Molecular interactions coordinating the development of the forebrain and face

Quantifying the relationship between cell proliferation and morphology during development of the face

Morphogenesis requires highly coordinated, complex interactions between cellular processes: proliferation, migration and apoptosis, along with physical tissue interactions. How these cellular and tissue dynamics drive morphogenesis remains elusive. Three dimensional (3D) microscopic imaging holds great promise, and generates elegant images, but generating even moderate throughput for quantified images is challenging for many reasons. As a result, the association between morphogenesis and cellular processes in 3D developing tissues has not been fully explored. To address this gap, we have developed an imaging and image analysis pipeline to enable 3D quantification of cellular dynamics along with 3D morphology for the same individual embryo. Specifically, we focus on how 3D distribution of proliferation relates to morphogenesis during mouse facial development. Our method involves imaging with light-sheet microscopy, automated segmentation of cells and tissues using machine learning-based tools, and quantification of external morphology by geometric morphometrics. Applying this framework, we show that changes in proliferation are tightly correlated with changes in morphology over the course of facial morphogenesis. These analyses illustrate the potential of this pipeline to investigate mechanistic relationships between cellular dynamics and morphogenesis during embryonic development.

Sonic hedgehog and fibroblast growth factor 8 regulate the evolution of amniote facial proportions

Amniote skulls are diverse in shape and skeletal composition, which is the basis of much adaptive diversification within this clade. Major differences in skull shape are established early in development, at a critical developmental interval spanning the initial outgrowth and fusion of the facial processes. In birds, this is orchestrated by domains of Shh and Fgf8 expression, known as the frontonasal ectodermal zone (FEZ). It is unclear whether this model of facial development applies to species with diverse facial skeletons, especially species possessing a skull morphology representative of early amniotes. By investigating facial morphogenesis in the lizard, Anolis sagrei, we show that reptilian skull development is driven by the same genes as mammals and birds, but the manner in which those genes regulate facial development is clade-specific. These genes are not expressed in the frontal-nasal prominence, the region of the avian FEZ. Downregulating Shh and Fgf8 signaling disrupts normal facial development, but in pathway-specific ways. Our results demonstrate that early facial morphogenesis in lizards does not conform to the FEZ model. Lizard skull development may be more representative of the ancestral amniote than other model species with highly derived facial skeletons suggesting that the FEZ may be an avian-specific novelty.

The role of sonic hedgehog signaling in the oropharyngeal epithelium during jaw development

Sonic hedgehog (Shh) is expressed in the oropharyngeal epithelium, including the frontonasal ectodermal zone (FEZ), which is defined as the boundary between Shh and Fgf8 expression domains in the frontonasal epithelium. To investigate the role of SHH signaling from the oropharyngeal epithelium, we generated mice in which Shh expression is specifically deleted in the oropharyngeal epithelium (Isl1-Cre; Shhf/f). In the mutant mouse, Shh expression was excised in the oropharyngeal epithelium as well as FEZ and ventral forebrain, consistent with the expression pattern of Isl1. Isl1-Cre; Shhf/f mice exhibited a complete loss of lower jaw components and a malformed upper jaw with defects in the cranial base and secondary palate. Massive cell death was observed in the mandibular process at embryonic day (E) 9.5 and E10.5, while mild cell death was observed in the lambdoidal region (the fusion area in the maxillary, lateral nasal, and medial nasal processes) at E10.5. An RNA-seq analysis revealed that Satb2, a gene involved in cell survival during jaw formation, was downregulated in the lambdoidal region in Isl1-Cre; Shhf/f mice. These results suggest that Shh expression in the FEZ is required for cell survival and skeletogenesis in the lambdoidal region during the development of the upper jaw and that the developmental control governed by SHH signaling is different between upper and lower jaws.

PBX1 and PBX3 transcription factors regulate SHH expression in the Frontonasal Ectodermal Zone through complementary mechanisms

Sonic hedgehog (SHH) signaling from the frontonasal ectodermal zone (FEZ) is a key regulator of craniofacial morphogenesis. Along with SHH, pre-B-cell leukemia homeobox (PBX) transcription factors regulate midfacial development. PBXs act in the epithelium during fusion of facial primordia, but their specific interactions with SHH have not been fully investigated. We hypothesized that PBX1/3 regulate SHH expression in the FEZ by activating or repressing transcription. The hypothesis was tested by manipulating PBX1/3 expression in chick embryos and profiling epigenomic landscapes at early developmental stages. PBX1/3 expression was perturbed in the chick face beginning at stage 10 (HH10) using RCAS viruses, and the resulting SHH expression was assessed at HH22. Overexpressing PBX1 expanded SHH expression, while overexpressing PBX3 decreased SHH expression. Conversely, reducing PBX1 expression decreased SHH expression, but reducing PBX3 induced ectopic SHH expression. We performed ATAC-seq and mapped binding of PBX1 and PBX3 with ChIP-seq on the FEZ at HH22 to assess direct interactions of PBX1/3 with the SHH locus. These multi-omics approaches uncovered a 400 bp PBX1-enriched element within intron 1 of SHH (chr2:8,173,222-8,173,621). Enhancer activity of this element was demonstrated by electroporation of reporter constructs in ovo and luciferase reporter assays in vitro . When bound by PBX1, this element upregulates transcription, while it downregulates transcription when bound by PBX3. The present study identifies a cis- regulatory element, named SFE1, that interacts with PBX1/3 to modulate SHH expression in the FEZ and establishes that PBX1 and PBX3 play complementary roles in SHH regulation during embryonic development.

Comparative analysis of craniofacial shape in two mouse models of Down syndrome: Ts65Dn and TcMAC21

Mouse models are central to studying and understanding the genotypic-to-phenotypic outcomes of Down syndrome (DS), a complex condition caused by an extra copy of the long arm of human chromosome 21. The recently developed TcMAC21-a transchromosomic mouse strain with comparable gene dosage to human chromosome 21 (Hsa21)-includes more Hsa21 genes than any other model of DS. Recent studies on TcMAC21 have provided valuable insight into the molecular, physiological, and neuroanatomical aspects of the model. However, relatively little is known about the craniofacial phenotype of TcMAC21 mice, particularly as it compares to the widely studied Ts65Dn model. Here we conducted a quantitative study of the cranial morphology of TcMAC21 and Ts65Dn mice and their respective unaffected littermates. Our comparative data comprise forty three-dimensional cranial measurements taken on micro-computed tomography scans of the heads of TcMAC21 and Ts65Dn mice. Our results show that TcMAC21 exhibit similar patterns of craniofacial change to Ts65Dn. However, the DS-specific morphology is more pronounced in Ts65Dn mice. Specifically, Ts65Dn present with more medio-lateral broadening and retraction of the snout compared to TcMAC21. Our findings reveal the complexity of potential gene interaction in the production of craniofacial phenotypes.

Shaping faces: genetic and epigenetic control of craniofacial morphogenesis

Major differences in facial morphology distinguish vertebrate species. Variation of facial traits underlies the uniqueness of human individuals, and abnormal craniofacial morphogenesis during development leads to birth defects that significantly affect quality of life. Studies during the past 40 years have advanced our understanding of the molecular mechanisms that establish facial form during development, highlighting the crucial roles in this process of a multipotent cell type known as the cranial neural crest cell. In this Review, we discuss recent advances in multi-omics and single-cell technologies that enable genes, transcriptional regulatory networks and epigenetic landscapes to be closely linked to the establishment of facial patterning and its variation, with an emphasis on normal and abnormal craniofacial morphogenesis. Advancing our knowledge of these processes will support important developments in tissue engineering, as well as the repair and reconstruction of the abnormal craniofacial complex.

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.

Anatomical network modules of the human central nervous-craniofacial skeleton system

Anatomical network analysis (AnNA) is a systems biological framework based on network theory that enables anatomical structural analysis by incorporating modularity to model structural complexity. The human brain and facial structures exhibit close structural and functional relationships, suggestive of a co-evolved anatomical network. The present study aimed to analyze the human head as a modular entity that comprises the central nervous system, including the brain, spinal cord, and craniofacial skeleton. An AnNA model was built using 39 anatomical nodes from the brain, spinal cord, and craniofacial skeleton. The linkages were identified using peripheral nerve supply and direct contact between structures. The Spinglass algorithm in the igraph software was applied to construct a network and identify the modules of the central nervous system-craniofacial skeleton anatomical network. Two modules were identified. These comprised an anterior module, which included the forebrain, anterior cranial base, and upper-middle face, and a posterior module, which included the midbrain, hindbrain, mandible, and posterior cranium. These findings may reflect the genetic and signaling networks that drive the mosaic central nervous system and craniofacial development and offer important systems biology perspectives for developmental disorders of craniofacial structures.

