Meis2 competes with the Groucho co-repressor Tle4 for binding to Otx2 and specifies tectal fate without induction of a secondary midbrain-hindbrain boundary organizer.

The transcription factor Otx2 is expressed throughout the anterior neuroectoderm and is required for the formation of all forebrain- and midbrain-derived structures. The molecular determinants that cooperate with Otx2 to subdivide its expression domain into distinct functional units are, however, poorly understood at present. We show here that the TALE-homeodomain protein Meis2 is expressed in the chick tectal anlage and is both necessary and sufficient for tectal development. Unlike known tectum-inducing genes, the ability of Meis2 to initiate tectal development does not involve the formation of a secondary midbrain-hindbrain boundary organizer, but instead requires direct interaction with Otx2. Using an Otx2-dependent reporter assay we demonstrate that Meis2 competes with the Groucho co-repressor Tle4 (Grg4) for binding to Otx2 and thereby restores Otx2 transcriptional activator function. Together, our data suggest a model in which the balance between a co-repressor and a co-activator, which compete for binding to Otx2 in the mesencephalic vesicle, provides spatial and temporal control over tectal development. Controlled formation of Meis2-containing higher order protein complexes might thus serve as a general mechanism to achieve subdivision of the anterior neuroectoderm into distinct functional units during embryogenesis.