Osr2 acts downstream of Pax9 and interacts with both Msx1 and Pax9 to pattern the tooth developmental field

Pax9 drives development of the upper jaw but not teeth in zebrafish

Loss of dentition has occurred repeatedly throughout vertebrate evolution. Cyprinid fish, including zebrafish, form teeth only deep within the pharynx, not on their oral jaws. However, zebrafish still robustly express transcription factors associated with mammalian tooth development in the neural crest-derived mesenchyme surrounding the mouth. We investigated whether this expression is vestigial or whether these factors contribute to the formation of non-tooth mesenchymal structures in the oral region, using Pax9 as a test case. Zebrafish homozygous for two different pax9 mutant alleles develop the normal complement of pharyngeal teeth but fail to form the premaxilla bone, most of the maxilla, and nasal and maxillary barbels. Lack of most of the upper jaw complex does not preclude effective feeding in the laboratory environment. We observe a significant deficit of sp7:EGFP + osteoblasts and adjacent alx4a:DsRed+ condensing mesenchyme around the maxilla, and no accumulation of either in the premaxillary domain. Loss of pax9 may prevent osteoprogenitors from maintaining the state of condensation required for full osteogenic differentiation. We conclude that Pax9 is not unequivocally required for all vertebrate tooth development but instead may be involved in the development of a variety of organs forming through mesenchymal condensation around the mouth.

Four Novel PAX9 Variants and the PAX9-Related Non-Syndromic Tooth Agenesis Patterns

The purpose of this research was to investigate and identify PAX9 gene variants in four Chinese families with non-syndromic tooth agenesis. We identified pathogenic gene variants by whole-exome sequencing (WES) and Sanger sequencing and then studied the effects of these variants on function by bioinformatics analysis and in vitro experiments. Four novel PAX9 heterozygous variants were identified: two missense variants (c.191G > T (p.G64V) and c.350T > G (p.V117G)) and two frameshift variants (c.352delC (p.S119Pfs*2) and c.648_649insC(p.Y217Lfs*100)). The bioinformatics analysis showed that these variants might be pathogenic. The tertiary structure analysis showed that these four variants could cause structural damage to PAX9 proteins. In vitro functional studies demonstrated that (1) the p.Y217Lfs*100 variant greatly affects mRNA stability, thereby affecting endogenous expression; (2) the p. S119Pfs* 2 variant impairs the subcellular localization of the nuclear expression of the wild-type PAX9 protein; and (3) the four variants (p.G64V, p.V117G, p.S119Pfs*2, and p.Y217Lfs*100) all significantly affect the downstream transcriptional activity of the BMP4 gene. In addition, we summarized and analyzed tooth missing positions caused by PAX9 variants and found that the maxillary second molar (84.11%) and mandibular second molar (84.11%) were the most affected tooth positions by summarizing and analyzing the PAX9-related non-syndromic tooth agenesis positions. Our results broaden the variant spectrum of the PAX9 gene related to non-syndromic tooth agenesis and provide useful information for future genetic counseling.

Genome-Wide Estimates of Runs of Homozygosity, Heterozygosity, and Genetic Load in Two Chinese Indigenous Goat Breeds

Runs of homozygosity (ROH) and heterozygosity (ROHet) are windows into population demographic history and adaptive evolution. Numerous studies have shown that deleterious mutations are enriched in the ROH of humans, pigs, cattle, and chickens. However, the relationship of deleterious variants to ROH and the pattern of ROHet in goats have been largely understudied. Here, 240 Guangfeng and Ganxi goats from Jiangxi Province, China, were genotyped using the Illumina GoatSNP50 BeadChip and genome-wide ROH, ROHet, and genetic load analyses were performed in the context of 32 global goat breeds. The classes with the highest percentage of ROH and ROHet were 0.5–2 Mb and 0.5–1 Mb, respectively. The results of inbreeding coefficients (based on SNP and ROH) and ROHet measurements showed that Guangfeng goats had higher genetic variability than most Chinese goats, while Ganxi goats had a high degree of inbreeding, even exceeding that of commercial goat breeds. Next, the predicted damaging homozygotes were more enriched in long ROHs, especially in Guangfeng goats. Therefore, we suggest that information on damaging alleles should also be incorporated into the design of breeding and conservation programs. A list of genes related to fecundity, growth, and environmental adaptation were identified in the ROH hotspots of two Jiangxi goats. A sense-related ROH hotspot (chromosome 12: 50.55–50.81 Mb) was shared across global goat breeds and may have undergone selection prior to goat domestication. Furthermore, an identical ROHet hotspot (chromosome 1: 132.21–132.54 Mb) containing two genes associated with embryonic development (STAG1 and PCCB) was detected in domestic goat breeds worldwide. Tajima’s D and BetaScan2 statistics indicated that this region may be caused by long-term balancing selection. These findings not only provide guidance for the design of conservation strategies for Jiangxi goat breeds but also enrich our understanding of the adaptive evolution of goats.

