Retinoic Acid Deficiency Underlies the Etiology of Midfacial Defects

Acute exposure to electronic cigarette components alters mRNA expression of pre-osteoblasts

The use of traditional nicotine delivery products such as tobacco has long been linked to detrimental health effects. However, little work to date has focused on the emerging market of aerosolized nicotine delivery known as electronic nicotine delivery systems (ENDS) or electronic cigarettes, and their potential for new effects on human health. Challenges studying these devices include heterogeneity in the formulation of the common components of most available ENDS, including nicotine and a carrier (commonly composed of propylene glycol and vegetable glycerin, or PG/VG). In the present study, we report on experiments interrogating the effects of major identified components in e-cigarettes. Specifically, the potential concomitant effects of nicotine and common carrier ingredients in commercial "vape" products are explored in vitro to inform the potential health effects on the craniofacial skeleton through novel vectors as compared to traditional tobacco products. MC3T3-E1 murine pre-osteoblast cells were cultured in vitro with clinically relevant liquid concentrations of nicotine, propylene glycol (PG), vegetable glycerin (VG), Nicotine+PG/VG, and the vape liquid of a commercial product (Juul). Cells were treated acutely for 24 h and RNA-Seq was utilized to determine segregating alteration in mRNA signaling. Influential gene targets identified with sparse partial least squares discriminant analysis (sPLS-DA) implemented in mixOmics were assessed using the PANTHER Classification system for molecular functions, biological processes, cellular components, and pathways of effect. Additional endpoint functional analyses were used to confirm cell cycle changes. The initial excitatory concentration (EC50) studied defined a target concentration of carrier PG/VG liquid that altered the cell cycle of the calvarial cells. Initial sPLS-DA analysis demonstrated the segregation of nicotine and non-nicotine exposures utilized in our in vitro modeling. Pathway analysis suggests a strong influence of nicotine exposures on cellular processes including metabolic processes and response to stimuli including autophagic flux. Further interrogation of the individual treatment conditions demonstrated segregation by treatment modality (Control, Nicotine, Carrier (PG+VG), Nicotine+PG/VG) along three dimensions best characterized by: latent variable 1 (PLSDA-1) showing strong segregation based on nicotine influence on cellular processes associated with cellular adhesion to collagen, osteoblast differentiation, and calcium binding and metabolism; latent variable 2 (PLSDA-2) showing strong segregation of influence based on PG+VG and Control influence on cell migration, survival, and cycle regulation; and latent variable 3 (PLSDA-3) showing strong segregation based on Nicotine and Control exposure influence on cell activity and growth and developmental processes. Further, gene co-expression network analysis implicates targets of the major pathway genes associated with bone growth and development, particularly craniofacial (FGF, Notch, TGFβ, WNT) and analysis of active subnetwork pathways found these additionally overrepresented in the Juul exposure relative to Nicotine+PG/VG. Finally, experimentation confirmed alterations in cell count, and increased evidence of cell stress (markers of autophagy), but no alteration in apoptosis. These data suggest concomitant treatment with Nicotine+PG/VG drives alterations in pre-osteoblast cell cycle signaling, specifically transcriptomic targets related to cell cycle and potentially cell stress. Although we suspected cell stress and well as cytotoxic effects of Nicotine+PG/VG, no great influence on apoptotic factors was observed. Further RNA-Seq analysis allowed for the direct interrogation of molecular targets of major pathways involved in bone and craniofacial development, each demonstrating segregation (altered signaling) due to e-cigarette-type exposure. These data have implications directed toward ENDS formulation as synergistic effects of Nicotine+PG/VG are evidenced here. Thus, future research will continue to interrogate how varied formulation of Nicotine+PG/VG affects overall cell functions in multiple vital systems.

