RaxIs a Selector Gene for Mediobasal Hypothalamic Cell Types

Abstract

The brain plays a central role in controlling energy, glucose, and lipid homeostasis, with specialized neurons within nuclei of the mediobasal hypothalamus, namely the arcuate (ARC) and ventromedial (VMH), tasked with proper signal integration. Exactly how the exquisite cytoarchitecture and underlying circuitry become established within these nuclei remains largely unknown, in part because hypothalamic developmental programs are just beginning to be elucidated. Here, we demonstrate that the Retinal and anterior neural fold homeobox (Rax) gene plays a key role in establishing ARC and VMH nuclei in mice. First, we show that Rax is expressed in ARC and VMH progenitors throughout development, and accordingly, genetic fate mapping studies reveal that Rax+ lineages give rise to VMH neurons. Second, the conditional ablation of Rax in a subset of VMH progenitors using a Shh∷Cre driver leads to a fate switch from a VMH neuronal phenotype to a hypothalamic but non-VMH identity, suggesting that Rax is a selector gene for VMH cellular fates. Finally, the broader elimination of Rax throughout ARC/VMH progenitors using Six3∷Cre leads to a severe loss of both VMH and ARC cellular phenotypes, demonstrating a role for Rax in both VMH and ARC fate specification. Combined, our study illustrates that Rax is required in ARC/VMH progenitors to specify neuronal phenotypes within this hypothalamic brain region. Rax thus provides a molecular entry point for further study of the ontology and establishment of hypothalamic feeding circuits.

Development of the hypothalamus: conservation, modification and innovation

“…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).…”

Differentiation of hypothalamic-like neurons from human pluripotent stem cells

“…RAX, VAX, and SIX3 are specifically expressed in rostral hypothalamic neuroepithelia (14,15). Moreover, Achaete-scute-like 1 (ASCL1, also called MASH1),…”

Notch/Rbpjκ signaling regulates progenitor maintenance and differentiation of hypothalamic arcuate neurons

“…The development of the Arc can be divided into three main events during embryogenesis: hypothalamic regionalization (before E10.5), specification and differentiation of Arc progenitors (E10.5-15.5), and organization of hypothalamic nuclei (E16.5-18.5) (Caqueret et al, 2005;McClellan et al, 2006;Shimada and Nakamura, 1973). Once regionalized, a subset of multipotent Sox2 and Rax Arc progenitors must exit the cell cycle and undergo neuronal differentiation (Lu et al, 2013). The expression of bHLH proneural genes such as Mash1 is necessary to promote differentiation of Arc neurons into neuronal subtypes (Kim et al, 2008;McNay et al, 2006) and generate a distinct nucleus.…”