Sonic hedgehog signaling and the developing tooth

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.

Oncogenic FGFR Fusions Produce Centrosome and Cilia Defects by Ectopic Signaling

A single primary cilium projects from most vertebrate cells to guide cell fate decisions. A growing list of signaling molecules is found to function through cilia and control ciliogenesis, including the fibroblast growth factor receptors (FGFR). Aberrant FGFR activity produces abnormal cilia with deregulated signaling, which contributes to pathogenesis of the FGFR-mediated genetic disorders. FGFR lesions are also found in cancer, raising a possibility of cilia involvement in the neoplastic transformation and tumor progression. Here, we focus on FGFR gene fusions, and discuss the possible mechanisms by which they function as oncogenic drivers. We show that a substantial portion of the FGFR fusion partners are proteins associated with the centrosome cycle, including organization of the mitotic spindle and ciliogenesis. The functions of centrosome proteins are often lost with the gene fusion, leading to haploinsufficiency that induces cilia loss and deregulated cell division. We speculate that this complements the ectopic FGFR activity and drives the FGFR fusion cancers.

Genetic interactions between the hedgehog co-receptors Gas1 and Boc regulate cell proliferation during murine palatogenesis

Abnormal regulation of Sonic hedgehog (Shh) signaling has been described in a variety of human cancers and developmental anomalies, which highlights the essential role of this signaling molecule in cell cycle regulation and embryonic development. Gas1 and Boc are membrane co-receptors for Shh, which demonstrate overlapping domains of expression in the early face. This study aims to investigate potential interactions between these co-receptors during formation of the secondary palate. Mice with targeted mutation in Gas1 and Boc were used to generate Gas1; Boc compound mutants. The expression of key Hedgehog signaling family members was examined in detail during palatogenesis via radioactive in situ hybridization. Morphometric analysis involved computational quantification of BrdU-labeling and cell packing; whilst TUNEL staining was used to assay cell death. Ablation of Boc in a Gas1 mutant background leads to reduced Shh activity in the palatal shelves and an increase in the penetrance and severity of cleft palate, associated with failed elevation, increased proliferation and reduced cell death. Our findings suggest a dual requirement for Boc and Gas1 during early development of the palate, mediating cell cycle regulation during growth and subsequent fusion of the palatal shelves.

The Role of Sonic Hedgehog in Craniofacial Patterning, Morphogenesis and Cranial Neural Crest Survival

Craniofacial defects (CFD) are a significant healthcare problem worldwide. Understanding both the morphogenetic movements which underpin normal facial development, as well as the molecular factors which regulate these processes, forms the cornerstone of future diagnostic, and ultimately, preventative therapies. The soluble morphogen Sonic hedgehog (Shh), a vertebrate orthologue of Drosophila hedgehog, is a key signalling factor in the regulation of craniofacial skeleton development in vertebrates, operating within numerous tissue types in the craniofacial primordia to spatiotemporally regulate the formation of the face and jaws. This review will provide an overview of normal craniofacial skeleton development, and focus specifically on the known roles of Shh in regulating the development and progression of the first pharyngeal arch, which in turn gives rise to both the upper jaw (maxilla) and lower jaw (mandible).

The role of APCDD1 in epithelial rearrangement in tooth morphogenesis

Adenomatosis polyposis coli downregulated 1 (APCDD1), a negative regulator of Wnt signaling, was examined to understand detailed mechanisms underlying Wnt signaling tooth development. In situ hybridization showed that Apcdd1 was expressed in the condensed mesenchyme at the bud stage, and in the inner enamel epithelium (IEE), including enamel knot (EK) at the cap stage. In vitro organ cultivation by using Apcdd1 antisense oligodeoxynucleotides was performed at E13.5 for 2 days to define the developmental functions of APCDD1 during tooth development. Analysis of histogenesis and cellular events such as cell adhesion, proliferation, apoptosis and epithelial rearrangement after Apcdd1 knockdown showed altered morphogenesis of the tooth germ with decreased cell proliferation and altered localization of cell adhesion molecules. Actin filament staining and 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) labeling of IEE cells showed that Apcdd1 knockdown enhanced epithelial rearrangement in the IEE and EK. To understand the precise signaling regulations of Apcdd1, we evaluated the altered expression patterns of signaling molecules, related with Wnt and enamel knot signalings using RT-qPCR. Tooth germs at cap stage were transplanted into the kidney capsules and were allowed to develop into calcified teeth for 3 weeks. Apcdd1 knockdown increased the number of ectopic cusps on the mesial side of the tooth. Our results suggested that APCDD1 modulates the gene expression of Wnt- and EK-related signaling molecules at the cap stage of tooth development, and is involved in tooth cusp patterning by modulating the epithelial rearrangement in the IEE.