Evolutionary mechanisms modulating the mammalian skull development

Mammals possess impressive craniofacial variation that mirrors their adaptation to diverse ecological niches, feeding behaviour, physiology and overall lifestyle. The spectrum of craniofacial geometries is established mainly during embryonic development. The formation of the head represents a sequence of events regulated on genomic, molecular, cellular and tissue level, with each step taking place under tight spatio-temporal control. Even minor variations in timing, position or concentration of the molecular drivers and the resulting events can affect the final shape, size and position of the skeletal elements and the geometry of the head. Our knowledge of craniofacial development increased substantially in the last decades, mainly due to research using conventional vertebrate model organisms. However, how developmental differences in head formation arise specifically within mammals remains largely unexplored. This review highlights three evolutionary mechanisms acknowledged to modify ontogenesis: heterochrony, heterotopy and heterometry. We present recent research that links changes in developmental timing, spatial organization or gene expression levels to the acquisition of species-specific skull morphologies. We highlight how these evolutionary modifications occur on the level of the genes, molecules and cellular processes, and alter conserved developmental programmes to generate a broad spectrum of skull shapes characteristic of the class Mammalia. This article is part of the theme issue 'The mammalian skull: development, structure and function'.

Quantifying the relationship between cell proliferation and morphology during development of the face

Morphogenesis requires highly coordinated, complex interactions between cellular processes: proliferation, migration, and apoptosis, along with physical tissue interactions. How these cellular and tissue dynamics drive morphogenesis remains elusive. Three dimensional (3D) microscopic imaging poses great promise, and generates elegant images. However, generating even moderate through-put quantified images is challenging for many reasons. As a result, the association between morphogenesis and cellular processes in 3D developing tissues has not been fully explored. To address this critical gap, we have developed an imaging and image analysis pipeline to enable 3D quantification of cellular dynamics along with 3D morphology for the same individual embryo. Specifically, we focus on how 3D distribution of proliferation relates to morphogenesis during mouse facial development. Our method involves imaging with light-sheet microscopy, automated segmentation of cells and tissues using machine learning-based tools, and quantification of external morphology via geometric morphometrics. Applying this framework, we show that changes in proliferation are tightly correlated to changes in morphology over the course of facial morphogenesis. These analyses illustrate the potential of this pipeline to investigate mechanistic relationships between cellular dynamics and morphogenesis during embryonic development.

Frem1 activity is regulated by Sonic hedgehog signaling in the cranial neural crest mesenchyme during midfacial morphogenesis

BACKGROUND

Frem1 has been linked to human face shape variation, dysmorphology, and malformation, but little is known about its regulation and biological role in facial development.

RESULTS

During midfacial morphogenesis in mice, we observed Frem1 expression in the embryonic growth centers that form the median upper lip, nose, and palate. Expansive spatial gradients of Frem1 expression in the cranial neural crest cell (cNCC) mesenchyme of these tissues suggested transcriptional regulation by a secreted morphogen. Accordingly, Frem1 expression paralleled that of the conserved Sonic Hedgehog (Shh) target gene Gli1 in the cNCC mesenchyme. Suggesting direct transcriptional regulation by Shh signaling, we found that Frem1 expression is induced by SHH ligand stimulation or downstream pathway activation in cNCCs and observed GLI transcription factor binding at the Frem1 transcriptional start site during midfacial morphogenesis. Finally, we found that FREM1 is sufficient to induce cNCC proliferation in a concentration-dependent manner and that Shh pathway antagonism reduces Frem1 expression during pathogenesis of midfacial hypoplasia.

CONCLUSIONS

By demonstrating that the Shh signaling pathway regulates Frem1 expression in cNCCs, these findings provide novel insight into the mechanisms underlying variation in midfacial morphogenesis.

A rare case of partial skull and brain duplication in a hatchling Alligator mississippiensis

Errors in development occur in all vertebrates. When severe, these anomalies are lethal and frequently escape attention. In rare cases, animals with profound malformations are born and can provide a glimpse into structures and their respective function that would otherwise go unnoticed. A rare abnormality in a hatchling Alligator mississippiensis is described in which duplication of the skull, face, and brain was incomplete. The rostral skull, face, and associated forebrain, including the olfactory apparatus, were duplicated. However, the caudal skull and brainstem were not. These observations were made with advanced imaging using both computed tomography and magnetic resonance coupled with gross brain dissections. These abnormal features emphasize the complex and intertwined relationship between the development of the brain, face, and skull which are influenced by certain signaling molecules, possible gene mutation(s), and potential environmental factors.

High-throughput detection of craniofacial defects in fluorescent zebrafish

Losses and malformations of cranial neural crest cell (cNCC) derivatives are a hallmark of several common brain and face malformations. Nevertheless, the etiology of these cNCC defects remains unknown for many cases, suggesting a complex basis involving interactions between genetic and/or environmental factors. However, the sheer number of possible factors (thousands of genes and hundreds of thousands of toxicants) has hindered identification of specific interactions. Here, we develop a high-throughput analysis that will enable faster identification of multifactorial interactions in the genesis of craniofacial defects. Zebrafish embryos expressing a fluorescent marker of cNCCs (fli1:EGFP) were exposed to a pathway inhibitor standard or environmental toxicant, and resulting changes in fluorescence were measured in high-throughput using a fluorescent microplate reader to approximate cNCC losses. Embryos exposed to the environmental Hedgehog pathway inhibitor piperonyl butoxide (PBO), a Hedgehog pathway inhibitor standard, or alcohol (ethanol) exhibited reduced fli1:EGFP fluorescence at one day post fertilization, which corresponded with craniofacial defects at five days post fertilization. Combining PBO and alcohol in a co-exposure paradigm synergistically reduced fluorescence, demonstrating a multifactorial interaction. Using pathway reporter transgenics, we show that the plate reader assay is sensitive at detecting alterations in Hedgehog signaling, a critical regulator of craniofacial development. We go on to demonstrate that this technique readily detects defects in other important cell types, namely neurons. Together, these findings demonstrate this novel in vivo platform can predict developmental abnormalities and multifactorial interactions in high-throughput.

Gene-alcohol interactions in birth defects

Most human birth defects are thought to result from complex interactions between combinations of genetic and environmental factors. This is true even for conditions that, at face value, may appear simple and straightforward, like fetal alcohol spectrum disorders (FASD). FASD describe the full range of structural and neurological disruptions that result from prenatal alcohol exposure. While FASD require alcohol exposure, evidence from human and animal model studies demonstrate that additional genetic and/or environmental factors can influence the embryo's susceptibility to alcohol. Only a limited number of alcohol interactions in birth defects have been identified, with many sensitizing genetic and environmental factors likely yet to be identified. Because of this, while unsatisfying, there is no definitively "safe" dose of alcohol for all pregnancies. Determining these other factors, as well as mechanistically characterizing known interactions, is critical for better understanding and preventing FASD and requires combined scrutiny of human and model organism studies.

Investigating the effects of compound paralogous EPHB receptor mutations on mouse facial development

BACKGROUND

Variation in facial shape may arise from the combinatorial or overlapping actions of paralogous genes. Given its many members, and overlapping expression and functions, the EPH receptor family is a compelling candidate source of craniofacial morphological variation. We performed a detailed morphometric analysis of an allelic series of E14.5 Ephb1-3 receptor mutants to determine the effect of each paralogous receptor gene on craniofacial morphology.

RESULTS

We found that Ephb1, Ephb2, and Ephb3 genotypes significantly influenced facial shape, but Ephb1 effects were weaker than Ephb2 and Ephb3 effects. Ephb2^-/- and Ephb3^-/- mutations affected similar aspects of facial morphology, but Ephb3^-/- mutants had additional facial shape effects. Craniofacial differences across the allelic series were largely consistent with predicted additive genetic effects. However, we identified a potentially important nonadditive effect where Ephb1 mutants displayed different morphologies depending on the combination of other Ephb paralogs present, where Ephb1^+/- , Ephb1^-/- , and Ephb1^-/- ; Ephb3^-/- mutants exhibited a consistent deviation from their predicted facial shapes.

