Hedgehog Signaling and Embryonic Craniofacial Disorders

Abstract

Since its initial discovery in a Drosophila mutagenesis screen, the Hedgehog pathway has been revealed to be instrumental in the proper development of the vertebrate face. Vertebrates possess three hedgehog paralogs: Sonic hedgehog (Shh), Indian hedgehog (Ihh), and Desert hedgehog (Dhh). Of the three, Shh has the broadest range of functions both in the face and elsewhere in the embryo, while Ihh and Dhh play more limited roles. The Hedgehog pathway is instrumental from the period of prechordal plate formation early in the embryo, until the fusion of the lip and secondary palate, which complete the major patterning events of the face. Disruption of Hedgehog signaling results in an array of developmental disorders in the face, ranging from minor alterations in the distance between the eyes to more serious conditions such as severe clefting of the lip and palate. Despite its critical role, Hedgehog signaling seems to be disrupted through a number of mechanisms that may either be direct, as in mutation of a downstream target of the Hedgehog ligand, or indirect, such as mutation in a ciliary protein that is otherwise seemingly unrelated to the Hedgehog pathway. A number of teratogens such as alcohol, statins and steroidal alkaloids also disrupt key aspects of Hedgehog signal transduction, leading to developmental defects that are similar, if not identical, to those of Hedgehog pathway mutations. The aim of this review is to highlight the variety of roles that Hedgehog signaling plays in developmental disorders of the vertebrate face.

Keywords: Hedgehog; Ihh; Shh; ciliopathy; craniofacial; holoprosencephaly.

Figure 1

Biogenesis of processed Hedgehog (Hh) ligand. Hh ligand is produced as a large precursor protein that undergoes a series of post-translational modifications prior to secretion. The N-terminal signaling domain is first cleaved from the C-terminal processing domain. Subsequent to cleavage, the N-terminal has a palmitate added to its N terminus and cholesterol added to its C terminus, resulting in a biologically active, processed Hh ligand that may be transported out of the cell. Ss—Signal sequence. Adapted from reference [56].

Figure 2

In the absence of Hh signal, Patched1 (Ptch1) inhibits the Hh pathway through the modulation of the Smoothened (Smo). Since Smo is inactivated, the Glioma-associated oncogene- Suppressor of Fused (Gli-Sufu) complex is retained in the cytosol, where it is exposed to phosphorylation by protein kinase A (PKA), thereby inhibiting signal transduction by Gli. When Hh signal is present (here represented by Shh), its binding to Ptch1 relives pathway inhibition, induces its translocation away from the cilium, allowing activated Smo and Gli-Sufu complex transport to the tip of the cilium, where Gli becomes dissociated from Sufu. Subsequent retrograde transport of Gli then brings it to the nucleus, where it regulates expression of target genes. Image adapted from Reference [76,77].

Figure 3

Fusion of the upper lip in human (A) and chicken (B) embryos reveals structural similarities. Across amniotes, the face is assembled from five main craniofacial prominences: the frontonasal mass (fnm), lateral nasal prominence (lnp), medial nasal prominence (mnp), maxillary prominences (mxp) and the mandibular prominences (md). The primary palate (upper lip/beak and nasal cavities/nasal pits [np]) forms from the fusion of the maxillary and lateral nasal prominences with the medial nasal prominence (highlighted in blue), although there is some variation in this process across different lineages [92].

Figure 4

Facial appearances and associated conditions. (A) Hypertelorism—abnormally increased distance between eyes, often accompanied by general broadening of midline facial structures. (B) Normal, proportionate individual. (C) Cyclopia—characterized by undivided or partially divided eye fields, with displacement of the proboscis (putative nose) above the eyes. D. Midline Facial Cleft—facial cleft where the frontonasal mass is reduced and the maxillary prominences do not meet. (E) Lateral Facial Cleft—facial cleft where frontonasal mass growth is not reduced, however maxillary prominences failed to fuse. (F) Palatal view of a lateral facial cleft. Image adapted from Reference [117].