Pax9's dual roles in modulating Wnt signaling during murine palatogenesis

BACKGROUND

Despite the strides made in understanding the complex network of key regulatory genes and cellular processes that drive palate morphogenesis, patients suffering from these conditions face treatment options that are limited to complex surgeries and multidisciplinary care throughout life. Hence, a better understanding of how molecular interactions drive palatal growth and fusion is critical for the development of treatment and preventive strategies for cleft palates in humans. Our previous work demonstrated that Pax9-dependent Wnt signaling is critical for the growth and fusion of palatal shelves. We showed that controlled intravenous delivery of small molecule Wnt agonists specifically blocks the action of Dkks (inhibitors of Wnt signaling) and corrects secondary palatal clefts in Pax9^-/- mice. While these data underscore the importance of the functional upstream relationship of Pax9 to the Wnt pathway, not much is known about how the genetic nature of Pax9's interactions in vivo and how it modulates the actions of these downstream effectors during palate formation.

RESULTS

Here, we show that the genetic reduction of Dkk1 during palatogenesis corrected secondary palatal clefts in Pax9^-/- mice with restoration of Wnt signaling activities. In contrast, genetically induced overexpression of Dkk1 mice phenocopied the defects in tooth and palate development visible in Pax9^-/- strains. Results of ChIP-qPCR assays showed that Pax9 can bind to regions near the transcription start sites of Dkk1 and Dkk2 as well as the intergenic region of Wnt9b and Wnt3 ligands that are downregulated in Pax9^-/- palates.

CONCLUSIONS

Taken together, these data suggest that the molecular mechanisms underlying Pax9's role in modulating Wnt signaling activity likely involve the inhibition of Dkk expression and the control of Wnt ligands during palatogenesis.

Trps1 Regulates Development of Craniofacial Skeleton and Is Required for the Initiation of Palatal Shelves Fusion

Trichorhinophalangeal syndrome (TRPS) is an autosomal dominant disorder resulting from heterozygous mutations of the TRPS1 gene. Common craniofacial abnormalities in TRPS patients include micrognathia, hypoplastic zygomatic arch, high-arched palate, and, occasionally, cleft palate. Studies have demonstrated that mice with a heterozygous Trps1 mutation (Trps1^+/− mice) have similar features to patients with TRPS, including high-arched palates. However, mice with a homozygous Trps1 mutation (Trps1^−/− mice) exhibit similar but more severe abnormalities, including cleft palate. Our study aimed to characterize the craniofacial phenotype to understand the role of Trps1 in craniofacial development and gain insight on the cleft palate pathogenesis in Trps1 deficiency. Whole-mount skeletal staining revealed hypoplastic skeletal and cartilaginous elements, steep nasal slope, and missing presphenoid in Trps1^−/− mice. Although several craniofacial skeleton elements were abnormal in Trps1^−/− mice, the Trps1 deficiency did not appear to disrupt cranial vault development. All Trps1^−/− mice presented with cleft palate. Analyses of Trps1 expression during palatogenesis detected Trps1 mRNA and protein in palatal mesenchyme and in specific regions of palatal epithelium, which suggested that Trps1 is involved in palatal fusion. Ex vivo culture experiments demonstrated that Trps1^−/− palatal shelves were unable to initiate the fusion process. On the molecular level, Trps1 deficiency resulted in decreased epithelial expression of proteins involved in palatal fusion, including chondroitin sulfate proteoglycan, transforming growth factor-beta 3, Twist1, and beta-catenin. Mesenchymal expression of chondroitin sulfate proteoglycan expression was unaffected, indicating a cell type-specific mechanism of Trps1 regulation on chondroitin sulfate proteoglycan. In conclusion, we demonstrated that Trps1 is involved in the development of craniofacial skeletal elements and in the initiation of the palatal shelves fusion. Furthermore, our studies uncovered that Trps1 is required for epithelial expression of several proteins involved in the palatal shelves fusion.

Genome-Wide mRNA-Seq Profiling Reveals that LEF1 and SMAD3 Regulate Epithelial-Mesenchymal Transition Through the Hippo Signaling Pathway During Palatal Fusion

BACKGROUND

Epithelial-mesenchymal transition (EMT) of the medial edge epithelium (MEE) occurs through fusion of the palatal shelves and is a crucial step in palatogenesis. The key genes, however, and the related signaling pathway of EMT are not yet fully understood. Therefore, the aim of this study was to reveal the key genes and the related signaling pathway of EMT during palatal fusion.

MATERIALS AND METHODS

C57BL/6J mice at embryonic gestation day 14.5 (E14.5; n = 6) were used to establish the cleft palate model for mRNA-Seq (HiSeq X Ten). The Gene Ontology (GO) and the Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses were performed for functional annotations of the differentially expressed genes. Quantitative polymerase chain reaction (qPCR) assays were used to validate the RNAseq data.

