Journal of Developmental Biology Review Cranial Neural Crest Cells and Their Role in the Pathogenesis of Craniofacial Anomalies and Coronal Craniosynostosis Erica M. Siismets 1 and Nan E. Hatch 2,* 1 Oral Health Sciences PhD Program, School of Dentistry, University of Michigan, Ann Arbor, MI 48109-1078, USA; [email protected] 2 Department of Orthodontics and Pediatric Dentistry, School of Dentistry, University of Michigan, Ann Arbor, MI 48109-1078, USA * Correspondence: [email protected]; Tel.: +1-734-647-6567 Received: 31 July 2020; Accepted: 7 September 2020; Published: 9 September 2020 Abstract: Craniofacial anomalies are among the most common of birth defects. The pathogenesis of craniofacial anomalies frequently involves defects in the migration, proliferation, and fate of neural crest cells destined for the craniofacial skeleton. Genetic mutations causing deficient cranial neural crest migration and proliferation can result in Treacher Collins syndrome, Pierre Robin sequence, and cleft palate. Defects in post-migratory neural crest cells can result in pre- or post-ossification defects in the developing craniofacial skeleton and craniosynostosis (premature fusion of cranial bones/cranial sutures). The coronal suture is the most frequently fused suture in craniosynostosis syndromes. It exists as a biological boundary between the neural crest-derived frontal bone and paraxial mesoderm-derived parietal bone. The objective of this review is to frame our current understanding of neural crest cells in craniofacial development, craniofacial anomalies, and the pathogenesis of coronal craniosynostosis. We will also discuss novel approaches for advancing our knowledge and developing prevention and/or treatment strategies for craniofacial tissue regeneration and craniosynostosis. Keywords: craniofacial; skull; craniofacial abnormalities; neural crest; craniosynostoses; growth and development 1. Introduction Craniofacial anomalies are among the most common of all birth defects. Like many congenital defects, most craniofacial anomalies occur due to a combination of genetic and environmental factors, the latter including maternal exposure to toxins (including tobacco and alcohol) and certain medications [1,2]. Craniofacial anomalies can occur in isolation (the anomaly occurs with no other defects and with no established genetic basis), as part of an established syndrome with a known constellation of defects due to single gene mutation or chromosomal abnormality, or in association with additional development defects, but without a known genetic basis. The pathogenesis of developmental craniofacial abnormalities commonly involves defects in the migration, proliferation, and fate of cranial neural crest cells and their derivatives. These neural crest developmental abnormalities lead to a variety of developmental defect syndromes, overall referred to as neurocristopathies, including those that are found in cleft palate [3], Treacher Collins syndrome [4], Pierre Robin sequence [5], and craniosynostosis [6]. The coronal cranial suture is prematurely fused in many individuals with syndromic craniosynostosis, and particularly in those with mutations in FGFR2 or Twist [7,8]. Unlike other cranial sutures, the coronal suture develops between mesoderm-derived parietal and neural crest-derived frontal bone rudiments, and it maintains the J. Dev. Biol. 2020, 8, 18; doi:10.3390/jdb8030018 www.mdpi.com/journal/jdb J. Dev. Biol. 2020, 8, 18 2 of 27 boundary between these bone tissues of different embryonic origin during growth [9,10]. Therefore, the role of neural crest cell defects in the pathogenesis of coronal craniosynostosis is of particular interest. In this review, we will first provide an overview of neural crest cells in mammalian craniofacial development and cranial neural crest cells in craniofacial anomalies. We will subsequently focus on cranial neural crest cells in mammalian skull development and the pathogenesis of craniosynostosis, with particular emphasis on premature fusion of the coronal suture, due to the large amount of literature that exists in this field. Finally, we will discuss novel approaches for prevention and treatment. 