Roles for Wnts and Their Receptors in Topographic Mapping and Laminar Termination

摘要

In topographic mapping, axons project to appropriate targets based upon their spatial positions, with adjacent cells projecting to adjacent targets. During development, retinal ganglion cells project to the optic tectum to map along the tectal surface based on the position of the originating soma in the retina. Retinotectal topographic map formation has been highly studied, but questions about the mechanisms of mapping along the mediolateral tectal axis remain. In this dissertation, I show that the activities and interactions of two sets of gradients, Wnt3-Ryk and ephrinB1-EphB, are necessary to form an accurate mediolateral map. During map formation, I show that the front of the tectal Wnt3 gradient shifts laterally between E10 and E12, while the ephrinB1 gradient remains stable, generating a laterally-shifting intersection between these two gradients at which mapping forces are balanced. The timing of this gradient movement corresponds to the similarly timed medial-to-lateral development of retinal axons and interstitial branches in the tectum. By overexpressing Ryk and EphB2 and downregulating Ryk via in ovo electroporation, I demonstrate that Ryk and EphB2 provide opposing mapping forces within retinal interstitial branches, with Ryk repulsing these branches laterally while EphB2 attracts them medially. These mediolateral direction choices occur near branch initiation without later direction reversals, suggesting that branches respond to specific balance points in a time-limited manner, defining the mediolateral map. This concept of a moving series of balance points which drives opposing forces within growth cones to define topographic positions in a time-specific manner comprises the Moving Gradient Model of topographic mapping.;Interstitial branches invade the tectal surface to select specific deeper tectal laminae for arbor stabilization and synapse formation after map formation. The mechanisms by which these laminae are selected or targeted are largely unknown. In this dissertation, I show that six Frizzleds are expressed in RGC subsets, while five Wnts appear in specific tectal laminae during map formation and tectal laminar targeting. To study how these Frizzleds may affect laminar targeting, I employ in ovo retinal electroporation and AChRbeta2 labeling to characterize small populations of retinal ganglion cells, as well as to overexpress Frizzled1 to observe targeting effects.