Topoisomerase IIβ is required for lamina-specific targeting of retinal ganglion cell axons and dendrites

The specific partnering of synaptically connected neurons is central to nervous system function. Proper wiring requires the interchange of signals between a postmitotic neuron and its environment, a distinct pattern of transcription in the nucleus, and deployment of guidance and adhesion cues to the cell surface. To identify genes involved in neurite targeting by retinal ganglion cells (GCs), their presynaptic partners in the retina, and their postsynaptic targets in the optic tectum, we undertook a forward genetic screen for mutations disrupting visual responses in zebrafish. This rapid primary screen was subsequently refined by immunohistochemical labeling of retinal and tectal neurites to detect patterning errors. From this unbiased screen, the notorious (noto) mutant exhibited the most specific phenotypes: intact retinal and tectal differentiation but multiple neurite targeting defects in the retinal inner plexiform layer (IPL) and tectal neuropil. Positional cloning and morpholino phenocopy revealed that the mutation disrupts Topoisomerase IIβ (Top2b), a broadly distributed nuclear protein involved in chromatin modifications during postmitotic differentiation. Top2b-DNA interactions are known to regulate transcription of developmentally important genes, including axon guidance factors and cell adhesion molecules, but a specific role in local synaptic targeting has not been previously described. The neurite targeting defects among GC axons are largely restricted to crossovers between sublaminae of a specific layer, SFGS, and were shown by mosaic analysis to be autonomous to the GC axons. The noto mutant provides the first example of the importance of an epigenetic regulator, Top2b, in the intricate series of events that lead to a properly wired visual system.