Essential function of the transcription factor Rax in the early patterning of the mammalian hypothalamus

Abstract

The hypothalamus is a region of the anterior forebrain that controls basic aspects of vertebrate physiology, but the genes involved in its development are still poorly understood. Here, we investigate the function of the homeobox gene Rax/Rx in early hypothalamic development using a conditional targeted inactivation strategy in the mouse. We found that lack of Rax expression prior to embryonic day 8.5 (E8.5) caused a general underdevelopment of the hypothalamic neuroepithelium, while inactivation at later timepoints had little effect. The early absence of Rax impaired neurogenesis and prevented the expression of molecular markers of the dorsomedial hypothalamus, including neuropeptides Proopiomelanocortin and Somatostatin. Interestingly, the expression domains of genes expressed in the ventromedial hypothalamus and infundibulum invaded dorsal hypothalamic territory, showing that Rax is needed for the proper dorsoventral patterning of the developing medial hypothalamus. The phenotypes caused by the early loss of Rax are similar to those of eliminating the expression of the morphogen Sonic hedgehog (Shh) specifically from the hypothalamus. Consistent with this similarity in phenotypes, we observed that Shh and Rax are coexpressed in the rostral forebrain at late head fold stages and that loss of Rax caused a downregulation of Shh expression in the dorsomedial portion of the hypothalamus.

A modulatory role of the Rax homeobox gene in mature pineal gland function: Investigating the photoneuroendocrine circadian system of a Rax conditional knockout mouse

“…Thus, the more severely affected SCN phenotype seen in the current study seem to reflect differences in development of the eye between the Rax CKO and the ZRDCT mouse strain. The diverse hypothalamic phenotypes of the ZRDCT mouse strain may also reflect reduced levels of RAX protein in the developing hypothalamus, where expression of Rax in the hypothalamic neuroepithelium, as in other parts of the forebrain, is required for proper neurogenesis (VanDunk et al 2011;Orquera et al 2016). However, in the current study the observed SCN phenotype is not a direct consequence of deleted Rax expression in the hypothalamus, since the Crx-driven Cre recombinase is expressed only in the retina and pineal gland (Nishida et al 2003;Omori et al 2011).…”

“…; Orquera et al . ). However, in the current study the observed SCN phenotype is not a direct consequence of deleted Rax expression in the hypothalamus, since the Crx ‐driven Cre recombinase is expressed only in the retina and pineal gland (Nishida et al .…”

Development of the hypothalamus: conservation, modification and innovation

“…Upon hypothalamic induction, genes encoding secreted morphogens are expressed within restricted regions of the hypothalamus, such as Bmp4, Fgf8, Fgf10 and Wnt8b in the caudal hypothalamus and Shh in the rostral hypothalamus (Lako et al, 1998;Orquera et al, 2016;Zhao et al, 2012), all of which may play a role in hypothalamic patterning. Indeed, a hypothalamic patterning defect was reported in mice lacking noggin, which encodes a Bmp antagonist (Davis and Camper, 2007).…”

“…Shh expression may also be mediated by Rx3/Rax, as in both zebrafish rx3 and mouse Rax mutants, Shh expression is lost accompanied by a hypothalamic patterning defect (Muthu et al, 2016;Orquera et al, 2016). Interestingly, this phenotype is only observed when Rax is knocked out prior to embryonic day (E) 8.5 in mouse embryos, whereas a more specific defect in specification and differentiation of hypothalamic neurons is revealed when Rax is knocked out later (Lu et al, 2013;Orquera et al, 2016).…”

Development of the basal hypothalamus through anisotropic growth

“…Do gene knockout studies provide insight into how the mouse hypothalamus is built? Previous gene knockout studies have suggested two major transcriptional programmes of hypothalamic development: a Fezf2/Olig2/Otp/Sim1 programme, which generates neurones of the PVN, PeVN, SON and SCN that occupy the anterior hypothalamus, and a Nkx2.1‐Shh‐Rx progenitor programme, which generates neurones of the tuberal hypothalamus, including those of the VMN and ARC; Placzek M., Fu T., Towers M. (submitted) . Thus, for example, initial reports suggested that the PVN/PeVN/SON/SCN, but not the ARC, can be detected in mice that lack functional Nkx2.1, whereas Lhx1 (a marker of the SCN) is still detected after genetic‐inactivation of Shh .…”