Regeneration in the ageing retina of the zebrafish.

The influence of processes of aging on the regenerative potential of the zebrafish retina

Senescence in the central nervous system (CNS) is a complex biological phenomenon, and the underlying mechanisms are subject to intensive study. Indeed, the natural process of ageing in the CNS leads to a drastic decline in sensory, motor and cognitive functions, which is mainly caused by significant neuronal cell death and strongly intertwined with the development of age-related pathologies, such as Alzheimer’s and Parkinson’s disease, multiple sclerosis and optic neuropathies. These disorders can have a drastic impact on the functional capacities and life quality of the aged population. An effective therapy for these neuronal diseases would comprise the accomplishment of neuroprotection, axonal regeneration and/or de novo neurogenesis. Although intensive research efforts have significantly contributed to new approaches and therapeutic strategies inducing neuroregeneration, functional recovery of the diseased mammalian CNS remains a challenge, especially in an aging environment.

Only recently was the zebrafish, already the focus of a myriad of comparative studies investigating neurogenesis, differentiation, axonal regeneration and remyelinisation, put forward as a gerontological model.  Herein the retinotectal system forms a powerful system to study neuronal damage and regeneration, and has already contributed significantly to our current knowledge of the multifactorial causes underlying the limited regeneration capacity of the adult mammalian CNS. Indeed, as the retinal structure and molecular mechanisms underlying neuroprotective and regenerative processes are widely conserved amongst vertebrates, studying zebrafish retinal and optic nerve regeneration offers great potential. However, both literature and preliminary data suggest that this robust regenerative potential decreases in the aged zebrafish. Therefore, the zebrafish optic system offers a valuable subject to study the effects of aging on neuroregenerative strategies, such as neuroprotection, axonal regeneration and de novo neurogenesis.

Within this study, we will try to confirm this decline in regenerative capacity in the aged zebrafish retina and identify proteins that underlie the expected decrease in axonal and neuronal regeneration. Therefore, we will pursue the following goals. As the senescent zebrafish eye is only scarcely described, the overall morphology of the aged retina, the distribution of cells and the organisation of the different layers will be studied. Specific hallmarks of aging, like the occurrence of oxidative stress, inflammation and degeneration will be visualised in young and aged fish. Next, the regenerative potential of the senescent zebrafish retina will be investigated by means of two validated damage paradigms. De novo neurogenesis of retinal ganglion cells (RGCs) will be studied after induction of RGC degeneration via the toxin ouabain, with a focus on the contribution of radial glia to the response process. Also, axonal regrowth of endogenous damaged neurons will be visualised in a more delicate injury model, namely optic nerve crush. Finally, a comparative proteomic analysis of young versus aged, either regenerating or naïve retinas, will allow us to identify important ‘aging proteins’, which underlie a diminished regenerative potential in the aged zebrafish retina. A selected number of proteins will be validated through Western blotting and in vivo experiments. Via transgenic/mutant zebrafish lines or loss-of-function strategies, their relative contribution to the regeneration response within the aged zebrafish retinotectal system will be determined.