Six3inactivation causes progressive caudalization and aberrant patterning of the mammalian diencephalon

Abstract

The homeobox gene Six3 represses Wnt1 transcription. It is also required in the anterior neural plate for the development of the mammalian rostral forebrain. We have now determined that at the 15-to 17-somite stage, the prospective diencephalon is the most-anterior structure in the Six3-null brain, and Wnt1 expression is anteriorly expanded. Consequently, the brain caudalizes, and at the 22-to 24-somite stage, the prospective thalamic territory is the most-anterior structure. At around E11.0, the pretectum replaces this structure. Analysis of Six3;Wnt1 double-null mice revealed that Six3-mediated repression of Wnt1 is necessary for the formation of the rostral diencephalon and that Six3 activity is required for the formation of the telencephalon. These results provide insight into the mechanisms that establish anteroposterior identity in the developing mammalian brain.

Brain diversity evolves via differences in patterning

“…The brain and gene expression phenotypes that we observed to differentiate mbuna cichlids (reduced rostrodorsal extent of wnt1-irx1b, more acute ZLI angle, smaller thalamus, larger telencephalon) from nonmbuna cichlids (greater rostrodorsal extent of wnt1-irx1b, more obtuse ZLI angle, larger thalamus, smaller telencephalon) ( Table 1 and Fig. 2) partially phenocopy zebrafish irx1 knockdown versus control embryos (23) and Wnt1 null versus control mice (26). Thus, we manipulated WNT signaling in vivo by treating cichlid embryos with nonlethal doses of the chemical agonist LiCl (Methods).…”

“…At stage 10, wnt1 is expressed in the MHB, as well as rostrally along the dorsal surface of the cichlid embryonic prosencephalon (midbrain plus forebrain). wnt1 is a posteriorizing signal that functions across the dorsal embryonic brain (3,23). As such, we wanted to calculate and compare the percentage of the dorsal prosencephalon under the influence of the wnt1 signal in mbuna and nonmbuna.…”

“…We focused on a gene circuit known to (i) establish anterior (e.g., telencephalon) versus posterior (e.g., thalamus) fate and (ii) position the signaling boundary ZLI within the forebrain. The transcription factor six3 is a WNT antagonist expressed from the ANR, required for the formation of the telencephalon and ZLI (23). wnt1 is a posteriorizing signal expressed from the MHB; knockout of Wnt1 in mouse results in a smaller thalamus, a posterior shift in the angle of the ZLI, and a larger telencephalon (23).…”

“…The transcription factor six3 is a WNT antagonist expressed from the ANR, required for the formation of the telencephalon and ZLI (23). wnt1 is a posteriorizing signal expressed from the MHB; knockout of Wnt1 in mouse results in a smaller thalamus, a posterior shift in the angle of the ZLI, and a larger telencephalon (23). six3 and wnt1 direct the activity of mutually repressive transcription factors fezf2 and irx1, which in turn set the AP position of the shh-positive ZLI (23)(24)(25).…”

“…wnt1 is a posteriorizing signal expressed from the MHB; knockout of Wnt1 in mouse results in a smaller thalamus, a posterior shift in the angle of the ZLI, and a larger telencephalon (23). six3 and wnt1 direct the activity of mutually repressive transcription factors fezf2 and irx1, which in turn set the AP position of the shh-positive ZLI (23)(24)(25). Knockdown of irx1 in zebrafish produces a posterior expansion of the ZLI at the expense of thalamus, along with a shortening of the wnt1 forebrain domain (25,26).…”