Role of Dlx genes in craniofacial morphogenesis: Dlx2 influences skeletal patterning by inducing ectomesenchymal aggregation in ovo

Propineb induced notochord deformity, craniofacial malformation, and osteoporosis in zebrafish through dysregulated reactive oxygen species generation

Dithiocarbamate (DTC) fungicides are contaminants that are ubiquitous in the environment. Exposure to DTC fungicides has been associated with a variety of teratogenic developmental effects. Propineb, a member of DTCs, was evaluated for the toxicological effects on notochord and craniofacial development, osteogenesis in zebrafish model. Embryos at 6 hours post-fertilization (hpf) were exposed to propineb at dosages of 1 and 4 μM. Morphological parameters were evaluated at exposure times of 24, 48, 72, and 120 hpf after propineb exposure. The survival and hatching rates as well as body length decreased at 1 and 4 μmol/L groups. Besides, transgenic zebrafish exposed to propineb showed abnormal vacuole biogenesis in notochord cells at the early stage of development. The expression of collagen type 2 alpha 1a (col2a1a), sonic hedgehog (shh), and heat shock protein family B member 11 (hspb11) measured by quantitative PCR and in situ hybridization experiment of col8a1a gene have consolidated the proposal process. Besides, Alcian blue, calcein, and alizarin red staining profiles displayed craniofacial malformations and osteoporosis were induced following propineb exposure. PPB exposure induced the changes in oxidative stress and reactive oxygen species inhibitor alleviated the deformities of PPB. Collectively, our data suggested that propineb exposure triggered bone abnormalities in different phenotypes of zebrafish. Therefore, propineb is a potential toxicant of high priority concern for aquatic organisms.

Developmental principles informing human pluripotent stem cell differentiation to cartilage and bone

Human pluripotent stem cells can differentiate into any cell type given appropriate signals and hence have been used to research early human development of many tissues and diseases. Here, we review the major biological factors that regulate cartilage and bone development through the three main routes of neural crest, lateral plate mesoderm and paraxial mesoderm. We examine how these routes have been used in differentiation protocols that replicate skeletal development using human pluripotent stem cells and how these methods have been refined and improved over time. Finally, we discuss how pluripotent stem cells can be employed to understand human skeletal genetic diseases with a developmental origin and phenotype, and how developmental protocols have been applied to gain a better understanding of these conditions.

A Neural Crest-specific Overexpression Mouse Model Reveals the Transcriptional Regulatory Effects of Dlx2 During Maxillary Process Development

Craniofacial morphogenesis is a complex process that requires precise regulation of cell proliferation, migration, and differentiation. Perturbations of this process cause a series of craniofacial deformities. Dlx2 is a critical transcription factor that regulates the development of the first branchial arch. However, the transcriptional regulatory functions of Dlx2 during craniofacial development have been poorly understood due to the lack of animal models in which the Dlx2 level can be precisely modulated. In this study, we constructed a Rosa26 site-directed Dlx2 gene knock-in mouse model Rosa26 ^{CAG-LSL-Dlx2-3xFlag} for conditionally overexpressing Dlx2. By breeding with wnt1 ^cre mice, we obtained wnt1 ^cre ; Rosa26 ^Dlx2/- mice, in which Dlx2 is overexpressed in neural crest lineage at approximately three times the endogenous level. The wnt1 ^cre ; Rosa26 ^Dlx2/- mice exhibited consistent phenotypes that include cleft palate across generations and individual animals. Using this model, we demonstrated that Dlx2 caused cleft palate by affecting maxillary growth and uplift in the early-stage development of maxillary prominences. By performing bulk RNA-sequencing, we demonstrated that Dlx2 overexpression induced significant changes in many genes associated with critical developmental pathways. In summary, our novel mouse model provides a reliable and consistent system for investigating Dlx2 functions during development and for elucidating the gene regulatory networks underlying craniofacial development.