The protoconid: a key cusp in lower molars. Evidence from a recent modern human population

BACKGROUND

The molar (M) size sequence in the genus Homo is decreasing and the general pattern in Homo sapiens is M1 > M2 > M3.

AIM

To gain a better understanding of the reduction patterns of molar components (cusps), we aim to assess the area of the protoconid, the phylogenetically oldest cusp of the lower molars.

SUBJECT AND METHODS

We measured the protoconid and the total crown area in the scaled photographs of a recent modern human sample of lower molars (76 males and 39 females). The values were statistically analysed.

RESULTS

The absolute size of the protoconid increases significantly between M1 and M2/M3, whereas the relative size of this cusp increases significantly from M1 to M3. In the latter, reduction or disappearance of the cusps of the talonid is common.

CONCLUSIONS

The results can be explained in the framework of the patterning cascade model. As the first cusp to appear developmentally, the protoconid forms in response to signals from the primary enamel knot, likely contributing to its stability. Inhibitory signals emitted during the protoconid formation may lead to the reduction or disappearance of the talonid cusps, if these do not have enough time to form before the end of the molar morphogenetic process.

Crucial Roles of microRNA-16-5p and microRNA-27b-3p in Ameloblast Differentiation Through Regulation of Genes Associated With Amelogenesis Imperfecta

Amelogenesis imperfecta is a congenital disorder within a heterogeneous group of conditions characterized by enamel hypoplasia. Patients suffer from early tooth loss, social embarrassment, eating difficulties, and pain due to an abnormally thin, soft, fragile, and discolored enamel with poor aesthetics and functionality. The etiology of amelogenesis imperfecta is complicated by genetic interactions. To identify mouse amelogenesis imperfecta-related genes (mAIGenes) and their respective phenotypes, we conducted a systematic literature review and database search and found and curated 70 mAIGenes across all of the databases. Our pathway enrichment analysis indicated that these genes were enriched in tooth development-associated pathways, forming four distinct groups. To explore how these genes are regulated and affect the phenotype, we predicted microRNA (miRNA)-gene interaction pairs using our bioinformatics pipeline. Our miRNA regulatory network analysis pinpointed that miR-16-5p, miR-27b-3p, and miR-23a/b-3p were hub miRNAs. The function of these hub miRNAs was evaluated through ameloblast differentiation assays with/without the candidate miRNA mimics using cultured mouse ameloblast cells. Our results revealed that overexpression of miR-16-5p and miR-27b-3p, but not miR-23a/b-3p, significantly inhibited ameloblast differentiation through regulation of mAIGenes. Thus, our study shows that miR-16-5p and miR-27b-3p are candidate pathogenic miRNAs for amelogenesis imperfecta.

Testing the inhibitory cascade model in a recent human sample

The Inhibitory Cascade Model was proposed by Kavanagh and colleagues (Nature, 449, 427-433 [2007]) after their experimental studies on the dental development of murine rodent species. These authors described an activator-inhibitor mechanism that has been employed to predict evolutionary size patterns of mammalian teeth, including hominins. In the present study, we measured the crown area of the three lower permanent molars (M1, M2, and M3) of a large recent modern human sample of male and female individuals from a collection preserved at the Institute of Anthropology of the University of Coimbra (Portugal). The main aim of the present study is to test if the size molar patterns observed in this human sample fits the Inhibitory Cascade Model. For this purpose, we first measured the crown area in those individuals preserving the complete molar series. Measurements were taken in photographs, using a planimeter and following well-tested techniques used in previous works. We then plot the M3 /M1 and M2 /M1  size ratios. Our results show that the premise of the Inhibitory Cascade Model, according to which the average of the crown area of M2 is approximately one-third of the sum of the crown area of the three molars, is fulfilled. However, our results also show that the individual values of a significant number of males and females are out of the 95% confidence interval predicted by the Inhibitory Cascade Model in rodents. As a result, the present analyses suggest that neither the sample of males, nor that of females, nor the pooled sample fits the Inhibitory Cascade Model. It is important to notice that, although this model has been successfully tested in a large number of current human populations, to the best of our knowledge this is the first study in which individual data have been obtained in a recent human population rather than using the average of the sample. Our results evince that, at the individual level, some factors not yet known could interfere with this model masking the modulation of the size on the molar series in modern humans. We suggest that the considerable delay in the onset of M3 formation in modern humans could be related to a weakening of the possible activation/inhibition process for this tooth. Finally, and in support of our conclusions, we have checked that the absolute and relative size of M1 and M2 is not related to the M3 agenesis in our sample. In line with other studies in primates, our results do not support the Inhibitory Cascade Model in a recent human sample. Further research is needed to better understand the genetic basis of this mechanism and its relationship to the phenotype. In this way, we may be able to find out which evolutionary changes may be responsible for the deviations observed in many species, including Homo sapiens.