Cyclosporine A up-regulates Sonic hedgehog in gingiva: role of the up-regulation on gingival cell proliferation

BACKGROUND AND OBJECTIVE

Sonic hedgehog protein (SHH) is a mitogen that stimulates cell proliferation. Cyclosporine A enhances the proliferation of gingival cells; however, the relationships of SHH to cyclosporine A or to cyclosporine A-enhanced gingival cell proliferation have not been described.

MATERIAL AND METHODS

Here, we investigated SHH expression in gingiva in vitro and in vivo after cyclosporine A treatment and tested the effect of SHH inhibition on cyclosporine A-enhanced gingival fibroblast proliferation in vitro.

RESULTS

In human gingival fibroblasts, cyclosporine A treatment increased the expression of SHH transcripts and SHH protein, and stimulated cell proliferation; the addition of cyclopamine, an SHH signaling inhibitor, suppressed cyclosporine A-enhanced cell proliferation. Up-regulated expression of SHH and up-regulation of proliferating cell nuclear antigen transcripts and protein were observed in the edentulous gingiva of cyclosporine A-treated rats.

CONCLUSION

Cyclosporine A up-regulates gingival SHH expression in vitro and in vivo, and the inhibition of the SHH pathway counteracts the stimulatory effect of cyclosporine A on gingival fibroblast proliferation. Therefore, we suggest that SHH mediates a novel molecular mechanism for cyclosporine A-induced gingival complications.

Molars and incisors: show your microarray IDs

Background

One of the key questions in developmental biology is how, from a relatively small number of conserved signaling pathways, is it possible to generate organs displaying a wide range of shapes, tissue organization, and function. The dentition and its distinct specific tooth types represent a valuable system to address the issues of differential molecular signatures. To identify such signatures, we performed a comparative transcriptomic analysis of developing murine lower incisors, mandibular molars and maxillary molars at the developmental cap stage (E14.5).

Results

231 genes were identified as being differentially expressed between mandibular incisors and molars, with a fold change higher than 2 and a false discovery rate lower than 0.1, whereas only 96 genes were discovered as being differentially expressed between mandibular and maxillary molars. Numerous genes belonging to specific signaling pathways (the Hedgehog, Notch, Wnt, FGF, TGFβ/BMP, and retinoic acid pathways), and/or to the homeobox gene superfamily, were also uncovered when a less stringent fold change threshold was used. Differential expressions for 10 out of 12 (mandibular incisors versus molars) and 9 out of 10 selected genes were confirmed by quantitative reverse transcription-PCR (qRT-PCR). A bioinformatics tool (Ingenuity Pathway Analysis) used to analyze biological functions and pathways on the group of incisor versus molar differentially expressed genes revealed that 143 genes belonged to 9 networks with intermolecular connections. Networks with the highest significance scores were centered on the TNF/NFκB complex and the ERK1/2 kinases. Two networks ERK1/2 kinases and tretinoin were involved in differential molar morphogenesis.

Conclusion

These data allowed us to build several regulatory networks that may distinguish incisor versus molar identity, and may be useful for further investigations of these tooth-specific ontogenetic programs. These programs may be dysregulated in transgenic animal models and related human diseases leading to dental anomalies.

Common developmental pathways link tooth shape to regeneration

In many non-mammalian vertebrates, adult dentitions result from cyclical rounds of tooth regeneration wherein simple unicuspid teeth are replaced by more complex forms. Therefore and by contrast to mammalian models, the numerical majority of vertebrate teeth develop shape during the process of replacement. Here, we exploit the dental diversity of Lake Malawi cichlid fishes to ask how vertebrates generally replace their dentition and in turn how this process acts to influence resulting tooth morphologies. First, we used immunohistochemistry to chart organogenesis of continually replacing cichlid teeth and discovered an epithelial down-growth that initiates the replacement cycle via a labial proliferation bias. Next, we identified sets of co-expressed genes from common pathways active during de novo, lifelong tooth replacement and tooth morphogenesis. Of note, we found two distinct epithelial cell populations, expressing markers of dental competence and cell potency, which may be responsible for tooth regeneration. Related gene sets were simultaneously active in putative signaling centers associated with the differentiation of replacement teeth with complex shapes. Finally, we manipulated targeted pathways (BMP, FGF, Hh, Notch, Wnt/β-catenin) in vivo with small molecules and demonstrated dose-dependent effects on both tooth replacement and tooth shape. Our data suggest that the processes of tooth regeneration and tooth shape morphogenesis are integrated via a common set of molecular signals. This linkage has subsequently been lost or decoupled in mammalian dentitions where complex tooth shapes develop in first generation dentitions that lack the capacity for lifelong replacement. Our dissection of the molecular mechanics of vertebrate tooth replacement coupled to complex shape pinpoints aspects of odontogenesis that might be re-evolved in the lab to solve problems in regenerative dentistry.

