The fast‐aging killifish: A unique vertebrate model to identify molecular targets for neuroreparative strategies in the aged retina
Authors: Moons Lieve , Bergmans Steven , Serneels Pieter‐Jan , De Schutter Julie , Vanhunsel Sofie , De Groef Lies 
Published: December 2022

The rise in life expectancy in our modern societies concurs with an increasing prevalence of age‐associated neurodegenerative disorders and central nervous system (CNS) trauma. Despite many research attempts trying to unravel aging processes and their role in functional decline, the mechanisms underlying healthy CNS aging remain elusive. Moreover, the reparative approaches proposed to combat loss of neurons due to damage or (age‐associated) disease are failing. One of the reasons might be that the currently used animal models often lack the capability to properly recapitulate the aged environment in which these pathologies manifest and the repair processes need to occur. We introduce the African turquoise killifish as an opportune model to study CNS aging as well as neuroreparative strategies. Indeed, while sharing adult neuroregenerative potential with other teleost fish like zebrafish, the short‐lived killifish displays many of the human aging hallmarks, making it a promising biogerontology model.

As vision decline manifests during human aging and in many neurodegenerative diseases, we performed a comprehensive characterization of the aged killifish visual system, comprising the retina, optic projections and optic tectum. Immunohistochemical, biochemical and molecular analyses revealed the presence of many aging traits in this system, including oxidative stress, genomic damage, cellular senescence, altered cellular communication, reduced neurogenic potential and even neurodegeneration. Specifically, microglia changed to a pro‐inflammatory secretion profile upon aging and macroglia became gliotic in the aging killifish visual system. Altogether, these age‐related changes led to a diminished visual performance in old killifish.

Using an optic nerve injury model, we further disclosed an age‐related decline in functional circuit restoration, with different phases of the repair process being affected depending on the age. While young adult killifish were able to functionally recover after injury, old fish did not regain vision at all. Here we observed for the first time in a regenerative teleost fish model, a glial scar that formed a mechanical barrier physically preventing axonal regrowth. Moreover, in the optic tectum the inflammatory response shifted from acute to chronic upon aging. Macroglia, which showed a transient gliotic response after injury in young adult fish, displayed a permanent response in old animals. Overall, as in mammals, both a reduced intrinsic growth potential and a non‐supportive cellular environment, orchestrated by activated micro‐ and macroglia, were found to underlie the declined repair potential in aged killifish.

To decode the cellular and molecular framework that defines visual system aging and the failed regenerative response in old fish, we are currently performing single‐cell RNA sequencing of the young adult and aged killifish retina. We envision our bioinformatic approaches to result in innovative druggable targets for future development of rejuvenative/curative therapies in the aged mammalian retina.

In summary, our study highlights the importance of a neuron–glia crosstalk in health, disease and repair. It puts forward the strengths of the killifish as a vertebrate model with mammalian‐like characteristics upon aging, that offers unique opportunities to identify rejuvenating and neuroreparative strategies ensuring healthy aging.