote the mechanistic understanding of their retinal pathogenicity. Stratifying the impact of pathological Aß aggregation in the retina may merit further investigation to decipher the pathophysiological relevance of processes of molecular self-assembly in retinal disorders.

Intravitreal Trimethoprim and Sulfamethoxazole Toxicity to the Retina of Albino Rabbits.

PURPOSE: To evaluate retinal toxicity of intravitreal trimethoprim-sulfamethoxazole (TMP-SMX) in an albino rabbit model. METHODS: Albino rabbits (N = 10) were treated in the right eye with the maximum intravitreal dose of TMP-SMX mixture (1600 µg/8000 µg /0.1 mL), while 0.1 mL saline was injected into the vitreous of the left eye. Clinical examination and electrophysiological (electroretinogram [ERG] and visual evoked potentials [VEPs]) testing were conducted before injection, 3 days, 1, 2, and 4 weeks postinjection. Retinal structure and expression of glial fibrillary acidic protein (GFAP) were assessed from histology and immunocytochemistry respectively at the end of the follow-up period. RESULTS: Clinical examination was normal throughout the follow-up period. ERG responses from the experimental eyes were similar to those recorded from the control eyes, but the sum of oscillatory potentials decreased in the experimental eyes at 2 weeks postinjection. The VEP responses, elicited by stimulation of the experimental eyes, were abnormal having reduced amplitude and prolonged implicit time. Histological damage in the experimental eyes was expressed by thickness reduction of whole, outer, and inner nuclear layers. GFAP was expressed in retinal Müller cells of all experimental eyes, but none of control eyes. CONCLUSIONS: A single intravitreal injection of TMP-SMX mixture (1600 µg/8000 µg, respectively) causes functional and structural damage to the inner retina and retinal output. Signs of retinal stress were also evident by GFAP expression in retinal Müller cells of all experimental eyes. Therefore, the use of TMP-SMX via intravitreal administration should be done with caution. TRANSLATIONAL RELEVANCE: These findings highlight the risk of retinal toxicity after intravitreal injection of trimethoprim-sulfamethoxazole and emphasize that this treatment should be carefully considered.

ATF3 Regulates the Expression of AChE During Stress.

Acetylcholinesterase (AChE) expresses in non-cholinergic cells, but its role(s) there remain unknown. We have previously attributed a pro-apoptotic role for AChE in stressed retinal photoreceptors, though by unknown mechanism. Here, we examined its promoter only to find that it includes a binding sequence for the activating transcription factor 3 (ATF3); a prototypical mediator of apoptosis. This suggests that expression of AChE could be regulated by ATF3 in the retina. Indeed, ATF3 binds the AChE-promoter to down-regulate its expressions in vitro. Strikingly, retinas of "blinded" mice display hallmarks of apoptosis, almost exclusively in the outer nuclear layer (ONL); coinciding with elevated levels of AChE and absence of ATF3. A mirror image is observed in the inner nuclear layer (INL), namely prominent levels of ATF3 and lack of AChE as well as lack of apoptosis. We conclude that segregated patterns of expressions of ATF3 reflect its ability to repress apoptosis in different layers of the retina-a novel mechanism behind apoptosis.