Individual Limb Muscle Bundles Are Formed through Progressive Steps Orchestrated by Adjacent Connective Tissue Cells during Primary Myogenesis

Although the factors regulating muscle cell differentiation are well described, we know very little about how differentiating muscle fibers are organized into individual muscle tissue bundles. Disruption of these processes leads to muscle hypoplasia or dysplasia, and replicating these events is vital in tissue engineering approaches. We describe the progressive cellular events that orchestrate the formation of individual limb muscle bundles and directly demonstrate the role of the connective tissue cells that surround muscle precursors in controlling these events. We show how disruption of gene activity within or genetic ablation of connective tissue cells impacts muscle precursors causing disruption of muscle bundle formation and subsequent muscle dysplasia and hypoplasia. We identify several markers of the populations of connective tissue cells that surround muscle precursors and provide a model for how matrix-modifying proteoglycans secreted by these cells may influence muscle bundle formation by effects on the local extracellular matrix (ECM) environment.

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Characterization of the events that prefigure the formation of individual muscle bundles

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Direct demonstration of the role of connective tissue cells in muscle morphogenesis

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Identification of markers of limb irregular connective tissue (ICT)

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Demonstration of molecularly distinct ICT subdomains in the limb

Innovative Molecular and Cellular Therapeutics in Cleft Palate Tissue Engineering

Clefts of the lip and/or palate are the most prevalent orofacial birth defects occurring in about 1:700 live human births worldwide. Early postnatal surgical interventions are extensive and staged to bring about optimal growth and fusion of palatal shelves. Severe cleft defects pose a challenge to correct with surgery alone, resulting in complications and sequelae requiring life-long, multidisciplinary care. Advances made in materials science innovation, including scaffold-based delivery systems for precision tissue engineering, now offer new avenues for stimulating bone formation at the site of surgical correction for palatal clefts. In this study, we review the present scientific literature on key developmental events that can go awry in palate development and the common surgical practices and challenges faced in correcting cleft defects. How key osteoinductive pathways implicated in palatogenesis inform the design and optimization of constructs for cleft palate correction is discussed within the context of translation to humans. Finally, we highlight new osteogenic agents and innovative delivery systems with the potential to be adopted in engineering-based therapeutic approaches for the correction of palatal defects. Impact statement Tissue-engineered scaffolds supplemented with osteogenic growth factors have attractive, largely unexplored possibilities to modulate molecular signaling networks relevant to driving palatogenesis in the context of congenital anomalies (e.g., cleft palate). Constructs that address this need may obviate current use of autologous bone grafts, thereby avoiding donor-site morbidity and other regenerative challenges in patients afflicted with palatal clefts. Combinations of biomaterials and drug delivery of diverse regenerative cues and biologics are currently transforming strategies exploited by engineers, scientists, and clinicians for palatal cleft repair.

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.

Primary cilia in murine palatal rugae development

Periodic patterning of iterative structures is a fundamental process during embryonic development, since these structures are diverse across the animal kingdom. Therefore, elucidating the molecular mechanisms in the formation of these structures promotes understanding of the process of organogenesis. Periodically patterned ridges, palatal rugae (situated on the hard palate of mammals), are an excellent experimental model to clarify the molecular mechanisms involved in the formation of periodic patterning of iterative structures. Primary cilia are involved in many biological events, including the regulation of signaling pathways such as Shh and non-canonical Wnt signaling. However, the role of primary cilia in the development of palatal rugae remains unclear. We found that primary cilia were localized to the oral cavity side of the interplacode epithelium of the palatal rugae, whereas restricted localization of primary cilia could not be detected in other regions. Next, we generated mice with a placodal conditional deletion of the primary cilia protein Ift88, using ShhCre mice (Ift88^fl/fl;ShhCre). Highly disorganized palatal rugae were observed in Ift88^fl/fl;ShhCre mice. Furthermore, by comparative in situ hybridization analysis, many Shh and non-canonical Wnt signaling-related molecules showed spatiotemporal expression patterns during palatal rugae development, including restricted expression in the epithelium (placodes and interplacodes) and mesenchyme. Some of these expression were found to be altered in Ift88^fl/fl;ShhCre mice. Primary cilia is thus involved in development of palatal rugae.

