Meis proteins are essential for hindbrain development in the zebrafish

Hox proteins together with their cofactors function in cell fate specification during embryogenesis. Of the Hox cofactors, Pbx and Meis proteins are believed to participate in Hox-containing transcriptional regulation. Recently, we reported that Meis proteins promote hindbrain fates at the expense of more anterior fore- and midbrain. To clearly demonstrate the function of Meis, we performed a dominant-negative approach with which all Meis family members can be interfered. We observed that expression of Pbx4 N-terminus efficiently sequesters Meis proteins in the cytoplasm, thereby keeping Meis out of transcriptional complex in the nucleus. Gene expression and neuronal specification in zebrafish hindbrain region were specifically impaired by the expression of Pbx N-terminus. Our results suggest that Meis proteins are essential for proper hindbrain patterning during embryonic development. On the other hand, even though a function of Meis proteins together with Pbx has been postulated to help improve DNAbinding specificity of Hox proteins, it is possible that Meis protein may have other in vivo activities than merely stabilizing Pbx/Hox complexes. Through a series of Meis deletion constructs and fusion constructs, we found that the Meinox domain of Meis protein is sufficient for its in vivo activity. More importantly, this activity is separable from Pbx binding. This result suggests that Meis proteins contribute an activity to the multimeric complex possibly by recruiting an unknown factor.