Partial duplication of the jaw: case reports and review of relevant publications

Craniofacial duplication is a rare congenital malformation with a wide phenotypic range. The signs and symptoms range from partial craniofacial duplication to bicephalus. We describe two cases of partial duplication of jaw: a girl with a duplication of the maxilla, and a boy with duplication of the mandible. We review the relevant publications and discuss the pathogenesis.

miR-539 mediates osteoblast mineralization by regulating Distal-less genes 2 in MC3T3-E1 cell line

microRNAs (miRNAs) play an important role in osteoblast differentiation. However, the mechanisms of miRNAs regulating osteoblast mineralization still needs to be further cleared. Distal-less genes 2 (Dlx2) plays an important role in osteoblast differentiation. We have found that miR-539 was significantly downregulated and Dlx2 was found to be inversely correlated with miR-539 in MC3T3-E1 cell line during osteoblast mineralization. The overexpression of miR-539 significantly decreased the expression level of Dlx2 and suppressed the osteogenic marker gene expression level, alkaline phosphatase activity and matrix mineralization. Our study showed that miR-539 was a negative regulator in osteoblast mineralization and that the targeting of Dlx2 gene partly contributes to this inhibitory effect exerted by miR-539.

Zebrafish models of orofacial clefts

Zebrafish is a model organism that affords experimental advantages toward investigating the normal function of genes associated with congenital birth defects. Here we summarize zebrafish studies of genes implicated in orofacial cleft (OFC). The most common use of zebrafish in this context has been to explore the normal function an OFC-associated gene product in craniofacial morphogenesis by inhibiting expression of its zebrafish ortholog. The most frequently deployed method has been to inject embryos with antisense morpholino oligonucleotides targeting the desired transcript. However, improvements in targeted mutagenesis strategies have led to widespread adoption of CRISPR/Cas9 technology. A second application of zebrafish has been for functional assays of gene variants found in OFC patients; such in vivo assays are valuable because the success of in silico methods for testing allele severity has been mixed. Finally, zebrafish have been used to test the tissue specificity of enhancers that harbor single nucleotide polymorphisms associated with risk for OFC. We review examples of each of these approaches in the context of genes that are implicated in syndromic and non-syndromic OFC. Developmental Dynamics 246:897-914, 2017. © 2017 Wiley Periodicals, Inc.

Rdh10 loss-of-function and perturbed retinoid signaling underlies the etiology of choanal atresia

Craniofacial development is a complex process that involves sequential growth and fusion of the facial prominences. When these processes fail, congenital craniofacial anomalies can occur. For example, choanal atresia (CA) is a congenital craniofacial anomaly in which the connection between the nasal airway and nasopharynx is completely blocked. CA occurs in approximately 1/5000 live births and is a frequent component of congenital disorders such as CHARGE, Treacher Collins, Crouzon and Pfeiffer syndromes. However, the detailed cellular and molecular mechanisms underpinning the etiology and pathogenesis of CA remain elusive. In this study, we discovered that mice with mutations in retinol dehydrogenase 10 (Rdh10), which perturbs Vitamin A metabolism and retinoid signaling, exhibit fully penetrant CA. Interestingly, we demonstrate Rdh10 is specifically required in non-neural crest cells prior to E10.5 for proper choanae formation, and that in the absence of Rdh10, Fgf8 is ectopically expressed in the nasal fin. Furthermore, we found that defects in choanae development are associated with decreased cell proliferation and increased cell death in the epithelium of the developing nasal cavity, which retards invagination of the nasal cavity, and thus appears to contribute to the pathogenesis of CA. Taken together, our findings demonstrate that RDH10 is essential during the early stages of facial morphogenesis for the formation of a functional nasal airway, and furthermore establish Rdh10 mutant mice as an important model system to study CA.

Dental and periodontal phenotypes of Dlx2 overexpression in mice

Distal-less homeobox 2 (Dlx2) is a member of the homeodomain family of transcription factors and is important for the development of cranial neural crest cells (CNCCs)-derived craniofacial tissues. Previous studies revealed that Dlx2 was expressed in the cementum and a targeted null mutation disrupted tooth development in mice. However, whether Dlx2 overexpression may impair in vivo tooth morphogenesis remains to be elucidated. The present study used a transgenic mouse model to specifically overexpress Dlx2 in neural crest cells in order to identify the dental phenotypes in mice by observation, micro-computed tomography and histological examination. The Dlx2-overexpressed mice exhibited tooth abnormalities including incisor cross-bite, shortened tooth roots, increased cementum deposition, periodontal ligament disorganization and osteoporotic alveolar bone. Therefore, Dlx2 overexpression may alter the alveolar bone, cementum and periodontal ligament (PDL) phenotypes in mice.

