Mechanisms of pioneer longitudinal axon guidance in developing mouse brain

Longitudinal axons project along the length of the neural axis connecting different regions of brain. To establish precise pathways, longitudinal axons need to choose their direction in the anterior-posterior axis as well as their position in dorsal-ventral axis. However, very little is known about the guidance of longitudinal axons. To investigate longitudinal axon guidance, we chose pioneer longitudinal axons which are the first ones to develop in the embryonic brain. Pioneer axons project through a relatively simple environment, without the confounding complexity of other axons.; In initial analyses, we have investigated formation of pioneer longitudinal axons in forebrain using axon tracing and antibody labeling techniques. We have characterized a system of early neurons establish the first to major longitudinal tracts in the embryonic mouse brain. The issue of how diverse axon populations navigated along the same pathways lead to an analysis of mutants for axonal defects. Analysis of Pax6 mutants revealed that both longitudinal tracts failed to form due to pathfinding errors in every population of early longitudinal axons. Thus, Pax6 is one of the key factors for pathfinding of forebrain axons. Investigation of the formation of pioneer forebrain tracts and role of Pax6 in their guidance increased our understanding in region specific regulation of axon navigation (Chapter II).; To identify more global mechanisms of axon guidance, we aimed to identify molecular cues involved in navigation of pioneer axons. Slits and their receptors Robo implicated in longitudinal axon guidance and might play a role in dorsal-ventral positioning of longitudinal axons in developing mouse brain. Our analysis showed that expression patterns of Slits and Robos correlate with spatially and temporally with longitudinal axon formation suggesting roles in longitudinal axon guidance. Co-culture assay of longitudinal axons with Slit expressing cells revealed that Slits are repellents for longitudinal axons. Analysis of Robo1 and Robo2 double mutants revealed that Slit/Robo signaling involved in almost every aspect of longitudinal axon guidance such as dorsal-ventral positioning, direction of growth and growth promotion (Chapter IV). Altogether, the information gained from this study increased our understanding of mechanisms longitudinal axon guidance.