07-P017 Sumoylation modulates Spalt activity

embryogenesis, the ventral telencephalon (VT) plays a major role in the early development of these tracts. First, two populations of pioneer neurons allow corticothalamic (CTA) and thalamocortical axons (TCA) to enter the VT. Secondly, the VT forms a permissive corridor which guides TCAs through the MGE. Finally, thalamortical and corticothalamic axons guide each other once they have met in the VT (‘‘handshake hypothesis’’). The Gli3 transcription factor is essential for regionalization of the forebrain but roles in the formation of axonal connections have not been investigated. Here, we present the analysis of thalamocortical tract development in the Gli3 hypomorphic mouse mutant Polydactyly Nagoja (Pdn). DiI injections and neurofilament (NF) staining showed that TCAs enter the VT but form a broader internal capsule. Also, the tract bifurcates with most of the caudal axons projecting ventrally in the MGE. Furthermore, the entry of CTAs into the VT is delayed which correlates with a failure to retrogradely label pioneer neurons in the mutant LGE. This defective development of pioneer neurons coincides with a severe size reduction of the mutant LGE which is caused by an increased cell cycle length of LGE progenitors and a reduced neuronal differentiation rate. Taken together, these analyses reveal novel roles for Gli3 in the development of thalamocortical and corticothalamic connections.