Overexpression of Dlx2 leads to postnatal condyle degradation

Distal-less homeobox 2 (Dlx2), a member of the Dlx family of transcription factors, is important for the development of craniofacial tissues. Previous studies based on knock-out mutant mice revealed that Dlx2 primarily disturbed the development of tissues from maxillary arch. The present study used a transgenic mouse model to specifically overexpress Dlx2 in neural crest cells in order to investigate the role of Dlx2 overexpression in post-natal condyle in mice. The model was constructed and the phenotype observed using gross observation, micro-CT scan and histological examination. The model determined that overexpression of Dlx2 may lead to postnatal condyle malformation, subchondral bone degradation and irregular histological structure of the condylar cartilage. In addition, the expression of osteocalcin in the condyle region was markedly downregulated, whereas expression of msh homeobox 2 was upregulated. The results of the present study suggest that Dlx2 overexpression in cranial neural crest cells would disrupt the development of post-natal condyle, which demonstrates that the expression level and the spatiotemporal expression patterns of Dlx2 may be important in regulating the development of post-natal condyle in mice, and also offered a possible temporal-mandibular joint osteoarthritis model animal for future studies.

A gain-of-function senescence bypass screen identifies the homeobox transcription factor DLX2 as a regulator of ATM-p53 signaling

Wang et al. performed a two-stage, gain-of function screen to select for the genes whose enhanced expression can bypass replicative senescence. Among the new genes that they identified, DLX2 reduces the protein components of the TTI1/TTI2/TEL2 complex, a key complex required for the proper folding and stabilization of ATM and other members of the PIKK family kinase, leading to reduced ATM–p53 signaling and senescence bypass.

Senescence stimuli activate multiple tumor suppressor pathways to initiate cycle arrest and a differentiation program characteristic of senescent cells. We performed a two-stage, gain-of-function screen to select for the genes whose enhanced expression can bypass replicative senescence. We uncovered multiple genes known to be involved in p53 and Rb regulation and ATM regulation, two components of the CST (CTC1–STN1–TEN1) complex involved in preventing telomere erosion, and genes such as REST and FOXO4 that have been implicated in aging. Among the new genes now implicated in senescence, we identified DLX2, a homeobox transcription factor that has been shown to be required for tumor growth and metastasis and is associated with poor cancer prognosis. Growth analysis showed that DLX2 expression led to increased cellular replicative life span. Our data suggest that DLX2 expression reduces the protein components of the TTI1/TTI2/TEL2 complex, a key complex required for the proper folding and stabilization of ATM and other members of the PIKK (phosphatidylinositol 3-kinase-related kinase) family kinase, leading to reduced ATM–p53 signaling and senescence bypass. We also found that the overexpression of DLX2 exhibited a mutually exclusive relationship with p53 alterations in cancer patients. Our functional screen identified novel players that may promote tumorigenesis by regulating the ATM–p53 pathway and senescence.

A gene expression map of the larval Xenopus laevis head reveals developmental changes underlying the evolution of new skeletal elements

The morphology of the vertebrate head skeleton is highly plastic, with the number, size, shape, and position of its components varying dramatically between groups. While this evolutionary flexibility has been key to vertebrate success, its developmental and genetic bases are poorly understood. The larval head skeleton of the frog Xenopus laevis possesses a unique combination of ancestral tetrapod features and anuran-specific novelties. We built a detailed gene expression map of the head mesenchyme in X. laevis during early larval development, focusing on transcription factor families with known functions in vertebrate head skeleton development. This map was then compared to homologous gene expression in zebrafish, mouse, and shark embryos to identify conserved and evolutionarily flexible aspects of vertebrate head skeleton development. While we observed broad conservation of gene expression between X. laevis and other gnathostomes, we also identified several divergent features that correlate to lineage-specific novelties. We noted a conspicuous change in dlx1/2 and emx2 expression in the second pharyngeal arch, presaging the differentiation of the reduced dorsal hyoid arch skeletal element typical of modern anamniote tetrapods. In the first pharyngeal arch we observed a shift in the expression of the joint inhibitor barx1, and new expression of the joint marker gdf5, shortly before skeletal differentiation. This suggests that the anuran-specific infrarostral cartilage evolved by partitioning of Meckel's cartilage with a new paired joint. Taken together, these comparisons support a model in which early patterning mechanisms divide the vertebrate head mesenchyme into a highly conserved set of skeletal precursor populations. While subtle changes in this early patterning system can affect skeletal element size, they do not appear to underlie the evolution of new joints or cartilages. In contrast, later expression of the genes that regulate skeletal element differentiation can be clearly linked to the evolution of novel skeletal elements. We posit that changes in the expression of downstream regulators of skeletal differentiation, like barx1 and gdf5, is one mechanism by which head skeletal element number and articulation are altered during evolution.