CONCLUSIONS

This study provides a detailed assessment of the effects of Ephb receptor gene paralogs on E14.5 mouse facial morphology and demonstrates how the loss of specific receptors contributes to facial dysmorphology.

Beyond the beak: Brain size and allometry in avian craniofacial evolution

Birds exhibit an enormous variety of beak shapes. Such remarkable variation, however, has distracted research from other important aspects of their skull evolution, the nature of which has been little explored. Key aspects of avian skull variation appear to be qualitatively similar to those of mammals, encompassing variation in the degree of cranial vaulting, cranial base flexure, and the proportions and orientations of the occipital and facial regions. The evolution of these traits has been studied intensively in mammals under the Spatial Packing Hypothesis (SPH), an architectural constraint so-called because the general anatomical organization and development of such skull parts makes them evolve predictably in response to changes in relative brain size. Such SPH predictions account for the different appearances of skull configurations across species, either in having longer or shorter faces, and caudally or ventrally oriented occiputs, respectively. This pattern has been morphometrically and experimentally proven in mammals but has not been examined in birds or other tetrapods, and so its generality remains unknown. We explored the SPH in an interspecific sample of birds using three-dimensional geometric morphometrics. Our results show that the dominant trend of evolutionary variation in the skull of crown-group birds can be predicted by the SPH, involving concomitant changes in the face, the cranial vault and the basicranium, and with striking similarities to craniofacial variation among mammals. Although craniofacial variation is significantly affected by allometry, these allometric effects are independent of the influence of the SPH on skull morphology, as are any effects of volumetric encephalization. Our results, therefore, validate the hypothesis that a general architectural constraint underlies skull homoplasy evolution of cranial morphology among avian clades, and possibly between birds and mammals, but they downplay encephalization and allometry as the only factors involved.

Environmental Thermal Stress Induces Neuronal Cell Death and Developmental Malformations in Reptiles

Every stage of organismal life history is being challenged by global warming. Many species are already experiencing temperatures approaching their physiological limits; this is particularly true for ectothermic species, such as lizards. Embryos are markedly sensitive to thermal insult. Here, we demonstrate that temperatures currently experienced in natural nesting areas can modify gene expression levels and induce neural and craniofacial malformations in embryos of the lizard Anolis sagrei. Developmental abnormalities ranged from minor changes in facial structure to significant disruption of anterior face and forebrain. The first several days of postoviposition development are particularly sensitive to this thermal insult. These results raise new concern over the viability of ectothermic species under contemporary climate change. Herein, we propose and test a novel developmental hypothesis that describes the cellular and developmental origins of those malformations: cell death in the developing forebrain and abnormal facial induction due to disrupted Hedgehog signaling. Based on similarities in the embryonic response to thermal stress among distantly related species, we propose that this developmental hypothesis represents a common embryonic response to thermal insult among amniote embryos. Our results emphasize the importance of adopting a broad, multidisciplinary approach that includes both lab and field perspectives when trying to understand the future impacts of anthropogenic change on animal development.

Ciliary Hedgehog signaling regulates cell survival to build the facial midline

Craniofacial defects are among the most common phenotypes caused by ciliopathies, yet the developmental and molecular etiology of these defects is poorly understood. We investigated multiple mouse models of human ciliopathies (including Tctn2, Cc2d2a, and Tmem231 mutants) and discovered that each displays hypotelorism, a narrowing of the midface. As early in development as the end of gastrulation, Tctn2 mutants displayed reduced activation of the Hedgehog (HH) pathway in the prechordal plate, the head organizer. This prechordal plate defect preceded a reduction of HH pathway activation and Shh expression in the adjacent neurectoderm. Concomitant with the reduction of HH pathway activity, Tctn2 mutants exhibited increased cell death in the neurectoderm and facial ectoderm, culminating in a collapse of the facial midline. Enhancing HH signaling by decreasing the gene dosage of a negative regulator of the pathway, Ptch1, decreased cell death and rescued the midface defect in both Tctn2 and Cc2d2a mutants. These results reveal that ciliary HH signaling mediates communication between the prechordal plate and the neurectoderm to provide cellular survival cues essential for development of the facial midline.

The neurodevelopmental basis of schizophrenia: clinical clues from craniofacial dysmorphology in northwest Ethiopia, 2020

Background

The neurodevelopmental speculation of schizophrenia states that the pathogenesis of schizophrenia starts with early fetal or neonatal neurocraniofacial development rather than youthful adulthood when manic signs and symptoms are evident. However, there is no direct evidence of a pre-or peri-natal lesion associated with schizophrenia, rather indirect evidence of impaired development can be seen in macroscopic anatomical variations as well as microscopic immunohistochemical anomalies. One approach to studying neurodevelopmental disturbances among schizophrenic patients is somatic physical evidence or neurodevelopmental markers. Thus Our study aimed to assess the neurodevelopmental basis of schizophrenia clinical clues from anthropometric assessment of craniofacial dysmorphology among schizophrenic patients in North West Ethiopia 2019–2020.

Method

Institutional-based comparative cross-sectional study design was conducted in Debre Markos comprehensive specialized hospitals in 190 schizophrenic patients, 190 1st-degree relatives, and 190 healthy controls. Data were collected using standard methods, entered into EpiData version 3.1, and exports to SPSS version 24 for analysis. Descriptive data were analyzed using descriptive statistics. Welch ANOVA and post hoc comparison, a Games-Howell test, were conducted. Significance was set at a p-value of α = 0.05. Read back analysis was also conducted for the conclusion.

Results

Five hundred seventy study samples, male 375(65.8%), and female 195 (34.2%), were included in this study. The Games-Howell test revealed that the coronal arc length and sagittal arc length among schizophrenic patients were statistically significantly longer than the healthy controls (p < 0.006; p < 0.001, respectively). However, the difference between schizophrenic and healthy control regarding head circumference was marginally significant (p = 0.056). Schizophrenic patients had a significantly shorter total facial height (p < 0.001) and upper facial height (p < 0.001) than healthy controls. Regarding facial depth, schizophrenic patients had significantly shallow upper facial depth (p < 0.001), middle facial depth (p = 0.046), and lower facial depth (p < 0.001).

Conclusion

our finding indicated indirect evidence for disturbed craniofacial development in schizophrenia patients, and close and read back analysis of the result supported the neurodevelopmental basis of disease.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12868-021-00663-y.

Exposure to ethanol leads to midfacial hypoplasia in a zebrafish model of FASD via indirect interactions with the Shh pathway

Background

Gene-environment interactions are likely to underlie most human birth defects. The most common known environmental contributor to birth defects is prenatal alcohol exposure. Fetal alcohol spectrum disorders (FASD) describe the full range of defects that result from prenatal alcohol exposure. Gene-ethanol interactions underlie susceptibility to FASD, but we lack a mechanistic understanding of these interactions. Here, we leverage the genetic tractability of zebrafish to address this problem.

Results

We first show that vangl2, a member of the Wnt/planar cell polarity (Wnt/PCP) pathway that mediates convergent extension movements, strongly interacts with ethanol during late blastula and early gastrula stages. Embryos mutant or heterozygous for vangl2 are sensitized to ethanol-induced midfacial hypoplasia. We performed single-embryo RNA-seq during early embryonic stages to assess individual variation in the transcriptional response to ethanol and determine the mechanism of the vangl2-ethanol interaction. To identify the pathway(s) that are disrupted by ethanol, we used these global changes in gene expression to identify small molecules that mimic the effects of ethanol via the Library of Integrated Network-based Cellular Signatures (LINCS L1000) dataset. Surprisingly, this dataset predicted that the Sonic Hedgehog (Shh) pathway inhibitor, cyclopamine, would mimic the effects of ethanol, despite ethanol not altering the expression levels of direct targets of Shh signaling. Indeed, we found that ethanol and cyclopamine strongly, but indirectly, interact to disrupt midfacial development. Ethanol also interacts with another Wnt/PCP pathway member, gpc4, and a chemical inhibitor of the Wnt/PCP pathway, blebbistatin, phenocopies the effect of ethanol. By characterizing membrane protrusions, we demonstrate that ethanol synergistically interacts with the loss of vangl2 to disrupt cell polarity required for convergent extension movements.

