This neurodevelopmental paper reports on the transcription factor Zic2 as critical regulator of visual target refinement. Establishing that Zic2 acts through the serotonin transporter SERT provides insight into a critical element of visual circuitry.
The development of the nervous system is a time-ordered and multi-stepped process that requires neural specification, axonal navigation and arbor refinement at the target tissues. Previous studies have demonstrated that the transcription factor Zic2 is necessary and sufficient for the specification of retinal ganglion cells (RGCs) that project ipsilaterally at the optic chiasm midline. Here, we report that, in addition, Zic2 controls the refinement of eye-specific inputs in the visual targets by regulating directly the expression of the serotonin transporter (Sert), which is involved in the modulation of activity-dependent mechanisms during the wiring of sensory circuits. In agreement with these findings, RGCs that express Zic2 ectopically show defects in axonal refinement at the visual targets and respond to pharmacological blockage of Sert, whereas Zic2-negative contralateral RGCs do not. These results link, at the molecular level, early events in neural differentiation with late activity-dependent processes and propose a mechanism for the establishment of eye-specific domains at the visual targets.