Constitutive activation of hedgehog signaling adversely affects epithelial cell fate during palatal fusion

Embryonic exposure to Smoothened Agonist disrupts tongue development in mice

The tongue is an important muscular organ for chewing and speech, and its development is regulated by multiple molecular signaling pathways, such as the hedgehog and TGF pathways. These pathways are particularly crucial in muscle patterning, which determines the structural and functional integrity of the tongue. Proper hedgehog signaling is essential for craniofacial tissue development, influencing muscle arrangement and differentiation critical for normal tongue morphology. In this study, we administered the Smoothened Agonist (SAG) 25 mg/kg to pregnant mice via intraperitoneal injection at E10.5 to further investigate the regulatory role of overexpressed hedgehog signaling in the early developmental process of the tongue. The intraperitoneal injection of SAG at E10.5 resulted in reduced tongue height and abnormal muscle development. Consequently, these alterations led to a midline cleft. Detection of cell proliferation using PHH3 and Ki67 showed that cell proliferation was significantly inhibited in the experimental group, while apoptosis showed no significant difference compared to the control group. At E11.5, the expression levels of downstream markers of hedgehog signaling, including Gli1, Ptch1, Foxf1, and Foxf2, were significantly elevated in the experimental group compared to the control group, whereas TGF-β2 mRNA expression was significantly downregulated(P < 0.05). Thus, overexpression of hedgehog signaling, induced by SAG, disrupts normal cellular processes by inhibiting proliferation and downregulating TGF-β2, ultimately leading to cleft tongue malformations.

Transcriptional factor ISL1 regulates palate development by tuning the SHH cascade

Cleft palate is one of the most common birth defects in humans, and palate morphogenesis depends on epithelial-mesenchymal interaction. In this study, we report that ablation of Isl1 in the epithelium leads to complete cleft palate. A significant reduction in mesenchymal cell proliferation was detected in the Isl1Pitx2Cre mutant palates, but there was no significant difference in apoptosis between wild-type and mutant embryos. Fewer rugae structures were observed in Isl1Pitx2Cre mutant embryos. Shh, Sox2, Foxe1, Foxd2, and Msx1 expression was downregulated in the developing palate in Isl1 mutant embryos. We found that ISL1 can directly regulate Shh expression in palatal epithelial cells, suggesting a critical role for ISL1 in epithelial-mesenchymal interactions during palate development. Remarkably, cleft palate defects due to Isl1 deletion were rescued by a conditional transgenic allele (Tg-pmes-Ihh), confirming the genetic integration of Hedgehog signaling. Our findings indicate that ISL1 controls palatal shelf morphogenesis by modulating epithelial-mesenchymal communication via SHH signaling.

p63: a crucial player in epithelial stemness regulation

Epithelial tissue homeostasis is closely associated with the self-renewal and differentiation behaviors of epithelial stem cells (ESCs). p63, a well-known marker of ESCs, is an indispensable factor for their biological activities during epithelial development. The diversity of p63 isoforms expressed in distinct tissues allows this transcription factor to have a wide array of effects. p63 coordinates the transcription of genes involved in cell survival, stem cell self-renewal, migration, differentiation, and epithelial-to-mesenchymal transition. Through the regulation of these biological processes, p63 contributes to, not only normal epithelial development, but also epithelium-derived cancer pathogenesis. In this review, we provide an overview of the role of p63 in epithelial stemness regulation, including self-renewal, differentiation, proliferation, and senescence. We describe the differential expression of TAp63 and ΔNp63 isoforms and their distinct functional activities in normal epithelial tissues and in epithelium-derived tumors. Furthermore, we summarize the signaling cascades modulating the TAp63 and ΔNp63 isoforms as well as their downstream pathways in stemness regulation.

Sonic hedgehog signaling in craniofacial development

Mutations in SHH and several other genes encoding components of the Hedgehog signaling pathway have been associated with holoprosencephaly syndromes, with craniofacial anomalies ranging in severity from cyclopia to facial cleft to midfacial and mandibular hypoplasia. Studies in animal models have revealed that SHH signaling plays crucial roles at multiple stages of craniofacial morphogenesis, from cranial neural crest cell survival to growth and patterning of the facial primordia to organogenesis of the palate, mandible, tongue, tooth, and taste bud formation and homeostasis. This article provides a summary of the major findings in studies of the roles of SHH signaling in craniofacial development, with emphasis on recent advances in the understanding of the molecular and cellular mechanisms regulating the SHH signaling pathway activity and those involving SHH signaling in the formation and patterning of craniofacial structures.