Conclusions

Our results show that the midfacial defects in ethanol-exposed vangl2 mutants are likely due to an indirect interaction between ethanol and the Shh pathway. Vangl2 functions as part of a signaling pathway that regulates coordinated cell movements during midfacial development. Ethanol exposure alters the position of a critical source of Shh signaling that separates the developing eye field into bilateral eyes, allowing the expansion of the midface. Collectively, our results shed light on the mechanism by which the most common teratogen can disrupt development.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01062-9.

Shared heritability of human face and brain shape

Evidence from model organisms and clinical genetics suggests coordination between the developing brain and face, but the role of this link in common genetic variation remains unknown. We performed a multivariate genome-wide association study of cortical surface morphology in 19,644 individuals of European ancestry, identifying 472 genomic loci influencing brain shape, of which 76 are also linked to face shape. Shared loci include transcription factors involved in craniofacial development, as well as members of signaling pathways implicated in brain-face cross-talk. Brain shape heritability is equivalently enriched near regulatory regions active in either forebrain organoids or facial progenitors. However, we do not detect significant overlap between shared brain-face genome-wide association study signals and variants affecting behavioral-cognitive traits. These results suggest that early in embryogenesis, the face and brain mutually shape each other through both structural effects and paracrine signaling, but this interplay may not impact later brain development associated with cognitive function.

Wnt Signaling Drives Correlated Changes in Facial Morphology and Brain Shape

Canonical Wnt signaling plays multiple roles critical to normal craniofacial development while its dysregulation is known to be involved in structural birth defects of the face. However, when and how Wnt signaling influences phenotypic variation, including those associated with disease, remains unclear. One potential mechanism is via Wnt signaling’s role in the patterning of an early facial signaling center, the frontonasal ectodermal zone (FEZ), and its subsequent regulation of early facial morphogenesis. For example, Wnt signaling may directly alter the shape and/or magnitude of expression of the sonic hedgehog (SHH) domain in the FEZ. To test this idea, we used a replication-competent avian sarcoma retrovirus (RCAS) encoding Wnt3a to modulate its expression in the facial mesenchyme. We then quantified and compared ontogenetic changes in treated to untreated embryos in the three-dimensional (3D) shape of both the SHH expression domain of the FEZ, and the morphology of the facial primordia and brain using iodine-contrast microcomputed tomography imaging and 3D geometric morphometrics (3DGM). We found that increased Wnt3a expression in early stages of head development produces correlated variation in shape between both structural and signaling levels of analysis. In addition, altered Wnt3a activation disrupted the integration between the forebrain and other neural tube derivatives. These results show that activation of Wnt signaling influences facial shape through its impact on the forebrain and SHH expression in the FEZ, and highlights the close relationship between morphogenesis of the forebrain and midface.

Mutation in the Ciliary Protein C2CD3 Reveals Organ-Specific Mechanisms of Hedgehog Signal Transduction in Avian Embryos

Primary cilia are ubiquitous microtubule-based organelles that serve as signaling hubs for numerous developmental pathways, most notably the Hedgehog (Hh) pathway. Defects in the structure or function of primary cilia result in a class of diseases called ciliopathies. It is well known that primary cilia participate in transducing a Hh signal, and as such ciliopathies frequently present with phenotypes indicative of aberrant Hh function. Interestingly, the exact mechanisms of cilia-dependent Hh signaling transduction are unclear as some ciliopathic animal models simultaneously present with gain-of-Hh phenotypes in one organ system and loss-of-Hh phenotypes in another. To better understand how Hh signaling is perturbed across different tissues in ciliopathic conditions, we examined four distinct Hh-dependent signaling centers in the naturally occurring avian ciliopathic mutant talpid² (ta²). In addition to the well-known and previously reported limb and craniofacial malformations, we observed dorsal-ventral patterning defects in the neural tube, and a shortened gastrointestinal tract. Molecular analyses for elements of the Hh pathway revealed that the loss of cilia impact transduction of an Hh signal in a tissue-specific manner at variable levels of the pathway. These studies will provide increased knowledge into how impaired ciliogenesis differentially regulates Hh signaling across tissues and will provide potential avenues for future targeted therapeutic treatments.

Physiological electric fields induce directional migration of mammalian cranial neural crest cells

During neurulation, cranial neural crest cells (CNCCs) migrate long distances from the neural tube to their terminal site of differentiation. The pathway traveled by the CNCCs defines the blueprint for craniofacial construction, abnormalities of which contribute to three-quarters of human birth defects. Biophysical cues like naturally occurring electric fields (EFs) have been proposed to be one of the guiding mechanisms for CNCC migration from the neural tube to identified position in the branchial arches. Such endogenous EFs can be mimicked by applied EFs of physiological strength that has been reported to guide the migration of amphibian and avian neural crest cells (NCCs), namely galvanotaxis or electrotaxis. However, the behavior of mammalian NCCs in external EFs has not been reported. We show here that mammalian CNCCs migrate towards the anode in direct current (dc) EFs. Reversal of the field polarity reverses the directedness. The response threshold was below 30 ​mV/mm and the migration directedness and displacement speed increased with increase in field strength. Both CNCC line (O9-1) and primary mouse CNCCs show similar galvanotaxis behavior. Our results demonstrate for the first time that the mammalian CNCCs respond to physiological EFs by robust directional migration towards the anode in a voltage-dependent manner.

Congenital Malformations in Sea Turtles: Puzzling Interplay between Genes and Environment

Simple Summary

Congenital malformations can lead to embryonic mortality in many species, and sea turtles are no exception. Genetic and/or environmental alterations occur during early development in the embryo, and may produce aberrant phenotypes, many of which are incompatible with life. Causes of malformations are multifactorial; genetic factors may include mutations, chromosomal aberrations, and inbreeding effects, whereas non-genetic factors may include nutrition, hyperthermia, low moisture, radiation, and contamination. It is possible to monitor and control some of these factors (such as temperature and humidity) in nesting beaches, and toxic compounds in feeding areas, which can be transferred to the embryo through their lipophilic properties. In this review, we describe possible causes of different types of malformations observed in sea turtle embryos, as well as some actions that may help reduce embryonic mortality.

Abstract

The completion of embryonic development depends, in part, on the interplay between genetic factors and environmental conditions, and any alteration during development may affect embryonic genetic and epigenetic regulatory pathways leading to congenital malformations, which are mostly incompatible with life. Oviparous reptiles, such as sea turtles, that produce numerous eggs in a clutch that is buried on the beach provide an opportunity to study embryonic mortality associated with malformations that occur at different times during development, or that prevent the hatchling from emerging from the nest. In sea turtles, the presence of congenital malformations frequently leads to mortality. A few years ago, a detailed study was performed on external congenital malformations in three species of sea turtles from the Mexican Pacific and Caribbean coasts, the hawksbill turtle, Eretmochelys imbricata (n = 23,559 eggs), the green turtle, Chelonia mydas (n = 17,690 eggs), and the olive ridley, Lepidochelys olivacea (n = 20,257 eggs), finding 63 types of congenital malformations, of which 38 were new reports. Of the three species, the olive ridley showed a higher incidence of severe anomalies in the craniofacial region (49%), indicating alterations of early developmental pathways; however, several malformations were also observed in the body, including defects in the carapace (45%) and limbs (33%), as well as pigmentation disorders (20%), indicating that deviations occurred during the middle and later stages of development. Although intrinsic factors (i.e., genetic mutations or epigenetic modifications) are difficult to monitor in the field, some environmental factors (such as the incubation temperature, humidity, and probably the status of feeding areas) are, to some extent, less difficult to monitor and/or control. In this review, we describe the aetiology of different malformations observed in sea turtle embryos, and provide some actions that can reduce embryonic mortality.

