Differential Expression of DSCAM Guides the Patterning of Retinal Axons Along Their Path and at Their Target in the Developing Xenopus Visual System
Abstract Background The Xenopus retinotectal circuit is organized topographically, where the dorsal-ventral axis of the retina maps respectively on to the ventral-dorsal axis of the tectum; axons from the nasal-temporal axis of the retina project respectively to the caudal-rostral axis of the tectum. Studies throughout the last two decades have shown that mechanisms involving molecular recognition of proper termination domains are at work guiding topographic organization. Such studies have shown that graded distribution of molecular cues is important for topographic mapping. However, the molecular cues organizing topography along the developing optic nerve, and as retinal axons cross the chiasm and navigate towards their target in the tectum, remain unknown. Down syndrome cell adhesion molecule (DSCAM) has been characterized as a key molecule in axon guidance, making it a strong candidate involved in the topographic organization of retinal fibers along the optic path.Methods Using a combination of whole-brain clearing and immunohistochemistry staining techniques we characterized DSCAM expression and the projection of ventral and dorsal retinal fibers starting from the eye, followed to the optic nerve into the chiasm, and into the terminal target in the optic tectum in Xenopus laevis tadpoles. We also assessed the effects of DSCAM on the establishment of retinotopic maps through spatially and temporally targeted DSCAM knockdown on retinal ganglion cells (RGCs) with axons innervating the optic tectum. Results Highest expression of DSCAM was localized to the ventral posterior region of the optic nerve and chiasm; this expression pattern coincides with ventral fibers derived from ventral RGCs. Downregulating DSCAM levels affected the segregation and proper sorting of medial axon fibers, derived from ventral RGCs, within the tectal neuropil, indicating that DSCAM plays a role in retinotopic organization. ConclusionThese findings together with the observation that DSCAM immunoreactivity accumulates on the primary dendrites of tectal neurons indicates that DSCAM exerts multiple roles in coordinating retinotopic order and connectivity in the developing vertebrate visual system.