Tbx2b is required for parapineal organogenesis

Abstract

The existence of anatomical differences along the left-right (L-R) axis in the brain and visceral organs is an evolutionarily conserved feature of the vertebrate lineage. Although much is known about the molecular pathways that lead to the development of visceral organ asymmetry, we are just starting to understand the molecular processes that lead to the development of brain laterality. The zebrafish epithalamus, which includes the pineal organ and a left sided accessory organ called the parapineal organ, offers a powerful model to study the events leading to the generation of brain asymmetry. A chemical mutagenesis screen was performed to identify mutants that disrupt laterality in the epithalamus. The from beyond (fby)/tbx2b mutation causes a brain-specific defect in laterality including a decrease in the total number of specified parapineal cells and an inability of parapineal cells to migrate to the left of the pineal organ. Time-lapse imaging of wild-type parapineal cells show several cells emerging from the pineal complex anlage and migrating as a group in a leftward direction. Similar time-lapse imaging of fby mutants shows that this leftward migration does not occur. Further, ablation of some parapineal cells in wild-type embryos show that reduced cell number does not prevent migration of the remaining cells in fby mutants. Characterization of a hypomorphic allele of tbx2b, lots-of-rods (lor), further supports the hypothesis that specification and migration defects found in tbx2b mutants are separable events that require different doses of tbx2b. Currently, the transcriptional regulators of tbx2b in the pineal complex are unknown; however a previous report had suggested that Floating Head (flh) protein controls tbx2b transcription in the epithalamus. Our studies suggest that this is not the case, and that Flh and Tbx2b act in parallel pathways. Fate mapping, in conjunction with efforts to identify upstream regulators and downstream targets of tbx2b activity in the developing pineal complex, will help determine the mechanism of parapineal organ formation.