A Highly ConservedShhEnhancer Coordinates Hypothalamic and Craniofacial Development

Abstract

Enhancers that are conserved deep in evolutionary time regulate characteristics held in common across taxonomic classes. Here, we deleted the highly conserved Shh enhancer SBE2 (Shh brain enhancer 2) which markedly reduced expression within the embryonic brain specifically in the rostral diencephalon; however, a putative secondary enhancer driving low levels of Shh partially compensated for the loss of SBE2. Further reduction of Shh levels, crossing the SBE2 deletion with the Shh null allele, disrupted brain and craniofacial tissue, linking SBE2 regulated expression to multiple defects and enabling analysis of development at different expression levels. Development of the hypothalamus, derived from the rostral diencephalon, was disrupted along both the anterior-posterior (AP) and the dorsal-ventral (DV) axes. DV patterning was highly sensitive to Shh expression levels, demonstrating a novel morphogenic context forShh. In addition, loss of SBE2 activity in the diencephalon disrupted neighbouring craniofacial development, including the mediolateral patterning of the bones along the cranial floor and viscerocranium. Thus, SHH signalling coordinates hypothalamic morphogenesis with the formation of the adjacent midline cranial bones that consequently protects the neural tissue.

GAS1 is required for NOTCH-dependent facilitation of SHH signaling in the ventral forebrain neuroepithelium

“…1C). These defects are consistent with SHH deficiency at early neurulation as seen in Shh mutant mouse embryos (Carreno et al, 2017;Corman et al, 2018;Crane-Smith et al, 2021;Szabo et al, 2009). However, contrary to prior analysis that focused on developmental stages after E9.5, our data uncovered that GAS1 is not required for initial establishment of the SHH domain in the RDVM at E8.5, but to sustain its activity at later stages of neurulation.…”

Insights Into the Complexity of Craniofacial Development From a Cellular Perspective

“…The spatially defined sources of morphogenetic signals, often represented by secreted ligands with long-range action potential (e.g., WNTs, FGFs, BMPs, and Hedgehogs), are referred to as signaling centers or organizers (Figure 1IIIA; Martinez Arias and Steventon, 2018;La Manno et al, 2020). The known signaling centers involved in craniofacial morphogenesis are located in the emerging nervous system, in the foregut endoderm and in spatially defined areas of the facial ectoderm (Marcucio et al, 2005;Wada et al, 2005;Brito et al, 2006;Eberhart et al, 2006;Foppiano et al, 2007;Szabo-Rogers et al, 2008Gitton et al, 2011;Reid et al, 2011;Bhullar et al, 2015;Kaucka et al, 2018;Crane-Smith et al, 2019). Tissueand/or time-specific ablation of morphogens or their sources, such as Sonic Hedgehog (Shh) or the olfactory placodes, showed that the condensations, and in turn the cartilage, are induced by different signaling centers at distinct time points as independent units, pointing at the composite character of the chondrocranium and the future skull (Figures 1 -IIIB,C ;McBratney-Owen et al, 2008;Kaucka et al, 2018).…”