Cellular senescence and reduced regeneration potential in aged zebrafish

Purpose: Today, a growing number of elderly suffers from neurodegenerative pathologies, seriously impacting the well being of these patients. Intensive research efforts are therefore focused on the stimulation of neuroregeneration, a capacity that is unfortunately absent in adult mammals and remains a challenge to induce, especially in an aging environment. Zebrafish however, posses high neurogenic and regenerative abilities, yet, are also subjected to gradual aging, similar as in humans. These teleost fish are therefore ideally suited to investigate the impact of aging processes on neuroregenerative potential. Methods: Within the young and aged zebrafish retinotectal system, - a powerful model to study regenerative capacities, - axonal regeneration was studied after applying optic nerve crush (ONC). Axonal outgrowth capacity was assessed by means of qRT-PCR for the growth-associated genes gap43 and tuba1. Also, growth speed and targed reinnervation of regenerating axons was visualized via biocytin tracing and quantified via morphometric analyses within the retina, optic nerve and tectum. In addition, visual recovery was assessed by longitudinally analysing the optokinetic response after axonal injury. Underlying mechanisms, 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 retino-tectal system, respectively. Results: Zebrafish subjected to ONC show a striking age-related decline in regenerative capacity, which is accompanied by reduced retinal expression of the regeneration-associated genes gap43 and tuba1. Detailed morphometric and immunohistochemical analyses of the aged zebrafish retinotectal system confirm a diminished outgrowth capacity of retinal ganglion cells, resulting in a significant delay in axonal regeneration and a retarded tectal innervation. In consequence, recovery of visual function is similarly affected. More importantly, preliminary data from in vitro outgrowth assays suggest that aged RGCs exhibit a cutback in their intrinsic outgrowth 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 modelling and comparative regeneration research. Our findings show that cellular senescence significantly delays axonal regeneration within this small vertebrate. These studies offer insights into the impact of aging on successful neuroregeneration, thereby contributing to the development of new therapeutic strategies for successful neuroregeneration within the aged mammalian CNS.status: publishe