Mandible and Tongue Development

Vascular endothelial growth factor control mechanisms in skeletal growth and repair

Vascular endothelial growth factor A (VEGF) is a critical regulator of vascular development and postnatal angiogenesis and homeostasis, and it is essential for bone development and repair. Blood vessels serve both as structural templates for bone formation and they provide essential cells, growth factors and minerals needed for synthesis and mineralization, as well as turnover, of the extracellular matrix in bone. Through its regulation of angiogenesis, VEGF contributes to coupling of osteogenesis to angiogenesis, and it directly controls the differentiation and function of osteoblasts and osteoclasts. In this review, we summarize the properties of VEGF and its receptors that are relevant to bone formation and repair; the roles of VEGF during development of endochondral and membranous bones; and the contributions of VEGF to bone healing during different phases of bone repair. Finally, we discuss contributions of altered VEGF function in inherited disorders with bone defects as part of their phenotypes, and we speculate on what will be required before therapeutic strategies based on VEGF modulation can be developed for clinical use to treat patients with bone growth disorders and/or compromised bone repair. Developmental Dynamics 246:227-234, 2017. © 2016 Wiley Periodicals, Inc.

Cilia-dependent GLI processing in neural crest cells is required for tongue development

Ciliopathies are a class of diseases caused by the loss of a ubiquitous, microtubule-based organelle called a primary cilium. Ciliopathies commonly result in defective development of the craniofacial complex, causing midfacial defects, craniosynostosis, micrognathia and aglossia. Herein, we explored how the conditional loss of primary cilia on neural crest cells (Kif3a^f/f;Wnt1-Cre) generated aglossia. On a cellular level, our data revealed that aglossia in Kif3a^f/f;Wnt1-Cre embryos was due to a loss of mesoderm-derived muscle precursors migrating into and surviving in the tongue anlage. To determine the molecular basis for this phenotype, we performed RNA-seq, in situ hybridization, qPCR and Western blot analyses. We found that transduction of the Sonic hedgehog (Shh) pathway, rather than other pathways previously implicated in tongue development, was aberrant in Kif3a^f/f;Wnt1-Cre embryos. Despite increased production of full-length GLI2 and GLI3 isoforms, previously identified GLI targets important for mandibular and glossal development (Foxf1, Foxf2, Foxd1 and Foxd2) were transcriptionally downregulated in Kif3a^f/f;Wnt1-Cre embryos. Genetic removal of GLI activator (GLIA) isoforms in neural crest cells recapitulated the aglossia phenotype and downregulated Fox gene expression. Genetic addition of GLIA isoforms in neural crest cells partially rescued the aglossia phenotype and Fox gene expression in Kif3a^f/f;Wnt1-Cre embryos. Together, our data suggested that glossal development requires primary cilia-dependent GLIA activity in neural crest cells. Furthermore, these data, in conjunction with our previous work, suggested prominence specific roles for GLI isoforms; with development of the frontonasal prominence relying heavily on the repressor isoform and the development of the mandibular prominence/tongue relying heavily on the activator isoform.

Histone methyltransferase Setdb1 is indispensable for Meckel's cartilage development

The histone methyltransferase Setdb1 represses gene expression by catalyzing lysine 9 of histone H3 trimethylation. Given that the conventional knockout of Setdb1 is embryo-lethal at the implantation stage, its role in craniofacial development is poorly understood. Here, we investigated the role of Setdb1, using conditional knockout mice-in which Setdb1 was deleted in the Meckel's cartilage (Setdb1 CKO)-and the mouse chondrogenic cell line ATDC5-in which Setdb1 was inhibited by siRNA. Deletion of Setdb1 in Meckel's cartilage, the supportive tissue in the embryonic mandible, led to its enlargement, instead of the degeneration that normally occurs. Chondrocytes from the Meckel's cartilage of Setdb1 CKO mice showed increased size. Furthermore, at embryonic days 16.5 and 18.5, part of the perichondrium was disrupted and mineralization was observed in the Meckel's cartilage. Proliferation analysis showed that inhibition of Setdb1 caused increased proliferation in chondrocytes in the Meckel's cartilage as well as in ATDC5 cells. Quantitative RT-PCR showed decreased expression of chondrogenic genes, such as Sox9, Mmp13, Collagen II, and Aggrecan, as a result of Setdb1 inhibition in ATDC5 cells. Along with these phenomenons, SMAD-dependent BMP signaling was significantly increased by the loss of Setdb1 in both the Meckel's cartilage of Setdb1 CKO mice and ATDC5 cells. Therefore, the abnormal development of Meckel's cartilage in Setdb1 CKO mice is partly due to the enhanced SMAD-dependent BMP signaling. Overall, to our knowledge, the present study is the first to show that epigenetic regulation by Setdb1 is indispensable for the embryonic development of Meckel's cartilage.

