Successful optic nerve regeneration in the senescent zebrafish despite age-related decline of cell intrinsic and extrinsic response processes

Abstract

Dysfunction of the central nervous system (CNS) in neurodegenerative diseases or after brain lesions seriously affects life quality of a growing number of elderly, since the adult CNS lacks the capacity to replace or repair damaged neurons. Despite intensive research efforts, full functional recovery after CNS disease and/or injury remains challenging, especially in an aging environment. As such, there is a rising need for an aging model in which the impact of aging on successful regeneration can be studied. Here, we introduce the senescent zebrafish retinotectal system as a valuable model to elucidate the cellular and molecular processes underlying age-related decline in axonal regeneration capacities. We found both intrinsic and extrinsic response processes to be altered in aged fish. Indeed, expression levels of growth-associated genes are reduced in naive and crushed retinas, and the injury-associated increase in innate immune cell density appears delayed, suggesting retinal inflammaging in old fish. Strikingly, however, despite a clear deceleration in regeneration onset and early axon outgrowth leading to an overall slowing of optic nerve regeneration, reinnervation of the optic tectum and recovery of visual function occurs successfully in the aged zebrafish retinotectal system.

Resident Immunity in Tissue Repair and Maintenance: The Zebrafish Model Coming of Age

“…As described above, an elegant recent study (Ferrero et al, 2018), showed the existence of a second wave of re-population of the brain’s microglia, which originate from HSCs – a process that occurs between 2 weeks and 3 months of age. Microglia can be detected by L-plastin (or LCP-1), a pan-leukocyte marker (Redd et al, 2006; Cvejic et al, 2008; Mathias et al, 2010; Van Houcke et al, 2017), by macrophage-expressed gene 1 ( mpeg1 ), labeling all mononuclear phagocytes (Ellett et al, 2011; Wittamer et al, 2011), by the 4c4 antibody (Becker and Becker, 2001)as well as Apolipoprotein E (ApoE) [Oosterhof et al (2015), see Table 1; Figure 1]. Note that, although ApoE specifically labels microglia if compared with the other markers mentioned, it has also been shown to be expressed astrocytes (Boyles et al, 1985; Poirier et al, 1991; Xu et al, 2006), oligodendrocytes (Stoll et al, 1989), as well as by some neurons albeit at lower levels (Han et al, 1994; Achariyar et al, 2016).…”

Complementary research models and methods to study axonal regeneration in the vertebrate retinofugal system

“…In the first model, the nerve is cut with scissors, thereby creating a proximal and distal nerve stump that are physically separated from each other. In contrast, in the ONC model, the nerve is crushed with forceps to separate the axons, but leaving the connective tissue around them intact, and thus holding the proximal and distal ends together (Becker et al 2000;McCurley and Callard 2010;Fleisch et al 2011;Lemmens et al 2015;Van Houcke et al 2017). As expected, the ONT model is somewhat more drastic and regeneration progresses slower as compared to ONC (Zou et al 2013).…”

“…To culture fish retinal explants, retinas are chopped into small squares and transferred to poly-L-lysine-coated well plates. After an incubation period of 1-4 days, the number of neurites, their length and/or density can be quantified (Becker and Becker 2002;Becker et al 2004;Veldman et al 2007Veldman et al , 2010Elsaeidi et al 2014;Ogai et al 2014;Welte et al 2015;Van Houcke et al 2017). …”

“…The latter approach offers the advantage of easily counting labeled RGC somata in the retina. Furthermore, it allows calculating the percentage of RGCs that contributes to regeneration (Van Houcke et al 2017). For retrograde labeling in zebrafish, mostly dextrans coupled to fluorophores are in use (Zou et al 2013;Elsaeidi et al 2014;Welte et al 2015;Williams et al 2015;Van Houcke et al 2017).…”

“…Furthermore, it allows calculating the percentage of RGCs that contributes to regeneration (Van Houcke et al 2017). For retrograde labeling in zebrafish, mostly dextrans coupled to fluorophores are in use (Zou et al 2013;Elsaeidi et al 2014;Welte et al 2015;Williams et al 2015;Van Houcke et al 2017). In addition, Zou and coworkers have developed a retrograde tracing method without the need for additional severing of the optic nerve (Zou et al 2014).…”