The shape of the human language-ready brain

Our core hypothesis is that the emergence of our species-specific language-ready brain ought to be understood in light of the developmental changes expressed at the levels of brain morphology and neural connectivity that occurred in our species after the split from Neanderthals–Denisovans and that gave us a more globular braincase configuration. In addition to changes at the cortical level, we hypothesize that the anatomical shift that led to globularity also entailed significant changes at the subcortical level. We claim that the functional consequences of such changes must also be taken into account to gain a fuller understanding of our linguistic capacity. Here we focus on the thalamus, which we argue is central to language and human cognition, as it modulates fronto-parietal activity. With this new neurobiological perspective in place, we examine its possible molecular basis. We construct a candidate gene set whose members are involved in the development and connectivity of the thalamus, in the evolution of the human head, and are known to give rise to language-associated cognitive disorders. We submit that the new gene candidate set opens up new windows into our understanding of the genetic basis of our linguistic capacity. Thus, our hypothesis aims at generating new testing grounds concerning core aspects of language ontogeny and phylogeny.

KDM6B epigenetically regulates odontogenic differentiation of dental mesenchymal stem cells

Mesenchymal stem cells (MSCs) have been identified and isolated from dental tissues, including stem cells from apical papilla, which demonstrated the ability to differentiate into dentin-forming odontoblasts. The histone demethylase KDM6B (also known as JMJD3) was shown to play a key role in promoting osteogenic commitment by removing epigenetic marks H3K27me3 from the promoters of osteogenic genes. Whether KDM6B is involved in odontogenic differentiation of dental MSCs, however, is not known. Here, we explored the role of KDM6B in dental MSC fate determination into the odontogenic lineage. Using shRNA-expressing lentivirus, we performed KDM6B knockdown in dental MSCs and observed that KDM6B depletion leads to a significant reduction in alkaline phosphate (ALP) activity and in formation of mineralized nodules assessed by Alizarin Red staining. Additionally, mRNA expression of odontogenic marker gene SP7 (osterix, OSX), as well as extracellular matrix genes BGLAP (osteoclacin, OCN) and SPP1 (osteopontin, OPN), was suppressed by KDM6B depletion. When KDM6B was overexpressed in KDM6B-knockdown MSCs, odontogenic differentiation was restored, further confirming the facilitating role of KDM6B in odontogenic commitment. Mechanistically, KDM6B was recruited to bone morphogenic protein 2 (BMP2) promoters and the subsequent removal of silencing H3K27me3 marks led to the activation of this odontogenic master transcription gene. Taken together, our results demonstrated the critical role of a histone demethylase in the epigenetic regulation of odontogenic differentiation of dental MSCs. KDM6B may present as a potential therapeutic target in the regeneration of tooth structures and the repair of craniofacial defects.

Morphologic relationship between the cranial base and the mandible in patients with facial asymmetry and mandibular prognathism

INTRODUCTION

This study was conducted to measure the dimensional changes in the cranial base and the mandible in patients with facial asymmetry and mandibular prognathism, and to examine the morphologic relationship between asymmetries of the cranial base and the mandible.

