Transcription Factors in Craniofacial Development: From Receptor Signaling to Transcriptional and Epigenetic Regulation

Transcriptome Analysis of Rheumatoid Arthritis Uncovers Genes Linked to Inflammation-Induced Pain

Autoimmune diseases such as rheumatoid arthritis (RA) can promote states of chronic Inflammation with accompanying tissue destruction and pain. RA can cause inflammatory synovitis in peripheral joints, particularly within the hands and feet, but can also sometimes trigger temporomandibular joint (TMJ) arthralgia. To better understand the effects of ongoing Inflammation-induced pain signaling, dorsal root ganglia (DRGs) were acquired from individuals with RA for transcriptomic study. We conducted RNA sequencing from the L5 DRGs because it contains the soma of the sensory neurons that innervate the affected joints in the foot. DRGs from 5 RA patients were compared with 9 non-arthritic controls. RNA-seq of L5 DRGs identified 128 differentially expressed genes (DEGs) that were dysregulated in the RA subjects as compared to the non-arthritic controls. The DRG resides outside the blood brain barrier and, as such, our initial transcriptome analysis detected signs of an autoimmune disorder including the upregulated expression of immunoglobulins and other immunologically related genes within the DRGs of the RA donors. Additionally, we saw the upregulation in genes implicated in neurogenesis that could promote pain hypersensitivity. overall, our DRG analysis suggests that there are upregulated inflammatory and pain signaling pathways that can contribute to chronic pain in RA.

Genetic architecture of non-syndromic skeletal class III malocclusion

Non-syndromic skeletal Class III malocclusion is a major craniofacial disorder characterized by genetic and environmental factors. Patients with severe skeletal Class III malocclusion require orthognathic surgery to obtain aesthetic facial appearance and functional occlusion. Recent studies have demonstrated that susceptible chromosomal regions and genetic variants of candidate genes play important roles in the etiology of skeletal Class III malocclusion. Here, we provide a comprehensive review of our current understanding of the genetic factors that affect non-syndromic skeletal Class III malocclusion, including the patterns of inheritance and multiple genetic approaches. We then summarize the functional studies on related loci and genes using cell biology and animal models, which will help to implement individualized therapeutic interventions.

Single-cell RNA-Seq reveals transcriptional regulatory networks directing the development of mouse maxillary prominence

During vertebrate embryonic development, neural crest-derived ectomesenchyme within the maxillary prominences undergoes precisely coordinated proliferation and differentiation to give rise to diverse craniofacial structures, such as tooth and palate. However, the transcriptional regulatory networks underpinning such an intricate process have not been fully elucidated. Here, we perform single-cell RNA-Seq to comprehensively characterize the transcriptional dynamics during mouse maxillary development from embryonic day (E) 10.5-E14.5. Our single-cell transcriptome atlas of ∼28,000 cells uncovers mesenchymal cell populations representing distinct differentiating states and reveals their developmental trajectory, suggesting that the segregation of dental from the palatal mesenchyme occurs at E11.5. Moreover, we identify a series of key transcription factors (TFs) associated with mesenchymal fate transitions and deduce the gene regulatory networks directed by these TFs. Collectively, our study provides important resources and insights for achieving a systems-level understanding of craniofacial morphogenesis and abnormality.

Integrative analysis of transcriptome dynamics during human craniofacial development identifies candidate disease genes

Craniofacial disorders arise in early pregnancy and are one of the most common congenital defects. To fully understand how craniofacial disorders arise, it is essential to characterize gene expression during the patterning of the craniofacial region. To address this, we performed bulk and single-cell RNA-seq on human craniofacial tissue from 4-8 weeks post conception. Comparisons to dozens of other human tissues revealed 239 genes most strongly expressed during craniofacial development. Craniofacial-biased developmental enhancers were enriched +/- 400 kb surrounding these craniofacial-biased genes. Gene co-expression analysis revealed that regulatory hubs are enriched for known disease causing genes and are resistant to mutation in the normal healthy population. Combining transcriptomic and epigenomic data we identified 539 genes likely to contribute to craniofacial disorders. While most have not been previously implicated in craniofacial disorders, we demonstrate this set of genes has increased levels of de novo mutations in orofacial clefting patients warranting further study.

Characterizing the role of Pdgfra in calvarial development

BACKGROUND

Mammalian calvarium is composed of flat bones developed from two origins, neural crest, and mesoderm. Cells from both origins exhibit similar behavior but express distinct transcriptomes. It is intriguing to ask whether genes shared by both origins play similar or distinct roles in development. In the present study, we have examined the role of Pdgfra, which is expressed in both neural crest and mesoderm, in specific lineages during calvarial development.