Apoptotic signaling in mouse odontogenesis

Apoptosis is an important morphogenetic event in embryogenesis as well as during postnatal life. In the last 2 decades, apoptosis in tooth development (odontogenesis) has been investigated with gradually increasing focus on the mechanisms and signaling pathways involved. The molecular machinery responsible for apoptosis exhibits a high degree of conservation but also organ and tissue specific patterns. This review aims to discuss recent knowledge about apoptotic signaling networks during odontogenesis, concentrating on the mouse, which is often used as a model organism for human dentistry. Apoptosis accompanies the entire development of the tooth and corresponding remodeling of the surrounding bony tissue. It is most evident in its role in the elimination of signaling centers within developing teeth, removal of vestigal tooth germs, and in odontoblast and ameloblast organization during tooth mineralization. Dental apoptosis is caspase dependent and proceeds via mitochondrial mediated cell death with possible amplification by Fas-FasL signaling modulated by Bcl-2 family members.

The homology of odontodes in gnathostomes: insights from Dlx gene expression in the dogfish, Scyliorhinus canicula

BACKGROUND

Teeth and tooth-like structures, together named odontodes, are repeated organs thought to share a common evolutionary origin. These structures can be found in gnathostomes at different locations along the body: oral teeth in the jaws, teeth and denticles in the oral-pharyngeal cavity, and dermal denticles on elasmobranch skin. We, and other colleagues, had previously shown that teeth in any location were serially homologous because: i) pharyngeal and oral teeth develop through a common developmental module; and ii) the expression patterns of the Dlx genes during odontogenesis were highly divergent between species but almost identical between oral and pharyngeal dentitions within the same species. Here we examine Dlx gene expression in oral teeth and dermal denticles in order to test the hypothesis of serial homology between these odontodes.

RESULTS

We present a detailed comparison of the first developing teeth and dermal denticles (caudal primary scales) of the dogfish (Scyliorhinus canicula) and show that both odontodes develop through identical stages that correspond to the common stages of oral and pharyngeal odontogenesis. We identified six Dlx paralogs in the dogfish and found that three showed strong transcription in teeth and dermal denticles (Dlx3, Dlx4 and Dlx5) whereas a weak expression was detected for Dlx1 in dermal denticles and teeth, and for Dlx2 in dermal denticles. Very few differences in Dlx expression patterns could be detected between tooth and dermal denticle development, except for the absence of Dlx2 expression in teeth.

CONCLUSIONS

Taken together, our histological and expression data strongly suggest that teeth and dermal denticles develop from the same developmental module and under the control of the same set of Dlx genes. Teeth and dermal denticles should therefore be considered as serial homologs developing through the initiation of a common gene regulatory network (GRN) at several body locations. This mechanism of heterotopy supports the 'inside and out' model that has been recently proposed for odontode evolution.

Sonic hedgehog stimulates proliferation of human periodontal ligament stem cells

Regulation of cell renewal in the periodontium is a critical cell function that has not been clarified. Sonic hedgehog (Shh) is a secreted signaling molecule that plays a key role during development and adult tissue homeostasis. In the present study, we have analyzed the role played by Shh in human periodontal ligament stem cell (HPLSC) proliferation. HPLSC were isolated with anti-STRO-1 antibodies. Shh increased the expression of GLI1 and PTC-1 and selectively stimulated cell proliferation in STRO-1(+) derived from adult periodontal ligament. Shh components were localized to primary cilia in STRO-1(+) cells after Shh stimulation. STRO-1(+) also expressed Shh, suggesting an autocrine-regulated phenomenon. Thus, we propose that Shh plays a critical role in the regulation of cell proliferation in STRO-1(+)/HPLSC.

Hedgehog signaling is required at multiple stages of zebrafish tooth development

Background

The accessibility of the developing zebrafish pharyngeal dentition makes it an advantageous system in which to study many aspects of tooth development from early initiation to late morphogenesis. In mammals, hedgehog signaling is known to be essential for multiple stages of odontogenesis; however, potential roles for the pathway during initiation of tooth development or in later morphogenesis are incompletely understood.