The Role of Sonic Hedgehog in Craniofacial Patterning, Morphogenesis and Cranial Neural Crest Survival

Craniofacial defects (CFD) are a significant healthcare problem worldwide. Understanding both the morphogenetic movements which underpin normal facial development, as well as the molecular factors which regulate these processes, forms the cornerstone of future diagnostic, and ultimately, preventative therapies. The soluble morphogen Sonic hedgehog (Shh), a vertebrate orthologue of Drosophila hedgehog, is a key signalling factor in the regulation of craniofacial skeleton development in vertebrates, operating within numerous tissue types in the craniofacial primordia to spatiotemporally regulate the formation of the face and jaws. This review will provide an overview of normal craniofacial skeleton development, and focus specifically on the known roles of Shh in regulating the development and progression of the first pharyngeal arch, which in turn gives rise to both the upper jaw (maxilla) and lower jaw (mandible).

Ephrin Ligands and Eph Receptors Show Regionally Restricted Expression in the Developing Palate and Tongue

The Eph family receptor-interacting (ephrin) ligands and erythropoietin-producing hepatocellular carcinoma (Eph) receptors constitute the largest known family of receptor tyrosine kinases. Ephrin ligands and their receptors form an important cell communication system with widespread roles in normal physiology and disease pathogenesis. In order to investigate potential roles of the ephrin-Eph system during palatogenesis and tongue development, we have characterized the cellular mRNA expression of family members EphrinA1-A3, EphA1–A8, and EphrinB2, EphB1, EphB4 during murine embryogenesis between embryonic day 13.5–16.5 using radioactive in situ hybridization. With the exception of EphA6 and ephrinA3, all genes were regionally expressed during the process of palatogenesis, with restricted and often overlapping domains. Transcripts were identified in the palate epithelium, localized at the tip of the palatal shelves, in the mesenchyme and also confined to the medial epithelium seam. Numerous Eph transcripts were also identified during tongue development. In particular, EphA1 and EphA2 demonstrated a highly restricted and specific expression in the tongue epithelium at all stages examined, whereas EphA3 was strongly expressed in the lateral tongue mesenchyme. These results suggest regulatory roles for ephrin-EphA signaling in development of the murine palate and tongue.

Ablation of the Sox11 Gene Results in Clefting of the Secondary Palate Resembling the Pierre Robin Sequence

Mouse gene inactivation has shown that the transcription factor Sox11 is required for mouse palatogenesis. However, whether Sox11 is primarily involved in the regulation of palatogenesis still remains elusive. In this study, we explored the role ofSox11in palatogenesis by analyzing the developmental mechanism in cleft palate formation in mutants deficient in Sox11. Sox11 is expressed both in the developing palatal shelf and in the surrounding structures, including the mandible. We found that cleft palate occurs only in the mutant in which Sox11is directly deleted. As in the wild type, the palatal shelves in the Sox11 mutant undergo outgrowth in a downward direction and exhibit potential for fusion and elevation. However, mutant palatal shelves encounter clefting, which is associated with a malpositioned tongue that results in physical obstruction of palatal shelf elevation at embryonic day 14.5 (E14.5). We found that loss of Sox11led to reduced cell proliferation in the developing mandibular mesenchyme via Cyclin D1, leading to mandibular hypoplasia, which blocks tongue descent. Extensive analyses of gene expression inSox11 deficiency identified FGF9 as a potential candidate target of Sox11 in the modulation of cell proliferation both in the mandible and the palatal shelf between E12.5 and E13.5. Finally we show, using in vitro assays, that Sox11 directly regulates the expression of Fgf9 and that application of FGF9 protein to Sox11-deficient palatal shelves restores the rate of BrdU incorporation. Taken together, the palate defects presented in the Sox11 loss mutant mimic the clefting in the Pierre Robin sequence in humans.