RESULTS

We found that in calvarial progenitor cells, Pdgfra is needed to maintain normal proliferation and migration of neural crest cells but only proliferation of mesoderm cells. Later in calvarial osteoblasts, we found that Pdgfra is necessary for both proliferation and differentiation of neural crest-derived cells, but not for differentiation of mesoderm-derived cells. We also examined the potential interaction between Pdgfra and other signaling pathway involved in calvarial osteoblasts but did not identify significant alteration of Wnt or Hh signaling activity in Pdgfra genetic models.

CONCLUSIONS

Pdgfra is required for normal calvarial development in both neural crest cells and mesoderm cells, but these lineages exhibit distinct responses to alteration of Pdgfra activity.

Klf4 haploinsufficiency in Sp7+ lineage leads to underdeveloped mandibles and insufficient elongation of mandibular incisor

The mandible is an important component of the craniofacial bones, whose development is regulated by complex molecular networks and involves the well-coordinated development of the bone, cartilage, and teeth. Previously, we demonstrated that Krüppel-like factor 4 (KLF4) promoted dentinogenesis and osteogenesis, but it was enigmatic whether Klf4 participated in the development of the mandible. In this study, the Sp7-Cre; Klf4^f/+ mice exhibited underdeveloped mandibles and insufficient elongation of the mandibular incisor when compared with Klf4^f/+ and Sp7-Cre mice. Moreover, morphological and molecular analysis showed that the alveolar bone mass was significantly decreased in KLF4 deficient mice, accompanied by reduced expression of osteoblast-related genes. Meanwhile, the KLF4 deficient mice had decreased expression of receptor activator of nuclear factor kappa-Β ligand (RANKL) and no significant change of osteoprotegerin (OPG) in the alveolar bone near the mandibular incisor. Simultaneously, the osteoclastogenesis in the alveolar bone of KLF4 deficient mice was attenuated, which was demonstrated by a diminished number of tartrate-resistant acid phosphatase positive (TRAP+), matrix metallopeptidase 9 positive (MMP9+), and cathepsin K positive (CTSK+) multinucleated osteoclasts, respectively. Collectively, our study suggested that Klf4 participated in mandibular development, and Klf4 in Sp7+ lineage affected osteogenesis directly and osteoclastogenesis indirectly.

Spatiotemporal Expression and Functional Analysis of miRNA-22 in the Developing Secondary Palate

OBJECTIVE

Normal development of the embryonic orofacial region requires precise spatiotemporal coordination between numerous genes. MicroRNAs represent small, single-stranded, non-coding molecules that regulate gene expression. This study examines the role of microRNA-22 (miR-22) in murine orofacial ontogeny.

METHODS

Spatiotemporal and differential expression of miR-22 (mmu-miR-22-3p) within the developing secondary palate was determined by in situ hybridization and quantitative real-time PCR, respectively. Bioinformatic approaches were used to predict potential mRNA targets of miR-22 and analyze their association with cellular functions indispensable for normal orofacial ontogeny. An in vitro palate organ culture system was used to assess the role of miR-22 in secondary palate development.

RESULTS

There was a progressive increase in miR-22 expression from GD12.5 to GD14.5 in palatal processes. On GD12.5 and GD13.5, miR-22 was expressed in the future oral, nasal, and medial edge epithelia. On GD14.5, miR-22 expression was observed in the residual midline epithelial seam (MES), the nasal epithelium and the mesenchyme, but not in the oral epithelium. Inhibition of miR-22 activity in palate organ cultures resulted in failure of MES removal. Bioinformatic analyses revealed potential mRNA targets of miR-22 that may play significant roles in regulating apoptosis, migration, and/or convergence/extrusion, developmental processes that modulate MES removal during palatogenesis.

CONCLUSIONS

Results from the current study suggest a key role for miR-22 in the removal of the MES during palatogenesis and that miR-22 may represent a potential contributor to the etiology of cleft palate.

Candidate positive targets of LHX6 and LHX8 transcription factors in the developing upper jaw

Craniofacial development is controlled by a large number of genes, which interact with one another to form a complex gene regulatory network (GRN). Key components of GRN are signaling molecules and transcription factors. Therefore, identifying targets of core transcription factors is an important part of the overall efforts toward building a comprehensive and accurate model of GRN. LHX6 and LHX8 are transcription factors expressed in the oral mesenchyme of the first pharyngeal arch (PA1), and they are crucial regulators of palate and tooth development. Previously, we performed genome-wide transcriptional profiling and chromatin immunoprecipitation to identify target genes of LHX6 and LHX8 in PA1, and described a set of genes repressed by LHX. However, there has not been any discussion of the genes positively regulated by LHX6 and LHX8. In this paper, we revisited the above datasets to identify candidate positive targets of LHX in PA1. Focusing on those with known connections to craniofacial development, we performed RNA in situ hybridization to confirm the changes in expression in Lhx6;Lhx8 mutant. We also confirmed the binding of LHX6 to several putative enhancers near the candidate target genes. Together, we have uncovered novel connections between Lhx and other important regulators of craniofacial development, including Eya1, Barx1, Rspo2, Rspo3, and Wnt11.

