Neurogenesis of Retinal Ganglion Cells Is Not Essential to Visual Functional Recovery after Optic Nerve Injury in Adult Zebrafish

Abstract

Zebrafish central nervous system (CNS) possesses a strong neural regeneration ability to restore visual function completely after optic nerve injury (ONI). However, whether neurogenesis of retinal ganglion cell (RGC) contributes to functional recovery remains controversial. Our quantitative analysis of RGCs in different ONI models showed that almost all RGCs survived in optic nerve crush (ONC) model; while over 90% of RGCs survived in the first 2 weeks with 75% remaining after 7 weeks in optic nerve transection (ONT) model. Retrograde labeling from tectum revealed a surprising regeneration rate, with over 90% and over 50% of RGCs regrowing axons to tectum at the first week in ONC and ONT model respectively. In the latter one, the number of regenerative RGCs after 4 weeks had no significant difference from the control group. As for neurogenesis, newborn RGCs were rarely detected either by double retrograde labeling or BrdU marker. Since few RGCs died, microglia number showed a temporary increase at 3 days post injury (dpi) and a decrease at 14 dpi. Finally, myelin structure within retina kept integrity and optomotor response (OMR) test demonstrated visual functional restoration at 5 weeks post injury (wpi). In conclusion, our results have directly shown that RGC survival and axon regrowth are responsible for functional recovery after ONI in adult zebrafish.

Zebrafish—on the move towards ophthalmological research

“…[17][18][19][20][21][22][23][24][25][26][27] The lens in zebrafish eyes is spheroid and not ellipsoid as compared with the human eye, and as a consequence, the zebrafish eye has a much lower volume of vitreous compared with the human eye ( Figure 1a). …”

“…28 In contrast to human ganglion cells where the retinal part of the axon is unmyelinated and the axon in the optic nerve is myelinated, in zebrafish the whole axon is myelinated to different extents. 26,27 The photoreceptor outer segment (POS) of zebrafish consists of rods and cones, comparable to the human retina. Anatomically cones are arranged in a mosaic pattern that can be categorized into four types: short single cones (SSCs)-ultraviolet (UV)-sensitive cones, long single cones (LSCs)-blue-sensitive cones, and double cones (DCs) consisting of short (green sensitive) and long (red sensitive) cones.…”

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

“…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).…”

“…Interestingly, almost all RGCs will survive the lesion (Kato et al 2013;Zou et al 2013), which is highly contrasting to the extensive cell death observed in rodent models (Berkelaar et al 1994;Kalesnykas et al 2012;Nadal-Nicolas et al 2015b;Yukita et al 2015). The subsequent regenerative process can be divided in different major phases, based on a combination of morphological, physiological, biochemical and behavioral methods (McCurley and Callard 2010; Kato et al 2013).…”

“…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).…”