Conserved roles of Rax/rx3 genes in hypothalamus and pituitary development.

Rax (Rx) genes encode paired-type homeodomain-containing transcription factors present in virtually all metazoan groups. In vertebrates, studies in fish, amphibian, chick and mouse models have revealed that these genes play important roles in the development of structures located at the anterior portion of the central nervous system, in particular the eyes, the hypothalamus and the pituitary gland. In addition, human patients with eye and brain defects carry mutations in the two human Rax paralogues, RAX and RAX2. Here, we review work done in the last years on Rax genes, focusing especially on the function that mouse Rax and its zebrafish homologue, rx3, play in hypothalamic and pituitary development. Work on both of these model organisms indicate that Rax genes are necessary for the patterning, growth and differentiation of the hypothalamus, in particular the ventro-tuberal and dorso-anterior hypothalamus, where they effect their action by controlling expression of the secreted signalling protein, Sonic hedgehog (Shh). In addition, Rax/rx3 mutations disturb the development of the pituitary gland, mimicking phenotypes observed in human subjects carrying mutations in the RAX gene. Thus, along with their crucial role in eye morphogenesis, Rax genes play a conserved role in the development of the hypothalamus and adjacent structures in the vertebrate clade.