METHODS

The patients were 60 adults with mandibular prognathism, divided into a symmetry group (menton deviation, <2 mm; n = 30) and an asymmetry group (menton deviation, >4 mm; n = 30) according to the degree of menton deviation. Three-dimensional computed tomography scans were obtained with a spiral scanner. Landmarks were designated on the reconstructed 3-dimensional surface models. Linear, angular, and volumetric measurements of the cranial base and mandibular variables were made.

RESULTS

In the asymmetry group, the hemi-base, anterior cranial base, and middle cranial base volumes were significantly larger (P <0.01), and crista galli to sphenoid, sphenoid to petrous ridge, anterior clinoid process to petrous ridge, and vomer to petrous ridge lengths were significantly longer (P <0.05) on the nondeviated side than on the deviated side. Menton deviation was significantly correlated with the difference in hemi-base volume, and ramal volume was significantly correlated with the difference in hemi-base and middle cranial base volumes between the nondeviated and deviated sides (P <0.05).

CONCLUSIONS

In patients with facial asymmetry and mandibular prognathism, cranial base volume increased on the nondeviated side and was also correlated with mandibular asymmetry.

Sequence analyses of the distal-less homeobox gene family in East African cichlid fishes reveal signatures of positive selection

BACKGROUND

Gen(om)e duplication events are hypothesized as key mechanisms underlying the origin of phenotypic diversity and evolutionary innovation. The diverse and species-rich lineage of teleost fishes is a renowned example of this scenario, because of the fish-specific genome duplication. Gene families, generated by this and other gene duplication events, have been previously found to play a role in the evolution and development of innovations in cichlid fishes - a prime model system to study the genetic basis of rapid speciation, adaptation and evolutionary innovation. The distal-less homeobox genes are particularly interesting candidate genes for evolutionary novelties, such as the pharyngeal jaw apparatus and the anal fin egg-spots. Here we study the dlx repertoire in 23 East African cichlid fishes to determine the rate of evolution and the signatures of selection pressure.

RESULTS

Four intact dlx clusters were retrieved from cichlid draft genomes. Phylogenetic analyses of these eight dlx loci in ten teleost species, followed by an in-depth analysis of 23 East African cichlid species, show that there is disparity in the rates of evolution of the dlx paralogs. Dlx3a and dlx4b are the fastest evolving dlx genes, while dlx1a and dlx6a evolved more slowly. Subsequent analyses of the nonsynonymous-synonymous substitution rate ratios indicate that dlx3b, dlx4a and dlx5a evolved under purifying selection, while signs of positive selection were found for dlx1a, dlx2a, dlx3a and dlx4b.

CONCLUSIONS

Our results indicate that the dlx repertoire of teleost fishes and cichlid fishes in particular, is shaped by differential selection pressures and rates of evolution after gene duplication. Although the divergence of the dlx paralogs are putative signs of new or altered functions, comparisons with available expression patterns indicate that the three dlx loci under strong purifying selection, dlx3b, dlx4a and dlx5a, are transcribed at high levels in the cichlids' pharyngeal jaw and anal fin. The dlx paralogs emerge as excellent candidate genes for the development of evolutionary innovations in cichlids, although further functional analyses are necessary to elucidate their respective contribution.

Signals and switches in Mammalian neural crest cell differentiation

Neural crest cells (NCCs) comprise a multipotent, migratory cell population that generates a diverse array of cell and tissue types during vertebrate development. These include cartilage and bone, tendons, and connective tissue, as well as neurons, glia, melanocytes, and endocrine and adipose cells; this remarkable lineage potential persists into adult life. Taken together with a limited capacity for self-renewal, neural crest cells bear the hallmarks of stem and progenitor cells and are considered to be synonymous with vertebrate evolution. The neural crest has provided a system for exploring the mechanisms that govern developmental processes such as morphogenetic induction, cell migration, and fate determination. Today, much of the focus on neural crest cells revolves around their stem cell-like characteristics and potential for use in regenerative medicine. A thorough understanding of the signals and switches that govern mammalian neural crest patterning is central to potential therapeutic application of these cells and better appreciation of the role that neural crest cells play in vertebrate evolution, development, and disease.

