The vertebrate brain is anatomically and functionally asymmetric; however, the molecular mechanisms that establish left-right brain patterning are largely unknown. In zebrafish, asymmetric left-sided Nodal signaling within the developing dorsal diencephalon is required for determining the direction of epithalamic asymmetries. Here, we show that Six3, a transcription factor essential for forebrain formation and associated with holoprosencephaly in humans, regulates diencephalic Nodal activity during initial establishment of brain asymmetry. Reduction of Six3 function causes brain-specific deregulation of Nodal pathway activity, resulting in epithalamic laterality defects. Based on misexpression and genetic epistasis experiments, we propose that Six3 acts in the neuroectoderm to establish a prepattern of bilateral repression of Nodal activity. Subsequently, Nodal signaling from the left lateral plate mesoderm alleviates this repression ipsilaterally. Our data reveal a Six3-dependent mechanism for establishment of correct brain laterality and provide an entry point to understanding the genetic regulation of Nodal signaling in the brain.
Bibliographical noteFunding Information:
We thank J. Gamse and S. Wilson for suggesting experiments; M. Carl and S. Wilson for discussions and sharing unpublished data; and S. Wilson, C. Wright, J. Gamse, B. Appel, G. Levkowitz, X. Geng, and LSK group members for critically reading the manuscript. We thank A. Bradshaw for technical assistance and our entire fish facility staff for excellent fish care. This work is supported by NIH and the Zebrafish Initiative—Vanderbilt University Academic Capital Venture Fund.