Dyrk1a is required for craniofacial development in Xenopus laevis

Loss of function variations in the dual specificity tyrosine-phosphorylation-regulated kinase 1 A (DYRK1A) gene are associated with craniofacial malformations in humans. Here we characterized the effects of deficient DYRK1A in craniofacial development using a developmental model, Xenopus laevis. Dyrk1a mRNA and protein were expressed throughout the developing head and both were enriched in the branchial arches which contribute to the face and jaw. Consistently, reduced Dyrk1a function, using dyrk1a morpholinos and pharmacological inhibitors, resulted in orofacial malformations including hypotelorism, altered mouth shape, slanted eyes, and narrower face accompanied by smaller jaw cartilage and muscle. Inhibition of Dyrk1a function resulted in misexpression of key craniofacial regulators including transcription factors and members of the retinoic acid signaling pathway. Two such regulators, sox9 and pax3 are required for neural crest development and their decreased expression corresponds with smaller neural crest domains within the branchial arches. Finally, we determined that the smaller size of the faces, jaw elements and neural crest domains in embryos deficient in Dyrk1a could be explained by increased cell death and decreased proliferation. This study is the first to provide insight into why craniofacial birth defects might arise in humans with variants of DYRK1A.

Dyrk1a is required for craniofacial development in Xenopus laevis

Loss of function mutations in the dual specificity tyrosine-phosphorylation-regulated kinase 1A (DYRK1A) gene are associated with craniofacial malformations in humans. Here we characterized the effects of deficient DYRK1A in craniofacial development using a developmental model, Xenopus laevis . Dyrk1a mRNA and protein was expressed throughout the developing head and was enriched in the branchial arches which contribute to the face and jaw. Consistently, reduced Dyrk1a function, using dyrk1a morpholinos and pharmacological inhibitors, resulted in orofacial malformations including hypotelorism, altered mouth shape, slanted eyes, and narrower face accompanied by smaller jaw cartilage and muscle. Inhibition of Dyrk1a function resulted in misexpression of key craniofacial regulators including transcription factors and members of the retinoic acid signaling pathway. Two such regulators, sox9 and pax3 are required for neural crest development and their decreased expression corresponds with smaller neural crest domains within the branchial arches. Finally, we determined that the smaller size of the faces, jaw elements and neural crest domains in embryos deficient in Dyrk1a could be explained by increased cell death and decreased proliferation. This study is the first to provide insight into why craniofacial birth defects might arise in humans with DYRK1A mutations.

TFAP2 paralogs regulate midfacial development in part through a conserved ALX genetic pathway

Cranial neural crest development is governed by positional gene regulatory networks (GRNs). Fine-tuning of the GRN components underlies facial shape variation, yet how those networks in the midface are connected and activated remain poorly understood. Here, we show that concerted inactivation of Tfap2a and Tfap2b in the murine neural crest, even during the late migratory phase, results in a midfacial cleft and skeletal abnormalities. Bulk and single-cell RNA-seq profiling reveal that loss of both TFAP2 family members dysregulates numerous midface GRN components involved in midface morphogenesis, patterning and differentiation. Notably, Alx1, Alx3 and Alx4 (ALX) transcript levels are reduced, whereas ChIP-seq analyses suggest TFAP2 family members directly and positively regulate ALX gene expression. Tfap2a, Tfap2b and ALX co-expression in midfacial neural crest cells of both mouse and zebrafish implies conservation of this regulatory axis across vertebrates. Consistent with this notion, tfap2a zebrafish mutants present with abnormal alx3 expression patterns, Tfap2a binds ALX loci and tfap2a-alx3 genetic interactions are observed. Together, these data demonstrate TFAP2 paralogs regulate vertebrate midfacial development in part by activating expression of ALX transcription factor genes.