Insights Into the Complexity of Craniofacial Development From a Cellular Perspective

The head represents the most complex part of the body and a distinctive feature of the vertebrate body plan. This intricate structure is assembled during embryonic development in the four-dimensional process of morphogenesis. The head integrates components of the central and peripheral nervous system, sensory organs, muscles, joints, glands, and other specialized tissues in the framework of a complexly shaped skull. The anterior part of the head is referred to as the face, and a broad spectrum of facial shapes across vertebrate species enables different feeding strategies, communication styles, and diverse specialized functions. The face formation starts early during embryonic development and is an enormously complex, multi-step process regulated on a genomic, molecular, and cellular level. In this review, we will discuss recent discoveries that revealed new aspects of facial morphogenesis from the time of the neural crest cell emergence till the formation of the chondrocranium, the primary design of the individual facial shape. We will focus on molecular mechanisms of cell fate specification, the role of individual and collective cell migration, the importance of dynamic and continuous cellular interactions, responses of cells and tissues to generated physical forces, and their morphogenetic outcomes. In the end, we will examine the spatiotemporal activity of signaling centers tightly regulating the release of signals inducing the formation of craniofacial skeletal elements. The existence of these centers and their regulation by enhancers represent one of the core morphogenetic mechanisms and might lay the foundations for intra- and inter-species facial variability.

The hedgehog co-receptor BOC differentially regulates SHH signaling during craniofacial development

The Hedgehog (HH) pathway controls multiple aspects of craniofacial development. HH ligands signal through the canonical receptor PTCH1, and three co-receptors: GAS1, CDON and BOC. Together, these co-receptors are required during embryogenesis to mediate proper HH signaling. Here, we investigated the individual and combined contributions of GAS1, CDON and BOC to HH-dependent mammalian craniofacial development. Notably, individual deletion of either Gas1 or Cdon results in variable holoprosencephaly phenotypes in mice, even on a congenic background. In contrast, we find that Boc deletion results in facial widening that correlates with increased HH target gene expression. In addition, Boc deletion in a Gas1 null background partially ameliorates the craniofacial defects observed in Gas1 single mutants; a phenotype that persists over developmental time, resulting in significant improvements to a subset of craniofacial structures. This contrasts with HH-dependent phenotypes in other tissues that significantly worsen following combined deletion of Gas1 and Boc Together, these data indicate that BOC acts as a multi-functional regulator of HH signaling during craniofacial development, alternately promoting or restraining HH pathway activity in a tissue-specific fashion.

Why Does the Face Predict the Brain? Neural Crest Induction, Craniofacial Morphogenesis, and Neural Circuit Development

Mesenchephalic and rhombencephalic neural crest cells generate the craniofacial skeleton, special sensory organs, and subsets of cranial sensory receptor neurons. They do so while preserving the anterior-posterior (A-P) identity of their neural tube origins. This organizational principle is paralleled by central nervous system circuits that receive and process information from facial structures whose A-P identity is in register with that in the brain. Prior to morphogenesis of the face and its circuits, however, neural crest cells act as "inductive ambassadors" from distinct regions of the neural tube to induce differentiation of target craniofacial domains and establish an initial interface between the brain and face. At every site of bilateral, non-axial secondary induction, neural crest constitutes all or some of the mesenchymal compartment for non-axial mesenchymal/epithelial (M/E) interactions. Thus, for epithelial domains in the craniofacial primordia, aortic arches, limbs, the spinal cord, and the forebrain (Fb), neural crest-derived mesenchymal cells establish local sources of inductive signaling molecules that drive morphogenesis and cellular differentiation. This common mechanism for building brains, faces, limbs, and hearts, A-P axis specified, neural crest-mediated M/E induction, coordinates differentiation of distal structures, peripheral neurons that provide their sensory or autonomic innervation in some cases, and central neural circuits that regulate their behavioral functions. The essential role of this neural crest-mediated mechanism identifies it as a prime target for pathogenesis in a broad range of neurodevelopmental disorders. Thus, the face and the brain "predict" one another, and this mutual developmental relationship provides a key target for disruption by developmental pathology.

Multifactorial Genetic and Environmental Hedgehog Pathway Disruption Sensitizes Embryos to Alcohol-Induced Craniofacial Defects

BACKGROUND

Prenatal alcohol exposure (PAE) is perhaps the most common environmental cause of human birth defects. These exposures cause a range of structural and neurological defects, including facial dysmorphologies, collectively known as fetal alcohol spectrum disorders (FASD). While PAE causes FASD, phenotypic outcomes vary widely. It is thought that multifactorial genetic and environmental interactions modify the effects of PAE. However, little is known of the nature of these modifiers. Disruption of the Hedgehog (Hh) signaling pathway has been suggested as a modifier of ethanol teratogenicity. In addition to regulating the morphogenesis of craniofacial tissues commonly disrupted in FASD, a core member of the Hh pathway, Smoothened, is susceptible to modulation by structurally diverse chemicals. These include environmentally prevalent teratogens like piperonyl butoxide (PBO), a synergist found in thousands of pesticide formulations.

METHODS

Here, we characterize multifactorial genetic and environmental interactions using a zebrafish model of craniofacial development.

RESULTS

We show that loss of a single allele of shha sensitized embryos to both alcohol- and PBO-induced facial defects. Co-exposure of PBO and alcohol synergized to cause more frequent and severe defects. The effects of this co-exposure were even more profound in the genetically susceptible shha heterozygotes.

CONCLUSIONS

Together, these findings shed light on the multifactorial basis of alcohol-induced craniofacial defects. In addition to further implicating genetic disruption of the Hh pathway in alcohol teratogenicity, our findings suggest that co-exposure to environmental chemicals that perturb Hh signaling may be important variables in FASD and related craniofacial disorders.

Disrupted Hypothalamo-Pituitary Axis in Association With Reduced SHH Underlies the Pathogenesis of NOTCH-Deficiency

CONTEXT

In human, Sonic hedgehog (SHH) haploinsufficiency is the predominant cause of holoprosencephaly, a structural malformation of the forebrain midline characterized by phenotypic heterogeneity and incomplete penetrance. The NOTCH signaling pathway has recently been associated with holoprosencephaly in humans, but the precise mechanism involving NOTCH signaling during early brain development remains unknown.

OBJECTIVE

The aim of this study was to evaluate the relationship between SHH and NOTCH signaling to determine the mechanism by which NOTCH dysfunction could cause midline malformations of the forebrain.

DESIGN

In this study, we have used a chemical inhibition approach in the chick model and a genetic approach in the mouse model. We also reported results obtained from the clinical diagnosis of a cohort composed of 141 holoprosencephaly patients.

RESULTS

We demonstrated that inhibition of NOTCH signaling in chick embryos as well as in mouse embryos induced a specific downregulation of SHH in the anterior hypothalamus. Our data in the mouse also revealed that the pituitary gland was the most sensitive tissue to Shh insufficiency and that haploinsufficiency of the SHH and NOTCH signaling pathways synergized to produce a malformed pituitary gland. Analysis of a large holoprosencephaly cohort revealed that some patients possessed multiple heterozygous mutations in several regulators of both pathways.

CONCLUSIONS

These results provided new insights into molecular mechanisms underlying the extreme phenotypic variability observed in human holoprosencephaly. They showed how haploinsufficiency of the SHH and NOTCH activity could contribute to specific congenital hypopituitarism that was associated with a sella turcica defect.

miR-199 family contributes to regulation of sonic hedgehog expression during craniofacial development

BACKGROUND

The frontonasal ectodermal zone (FEZ) is a signaling center that regulates patterned development of the upper jaw, and Sonic hedgehog (SHH) mediates FEZ activity. Induction of SHH expression in the FEZ results from SHH-dependent signals from the brain and neural crest cells. Given the role of miRNAs in modulating gene expression, we investigated the extent to which miRNAs regulate SHH expression and FEZ signaling.

RESULTS

In the FEZ, the miR-199 family appears to be regulated by SHH-dependent signals from the brain; expression of this family increased from HH18 to HH22, and upon activation of SHH signaling in the brain. However, the miR-199 family is more broadly expressed in the mesenchyme of the frontonasal process and adjacent neuroepithelium. Downregulating the miR-199 genes expanded SHH expression in the FEZ, resulting in wider faces, while upregulating miR-199 genes resulted in decreased SHH expression and narrow faces. Hypoxia inducible factor 1 alpha (HIF1A) and mitogen-activated protein kinase kinase kinase 4 (MAP3K4) appear to be potential targets of miR-199b. Reduction of MAP3K4 altered beak development but increased apoptosis, while reducing HIF1A reduced expression of SHH in the FEZ and produced malformations independent of apoptosis.

CONCLUSIONS

Our results demonstrate that this miRNA family appears to participate in regulating SHH expression in the FEZ; however, specific molecular mechanisms remain unknown.