The Function and Regulatory Network of Pax9 Gene in Palate Development

Cleft palate, a common congenital deformity, can arise from disruptions in any stage of palatogenesis, including palatal shelf growth, elevation, adhesion, and fusion. Paired box gene 9 (Pax9) is recognized as a vital regulator of palatogenesis with great relevance to cleft palate in humans and mice. Pax9-deficient murine palatal shelves displayed deficient elongation, postponed elevation, failed contact, and fusion. Pax9 is expressed in epithelium and mesenchyme, exhibiting a dynamic expression pattern that changes according to the proceeding of palatogenesis. Recent studies highlighted the Pax9-related genetic interactions and their critical roles during palatogenesis. During palate growth, PAX9 interacts with numerous molecules and members of pathways (e.g., OSR2, FGF10, SHOS2, MSX1, BARX1, TGFβ3, LDB1, BMP, WNT β-catenin dependent, and EDA) in the mesenchyme and functions as a key mediator in epithelial-mesenchymal communications with FGF8, TBX1, and the SHH pathway. During palate elevation, PAX9 is hypothesized to mediate the time point of the elevation event in the anterior and posterior parts of the palatal shelves. The delayed elevation of Pax9 mutant palatal shelves probably results from abnormal expressions of a series of genes ( Osr2 and Bmpr1a) leading to deficient palate growth, abnormal tongue morphology, and altered hyaluronic acid distribution. The interactions between PAX9 and genes encoding the OSR2, TGFβ3, and WNT β-catenin-dependent pathways provide evidence that PAX9 might participate in the regulation of palate fusion. This review summarizes the current understanding of PAX9’s functions and emphasizes the interactions between PAX9 and vital genes during palatogenesis. We hope to provide some clues for further exploration of the function and mechanism of PAX9, especially during palate elevation and fusion events.

Upper jaw development in the absence of teeth: New insights for craniodental evo-devo integration

In p63-null mice (p63^-/- ), teeth fail to form but the mandible forms normally; conversely, the upper jaw skeleton is malformed. Here we explored whether lack of dental tissues contributed to midfacial dysmorphologies in p63^-/- mice by testing if facial prominence defects appeared before odontogenesis failed. We also investigated gene dose effects by testing if one wild type (WT) p63 allele (p63^+/- ) was sufficient for normal upper jaw skeleton formation. We micro-CT scanned PFA-fixed p63^-/- , p63^+/- , and WT (p63^+/+ ) adult and embryonic mice aged E10-E14. Next, we landmarked mandibular (MdP), maxillary (MxP) and nasal prominences (NPs), and facial bones. 3D landmark data were assessed using Principal Component, Canonical Variate, Partial Least Squares, and other statistical analyses. The p63^-/- embryos showed MxP and NP malformations by E12, despite the presence of dental tissues. MdP shape was comparable among p63^-/- , p63^+/- , and p63^+/+ embryos. Upper jaw shape was comparable between p63^+/+ and p63^+/- adults. The upper jaw and its dentition both require p63 signaling, but not each other's presence, to form properly. One WT p63 allele enables normal midfacial morphogenesis; gene dose may be a target for jaw macroevolution. Jaw-specific genetic mechanisms likely integrate the evo-devo of dentitions with upper versus lower jaws.

Gene datasets associated with mouse cleft palate

This article presents data on genes associated with cleft palate (CP), retrieved through both a full-text systematic review and a mouse genome informatics (MGI) database search. In order to group CP-associated genes according to function, pathway, biological process, and cellular component, the genes were analyzed using category enrichment bioinformatics tools, the Kyoto Encyclopedia of Genes and Genomes (KEGG) and Gene Ontology (GO). This approach provides invaluable opportunities for the identification of candidate pathways and genes in CP research.

