The Xenopus Irx genes are essential for neural patterning and define the border between prethalamus and thalamus through mutual antagonism with the anterior repressors Fezf and Arx

Abstract

The Iroquois (Irx) genes encode homeoproteins conserved during evolution. Vertebrate genomes contain six Irx genes organized in two clusters, IrxA (which harbors Irx1, Irx2 and Irx4) and IrxB (which harbors Irx3, Irx5 and Irx6). To determine the precise role of these genes during development and their putative redundancies, we conducted a comparative expression analysis and a comprehensive loss-of-function study of all the early expressed Irx genes (Irx1-5) using specific morpholinos in Xenopus. We found that the five Irx genes display largely overlapping expression patterns and contribute to neural patterning. All Irx genes are required for proper formation of posterior forebrain, midbrain, hindbrain and, to a lesser an extent, spinal cord. Nevertheless, Irx1 and Irx3 seem to have a predominant role during regionalization of the neural plate. In addition, we find that the common anterior limit of Irx gene expression, which will correspond to the future border between the prethalamus and thalamus, is defined by mutual repression between Fezf and Irx proteins. This mutual repression is likely direct. Finally, we show that Arx, another anteriorly expressed repressor, also contribute to delineate the anterior border of Irx expression.

An evolutionarily conserved three-dimensional structure in the vertebrate Irx clusters facilitates enhancer sharing and coregulation

“…1 ), a proportion in line with our previous analysis of HCNRs from the IrxB cluster 3 . Th ese HCNRs drove reporter expression within sub-domains of the endogenous pattern of IrxA genes at this stage 14 , such as the midbrain, hindbrain, spinal cord, kidney and otic vesicle ( Fig. 1 ).…”

“…Th e expression of the third gene in each cluster, Irx4 or Irx6 , is usually more divergent. However, in some tissues, all the genes of a cluster, or even of both clusters, are identically expressed 3,7,10,14 .…”

Segregation of eye and antenna fates maintained by mutual antagonism in Drosophila

“…The maintenance of such division by mutual antagonism has been reported before. For example, the division between the presumptive thalamus and prethalamus in Xenopus is due to the mutual repression by the Irx homeodomain proteins and the Fezf zinc-finger proteins (Rodríguez-Seguel et al, 2009). The boundary between optic cup and optic vesicle is maintained by mutual transcriptional repression between Pax6 and Pax2 (Schwarz et al, 2000).…”

Brain diversity evolves via differences in patterning

“…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).…”