Results

We have identified mRNA expression of the hedgehog ligands shha and the receptors ptc1 and ptc2 during zebrafish pharyngeal tooth development. We looked for, but did not detect, tooth germ expression of the other known zebrafish hedgehog ligands shhb, dhh, ihha, or ihhb, suggesting that as in mammals, only Shh participates in zebrafish tooth development. Supporting this idea, we found that morphological and gene expression evidence of tooth initiation is eliminated in shha mutant embryos, and that morpholino antisense oligonucleotide knockdown of shha, but not shhb, function prevents mature tooth formation. Hedgehog pathway inhibition with the antagonist compound cyclopamine affected tooth formation at each stage in which we applied it: arresting development at early stages and disrupting mature tooth morphology when applied later. These results suggest that hedgehog signaling is required continuously during odontogenesis. In contrast, over-expression of shha had no effect on the developing dentition, possibly because shha is normally extensively expressed in the zebrafish pharyngeal region.

Conclusion

We have identified previously unknown requirements for hedgehog signaling for early tooth initiation and later morphogenesis. The similarity of our results with data from mouse and other vertebrates suggests that despite gene duplication and changes in the location of where teeth form, the roles of hedgehog signaling in tooth development have been largely conserved during evolution.

Signaling pathways regulating the expression of Prx1 and Prx2 in the chick mandibular mesenchyme

Prx1 and Prx2 are members of the aristaless-related homeobox genes shown to play redundant but essential roles in morphogenesis of the mandibular processes. To gain insight into the signaling pathways that regulate expression of Prx genes in the mandibular mesenchyme, we used the chick as a model system. We examined the patterns of gene expression in the face and the roles of signals derived from the epithelium on the expression of Prx genes in the mandibular mesenchyme. Our results demonstrated stage-dependent roles of mandibular epithelium on the expression of Prx in the mandibular mesenchyme and provide evidence for positive roles of members of the fibroblast and hedgehog families derived from mandibular epithelium on the expression of Prx genes in the mandibular mesenchyme. Our studies suggest that endothelin-1 signaling derived from the mesenchyme is involved in restricting the expression of Prx2 to the medial mandibular mesenchyme.

Essential role of ADAM28 in regulating the proliferation and differentiation of human dental papilla mesenchymal cells (hDPMCs)

Dental papilla mesenchymal cells (DPMCs) have been supposed to possess the relatively independent and critical role for tooth development and morphogenesis. Here, we characterized the role of ADAM28, a member of a disintegrin and metalloproteinase (ADAM) family, in the regulative mechanisms of odontogenic capability of hDPMCs. Immunofluorescence staining showed the ubiquitous expression of ADAM28 in multiple human dental mesenchymal and epithelial cells. After confirming the effect of eukaryotic expression plasmid containing ADAM28 coding region and ADAM28 antisense oligodeoxynucleotide (AS-ODN), we respectively transfected them into hDPMCs and observed the biological markers for proliferation and differentiation. Overexpression of ADAM28 favored the proliferation and lineage-specific differentiation of hDPMCs, while blockage of ADAM28 exerted the opposite effects and induced apoptosis. These results identified an unrecognized hypothesis that ADAM28 may function as positive regulator of growth and differentiation of hDPMCs and act as an important molecule mediating reciprocal epithelial-mesenchymal signaling during tooth organ development.

Genetische Störungen der Zahnentwicklung und Dentition

Die Zähne sind Organe, die aus ektodermalen epithelialen Aussackungen im Bereich des 1. Kiemenbogens entstehen, gesteuert von epitheliomesenchymalen Interaktionen. Dabei spielen zahlreiche Signalmoleküle speziell der 4 großen Familien TGF-β, FGF, Hedgehog und WNT sowie diverse Transkriptionsfaktoren eine Rolle. Eine Beteiligung der Retinoide an der Odontogenese ist durch umfangreiche Befunde belegt, auch wenn die Inaktivierung relevanter Gene in Mausmodellen meist keine Zahnanomalien verursacht. Die Zahnentwicklung wird klassischerweise in verschiedene Stadien eingeteilt: Entstehung der Zahnleiste, der Zahnknospe, der Schmelzkappe, der Schmelzglocke, die Wurzelbildung und der Zahndurchbruch. Anomalien der Zahnentwicklung können isoliert oder gemeinsam mit anderen Symptomen im Zusammenhang mit Syndromen auftreten. Sie können genetisch bedingt sein oder unter Einwirkung teratogener Stoffe während der Bildung und Mineralisierung der Zahnkeime zustande kommen. Dentibukkale Entwicklungsanomalien treten im Kontext seltener Erkrankungen auf und finden zunehmend Beachtung, da sie bei bestimmten Erkrankungen in der Diagnostik und als prädikative Faktoren wichtige Anhaltspunkte geben können. Allerdings ist hierfür eine interdisziplinäre und internationale Kooperation notwendig, die bislang erst in Ansätzen verwirklicht wurde.