Differential Expression of Long Noncoding RNAs in Normal and Inflamed Human Dental Pulp

INTRODUCTION

Dental pulp inflammation is an excellent model for the interaction between tissue inflammation and regenerative processes. It is worthwhile to better understand molecular signaling of repair and regeneration in inflammatory processes. Emerging evidence suggests that long noncoding RNA (lncRNA) participates in immune system inflammatory processes. Here we investigate the expression of lncRNAs in pulpitis, the inflammation of dental pulp tissue, and identify lncRNAs that possibly participate in inflammation responses and odontogenesis.

METHODS

Integrated comparative lncRNA microarray was used to examine lncRNA and mRNA expression between inflamed and normal human pulp tissue. The differential expression of lncRNAs and mRNAs was then validated by quantitative real-time polymerase chain reaction. A bioinformatics analysis was performed to predict the function of differentially expressed lncRNAs and mRNAs.

RESULTS

Our data indicated 752 lncRNAs were significantly differentially expressed (fold change > 2.0; P < .05) in inflamed pulp tissues compared with normal pulp tissues, including 338 upregulated and 414 downregulated lncRNAs. Among the 646 differentially expressed mRNAs (fold change > 2.0; P < .05), 460 were upregulated, and 186 were downregulated. The differentially downregulated genes are enriched for gene ontology terms related to odontogenesis and cell development in biological processes, whereas the differentially upregulated genes are related to immune and inflammation processes.

CONCLUSIONS

LncRNAs are differentially expressed in inflamed human pulp tissue compared with normal pulp tissue, indicating that lncRNAs may play key roles in pulpitis pathogenesis and development.

Small-molecule Wnt agonists correct cleft palates in Pax9 mutant mice in utero

Clefts of the palate and/or lip are among the most common human craniofacial malformations and involve multiple genetic and environmental factors. Defects can only be corrected surgically and require complex life-long treatments. Our studies utilized the well-characterized Pax9^-/- mouse model with a consistent cleft palate phenotype to test small-molecule Wnt agonist therapies. We show that the absence of Pax9 alters the expression of Wnt pathway genes including Dkk1 and Dkk2, proven antagonists of Wnt signaling. The functional interactions between Pax9 and Dkk1 are shown by the genetic rescue of secondary palate clefts in Pax9^-/-Dkk1^f/+;Wnt1Cre embryos. The controlled intravenous delivery of small-molecule Wnt agonists (Dkk inhibitors) into pregnant Pax9^+/- mice restored Wnt signaling and led to the growth and fusion of palatal shelves, as marked by an increase in cell proliferation and osteogenesis in utero, while other organ defects were not corrected. This work underscores the importance of Pax9-dependent Wnt signaling in palatogenesis and suggests that this functional upstream molecular relationship can be exploited for the development of therapies for human cleft palates that arise from single-gene disorders.

Anti-EDAR Agonist Antibody Therapy Resolves Palate Defects in Pax9-/- Mice

To date, surgical interventions are the only means by which craniofacial anomalies can be corrected so that function, esthetics, and the sense of well-being are restored in affected individuals. Unfortunately, for patients with cleft palate-one of the most common of congenital birth defects-treatment following surgery is prolonged over a lifetime and often involves multidisciplinary regimens. Hence, there is a need to understand the molecular pathways that control palatogenesis and to translate such information for the development of noninvasive therapies that can either prevent or correct cleft palates in humans. Here, we use the well-characterized model of the Pax9^-/- mouse, which displays a consistent phenotype of a secondary cleft palate, to test a novel therapeutic. Specifically, we demonstrate that the controlled intravenous delivery of a novel mouse monoclonal antibody replacement therapy, which acts as an agonist for the ectodysplasin (Eda) pathway, can resolve cleft palate defects in Pax9^-/- embryos in utero. Such pharmacological interventions did not reverse the arrest in tooth, thymus, and parathyroid gland development, suggesting that the relationship of Pax9 to the Eda/Edar pathway is both unique and essential for palatogenesis. Expression analyses and unbiased gene expression profiling studies offer a molecular explanation for the resolution of palatal defects, showing that Eda and Edar-related genes are expressed in normal palatal tissues and that the Eda/Edar signaling pathway is downstream of Pax9 in palatogenesis. Taken together, our data uncover a unique relationship between Pax9 and the Eda/Edar signaling pathway that can be further exploited for the development of noninvasive, safe, and effective therapies for the treatment of cleft palate conditions and other single-gene disorders affecting the craniofacial complex.