Comparative growth in the olfactory system of the developing chick with considerations for evolutionary studies

Despite the long-held assumption that olfaction plays a relatively minor role in the behavioral ecology of birds, crown-group avians exhibit marked phylogenetic variation in the size and form of the olfactory apparatus. As part of a larger effort to better understand the role of olfaction and olfactory tissues in the evolution and development of the avian skull, we present the first quantitative analysis of ontogenetic scaling between olfactory features [olfactory bulbs (OBs) and olfactory turbinates] and neighboring structures (cerebrum, total brain, respiratory turbinates) based on the model organism Gallus gallus. The OB develops under the predictions of a concerted evolutionary model with rapid early growth that is quickly overcome by the longer, sustained growth of the larger cerebrum. A similar pattern is found in the nasal cavity where the morphologically simple (non-scrolled) olfactory turbinates appear and mature early, with extended growth characterizing the larger and scrolled respiratory turbinates. Pairwise regressions largely recover allometric relationships among the examined structures, with a notable exception being the isometric trajectory of the OB and olfactory turbinate. Their parallel growth suggests a unique regulatory pathway that is likely driven by the morphogenesis of the olfactory nerve, which serves as a structural bridge between the two features. Still, isometry was not necessarily expected given that the olfactory epithelium covers more than just the turbinate. These data illuminate a number of evolutionary hypotheses that, moving forward, should inform tradeoffs and constraints between the olfactory and neighboring systems in the avian head.

Is it possible to recognize children diagnosed with ADHD from their facial anthropometric measures? A case-control study

The recent main focus of the researches on Attention-deficit Hyperactivity Disorder is on identifying behavioral phenotypes. For this purpose, neuroanatomical factors have recently become a focus. This study aimed to investigate whether the individuals diagnosed with Attention-deficit Hyperactivity Disorder differ from healthy individuals in terms of facial anthropometric measurements. Forty children, diagnosed with Attention-deficit Hyperactivity Disorder, were included in the study as the case group, and forty healthy children were included in the study as the control group. Two photographs were taken from the facial region, and anthropometric measurements were performed using the computer program "Image J" in the computer environment. It was found that a strong relationship between Attention-deficit Hyperactivity Disorder and nasal width, ear length and upper face debt length. The results obtained from the research support the knowledge that there is a close relationship between the forebrain development process and the facial development process during the embryonic development process.

SHH Signaling Pathway Drives Pediatric Bone Sarcoma Progression

Primary bone tumors can be divided into two classes, benign and malignant. Among the latter group, osteosarcoma and Ewing sarcoma are the most prevalent malignant primary bone tumors in children and adolescents. Despite intensive efforts to improve treatments, almost 40% of patients succumb to the disease. Specifically, the clinical outcome for metastatic osteosarcoma or Ewing sarcoma remains poor; less than 30% of patients who present metastases will survive 5 years after initial diagnosis. One common and specific point of these bone tumors is their ability to deregulate bone homeostasis and remodeling and divert them to their benefit. Over the past years, considerable interest in the Sonic Hedgehog (SHH) pathway has taken place within the cancer research community. The activation of this SHH cascade can be done through different ways and, schematically, two pathways can be described, the canonical and the non-canonical. This review discusses the current knowledge about the involvement of the SHH signaling pathway in skeletal development, pediatric bone sarcoma progression and the related therapeutic options that may be possible for these tumors.

Aberrant cell segregation in the craniofacial primordium and the emergence of facial dysmorphology in craniofrontonasal syndrome

Craniofrontonasal syndrome (CFNS) is a rare X-linked disorder characterized by craniofacial, skeletal, and neurological anomalies and is caused by mutations in EFNB1. Heterozygous females are more severely affected by CFNS than hemizygous males, a phenomenon called cellular interference that results from EPHRIN-B1 mosaicism. In Efnb1 heterozygous mice, mosaicism for EPHRIN-B1 results in cell sorting and more severe phenotypes than Efnb1 hemizygous males, but how craniofacial dysmorphology arises from cell segregation is unknown and CFNS etiology therefore remains poorly understood. Here, we couple geometric morphometric techniques with temporal and spatial interrogation of embryonic cell segregation in mouse mutant models to elucidate mechanisms underlying CFNS pathogenesis. By generating EPHRIN-B1 mosaicism at different developmental timepoints and in specific cell populations, we find that EPHRIN-B1 regulates cell segregation independently in early neural development and later in craniofacial development, correlating with the emergence of quantitative differences in face shape. Whereas specific craniofacial shape changes are qualitatively similar in Efnb1 heterozygous and hemizygous mutant embryos, heterozygous embryos are quantitatively more severely affected, indicating that Efnb1 mosaicism exacerbates loss of function phenotypes rather than having a neomorphic effect. Notably, neural tissue-specific disruption of Efnb1 does not appear to contribute to CFNS craniofacial dysmorphology, but its disruption within neural crest cell-derived mesenchyme results in phenotypes very similar to widespread loss. EPHRIN-B1 can bind and signal with EPHB1, EPHB2, and EPHB3 receptor tyrosine kinases, but the signaling partner(s) relevant to CFNS are unknown. Geometric morphometric analysis of an allelic series of Ephb1; Ephb2; Ephb3 mutant embryos indicates that EPHB2 and EPHB3 are key receptors mediating Efnb1 hemizygous-like phenotypes, but the complete loss of EPHB1-3 does not fully recapitulate the severity of CFNS-like Efnb1 heterozygosity. Finally, by generating Efnb1+/Δ; Ephb1; Ephb2; Ephb3 quadruple knockout mice, we determine how modulating cumulative receptor activity influences cell segregation in craniofacial development and find that while EPHB2 and EPHB3 play an important role in craniofacial cell segregation, EPHB1 is more important for cell segregation in the brain; surprisingly, complete loss of EPHB1-EPHB3 does not completely abrogate cell segregation. Together, these data advance our understanding of the etiology and signaling interactions underlying CFNS dysmorphology.

Lobar holoprosencephaly with craniofacial defects in a Friesian calf: A case report

BACKGROUND

Holoprosencephaly is a forebrain deformity that results from varying degrees of separation failure of cerebral hemispheres. The condition is classified based on the degree of non-separation of the hemispheres which, in turn, determines its severity. Holoprosencephaly is usually accompanied by craniofacial defects whose severity tends to reflect the extent of brain deformities. In humans, holoprosencephaly is one of the commonest congenital brain anomalies but in animals, reported cases are scarce. The condition has multifactorial aetiology that involves interactions between several genetic and environmental factors.

CASE PRESENTATION

A 4-day-old female Friesian calf with a deformed face was reported to the Faculty of veterinary medicine and surgery, Egerton University. The calf and the dam were sired by the same bull. On clinical and radiographic examination, the calf had a short snout that curved dorsally with bilateral cleft lip, right-sided cleft jaw and a largely absent primary palate. Anatomopathological examination revealed brain deformities which included ventral fusion of frontal lobes of cerebral hemispheres, large merged lateral ventricles without septum pellucidum and fornix, hypoplastic corpus callosum, high degree of non-separation between diencephalic structures, poorly developed hippocampal formation and hypoplastic olfactory lobe, optic chiasma, and nerve.

CONCLUSION

The case was confirmed as lobar holoprosencephaly based on characteristic anatomopathological findings. The aetiology of the defects in the present case could not be determined though they are thought to be either a result of recessive inheritance or exposure to teratogenic steroid alkaloids through materials fed to the dam during early pregnancy.

Finding the Unicorn, a New Mouse Model of Midfacial Clefting

Human midfacial clefting is a rare subset of orofacial clefting and in severe cases, the cleft separates the nostrils splitting the nose into two independent structures. To begin to understand the morphological and genetic causes of midfacial clefting we recovered the Unicorn mouse line. Unicorn embryos develop a complete midfacial cleft through the lip, and snout closely modelling human midfacial clefting. The Unicorn mouse line has ethylnitrosourea (ENU)-induced missense mutations in Raldh2 and Leo1. The mutations segregate with the cleft face phenotype. Importantly, the nasal cartilages and surrounding bones are patterned and develop normal morphology, except for the lateral displacement because of the cleft. We conclude that the midfacial cleft arises from the failure of the medial convergence of the paired medial nasal prominences between E10.5 to E11.5 rather than defective cell proliferation and death. Our work uncovers a novel mouse model and mechanism for the etiology of midfacial clefting.

