nox2/cybb Deficiency Affects Zebrafish Retinotectal Connectivity

NADPH oxidase (Nox)-derived reactive oxygen species (ROS) have been linked to neuronal polarity, axonal outgrowth, cerebellar development, regeneration of sensory axons, and neuroplasticity. However, the specific roles that individual Nox isoforms play during nervous system development in vivo remain unclear. To address this problem, we investigated the role of Nox activity in the development of retinotectal connections in zebrafish embryos. Zebrafish broadly express four nox genes (nox1, nox2/cybb, nox5, and duox) throughout the CNS during early development. Application of a pan-Nox inhibitor, celastrol, during the time of optic nerve (ON) outgrowth resulted in significant expansion of the ganglion cell layer (GCL), thinning of the ON, and a decrease in retinal axons reaching the optic tectum (OT). With the exception of GCL expansion, these effects were partially ameliorated by the addition of H₂O₂, a key ROS involved in Nox signaling. To address isoform-specific Nox functions, we used CRISPR/Cas9 to generate mutations in each zebrafish nox gene. We found that nox2/cybb chimeric mutants displayed ON thinning and decreased OT innervation. Furthermore, nox2/cybb homozygous mutants (nox2/cybb^–/–) showed significant GCL expansion and mistargeted retinal axons in the OT. Neurite outgrowth from cultured zebrafish retinal ganglion cells was reduced by Nox inhibitors, suggesting a cell-autonomous role for Nox in these neurons. Collectively, our results show that Nox2/Cybb is important for retinotectal development in zebrafish.

SIGNIFICANCE STATEMENT Most isoforms of NADPH oxidase (Nox) only produce reactive oxygen species (ROS) when activated by an upstream signal, making them ideal candidates for ROS signaling. Nox enzymes are present in neurons and their activity has been shown to be important for neuronal development and function largely by in vitro studies. However, whether Nox is involved in the development of axons and formation of neuronal connections in vivo has remained unclear. Using mutant zebrafish embryos, this study shows that a specific Nox isoform, Nox2/Cybb, is important for the establishment of axonal connections between retinal ganglion cells and the optic tectum.