Identification of Osr2 Transcriptional Target Genes in Palate Development

Previous studies have identified the odd-skipped related 2 (Osr2) transcription factor as a key intrinsic regulator of palatal shelf growth and morphogenesis. However, little is known about the molecular program acting downstream of Osr2 in the regulation of palatogenesis. In this study, we isolated palatal mesenchyme cells from embryonic day 12.5 (E12.5) and E13.5 Osr2^RFP/+ and Osr2^RFP/- mutant mouse embryos and performed whole transcriptome RNA sequencing analyses. Differential expression analysis of the RNA sequencing datasets revealed that expression of 70 genes was upregulated and expression of 61 genes was downregulated by >1.5-fold at both E12.5 and E13.5 in the Osr2^RFP/- palatal mesenchyme cells, in comparison with Osr2^RFP/+ littermates. Gene ontology analysis revealed enrichment of signaling molecules and transcription factors crucial for skeletal development and osteoblast differentiation among those significantly upregulated in the Osr2 mutant palatal mesenchyme. Using quantitative real-time polymerase chain reaction (RT-PCR)and in situ hybridization assays, we validated that the Osr2^-/- embryos exhibit significantly increased and expanded expression of many osteogenic pathway genes, including Bmp3, Bmp5, Bmp7, Mef2c, Sox6, and Sp7 in the developing palatal mesenchyme. Furthermore, we demonstrate that expression of Sema3a, Sema3d, and Sema3e, is ectopically activated in the developing palatal mesenchyme in Osr2^-/- embryos. Through chromatin immunoprecipitation, followed by RT-PCR analysis, we demonstrate that endogenous Osr2 protein binds to the promoter regions of the Sema3a and Sema3d genes in the embryonic palatal mesenchyme. Moreover, Osr2 expression repressed the transcription from the Sema3a and Sema3d promoters in cotransfected cells. Since the Sema3 subfamily of signaling molecules plays diverse roles in the regulation of cell proliferation, migration, and differentiation, these data reveal a novel role for Osr2 in regulation of palatal morphogenesis through preventing aberrant activation of Sema3 signaling. Together, these data indicate that Osr2 controls multiple molecular pathways, including BMP and Sema3 signaling, in palate development.

Modulating Wnt Signaling Rescues Palate Morphogenesis in Pax9 Mutant Mice

Cleft palate is a common birth defect caused by disruption of palatogenesis during embryonic development. Although mutations disrupting components of the Wnt signaling pathway have been associated with cleft lip and palate in humans and mice, the mechanisms involving canonical Wnt signaling and its regulation in secondary palate development are not well understood. Here, we report that canonical Wnt signaling plays an important role in Pax9-mediated regulation of secondary palate development. We found that cleft palate pathogenesis in Pax9-deficient embryos is accompanied by significantly reduced expression of Axin2, an endogenous target of canonical Wnt signaling, in the developing palatal mesenchyme, particularly in the posterior regions of the palatal shelves. We found that expression of Dkk2, encoding a secreted Wnt antagonist, is significantly increased whereas the levels of active β-catenin protein, the essential transcriptional coactivator of canonical Wnt signaling, is significantly decreased in the posterior regions of the palatal shelves in embryonic day 13.5 Pax9-deficent embryos in comparison with control littermates. We show that small molecule-mediated inhibition of Dickkopf (DKK) activity in utero during palatal shelf morphogenesis partly rescued secondary palate development in Pax9-deficient embryos. Moreover, we found that genetic inactivation of Wise, which is expressed in the developing palatal shelves and encodes another secreted antagonist of canonical Wnt signaling, also rescued palate morphogenesis in Pax9-deficient mice. Furthermore, whereas Pax9^del/del embryos exhibit defects in palatal shelf elevation/reorientation and significant reduction in accumulation of hyaluronic acid-a high molecular extracellular matrix glycosaminoglycan implicated in playing an important role in palatal shelf elevation-80% of Pax9^del/del;Wise^-/- double-mutant mouse embryos exhibit rescued palatal shelf elevation/reorientation, accompanied by restored hyaluronic acid accumulation in the palatal mesenchyme. Together, these data identify a crucial role for canonical Wnt signaling in acting downstream of Pax9 to regulate palate morphogenesis.