RNA-binding protein Elavl1/HuR is required for maintenance of cranial neural crest specification

While neural crest development is known to be transcriptionally controlled via sequential activation of gene regulatory networks (GRNs), recent evidence increasingly implicates a role for post-transcriptional regulation in modulating the output of these regulatory circuits. Using available single-cell RNA-sequencing datasets from avian embryos to identify potential post-transcriptional regulators, we found that Elavl1, which encodes for an RNA-binding protein with roles in transcript stability, was enriched in the premigratory cranial neural crest. Perturbation of Elavl1 resulted in premature neural crest delamination from the neural tube as well as significant reduction in transcripts associated with the neural crest specification GRN, phenotypes that are also observed with downregulation of the canonical Wnt inhibitor Draxin. That Draxin is the primary target for stabilization by Elavl1 during cranial neural crest specification was shown by RNA-sequencing, RNA immunoprecipitation, RNA decay measurement, and proximity ligation assays, further supporting the idea that the downregulation of neural crest specifier expression upon Elavl1 knockdown was largely due to loss of Draxin. Importantly, exogenous Draxin rescued cranial neural crest specification defects observed with Elavl1 knockdown. Thus, Elavl1 plays a critical a role in the maintenance of cranial neural crest specification via Draxin mRNA stabilization. Together, these data highlight an important intersection of post-transcriptional regulation with modulation of the neural crest specification GRN.

A tale of two cities: The genetic mechanisms governing calvarial bone development

The skull bones must grow in a coordinated, three-dimensional manner to coalesce and form the head and face. Mammalian skull bones have a dual embryonic origin from cranial neural crest cells (CNCC) and paraxial mesoderm (PM) and ossify through intramembranous ossification. The calvarial bones, the bones of the cranium which cover the brain, are derived from the supraorbital arch (SOA) region mesenchyme. The SOA is the site of frontal and parietal bone morphogenesis and primary center of ossification. The objective of this review is to frame our current in vivo understanding of the morphogenesis of the calvarial bones and the gene networks regulating calvarial bone initiation in the SOA mesenchyme.

Protein Arginine Methyltransferase PRMT1 Is Essential for Palatogenesis

Cleft palate is among the most common birth defects. Currently, only 30% of cases have identified genetic causes, whereas the etiology of the majority remains to be discovered. We identified a new regulator of palate development, protein arginine methyltransferase 1 (PRMT1), and demonstrated that disruption of PRMT1 function in neural crest cells caused complete cleft palate and craniofacial malformations. PRMT1 is the most highly expressed of the protein arginine methyltransferases, enzymes responsible for methylation of arginine motifs on histone and nonhistone proteins. PRMT1 regulates signal transduction and transcriptional activity that affect multiple signal pathways crucial in craniofacial development, such as the BMP, TGFβ, and WNT pathways. We demonstrated that Wnt1-Cre;Prmt1 ^fl/fl mice displayed a decrease in palatal mesenchymal cell proliferation and failure of palatal shelves to reach the midline. Further analysis in signal pathways revealed that loss of Prmt1 in mutant mice decreased BMP signaling activation and reduced the deposition of H4R3me2a mark. Collectively, our study demonstrates that Prmt1 is crucial in palate development. Our study may facilitate the development of a better strategy to interrupt the formation of cleft palate through manipulation of PRMT1 activity.