Bony Patchwork: Mosaic Patterns of Evolution in the Skull of Electric Fishes (Apteronotidae: Gymnotiformes)

Mosaic evolution refers to the pattern whereby different organismal traits exhibit differential rates of evolution typically due to reduced levels of trait covariation through deep time (i.e., modularity). These differences in rates can be attributed to variation in responses to selective pressures between individual traits. Differential responses to selective pressures also have the potential to facilitate functional specialization, allowing certain traits to track environmental stimuli more closely than others. The teleost skull is a multifunctional structure comprising a complex network of bones and thus an excellent system for which to study mosaic evolution. Here we construct an ultrametric phylogeny for a clade of Neotropical electric fishes (Apteronotidae: Gymnotiformes) and use three-dimensional geometric morphometrics to investigate patterns of mosaic evolution in the skull and jaws. We find strong support for a developmental, three-module hypothesis that consists of the face, braincase, and mandible, and we find that the mandible has evolved four times faster than its neighboring modules. We hypothesize that the functional specialization of the mandible in this group of fishes has allowed it to outpace the face and braincase and evolve in a more decoupled manner. We also hypothesize that this pattern of mosaicism may be widespread across other clades of teleost fishes.

A forebrain undivided: Unleashing model organisms to solve the mysteries of holoprosencephaly

Evolutionary conservation and experimental tractability have made animal model systems invaluable tools in our quest to understand human embryogenesis, both normal and abnormal. Standard genetic approaches, particularly useful in understanding monogenic diseases, are no longer sufficient as research attention shifts toward multifactorial outcomes. Here, we examine this progression through the lens of holoprosencephaly (HPE), a common human malformation involving incomplete forebrain division, and a classic example of an etiologically complex outcome. We relate the basic underpinning of HPE pathogenesis to critical cell-cell interactions and signaling molecules discovered through embryological and genetic approaches in multiple model organisms, and discuss the role of the mouse model in functional examination of HPE-linked genes. We then outline the most critical remaining gaps to understanding human HPE, including the conundrum of incomplete penetrance/expressivity and the role of gene-environment interactions. To tackle these challenges, we outline a strategy that leverages new and emerging technologies in multiple model systems to solve the puzzle of HPE.

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.

Are endocasts good proxies for brain size and shape in archosaurs throughout ontogeny?

Cranial endocasts, or the internal molds of the braincase, are a crucial correlate for investigating the neuroanatomy of extinct vertebrates and tracking brain evolution through deep time. Nevertheless, the validity of such studies pivots on the reliability of endocasts as a proxy for brain morphology. Here, we employ micro-computed tomography imaging, including diffusible iodine-based contrast-enhanced CT, and a three-dimensional geometric morphometric framework to examine both size and shape differences between brains and endocasts of two exemplar archosaur taxa - the American alligator (Alligator mississippiensis) and the domestic chicken (Gallus gallus). With ontogenetic sampling, we quantitatively evaluate how endocasts differ from brains and whether this deviation changes during development. We find strong size and shape correlations between brains and endocasts, divergent ontogenetic trends in the brain-to-endocast correspondence between alligators and chickens, and a comparable magnitude between brain-endocast shape differences and intraspecific neuroanatomical variation. The results have important implications for paleoneurological studies in archosaurs. Notably, we demonstrate that the pattern of endocranial shape variation closely reflects brain shape variation. Therefore, analyses of endocranial morphology are unlikely to generate spurious conclusions about large-scale trends in brain size and shape. To mitigate any artifacts, however, paleoneurological studies should consider the lower brain-endocast correspondence in the hindbrain relative to the forebrain; higher size and shape correspondences in chickens than alligators throughout postnatal ontogeny; artificially 'pedomorphic' shape of endocasts relative to their corresponding brains; and potential biases in both size and shape data due to the lack of control for ontogenetic stages in endocranial sampling.

Increased Risk for Neurodevelopmental Disorders in Children With Orofacial Clefts

OBJECTIVE

Children with orofacial clefts (OFC) may have an increased risk of poor mental health. This study aimed to investigate the risk of psychiatric diagnoses in individuals with OFC, stratified by cleft type.

METHOD

A nationwide register-based cohort of all individuals born with nonsyndromic OFC in Sweden between 1973 and 2012 (n = 7,842) was compared to a matched cohort (n = 78,409) as well as to their unaffected siblings (n = 9,637). The risk of psychiatric diagnoses, suicide attempts, and suicides was examined by crude and adjusted Cox regression models. Effect modification by sex was investigated with interaction terms in the models.

RESULTS

Children with cleft lip (CL) had a significantly higher risk of any psychiatric disorder, intellectual disability, and language disorders; children with cleft lip and palate (CLP) had, in addition, an increased risk of autism spectrum disorder (ASD). Children with cleft palate only (CPO) had risk increases for the same diagnoses as children with CL and CLP, but with higher hazard ratios, and also for psychotic disorders, attention-deficit/hyperactivity disorder (ADHD), and other behavioral or emotional disorders in childhood. Sex stratification indicated higher risk increases among females in CL and CLP but not in CPO. Siblings without OFC were less likely to be diagnosed with any psychiatric disorder, intellectual disability, language disorder, ASD, or ADHD compared to their siblings with OFC.

CONCLUSION

Children with nonsyndromic clefts had a significantly higher risk of neurodevelopmental disorders. This risk is unlikely to be explained by familial influences such as inherited genetic or shared environmental factors.

FoxO6 regulates Hippo signaling and growth of the craniofacial complex

The mechanisms that regulate post-natal growth of the craniofacial complex and that ultimately determine the size and shape of our faces are not well understood. Hippo signaling is a general mechanism to control tissue growth and organ size, and although it is known that Hippo signaling functions in neural crest specification and patterning during embryogenesis and before birth, its specific role in postnatal craniofacial growth remains elusive. We have identified the transcription factor FoxO6 as an activator of Hippo signaling regulating neonatal growth of the face. During late stages of mouse development, FoxO6 is expressed specifically in craniofacial tissues and FoxO6-/- mice undergo expansion of the face, frontal cortex, olfactory component and skull. Enlargement of the mandible and maxilla and lengthening of the incisors in FoxO6-/- mice are associated with increases in cell proliferation. In vitro and in vivo studies demonstrated that FoxO6 activates Lats1 expression, thereby increasing Yap phosphorylation and activation of Hippo signaling. FoxO6-/- mice have significantly reduced Hippo Signaling caused by a decrease in Lats1 expression and decreases in Shh and Runx2 expression, suggesting that Shh and Runx2 are also linked to Hippo signaling. In vitro, FoxO6 activates Hippo reporter constructs and regulates cell proliferation. Furthermore PITX2, a regulator of Hippo signaling is associated with Axenfeld-Rieger Syndrome causing a flattened midface and we show that PITX2 activates FoxO6 expression. Craniofacial specific expression of FoxO6 postnatally regulates Hippo signaling and cell proliferation. Together, these results identify a FoxO6-Hippo regulatory pathway that controls skull growth, odontogenesis and face morphology.

FGF8 and Shh promote the survival and maintenance of multipotent neural crest progenitors

The developmental mechanisms that control the building of the complex head of vertebrates and particularly, facial skeletogenesis, remain poorly known. Progenitor cells derived from the embryonic neural crest (NC) are the major constituents and players of facial tissue development. Deciphering the cellular and molecular machinery that controls NC cell (NCC) differentiation into bone, cartilage, fat and other mesenchymal tissues, is thus a main issue for understanding vertebrate facial variations. In this work, we investigated the effects of fibroblast growth factor 8 (FGF8) and Sonic Hedgehog (Shh), two signaling molecules essential for craniofacial development, on the in vitro differentiation and multipotentiality of mesencephalic NCCs (MNCCs) isolated from the quail embryo. Comparison of distinct temporal treatments with FGF8 and/or Shh showed that both promoted chondrogenesis of MNCCs by increasing the amount and size of cartilage nodules. Higher rates of chondrogenesis were observed when MNCCs were treated with FGF8 during the migration phase, thus mimicking the in vivo exposure of migrating NCCs to FGF8 secreted by the isthmic brain signaling center. An in vitro cell cloning assay revealed that, after concomitant treatment with FGF8 and Shh, about 80% of NC progenitors displayed chondrogenic potential, while in untreated cultures, only 18% exhibited this potential. In addition, colony analysis showed for the first time the existence of a highly multipotent progenitor able to clonally give rise to adipocytes in addition to other cephalic NC phenotypes (i.e. glial cells, neurons, melanocytes, smooth muscle cells and chondrocytes) (GNMFCA progenitor). This progenitor was observed only when clonal cultures were treated with both FGF8 and Shh. Several other types of multipotent cells, which generated four, five or six distinct phenotypes, accounted for 55% of the progenitors in FGF8 and Shh treated cultures, versus 13,5% in the untreated ones. Together, these data reveal an essential role for both FGF8 and Shh together in maintenance of MNCC multipotentiality by favoring the development of NC progenitors endowed with a broad array of mesectodermal potentials.