Detangling the evolutionary developmental integration of dentate jaws: evidence that a p63 gene network regulates odontogenesis exclusive of mandible morphogenesis

Vertebrate jaws and dentitions fit and function together, yet the genetic processes that coordinate cranial and dental morphogenesis and evolution remain poorly understood. Teeth but not jaws fail to form in the edentate p63^-/- mouse mutant, which we used here to identify genes important to odontogenesis, but not jaw morphogenesis, and that may allow dentitions to change during development and evolution without necessarily affecting the jaw skeleton. With the working hypothesis that tooth and jaw development are autonomously controlled by discreet gene regulatory networks, using gene expression microarray assays validated by quantitative reverse-transcription PCR we contrasted expression in mandibular prominences at embryonic days (E) 10-13 of mice with normal lower jaw development but either normal (p63^+/- , p63^+/+ ) or arrested (p63^-/- ) tooth development. The p63^-/- mice showed significantly different expression (fold change ≥2, ≤-2; P ≤ 0.05) of several genes. Some of these are known to help regulate odontogenesis (e.g., p63, Osr2, Cldn3/4) and/or to be targets of p63 (e.g., Jag1/2, Fgfr2); other genes have no previously reported roles in odontogenesis or the p63 pathway (e.g., Fermt1, Cbln1, Pltp, Krt8). As expected, from E10 to E13, few genes known to regulate mandible morphogenesis differed in expression between mouse strains. This study newly links several genes to odontogenesis and/or to the p63 signaling network. We propose that these genes act in a novel odontogenic network that is exclusive of lower jaw morphogenesis, and posit that this network evolved in oral, not pharyngeal, teeth.

Activin and Bmp4 Signaling Converge on Wnt Activation during Odontogenesis

Previous studies show that both activin and Bmp4 act as crucial mesenchymal odontogenic signals during early tooth development. Remarkably, mice lacking activin-βA ( Inhba^-/-) and mice with neural crest-specific inactivation of Bmp4 ( Bmp4^ncko/ncko) both exhibit bud-stage developmental arrest of the mandibular molar tooth germs while their maxillary molar tooth germs completed morphogenesis. In this study, we found that, whereas expression of Inhba and Bmp4 in the developing tooth mesenchyme is independent of each other, Bmp4^ncko/nckoInhba^-/- compound mutant mice exhibit early developmental arrest of all tooth germs. Moreover, genetic inactivation of Osr2, a negative regulator of the odontogenic function of the Bmp4-Msx1 signaling pathway, rescues mandibular molar morphogenesis in Inhba^-/- embryos. We recently reported that Osr2 and the Bmp4-Msx1 pathway control the bud-to-cap transition of tooth morphogenesis through antagonistic regulation of expression of secreted Wnt antagonists, including Dkk2 and Sfrp2, in the developing tooth mesenchyme. We show here that expression of Dkk2 messenger RNAs was significantly upregulated and expanded into the tooth bud mesenchyme in Inhba^-/- embryos in comparison with wild-type littermates. Furthermore, in utero treatment with either lithium chloride, an agonist of canonical Wnt signaling, or the DKK inhibitor IIIC3a rescued mandibular molar tooth morphogenesis in Inhba^-/- embryos. Together with our finding that the developing mandibular molar tooth bud mesenchyme expresses significantly higher levels of Dkk2 than the developing maxillary molar tooth mesenchyme, these data indicate that Bmp4 and activin signaling pathways converge on activation of the Wnt signaling pathway to promote tooth morphogenesis through the bud-to-cap transition and that the differential effects of loss of activin or Bmp4 signaling on maxillary and mandibular molar tooth morphogenesis are mainly due to the differential expression of Wnt antagonists, particularly Dkk2, in the maxillary and mandibular tooth mesenchyme.

Golga5 is dispensable for mouse embryonic development and postnatal survival

Golgins are a family of coiled-coil proteins located at the cytoplasmic surface of the Golgi apparatus and have been implicated in maintaining Golgi structural integrity through acting as tethering factors for retrograde vesicle transport. Whereas knockdown of several individual golgins in cultured cells caused Golgi fragmentation and disruption of vesicle trafficking, analysis of mutant mouse models lacking individual golgins have discovered tissue-specific developmental functions. Recently, homozygous loss of function of GOLGA2, of which previous in vitro studies suggested an essential role in maintenance of Golgi structure and in mitosis, has been associated with a neuromuscular disorder in human patients, which highlights the need for understanding the developmental roles of the golgins in vivo. We report here generation of Golga5-deficient mice using CRISPR/Cas9-mediated genome editing. Although knockdown studies in cultured cells have implicated Golga5 in maintenance of Golgi organization, we show that Golga5 is not required for mouse embryonic development, postnatal survival, or fertility. Moreover, whereas Golga5 is structurally closely related to Golgb1, we show that inactivation of Golga5 does not enhance the severity of developmental defects in Golgb1-deficient mice. The Golga5-deficient mice enable further investigation of the roles and functional specificity of golgins in development and diseases.

