The impact of senescence on the zebrafish’s neuroregenerative capacities

Purpose: Developing effective treatments for age-related neuropathies is one of the major challenges in modern medical research. Intensive comparative studies are focused on finding the underlying mechanisms of successful neuroregeneration and subsequent recovery of the diseased CNS. In contrast to mammals, zebrafish retain a high regenerative potential through adulthood. As they exhibit gradual aging similar to humans, they are an ideal species to unravel the effects of senescence on regeneration capacities. We investigate the retinotectal system, a powerful system to study neurogenesis, neuronal survival and axonal regrowth after damage. Methods: Immunohistochemistry methods (IHC) were used to evaluate apoptosis, synaptic integrity, inflammation and neurogenesis within the retina and optic nerve of aged zebrafish. In addition, axonal regeneration was followed by means of GAP-43 IHC and biocytin tracing within the retinotectal system after applying optic nerve crush (ONC). Underlying processes such as senescence of the retinal ganglion cells (RGCs) and responding innate immune cells were analysed via outgrowth assays within retinal explants and IHC analyses within the retina, optic nerve and tectum, respectively. Results: Hallmarks of aging, such as retinal atrophy and inflammaging, are indeed present in the aged zebrafish retinotectal system and seem to influence neurogenic capacities, since RGC neurogenesis diminishes with age. Moreover, after optic nerve crush, axonal regeneration is significantly delayed as the growth rate of damaged axons is decreased in aged zebrafish, resulting in a retarded tectal reinnervation. Currently, we are investigating if recovery of visual function is correspondingly affected. Preliminary data from in vitro outgrowth assays suggest that aged RGCs exhibit a cutback in their intrinsic regeneration potential. In addition, altered morphological and functional changes of innate immune cells indicate that senescent microglia might also underlie the observed decline in regeneration capacity. Conclusions: Zebrafish form an excellent species for disease modeling and comparative regeneration research. We show that senescence significantly affects neurogenic and axonal regeneration capacities within this small vertebrate. These studies can lead to developing new therapeutic strategies for successful neuroregeneration within the aged mammalian CNS.status: publishe