Quantitative analyses link modulation of sonic hedgehog signaling to continuous variation in facial growth and shape

Abstract

SUMMARYVariation is an intrinsic feature of biological systems, yet developmental biology does not frequently address population-level phenomena. Sonic hedgehog (SHH) signaling activity in the vertebrate forebrain and face is thought to contribute to continuous variation in the morphology of the upper jaw, but despite its potential explanatory power, this idea has never been quantitatively assessed. Here, we test this hypothesis with an experimental design that is explicitly focused on the generation and measurement of variation in multivariate shape, tissue growth, cellular behavior and gene expression. We show that the majority of upper jaw shape variation can be explained by progressive changes in the spatial organization and mitotic activity of midfacial growth zones controlled by SHH signaling. In addition, nonlinearity between our treatment doses and phenotypic outcomes suggests that threshold effects in SHH signaling may play a role in variability in midfacial malformations such as holoprosencephaly (HPE). Together, these results provide novel insight into the generation of facial morphology, and demonstrate the value of quantifying variation for our understanding of development and disease.

Apaf1 apoptotic function critically limits Sonic hedgehog signaling during craniofacial development

“…Alterations in Shh activity in either the forebrain or facial mesenchyme are known to affect the spatial organization and mitotic activity of midfacial growth zones, resulting in a continuous phenotypic spectrum from cyclopia to hypertelorism. 33,46 Therefore, our data are consistent with the loss of Apaf1 cell-death function causing an increase in Shh signaling that results in the craniofacial development defects we see in the Apaf1 yautja mutants ( Figure 6). …”

“…[47][48] The current understanding is that the size of the Shh expression region is strictly controlled to coordinate craniofacial development. 33,46 We found increased cell proliferation in the FNP mesenchyme, as would be predicted by increased or persistent Shh signaling from the ventral forebrain signaling center. The broadened, diffuse Shh expression patterns we saw in yautja mutants are concordant with published results in which ectopically expanded Shh activity resulted Figure 5 Craniofacial defects: getting at a mechanism.…”

“…The box in the first panel of each pair is enlarged in the second panel to show detail of the floor plate and Shh expression (arrows) in a widened midface. 33 Taken together, our data suggest that decreased cell death resulting from loss of Apaf1 function leads to an expansion of Shh-expressing cells and cell proliferation that results in the wide FNP and split-face phenotype (Figure 6b).…”

“…[25][26] Directions from these signaling centers drive bone morphogenetic protein (Bmp), retinoid and wingless (Wnt) expression in the mesenchyme and induce outgrowth of the facial prominences. [27][28][29][30][31][32] Shh is important not only for establishing craniofacial signaling centers, but also for controlling proliferation and directing cell migration, 33 which make this signaling molecule a key player in craniofacial development.…”

NODAL and SHH dose-dependent double inhibition promotes an HPE-like phenotype in chick embryos

“…However, several studies have indicated that the NODAL pathway regulates ventral forebrain patterning through SHH-independent mechanisms (Rohr et al, 2001;Monuki, 2007). SHH might have to reach a threshold concentration to ensure proper forebrain development (Young et al, 2010). The dose-and stage-dependent relationship we observe here is consistent with a threshold effect: slightly reduced NODAL signaling might cause a small downregulation of SHH, which produces a HPE phenotype only when associated with another small decrease in SHH signaling.…”