A392V and R945X mutations cause orofacial clefts via impairing PTCH1 function

The Hedgehog (HH) signaling plays key roles in embryogenesis and organogenesis, and its dysfunction causes a variety of human birth defects. Orofacial cleft (OFC) is one of the most common congenital craniofacial defects, and its etiology is closely related to mutations in multiple components in the HH pathway, including the PTCH1 receptor. A quantity of PTCH1 variants have been associated with OFC, but the pathogenicity and underlying mechanism of these variants have not been functionally validated. In our previous studies, we identified two PTCH1 variants (A392V and R945X) in two families with hereditary OFC. Here we explore the functional consequences of these two variants. In zebrafish embryos, microinjection of wild type PTCH1 mRNA causes curved body axis and craniofacial anomalies. In contrast, microinjection of A392V and R945X PTCH1 mRNAs results in much milder phenotypes, suggesting these two variants are loss-of-function mutations. In mammalian cells, A392V and R945X mutations reverse the inhibitory effect of PTCH1 on HH signaling. Biochemically, the two mutants PTCH1 show lower expression levels and shortened half-life, indicting these mutations decrease the stability of PTCH1. A392V and R945X mutations also appear to cause PTCH1 to localize away from vesicles. Taken together, our findings indicate that A392V and R945X variants are loss-of-function mutations that disrupt the function of PTCH1 and thus cause dysregulation of HH signaling, leading to the pathogenesis of OFC.

Involvement of homeobox transcription factor Mohawk in palatogenesis

Palatogenesis is affected by many factors, including gene polymorphisms and exposure to toxic chemicals during sensitive developmental periods. Cleft palate is one of the most common congenital anomalies, and ongoing efforts to elucidate the molecular mechanisms underlying palatogenesis are providing useful insights to reduce the risk of this disorder. To identify novel potential regulators of palatogenesis, we analyzed public transcriptome datasets from a mouse model of cleft palate caused by selective deletion of transforming growth factor-β (TGFβ) receptor type 2 in cranial neural crest cells. We identified the homeobox transcription factor Mohawk (Mkx) as a gene downregulated in the maxilla of TGFβ knockout mice compared with wild-type mice. To examine the role of mkx in palatogenesis, we used CRISPR/Cas9 editing to generate zebrafish with impaired expression of mkxa and mkxb, the zebrafish homologs of Mkx. We found that mkx crispants expressed reduced levels of gli1, a critical transcription factor in the Sonic hedgehog (SHH) signaling pathway that plays an important role in the regulation of palatogenesis. Furthermore, we found that mkxa^-/- zebrafish were more susceptible than mkxa^+/+ zebrafish to the deleterious effects of cyclopamine, an inhibitor of SHH signaling, on upper jaw development. These results suggest that Mkx may be involved in palatogenesis regulated by TGFβ and SHH signaling, and that impairment in Mkx function may be related to the etiology of cleft palate.

Familial cleft tongue caused by a unique translation initiation codon variant in TP63

Variants in transcription factor p63 have been linked to several autosomal dominantly inherited malformation syndromes. These disorders show overlapping phenotypic characteristics with various combinations of the following features: ectodermal dysplasia, split-hand/foot malformation/syndactyly, lacrimal duct obstruction, hypoplastic breasts and/or nipples, ankyloblepharon filiforme adnatum, hypospadias and cleft lip/palate. We describe a family with six individuals presenting with a striking novel phenotype characterized by a furrowed or cleft tongue, a narrow face, reddish hair, freckles and various foot deformities. Whole-exome sequencing (WES) identified a novel heterozygous variant, c.3G>T, in TP63 affecting the translation initiation codon (p.1Met?). Sanger sequencing confirmed dominant inheritance of this unique variant in all six affected family members. In summary, our findings indicate that heterozygous variants in TP63 affecting the first translation initiation codon result in a novel phenotype dominated by a cleft tongue, expanding the complex genotypic and phenotypic spectrum of TP63-associated disorders.

Genetics and signaling mechanisms of orofacial clefts

Craniofacial development involves several complex tissue movements including several fusion processes to form the frontonasal and maxillary structures, including the upper lip and palate. Each of these movements are controlled by many different factors that are tightly regulated by several integral morphogenetic signaling pathways. Subject to both genetic and environmental influences, interruption at nearly any stage can disrupt lip, nasal, or palate fusion and result in a cleft. Here, we discuss many of the genetic risk factors that may contribute to the presentation of orofacial clefts in patients, and several of the key signaling pathways and underlying cellular mechanisms that control lip and palate formation, as identified primarily through investigating equivalent processes in animal models, are examined.

