The zebrafish Iroquois geneiro7positions the r4/r5 boundary and controls neurogenesis in the rostral hindbrain

Abstract

Early brain regionalisation involves the activation of genes coding for transcription factors in distinct domains of the neural plate. The limits of these domains often prefigure morphological boundaries. In the hindbrain,anteroposterior patterning depends on a segmentation process that leads to the formation of seven bulges called rhombomeres (r). The molecular cues involved in the early subdivision of the hindbrain and in rhombomere formation are not well understood. We show that iro7, a zebrafish gene coding for a transcription factor of the Iroquois family, is expressed at the end of gastrulation in the future midbrain and hindbrain territories up to the prospective r4/r5 boundary. This territory is strictly complementary to the expression domain of another homeobox gene, vhnf1, in the caudal neural plate. We demonstrate that Iro7 represses vhnf1 expression anterior to their common border and that, conversely, vHnf1 represses iro7 expression caudal to it. This suggests that the r4/r5 boundary is positioned by mutual repression between these two transcription factors. In addition, iro7 is involved in the specification of primary neurons in the rostral hindbrain. In particular, it is essential for the formation of the Mauthner neurons in r4. We propose that iro7 has a dual function in the hindbrain of the zebrafish embryo: it is required for the proper positioning of the prospective r4/r5 boundary and it promotes neurogenesis in the anterior hindbrain.

Otx1l, Otx2 and Irx1b establish and position the ZLI in the diencephalon

“…1A). At the same stage, irx1b is expressed in the posterior diencephalon, marking the thalamic anlage (Lecaudey et al, 2004), whereas the otx2-positive pZLI was negative for irx1b (Fig. 1B).…”

Cyp26 enzymes generate the retinoic acid response pattern necessary for hindbrain development

“…A third possibility is that spatially restricted transcription factors repress RA-responsive gene expression even when ligand and receptor are present. Iro7 (also known as Irx7 -Zebrafish Information Network) is a TALE homeodomain protein expressed in the anterior hindbrain that represses vhnf1 expression anterior to the r4-r5 boundary (Lecaudey et al, 2004). Other TALE homeodomain proteins have been shown to repress transcription from retinoid-responsive elements by binding to RXR retinoid receptors and recruiting general corepressors to the complex (Bartholin et al, 2006).…”

“…cyp26b1 and cyp26c1 expression is also independent of the prior establishment of hindbrain boundaries by Cyp26a1, because both genes are expressed normally in cyp26a1 mutants and in embryos in which cyp26a1 expression is globally up-regulated by sub-teratogenic concentrations of RA. The early expression domain of cyp26b1 and cyp26c1 in r3 and r4 is similar to that of iro7, suggesting that they may be downstream of, or co-regulated with, iro7 (Lecaudey et al, 2004). Modern genetic and genomic resources available for the zebrafish will allow the important mystery of cyp26 regulation to be addressed in the future.…”

Mutual repression betweenmshand Iro-C is an essential component of the boundary between body wall and wing inDrosophila

“…In vertebrates, although to our knowledge no instance of mutual repression between homologs of Iro-C and msh has been described, members of each family participate in establishing borders by repression with other genes in the spinal cord, the brain (reviewed by Gómez-Skarmeta et al, 2003) and between rhombomeres (Lecaudey et al, 2004). Clearly, both genes are used frequently to subdivide territories and establish alternative differentiation pathways at each side of the border that separates them.…”