Robo1 and Robo2 control the development of the lateral olfactory tract

The development of olfactory bulb projections that form the lateral olfactory tract (LOT) is still poorly understood. It is known that the septum secretes Slit1 and Slit2 which repel olfactory axons in vitro and that in Slit1(-/-); Slit2(-/-) mutant mice, the LOT is profoundly disrupted. However, the involvement of Slit receptors, the roundabout (Robo) proteins, in guiding LOT axons has not been demonstrated. We show here that both Robo1 and Robo2 receptors are expressed on early developing LOT axons, but that only Robo2 is present at later developmental stages. Olfactory bulb axons from Robo1(-/-); Robo2(-/-) double-mutant mice are not repelled by Slit in vitro. The LOT develops normally in Robo1(-/-) mice, but is completely disorganized in Robo2(-/-) and Robo1(-/-); Robo2(-/-) double-mutant embryos, with many LOT axons spreading along the ventral surface of the telencephalon. Finally, the position of lot1-expressing cells, which have been proposed to be the LOT guidepost cells, appears unaffected in Slit1(-/-); Slit2(-/-) mice and in Robo1(-/-); Robo2(-/-) mice. Together, our results indicate that Robo1 and Robo2 directly mediate the repulsive activity of Slit receptors on LOT axons, and are required for normal guidance of these axons in vivo.