Slit inhibition of retinal axon growth and its role in retinal axon pathfinding and innervation patterns in the diencephalon

We have analyzed the role of the Slit family of repellent axon guidance mol ecules in the patterning of the axonal projections of retinal ganglion cell s (RGCs) within the embryonic rat diencephalon and whether the slits can ac count for a repellent activity for retinal axons released by hypothalamus a nd epithalamus. At the time RGC axons extend over the diencephalon, slit1 a nd slit2 are expressed in hypothalamus and epithalamus but not in the later al part of dorsal thalamus, a retinal target. slit3 expression is low or un detectable. The Slit receptors robo2, and to a limited extent robo1, are ex pressed in the RGC layer, as are slit1 and slit2. In collagen gels, axon ou tgrowth from rat retinal explants is biased away from slit2-transfected 293 T cells, and the number and length of axons are decreased on the explant si de facing the cells. In addition, in the presence of Slit2, overall axon ou tgrowth is decreased, and bundles of retinal axons are more tightly fascicu lated. This action of Slit2 as a growth inhibitor of retinal axons and the expression patterns of slit1 and slit2 correlate with the fasciculation and innervation patterns of RGC axons within the diencephalon and implicate th e Slits as components of the axon repellent activity associated with the hy pothalamus and epithalamus. Our findings suggest that in vivo the Slits con trol RGC axon pathfinding and targeting within the diencephalon by regulati ng their fasciculation, preventing them or their branches from invading non target tissues, and steering them toward their most distal target, the supe rior colliculus.