2. Neural CrestJ. Dev. Cells Biol. 2020, 8, x 2 of 28 Neural crestneural cells crest arecell defects a transient in the pathogenesis stem cell of coronal population craniosynostosis characterized is of particular byinterest. their In this strong migratory review, we will first provide an overview of neural crest cells in mammalian craniofacial potential and multipotency.development and The cranial neural neural crest cells is uniquein craniofacial to vertebrates anomalies. We and will subsequently it can be divided focus on into four major sub-populations:cranial cranial, neural crest cardiac, cells in mammalian vagal, and skull trunk. development Each and defined the pathogenesis sub-population of craniosynostosis, of neural crest cells with particular emphasis on premature fusion of the coronal suture, due to the large amount of differentiates intoliterature diverse, that exists specialized in this field. cells, Finally, and we tissueswill discuss particular novel approaches to the for axial prevention level and of origin. Neural crest cells arisetreatment. in the embryonic ectodermal germ layer at the neural plate border during gastrulation. A precise combination2. Neural Crest ofBMP, Cells FGF, Wnt, and Notch inductive signals from the ectoderm triggers an epithelial-to-mesenchymalNeural crest transition cells are a transient (EMT) stem in thecell popu neurallation plate characterized border by territory their strong [ 11migratory] (Figure 1). EMT is a complex, multi-steppotential process and multipotency. required The to neural transform crest is unique adherent to vertebrates neuroepithelial and it can be divided cells into into four migratory neural crest cells. Thismajor transformation sub-populations: is cranial, induced cardiac, through vagal, and the trunk. dissolution Each defined of sub-population adherens junctions of neural crest through cadherin cells differentiates into diverse, specialized cells, and tissues particular to the axial level of origin. expression changes.Neural EMTcrest cells is regulatedarise in the embryonic by Wnt /ectodermalβ-catenin germ signaling layer at the and neural it can plate be border thought during of as a spectrum of epithelial andgastrulation. mesenchymal A precise cell combination fates, rather of BMP, than FGF, an Wnt, exact and binary Notch switchinductive [ 12signals]. The from first the cadherin switch ectoderm triggers an epithelial-to-mesenchymal transition (EMT) in the neural plate border territory allows neural crest[11] (Figure cells to1). delaminateEMT is a complex, from multi-step the neural process tuberequired and to transform the second adherent switch neuroepithelial grants neural crest cells their migratorycells potential into migratory [13]. neural The onsetcrest cells. of EMTThis transformation in mouse embryosis induced through is controlled the dissolution by glycogen of synthase adherens junctions through βcadherin expression changes. EMT is regulated by Wnt/β-catenin kinase 3 (GSK3),signaling a component and it can be of thought the of-catenin as a spectrum destruction of epithelial complexand mesenchymal [14,15 cell]. fates, GSK3 rather phosphorylates than Snail and Twist in premigratoryan exact binary switch neural [12]. crest The first cells cadherin (Lander), switch allows which neural along crest withcells to other delaminate neural from crestthe transcription factors FoxD3, neuralSip1/Zeb2 tube and, and the secondSox9 /switch10, will grants downregulate neural crest cells their epithelial migratory cadherins potential [13]. and The onset cadherin6B [16,17]. of EMT in mouse embryos is controlled by glycogen synthase kinase 3 (GSK3), a component of the β- Epithelial cadherincatenin signatures destruction comple are replacedx [14,15]. GSK3 with phosphorylates mesenchymal Snail cadherin-7and Twist in premigratoryand cadherin-11 neural , as neural crest cells become migratorycrest cells (Lander), [18]. Metalloproteinases which along with other neural (including crest transcription MMP-2, factors -9, -14FoxD3 and, Sip1/Zeb2 ADAM-10,, and -13, and -19) Sox9/10, will downregulate epithelial cadherins and cadherin6B [16,17]. Epithelial cadherin are required forsignatures neural are crest replaced delamination with mesenchymal and migrationcadherin-7 andto cadherin-11 degrade, as cadherinsneural crest cells within become and extracellular matrix proteinsmigratory around [18]. neural Metalloproteinases crest cells (including [12,19–21 MMP-2,]. Defects -9, -14 inand components ADAM-10, -13, thatand -19) are are involved in EMT required for neural crest delamination and migration to degrade cadherins within and extracellular can affect craniofacialmatrix proteins development. around neural crest For cells example, [12,19–21]. the Defects loss in ofcomponents GSK3 inthat mouse are involved embryos in EMT results in cleft palate [22], reducedcan affect frontal craniofacial bone development. size, and For enlarged example, the frontal loss of fontanelleGSK3 in mouse [23 embryos]. results in cleft palate [22], reduced frontal bone size, and enlarged frontal fontanelle [23]. Figure 1. Early neural crest cell development in mouse embryos. (a) Gastrulation: neural crest cells Figure 1. Earlyare neural inducedcrest at the neural cell developmentplate border (NPB) between in mouse the neural embryos. plate (NP) ( aand) Gastrulation: non-neural ectoderm