RA-induced prominence-specific response resulted in distinctive regulation of Wnt and osteogenesis

Proper retinoic acid (RA) signaling is essential for normal craniofacial development. Both excessive RA and RA deficiency in early embryonic stage may lead to a variety of craniofacial malformations, for example, cleft palate, which have been investigated extensively. Dysregulated Wnt and Shh signaling were shown to underlie the pathogenesis of RA-induced craniofacial defects. In our present study, we showed a spatiotemporal-specific effect of RA signaling in regulating early development of facial prominences. Although inhibited Wnt activities was observed in E12.5/E13.5 mouse palatal shelves, early exposure of excessive RA induced Wnt signaling and Wnt-related gene expression in E11.5/E12.5 mouse embryonic frontonasal/maxillary processes. A conserved regulatory network of miR-484-Fzd5 was identified to play critical roles in RA-regulated craniofacial development using RNA-seq. In addition, subsequent osteogenic/chondrogenic differentiation were differentially regulated in discrete mouse embryonic facial prominences in response to early RA induction, demonstrated using both in vitro and in vivo analyses.

TFAP2 paralogs regulate midfacial development in part through a conserved ALX genetic pathway

Cranial neural crest development is governed by positional gene regulatory networks (GRNs). Fine-tuning of the GRN components underly facial shape variation, yet how those in the midface are connected and activated remain poorly understood. Here, we show that concerted inactivation of Tfap2a and Tfap2b in the murine neural crest even during the late migratory phase results in a midfacial cleft and skeletal abnormalities. Bulk and single-cell RNA-seq profiling reveal that loss of both Tfap2 members dysregulated numerous midface GRN components involved in midface fusion, patterning, and differentiation. Notably, Alx1/3/4 (Alx) transcript levels are reduced, while ChIP-seq analyses suggest TFAP2 directly and positively regulates Alx gene expression. TFAP2 and ALX co-expression in midfacial neural crest cells of both mouse and zebrafish further implies conservation of this regulatory axis across vertebrates. Consistent with this notion, tfap2a mutant zebrafish present abnormal alx3 expression patterns, and the two genes display a genetic interaction in this species. Together, these data demonstrate a critical role for TFAP2 in regulating vertebrate midfacial development in part through ALX transcription factor gene expression.

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.

The role of Hedgehog and Notch signaling pathway in cancer

Notch and Hedgehog signaling are involved in cancer biology and pathology, including the maintenance of tumor cell proliferation, cancer stem-like cells, and the tumor microenvironment. Given the complexity of Notch signaling in tumors, its role as both a tumor promoter and suppressor, and the crosstalk between pathways, the goal of developing clinically safe, effective, tumor-specific Notch-targeted drugs has remained intractable. Drugs developed against the Hedgehog signaling pathway have affirmed definitive therapeutic effects in basal cell carcinoma; however, in some contexts, the challenges of tumor resistance and recurrence leap to the forefront. The efficacy is very limited for other tumor types. In recent years, we have witnessed an exponential increase in the investigation and recognition of the critical roles of the Notch and Hedgehog signaling pathways in cancers, and the crosstalk between these pathways has vast space and value to explore. A series of clinical trials targeting signaling have been launched continually. In this review, we introduce current advances in the understanding of Notch and Hedgehog signaling and the crosstalk between pathways in specific tumor cell populations and microenvironments. Moreover, we also discuss the potential of targeting Notch and Hedgehog for cancer therapy, intending to promote the leap from bench to bedside.

Gene-environment interactions in the pathogenesis of common craniofacial anomalies

Craniofacial anomalies often exhibit phenotype variability and non-mendelian inheritance due to their multifactorial origin, involving both genetic and environmental factors. A combination of epidemiologic studies, genome-wide association, and analysis of animal models have provided insight into the effects of gene-environment interactions on craniofacial and brain development and the pathogenesis of congenital disorders. In this chapter, we briefly summarize the etiology and pathogenesis of common craniofacial anomalies, focusing on orofacial clefts, hemifacial microsomia, and microcephaly. We then discuss how environmental risk factors interact with genes to modulate the incidence and phenotype severity of craniofacial anomalies. Identifying environmental risk factors and dissecting their interaction with different genes and modifiers is central to improved strategies for preventing craniofacial anomalies.