Prmt1 regulates craniofacial bone formation upstream of Msx1

Protein arginine methylation has been recently identified as an important form of post-translational modification (PTM). It is carried out by the protein arginine methyltransferase (PRMT) family of enzymes, which in mammals consists of nine members. Among them, PRMT1 is the major arginine methyltransferase and participates in transcription, signal transduction, development and cancer. The function of PRMT1 in craniofacial development remains unclear. We generated Wnt1-Cre;Prmt1^fl/fl mice with cranial neural crest (CNC)-specific deletion of Prmt1 and compared CNC-derived craniofacial bones from newborn control and Wnt1-Cre;Prmt1^fl/fl mice. The size, surface area and volume of the premaxilla, maxilla, palatine bone, frontal bone, and mandible were analyzed using three-dimensional (3D) micro-computed tomography (microCT). We found that Prmt1 deficiency led to alterations in craniofacial bones including the premaxilla, maxilla, palatine bone, frontal bone, and mandible, as well as defects in the incisor and alveolar bone, recapitulating changes seen in Msx1-deficient mice. We further determined that Prmt1 depletion resulted in significant downregulation of Msx1 in calvaria-derived preosteoblast and primordium of frontal bone and mandible. Our study reveals critical roles of PRMT1 in the formation of CNC-derived craniofacial bones and suggests that Prmt1 is an upstream regulator of Msx1 in craniofacial bone development.

Genes and microRNAs associated with mouse cleft palate: A systematic review and bioinformatics analysis

Cleft palate (CP) is the most prevalent craniofacial deformity, with ethnic and geographic variation in prevalence in humans. Mice have been used as an animal model to study the cause(s) of CP by several approaches, including genetic and chemical-induced approaches. Mouse genetic approaches revealed that significant amounts of genes are involved in the CP pathology. The aim of this study was to identify common features of CP-associated genes and to explore the roles of microRNAs (miRNAs) as important post-transcriptional regulators that may be involved in the regulation of CP genes. To generate an accurate list of genes associated with CP, we first conducted systematic literature searches through main databases such as Medline, Embase, and PubMed, as well as other sources such as Scopus and Mouse Genome Informatics. We found that 195 mouse strains with single-gene mutations and 140 mouse strains with compound-gene mutations were reported to have CP. The CP genes were categorized by functions and pathways using the Kyoto Encyclopedia of Genes and Genomes and Gene Ontology annotations, highlighting the contribution of cellular metabolism to CP. A total of 18 miRNAs were involved in the regulation of multiple CP genes. Human genotype-phenotype analysis revealed that variants in five human homologous CP genes (IRF6, FOXE1, VAX1, WNT9B, and GAD1) significantly contributed to the human CP phenotype. Thus, our results suggest that cellular metabolism and miRNAs play an important role in the regulation of genetic pathways and networks crucial for palatal formation.

Differential methylation is associated with non-syndromic cleft lip and palate and contributes to penetrance effects

Non-syndromic cleft lip and/or palate (NSCLP) is a common congenital malformation with a multifactorial model of inheritance. Although several at-risk alleles have been identified, they do not completely explain the high heritability. We postulate that epigenetic factors as DNA methylation might contribute to this missing heritability. Using a Methylome-wide association study in a Brazilian cohort (67 NSCLP, 59 controls), we found 578 methylation variable positions (MVPs) that were significantly associated with NSCLP. MVPs were enriched in regulatory and active regions of the genome and in pathways already implicated in craniofacial development. In an independent UK cohort (171 NSCLP, 177 controls), we replicated 4 out of 11 tested MVPs. We demonstrated a significant positive correlation between blood and lip tissue DNA methylation, indicating blood as a suitable tissue for NSCLP methylation studies. Next, we quantified CDH1 promoter methylation levels in CDH1 mutation-positive families, including penetrants, non-penetrants or non-carriers for NSCLP. We found methylation levels to be significantly higher in the penetrant individuals. Taken together, our results demonstrated the association of methylation at specific genomic locations as contributing factors to both non-familial and familial NSCLP and altered DNA methylation may be a second hit contributing to penetrance.

Negative regulation of endothelin signaling by SIX1 is required for proper maxillary development

Jaw morphogenesis is a complex event mediated by inductive signals that establish and maintain the distinct developmental domains required for formation of hinged jaws, the defining feature of gnathostomes. The mandibular portion of pharyngeal arch 1 is patterned dorsally by Jagged-Notch signaling and ventrally by endothelin receptor A (EDNRA) signaling. Loss of EDNRA signaling disrupts normal ventral gene expression, the result of which is homeotic transformation of the mandible into a maxilla-like structure. However, loss of Jagged-Notch signaling does not result in significant changes in maxillary development. Here we show in mouse that the transcription factor SIX1 regulates dorsal arch development not only by inducing dorsal Jag1 expression but also by inhibiting endothelin 1 (Edn1) expression in the pharyngeal endoderm of the dorsal arch, thus preventing dorsal EDNRA signaling. In the absence of SIX1, but not JAG1, aberrant EDNRA signaling in the dorsal domain results in partial duplication of the mandible. Together, our results illustrate that SIX1 is the central mediator of dorsal mandibular arch identity, thus ensuring separation of bone development between the upper and lower jaws.