Stem cells and tooth tissue engineering

The notion that teeth contain stem cells is based on the well-known repairing ability of dentin after injury. Dental stem cells have been isolated according to their anatomical locations, colony-forming ability, expression of stem cell markers, and regeneration of pulp/dentin structures in vivo. These dental-derived stem cells are currently under increasing investigation as sources for tooth regeneration and repair. Further attempts with bone marrow mesenchymal stem cells and embryonic stem cells have demonstrated the possibility of creating teeth from non-dental stem cells by imitating embryonic development mechanisms. Although, as in tissue engineering of other organs, many challenges remain, stem-cell-based tissue engineering of teeth could be a choice for the replacement of missing teeth in the future.

The primary enamel knot determines the position of the first buccal cusp in developing mice molars

The enamel knot (EK), which is located in the center of bud and cap stage tooth germs, is a transitory cluster of non-dividing epithelial cells. The EK acts as a signaling center that provides positional information for tooth morphogenesis and regulates the growth of tooth cusps by inducing secondary EKs. The morphological, cellular, and molecular events leading to the relationship between the primary and secondary EKs have not been described clearly. This study investigated the relationship between the primary and secondary EKs in the maxillary and mandibular first molars of mice. The location of the primary EK and secondary EKs was investigated by chasing Fgf4 expression patterns in tooth germ at some intervals of in vitro culture, and the relationship between the primary EK and secondary EK was examined by tracing the primary EK cells in the E13.5 tooth germs which were frontally half sliced to expose the primary EK. After 48 hr, the primary EK cells in the sliced tooth germs were located on the buccal secondary EKs, which correspond to the future paracone in maxilla and protoconid in mandible. The Bmp4 expression in buccal part of the dental mesenchyme might be related with the lower growth in buccal epithelium than in lingual epithelium, and the Msx2 expressing area in epithelium was overlapped with the enamel cord (or septum) and cell dense area. The enamel cord might connect the primary EK with enamel navel to fix the location of the primary EK in the buccal side during the cap to bell stages. Overall, these results suggest that primary EK cells strictly contribute to form the paracone or protoconid, which are the main cusps of the tooth in the maxilla or mandible.

Differentiation of an ameloblast-lineage cell line (ALC) is induced by Sonic hedgehog signaling

Sonic hedgehog (Shh), one of the essential molecules for embryogenesis and organogenesis, is strongly expressed in the enamel knot, which represents the signaling center for odontogenesis due to the presence of essential secretory molecules. Here, we investigated the effects of Shh on ameloblast differentiation using a mouse ameloblast-lineage cell line (ALC). In the presence of Shh, the expression of amelogenin and ameloblastin was up-regulated, and their promoter activities were also elevated. Subcutaneous implantation of Shh-treated ALC into the dorsa of C57/Bl6J mouse caused increased expression of amelogenin and ameloblastin. Forced expression of Gli1, a major transcription factor in Shh signaling, up-regulated both amelogenin and ameloblastin, while Gli1-specific RNAi blocked their expression. These results suggest that Shh directly promotes ALC differentiation, and that Gli1 may be responsible for the signal mediation of Shh.

CD 95 mediated apoptosis in embryogenesis: implication in tooth development

INTRODUCTION

Understanding of apoptotic mechanisms involved in tissue shaping is of particular interest because of possible targeted modulation of the development of organ structures such as teeth. Research of CD 95 mediated apoptosis has been focused particularly on cell death in the immune system and related disorders. However, CD 95 mediated apoptosis is also involved in embryogenesis of many organs as the kidney, the lung, the intestine and tissue networks such as the nervous system.

DESIGN

Narrative review.

RESULTS

This review briefly summarizes the current knowledge of CD 95 mediated apoptosis in embryogenesis with possible implication in tooth development. CD 95 receptor and CD 95 ligand are found at early stages of tooth development. The data suggest some positive correlations with dental apoptosis distribution, particularly in the primary enamel knot where apoptosis occurs during elimination of this structure. CD 95 deficient (lpr) adult mouse tooth phenotype, however, did not show any alterations in final tooth pattern and morphology.

CONCLUSION

To date studies of apoptotic machinery during tooth development show spatial localization of many of the components together with precise and localized timing of cell death. There is still much to be learned about the regulation and importance of apoptosis in tooth development. Nevertheless, the involvement of apoptotic regulatory mechanisms interplaying with other molecules participates to the cellular cross-talk in developing tissues, which opens possible targeted modulations as suggested, e.g. for future molecular dentistry.