Transforming Growth Factor-Beta and Sonic Hedgehog Signaling in Palatal Epithelium Regulate Tenascin-C Expression in Palatal Mesenchyme During Soft Palate Development

During palatogenesis, the palatal shelves first grow vertically on either side of the tongue before changing their direction of growth to horizontal. The extracellular matrix (ECM) plays an important role in these dynamic changes in palatal shelf morphology. Tenascin-C (TNC) is an ECM glycoprotein that shows unique expression in the posterior part of the palatal shelf, but little is known about the regulation of TNC expression. Since transforming growth factor-beta-3 (TGF-β3) and sonic hedgehog (SHH) signaling are known to play important roles in palatogenesis, we investigated whether TGF-β3 and SHH are involved in the regulation of TNC expression in the developing palate. TGF-β3 increased the expression of TNC mRNA and protein in primary mouse embryonic palatal mesenchymal cells (MEPM) obtained from palatal mesenchyme dissected at embryonic day 13.5-14.0. Interestingly, immunohistochemistry experiments revealed that TNC expression was diminished in K14-cre;Tgfbr2 ^fl/fl mice that lack the TGF-β type II receptor in palatal epithelial cells and exhibit cleft soft palate, whereas TNC expression was maintained in Wnt1-cre;Tgfbr2 ^fl/fl mice that lack the TGF-β type II receptor in palatal mesenchymal cells and exhibit a complete cleft palate. SHH also increased the expression of TNC mRNA and protein in MEPM cells. However, although TGF-β3 up-regulated TNC mRNA and protein expression in O9-1 cells (a cranial neural crest cell line), SHH did not. Furthermore, TGF-β inhibited the expression of osteoblastic differentiation markers (osterix and alkaline phosphatase) and induced the expression of fibroblastic markers (fibronectin and periostin) in O9-1 cells, whereas SHH did not affect the expression of osteoblastic and fibroblastic markers in O9-1 cells. However, immunohistochemistry experiments showed that TNC expression was diminished in the posterior palatal shelves of Shh^-/+ ;MFCS4 ^+/- mice, which have deficient SHH signaling in the posterior palatal epithelium. Taken together, our findings support the proposal that TGF-β and SHH signaling in palatal epithelium co-ordinate the expression of TNC in the posterior palatal mesenchyme through a paracrine mechanism. This signal cascade may work in the later stage of palatogenesis when cranial neural crest cells have differentiated into fibroblast-like cells. The spatiotemporal regulation of ECM-related proteins by TGF-β and SHH signaling may contribute not only to tissue construction but also to cell differentiation or determination along the anterior-posterior axis of the palatal shelves.

Regulatory Mechanisms of Soft Palate Development and Malformations

Orofacial clefting is the most common congenital craniofacial malformation, appearing in approximately 1 in 700 live births. Orofacial clefting includes several distinct anatomic malformations affecting the upper lip and hard and soft palate. The etiology of orofacial clefting is multifactorial, including genetic or environmental factors or their combination. A large body of work has focused on the molecular etiology of cleft lip and clefts of the hard palate, but study of the underlying etiology of soft palate clefts is an emerging field. Recent advances in the understanding of soft palate development suggest that it may be regulated by distinct pathways from those implicated in hard palate development. Soft palate clefting leads to muscle misorientation and oropharyngeal deficiency and adversely affects speech, swallowing, breathing, and hearing. Hence, there is an important need to investigate the regulatory mechanisms of soft palate development. Significantly, the anatomy, function, and development of soft palatal muscles are similar in humans and mice, rendering the mouse an excellent model for investigating molecular and cellular mechanisms of soft palate clefts. Cranial neural crest-derived cells provide important regulatory cues to guide myogenic progenitors to differentiate into muscles in the soft palate. Signals from the palatal epithelium also play key roles via tissue-tissue interactions mediated by Tgf-β, Wnt, Fgf, and Hh signaling molecules. Additionally, mutations in transcription factors, such as Dlx5, Tbx1, and Tbx22, have been associated with soft palate clefting in humans and mice, suggesting that they play important regulatory roles during soft palate development. Finally, we highlight the importance of distinguishing specific types of soft palate defects in patients and developing relevant animal models for each of these types to improve our understanding of the regulatory mechanism of soft palate development. This knowledge will provide a foundation for improving treatment for patients in the future.

Hedgehog Signaling and Embryonic Craniofacial Disorders

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.