The age factor in optic nerve regeneration: Intrinsic and extrinsic barriers hinder successful recovery in the short‐living killifish

Abstract

As the mammalian central nervous system matures, its regenerative ability decreases, leading to incomplete or non‐recovery from the neurodegenerative diseases and central nervous system insults that we are increasingly facing in our aging world population. Current neuroregenerative research is largely directed toward identifying the molecular and cellular players that underlie central nervous system repair, yet it repeatedly ignores the aging context in which many of these diseases appear. Using an optic nerve crush model in a novel biogerontology model, that is, the short‐living African turquoise killifish, the impact of aging on injury‐induced optic nerve repair was investigated. This work reveals an age‐related decline in axonal regeneration in female killifish, with different phases of the repair process being affected depending on the age. Interestingly, as in mammals, both a reduced intrinsic growth potential and a non‐supportive cellular environment seem to lie at the basis of this impairment. Overall, we introduce the killifish visual system and its age‐dependent regenerative ability as a model to identify new targets for neurorepair in non‐regenerating individuals, thereby also considering the effects of aging on neurorepair.

Tissue stretching is a confounding factor for the evaluation of neurodegeneration in the ever-growing killifish

“…As there are no antibodies available to label killi sh RGCs, we used a retrograde tracing to e ciently and truthfully label retinal ganglion cells (RGCs) (Vanhunsel et al 2022). Brie y, sh were anaesthetized as described above and positioned on their side under a stereomicroscope.…”

Rapid and precise genome engineering in a naturally short-lived vertebrate

“…Tools to advance genetic interrogation of the killifish have been developed, including a sequenced genome (Reichwald et al, 2015; Valenzano et al, 2015) and Tol2 transgenesis (Allard et al, 2013; Hartmann and Englert, 2012; Valenzano et al, 2011), as well as CRISPR/Cas9-mediated knock-out (Harel et al, 2015) and CRISPR/Cas13-mediated knock-down (Kushawah et al, 2020). This genetic toolkit has enabled discoveries about the mechanisms of aging (Astre et al, 2022a; Bradshaw et al, 2022; Chen et al, 2022; Harel et al, 2022; Louka et al, 2022; Matsui et al, 2019; Smith et al, 2017; Van Houcke et al, 2021b), regeneration (Vanhunsel et al, 2022a; Vanhunsel et al, 2021; Vanhunsel et al, 2022b; Wang et al, 2020), evolution (Cui et al, 2019; Sahm et al, 2017; Singh et al, 2021; Willemsen et al, 2020), development (Abitua et al, 2021; Dolfi et al, 2019), and ‘suspended animation’ (Hu et al, 2020; Singh et al, 2021).…”

Killifish switch towards mammalian-like regeneration upon aging

“…Interestingly, killifish appear to pay a price for their fast growth and aging. In contrast to zebrafish –that maintain their neuroreparative ability albeit regenerate less efficiently at old age [ 2 – 4 ] –, killifish completely lose their regeneration capacity at old age and are unable to fully recover from CNS injury [ 5 , 6 ]. Using an optic nerve crush injury model in killifish of different ages, we indeed revealed that, in contrast to young fish, aged animals do not regain vision following damage.…”

“…Using an optic nerve crush injury model in killifish of different ages, we indeed revealed that, in contrast to young fish, aged animals do not regain vision following damage. An inadequate intrinsic capacity of aged retinal ganglion cells (RGCs) to revert to a “regenerative state” as well as a growth-inhibiting neuron-extrinsic environment seem to contribute to this impairment [ 6 ], similar to what has been described for (young) adult mammals. We postulate that age-associated changes within neurons and their glial environment –already manifesting before damage occurs– negatively affect the regeneration potential of the killifish CNS, which then leads to a mammalian-like regenerative response upon injury.…”

“…Strikingly, the exaggerated neuroinflammatory events then result in the formation of a long-term glial scar, which has never been shown in any other regeneration-competent model before. Our killiteam at KU Leuven is the first to reveal evidence for such glial scarring within the CNS of aged killifish when subjected to either optic nerve crush or stab injury in the telencephalon [ 5 , 6 ]. This suggests that, in addition to intrinsic factors and altered glial responses, neurorepair in the old killifish CNS is hindered chemically as well as mechanically by glial scar tissue.…”