Identification of sonic hedgehog-regulated genes and biological processes in the cranial neural crest mesenchyme by comparative transcriptomics

Background

The evolutionarily conserved Sonic Hedgehog (Shh) signaling pathway is essential for embryogenesis and orofacial development. SHH ligand secreted from the surface ectoderm activates pathway activity in the underlying cranial neural crest cell (cNCC)-derived mesenchyme of the developing upper lip and palate. Disruption of Shh signaling causes orofacial clefts, but the biological action of Shh signaling and the full set of Shh target genes that mediate normal and abnormal orofacial morphogenesis have not been described.

Results

Using comparative transcriptional profiling, we have defined the Shh-regulated genes of the cNCC-derived mesenchyme. Enrichment analysis demonstrated that in cultured cNCCs, Shh-regulated genes are involved in smooth muscle and chondrocyte differentiation, as well as regulation of the Forkhead family of transcription factors, G1/S cell cycle transition, and angiogenesis. Next, this gene set from Shh-activated cNCCs in vitro was compared to the set of genes dysregulated in the facial primordia in vivo during the initial pathogenesis of Shh pathway inhibitor-induced orofacial clefting. Functional gene annotation enrichment analysis of the 112 Shh-regulated genes with concordant expression changes linked Shh signaling to interdependent and unique biological processes including mesenchyme development, cell adhesion, cell proliferation, cell migration, angiogenesis, perivascular cell markers, and orofacial clefting.

Conclusions

We defined the Shh-regulated transcriptome of the cNCC-derived mesenchyme by comparing the expression signatures of Shh-activated cNCCs in vitro to primordial midfacial tissues exposed to the Shh pathway inhibitor in vivo. In addition to improving our understanding of cNCC biology by determining the identity and possible roles of cNCC-specific Shh target genes, this study presents novel candidate genes whose examination in the context of human orofacial clefting etiology is warranted.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4885-5) contains supplementary material, which is available to authorized users.

Neural crest and the origin of species-specific pattern

For well over half of the 150 years since the discovery of the neural crest, the special ability of these cells to function as a source of species-specific pattern has been clearly recognized. Initially, this observation arose in association with chimeric transplant experiments among differentially pigmented amphibians, where the neural crest origin for melanocytes had been duly noted. Shortly thereafter, the role of cranial neural crest cells in transmitting species-specific information on size and shape to the pharyngeal arch skeleton as well as in regulating the timing of its differentiation became readily apparent. Since then, what has emerged is a deeper understanding of how the neural crest accomplishes such a presumably difficult mission, and this includes a more complete picture of the molecular and cellular programs whereby neural crest shapes the face of each species. This review covers studies on a broad range of vertebrates and describes neural-crest-mediated mechanisms that endow the craniofacial complex with species-specific pattern. A major focus is on experiments in quail and duck embryos that reveal a hierarchy of cell-autonomous and non-autonomous signaling interactions through which neural crest generates species-specific pattern in the craniofacial integument, skeleton, and musculature. By controlling size and shape throughout the development of these systems, the neural crest underlies the structural and functional integration of the craniofacial complex during evolution.

Modeling the complex etiology of holoprosencephaly in mice

Holoprosencephaly (HPE) is a common developmental defect caused by failure to define the midline of the forebrain and/or midface. HPE is associated with heterozygous mutations in Nodal and Sonic hedgehog (SHH) pathway components, but clinical presentation is highly variable, and many mutation carriers are unaffected. It is therefore thought that such mutations interact with more common modifiers, genetic and/or environmental, to produce severe patterning defects. Modifiers are difficult to identify, as their effects are context-dependent and occur within the complex genetic and environmental landscapes that characterize human populations. This has made a full understanding of HPE etiology challenging. We discuss here the use of mice, a genetically tractable model sensitive to teratogens, as a system to address this challenge. Mice carrying mutations in human HPE genes often display wide variations in phenotypic penetrance and expressivity when placed on different genetic backgrounds, demonstrating the existence of silent HPE modifier genes. Studies with mouse lines carrying SHH pathway mutations on appropriate genetic backgrounds have led to identification of both genetic and environmental modifiers that synergize with the mutations to produce a spectrum of HPE phenotypes. These models favor a scenario in which multiple modifying influences-both genetic and environmental, sensitizing and protective-interact with bona fide HPE mutations to grade phenotypic outcomes. Despite the complex interplay of HPE risk factors, mouse models have helped establish some clear concepts in HPE etiology. A combination of mouse and human cohort studies should improve our understanding of this fascinating and medically important issue.

Common basis for orofacial clefting and cortical interneuronopathy

Orofacial clefts (OFCs) of the lip and/or palate are among the most common human birth defects. Current treatment strategies focus on functional and cosmetic repair but even when this care is available, individuals born with OFCs are at high risk for persistent neurobehavioral problems. In addition to learning disabilities and reduced academic achievement, recent evidence associates OFCs with elevated risk for a constellation of psychiatric outcomes including anxiety disorders, autism spectrum disorder, and schizophrenia. The relationship between these outcomes and OFCs is poorly understood and controversial. Recent neuroimaging studies in humans and mice demonstrate subtle morphological brain abnormalities that co-occur with OFCs but specific molecular and cellular mechanisms have not been investigated. Here, we provide the first evidence directly linking OFC pathogenesis to abnormal development of GABAergic cortical interneurons (cINs). Lineage tracing revealed that the structures that form the upper lip and palate develop in molecular synchrony and spatiotemporal proximity to cINs, suggesting these populations may have shared sensitivity to genetic and/or teratogenic insult. Examination of cIN development in a mouse model of nonsyndromic OFCs revealed significant disruptions in cIN proliferation and migration, culminating in misspecification of the somatostatin-expressing subgroup. These findings reveal a unified developmental basis for orofacial clefting and disrupted cIN development, and may explain the significant overlap in neurobehavioral and psychiatric outcomes associated with OFCs and cIN dysfunction. This emerging mechanistic understanding for increased prevalence of adverse neurobehavioral outcomes in OFC patients is the entry-point for developing evidence-based therapies to improve patient outcomes.

Academic Achievement Among Children With Nonsyndromic Orofacial Clefts : A Population-Based Study

OBJECTIVE

Children with orofacial clefts (OFCs) may experience poor reading proficiency, learning disabilities, and academic underachievement. We examined the association between nonsyndromic (NS) OFCs and end-of-grade (EOG) performance in reading and math from third through eighth grade in a sample subgroup.

PARTICIPANTS

We identified a cohort of 559 children with NS-OFCs and 6822 children without birth defects, classifying cleft type by cleft lip alone, with or without cleft alveolar ridge (CL); cleft lip with cleft palate (CL+P); and cleft palate only (CP).

MAIN OUTCOME MEASURES

Using logistic regression, we estimated the odds of not meeting grade-level standards among children with NS-OFCs compared to unaffected peers. Using longitudinal analyses, we estimated the odds of not meeting grade-level standards and average change in test scores through eighth grade.

RESULTS

Children with NS-OFCs were 1.22 (95% CI: 0.96, 1.83) times as likely not to meet grade-level standards in reading compared to unaffected peers. The effect was similar for math (OR: 1.17; 95% CI: 0.92, 1.48). Children with CL+P were 1.33 (95% CI: 0.86, 1.83) and 1.74 (95% CI: 1.19, 2.56) times as likely not to meet grade-level standard in reading and in both subjects, respectively, compared to unaffected peers. The average rate of change in both scores was similar for children with and without OFCs.

CONCLUSIONS

Poor academic performance appears greatest for children with CL+P, a finding compatible with previous observations and hypothesized mechanisms associating orofacial clefts with subtle abnormalities in brain development. Academic performance monitoring and referral for academic assistance is warranted.