COOPERATION OF PAX2 AND PAX5 IN MIDBRAIN AND CEREBELLUM DEVELOPMENT

Midbrain and cerebellum development depends on an organizing center th at is located at the midbrain-hindbrain junction of the vertebrate emb ryo. Expression of the two closely related transcription factors Pax2 and Pax5 overlaps spatially and temporally in this region of the devel oping central nervous system, To study a possible interaction of these transcription factors in midbrain and cerebellum patterning, we have generated Pax5, Krd double mutant mice, The transgene-induced Krd muta tion corresponds to an approximate to 7-centimorgan chromosome 19 dele tion that eliminates the entire Pax2 locus, The heterozygous Krd mutat ion deleting one Pax2 allele had no effect on midbrain and cerebellum development, Moreover, only minor developmental defects were previousl y observed at the midline of the inferior colliculus and anterior cere bellum in mice that were homozygous for a targeted Pax5 mutation, Simi lar morphological alterations were observed in 80% of all compound het erozygous Pax5 (+/-) Krd (+/-) mice. However, in the remaining 20% of compound heterozygotes, the inferior colliculi were missing, and the v ermis of the cerebellum was severely disrupted due to the failure of t he cerebellar primordia to fuse at the midline, Inactivation of the se cond Pax5 allele in Pax5 (-/-) Krd (+/-) mice resulted in complete los s of the posterior mid brain and cerebellum, as the tissue originating from the midbrain-hindbrain boundary region was deleted in the embryo as early as day 9.5, On the basis of these data, we propose that the cooperation of Pax2 and Pax5 is essential for normal functioning of th e organizing center at the midbrain-hindbrain junction.