RESULTS

A total of 936 differentially expressed genes, including 558 upregulated and 378 downregulated genes were identified in cases versus controls, respectively. Among these genes, the GO analysis showed that Lymphoid Enhancer-Binding Factor 1 (LEF1) and SMAD Family Member 3 (SMAD3) significantly enriched biological processes, which were EMT related. The KEGG analysis showed that these genes regulated EMT through the Hippo signaling pathway. LEF1 and SMAD3 were downregulated, and the qPCR results corroborated the RNA-seq data.

CONCLUSIONS

These results demonstrate that LEF1 and SMAD3 inhibits EMT at the MEE through the Hippo signaling pathway; and that this could contribute to cleft palate formation in embryonic palatal fusion at E 14.5.

TGF-β Signaling and the Epithelial-Mesenchymal Transition during Palatal Fusion

Signaling by transforming growth factor (TGF)-β plays an important role in development, including in palatogenesis. The dynamic morphological process of palatal fusion occurs to achieve separation of the nasal and oral cavities. Critically and specifically important in palatal fusion are the medial edge epithelial (MEE) cells, which are initially present at the palatal midline seam and over the course of the palate fusion process are lost from the seam, due to cell migration, epithelial-mesenchymal transition (EMT), and/or programed cell death. In order to define the role of TGF-β signaling during this process, several approaches have been utilized, including a small interfering RNA (siRNA) strategy targeting TGF-β receptors in an organ culture context, the use of genetically engineered mice, such as Wnt1-cre/R26R double transgenic mice, and a cell fate tracing through utilization of cell lineage markers. These approaches have permitted investigators to distinguish some specific traits of well-defined cell populations throughout the palatogenic events. In this paper, we summarize the current understanding on the role of TGF-β signaling, and specifically its association with MEE cell fate during palatal fusion. TGF-β is highly regulated both temporally and spatially, with TGF-β3 and Smad2 being the preferentially expressed signaling molecules in the critical cells of the fusion processes. Interestingly, the accessory receptor, TGF-β type 3 receptor, is also critical for palatal fusion, with evidence for its significance provided by Cre-lox systems and siRNA approaches. This suggests the high demand of ligand for this fine-tuned signaling process. We discuss the new insights in the fate of MEE cells in the midline epithelial seam (MES) during the palate fusion process, with a particular focus on the role of TGF-β signaling.

Genome-Wide DNA Methylation Profile of Gene cis-Acting Element Methylations in All-trans Retinoic Acid-Induced Mouse Cleft Palate

DNA methylation epigenetically regulates gene expression. This study is aimed to investigate genome-wide DNA methylations involved in the regulation of palatal fusion in the all-trans retinoic acid-induced mouse cleft palate model. There were 4,718,556 differentially CCGG methylated sites and 367,504 CCWGG methylated sites for 1497 genes between case and control embryonic mouse palatal tissues. The enhancers (HDAC4 and SMAD3) and promoter (MID1) of these three genes had cis-acting element methylation. HDAC4 is localized within the CCWGG, while MID1 and SMAD3 are localized within the CCGG of the gene intron. The methylation-specific polymerase chain reaction data confirmed the MethylRAD-seq results, while the quantitative reverse transcriptase-polymerase chain reaction result showed that changes in gene expression inversely were associated with the cis-acting element methylation of the gene during retinoic acid-induced palatal fusion. The GO and KEGG data showed that these three genes could regulate cell proliferation, skeletal muscle fiber development, and development-related gene signaling or activity. The cis-acting element methylation of HDAC4, SMAD3, and MID1 may play a regulatory role during palatal fusion. Further research is needed to verify these novel epigenetic biomarkers for cleft palate.

Analysis of Snail1 function and regulation by Twist1 in palatal fusion

Palatal fusion is a tightly controlled process which comprises multiple cellular events, including cell movement and differentiation. Midline epithelial seam (MES) degradation is essential to palatal fusion. In this study, we analyzed the function of Snail1 during the degradation of the MES. We also analyzed the mechanism regulating the expression of the Snail1 gene in palatal shelves. Palatal explants treated with Snail1 siRNA did not degrade the MES and E-cadherin was not repressed leading to failure of palatal fusion. Transforming growth factor beta 3 (Tgfβ3) regulated Snail1 mRNA, as Snail1 expression decreased in response to Tgfβ3 neutralizing antibody and a PI-3 kinase (PI3K) inhibitor. Twist1, in collaboration with E2A factors, regulated the expression of Snail1. Twist1/E47 dimers bond to the Snail1 promoter to activate expression. Without E47, Twist1 repressed Snail1 expression. These results support the hypothesis that Tgfβ3 may signal through Twist1 and then Snail1 to downregulate E-cadherin expression during palatal fusion.