What can pharmacological models of retinal degeneration tell us?

Animal models with pharmacologically induced retinal degeneration including sodium iodate (NaIO3) and N-methyl-N-nitrosourea (MNU) have been extensively used in ophthalmic research to investigate retinal degeneration. NaIO3 induces degeneration of the retinal pigment epithelium (RPE) followed by photoreceptor (PRC) cell death, mimicking features of age-related macular degeneration. In contrast, MNU leads to rapid destruction of the PRCs only, enabling the use of the MNU model to investigate degeneration induced in retinitis pigmentosa. It has been shown that multiple cell death pathways are involved in the cell-specific effects of the toxins. Necrosis has been identified as the cause of the NaIO3-induced RPE loss. PRC degeneration in the described models is mainly induced by programmed cell death, indicated by the upregulation of conventional apoptosis initiator and effector caspases. However, recent research points to the additional involvement of caspase-independent processes as endoplasmic reticulum stress and calpain activation. Since there is still a substantial amount of contradictory hypotheses concerning triggers of cell death, the use of pharmacological models is controversial. Thereby, the advantages of such models like the application reaching across species and strains as well as modulation of onset and severity of damage are not exploited to a full extent. Thus, the present review aims to give more insight into the involved cell death pathways and discusses recent findings in the most widely used retinal degeneration models. It might facilitate further studies aiming to develop putative therapeutic approaches for retinal degenerative diseases including combinatory treatment with cell death inhibitors and cell transplantation therapy.