Retinoic acid expands the evolutionarily reduced dentition of zebrafish

Zebrafish lost anterior teeth during evolution but retain a posterior pharyngeal dentition that requires retinoic acid (RA) cell-cell signaling for its development. The purposes of this study were to test the sufficiency of RA to induce tooth development and to assess its role in evolution. We found that exposure of embryos to exogenous RA induces a dramatic anterior expansion of the number of pharyngeal teeth that later form and shifts anteriorly the expression patterns of genes normally expressed in the posterior tooth-forming region, such as pitx2 and dlx2b. After RA exposure, we also observed a correlation between cartilage malformations and ectopic tooth induction, as well as abnormal cranial neural crest marker gene expression. Additionally, we observed that the RA-induced zebrafish anterior teeth resemble in pattern and number the dentition of fish species that retain anterior pharyngeal teeth such as medaka but that medaka do not express the aldh1a2 RA-synthesizing enzyme in tooth-forming regions. We conclude that RA is sufficient to induce anterior ectopic tooth development in zebrafish where teeth were lost in evolution, potentially by altering neural crest cell development, and that changes in the location of RA synthesis correlate with evolutionary changes in vertebrate dentitions.

Laser ablation of the sonic hedgehog-a-expressing cells during fin regeneration affects ray branching morphogenesis

The zebrafish fin is an excellent system to study the mechanisms of dermal bone patterning. Fin rays are segmented structures that form successive bifurcations both during ontogenesis and regeneration. Previous studies showed that sonic hedgehog (shha) may regulate regenerative bone patterning based on its expression pattern and functional analysis. The present study investigates the role of the shha-expressing cells in the patterning of fin ray branches. The shha expression domain in the basal epidermis of each fin ray splits into two prior to ray bifurcation. In addition, the osteoblast proliferation profile follows the dynamic expression pattern of shha. A zebrafish transgenic line, 2.4shh:gfpABC#15, in which GFP expression recapitulates the endogenous expression of shha, was used to specifically ablate shha-expressing cells with a laser beam. Such ablations lead to a delay in the sequence of events leading to ray bifurcation without affecting the overall growth of the fin ray. These results suggest that shha-expressing cells direct localized osteoblast proliferation and thus regulate branching morphogenesis. This study reveals the fin ray as a new accessible system to investigate epithelial-mesenchymal interactions leading to organ branching.

Altered gene expression in rat cranial neural crest cells exposed to a teratogenic glucose concentration in vitro: paradoxical downregulation of antioxidative defense genes

BACKGROUND

Diabetic pregnancy is associated with increased risk of malformation in the infant. Diabetes-induced anomalies of the face and heart are strongly correlated with neural crest cell (NCC) maldevelopment. We aimed to study glucose-induced alterations of mRNA levels in cranial and trunk NCCs isolated from rat embryos with increased risk of developing mandibular and cardiac malformations in diabetic pregnancy.

METHODS

Inbred Sprague-Dawley rat embryos were used for NCC isolation from neural tube explants. The migrating cells were exposed to 5.5 or 30 mmol/l glucose concentration for 48 hr, harvested, and prepared for gene expression measurement by RT-PCR or immunostaining with either distal-less (Dlx) or AP-2-α antibodies.

RESULTS

Evaluation of the immunostained slides showed that approximately 75% of the cells were of NCC origin. Exposure to 30 mM glucose decreased mRNA levels of Copper-Zinc superoxide dismutase, manganese superoxide dismutase, extracellular superoxide dismutase, Catalase, Gpx-1, Nrf2, poly-ADP ribose polymerase, B-cell leukemia/lymphoma protein 2, and β-Catenin genes in cranial neural crest explant cultures. In addition, Pax-3, Pax-6, Wnt3a, and Apc mRNA levels were decreased by high glucose exposure in both cranial and trunk neural crest explant cultures.

CONCLUSION

Cranial NCCs diminish their mRNA levels of antioxidative enzymes and the Nrf2 response factor, as well as the antiapoptotic B-cell leukemia/lymphoma protein 2 gene, in response to increased ambient glucose concentration. Furthermore, both cranial and trunk NCC decrease the mRNA levels of the transcription factors Pax-3 and Pax-6, as well as key components of the Wnt pathway. These patterns of glucose-altered gene expression in a developmentally important cell population may be of etiological importance for NCC-associated malformations in diabetic pregnancy.