Downstream mediators of Ten-m3 signalling in the developing visual pathway

Background: The formation of visuotopically-aligned projections in the brain is required for the generation of functional binocular circuits. The mechanisms which underlie this process are unknown. Ten-m3 is expressed in a broad high-ventral to low-dorsal gradient across the retina and in topographically-corresponding gradients in primary visual centres. Deletion of Ten-m3 causes profound disruption of binocular visual alignment and function. Surprisingly, one of the most apparent neuroanatomical changes—dramatic mismapping of ipsilateral, but not contralateral, retinal axons along the representation of the nasotemporal retinal axis—does not correlate well with Ten-m3’s expression pattern, raising questions regarding mechanism. The aim of this study was to further our understanding of the molecular interactions which enable the formation of functional binocular visual circuits.

Methods: Anterograde tracing, gene expression studies and protein pull-down experiments were performed. Statistical signifcance was tested using a Kolmogorov–Smirnov test, pairwise-fxed random reallocation tests and univariate ANOVAs.

Results: We show that the ipsilateral retinal axons in Ten-m3 knockout mice are mismapped as a consequence of early axonal guidance defects. The aberrant invasion of the ventral-most region of the dorsal lateral geniculate nucleus by ipsilateral retinal axons in Ten-m3 knockouts suggested changes in the expression of other axonal guidance molecules, particularly members of the EphA–ephrinA family. We identifed a consistent down-regulation of EphA7, but none of the other EphA–ephrinA genes tested, as well as an up-regulation of ipsilateral-determinants Zic2 and EphB1 in visual structures. We also found that Zic2 binds specifcally to the intracellular domain of Ten-m3 in vitro.

Conclusion: Our fndings suggest that Zic2, EphB1 and EphA7 molecules may work as efectors of Ten-m3 signalling, acting together to enable the wiring of functional binocular visual circuits.