Genetic Basis of Nonsyndromic and Syndromic Tooth Agenesis

Human dentition development is a long and complex process which involves a series of reciprocal and sequential interactions between the embryonic stomodeal epithelium and the underlying neural crest-derived mesenchyme. Despite environment disturbances, tooth development is predominantly genetically controlled. To date, more than 200 genes have been identified in tooth development. These genes implied in various signaling pathways such as the bone morphogenetic protein, fibroblast growth factor, sonic hedgehog homolog, ectodysplasin A, wingless-type MMTV integration site family (Wnt), and transform growth factor pathways. Mutations in any of these strictly balanced signaling cascades may cause arrested odontogenesis and/or other dental defects. This article aims to review current knowledge about the genetic mechanisms responsible for selective nonsyndromic tooth agenesis in humans and to present a detailed summary of syndromes with hypodontia as regular features and their causative genes.

Species-specific modifications of mandible shape reveal independent mechanisms for growth and initiation of the coronoid

Background

The variation in mandibular morphology of mammals reflects specialisations for different diets. Omnivorous and carnivorous mammals posses large mandibular coronoid processes, while herbivorous mammals have proportionally smaller or absent coronoids. This is correlated with the relative size of the temporalis muscle that forms an attachment to the coronoid process. The role of this muscle attachment in the development of the variation of the coronoid is unclear.

Results

By comparative developmental biology and mouse knockout studies, we demonstrate here that the initiation and growth of the coronoid are two independent processes, with initiation being intrinsic to the ossifying bone and growth dependent upon the extrinsic effect of muscle attachment. A necessary component of the intrinsic patterning is identified as the paired domain transcription factor Pax9. We also demonstrate that Sox9 plays a role independent of chondrogenesis in the growth of the coronoid process in response to muscle interaction.

Conclusions

The mandibular coronoid process is initiated by intrinsic factors, but later growth is dependent on extrinsic signals from the muscle. These extrinsic influences are hypothesised to be the basis of the variation in coronoid length seen across the mammalian lineage.

Cellular and Molecular Mechanisms of Palatogenesis

Palatogenesis involves the initiation, growth, morphogenesis, and fusion of the primary and secondary palatal shelves from initially separate facial prominences during embryogenesis to form the intact palate separating the oral cavity from the nostrils. The palatal shelves consist mainly of cranial neural crest-derived mesenchymal cells covered by a simple embryonic epithelium. The growth and patterning of the palatal shelves are controlled by reciprocal epithelial-mesenchymal interactions regulated by multiple signaling pathways and transcription factors. During palatal shelf outgrowth, the embryonic epithelium develops a "teflon" coat consisting of a single, continuous layer of periderm cells that prevents the facial prominences and palatal shelves from forming aberrant interepithelial adhesions. Palatal fusion involves not only spatiotemporally regulated disruption of the periderm but also dynamic cellular and molecular processes that result in adhesion and intercalation of the palatal medial edge epithelia to form an intershelf epithelial seam, and subsequent dissolution of the epithelial seam to form the intact roof of the oral cavity. The complexity of regulation of these morphogenetic processes is reflected by the common occurrence of cleft palate in humans. This review will summarize major recent advances and discuss major remaining gaps in the understanding of cellular and molecular mechanisms controlling palatogenesis.

Deletion of Osr2 Partially Rescues Tooth Development in Runx2 Mutant Mice

Tooth organogenesis depends on genetically programmed sequential and reciprocal inductive interactions between the dental epithelium and neural crest-derived mesenchyme. Previous studies showed that the Msx1 and Runx2 transcription factors are required for activation of odontogenic signals, including Bmp4 and Fgf3, in the early tooth mesenchyme to drive tooth morphogenesis through the bud-to-cap transition and that Runx2 acts downstream of Msx1 to activate Fgf3 expression. Recent studies identified Osr2 as a repressor of tooth development and showed that inactivation of Osr2 rescued molar tooth morphogenesis in the Msx1(-/-) mutant mice as well as in mice with neural crest-specific inactivation of Bmp4. Here we show that Runx2 expression is expanded in the tooth bud mesenchyme in Osr2(-/-) mutant mouse embryos and is partially restored in the tooth mesenchyme in Msx1(-/-)Osr2(-/-) mutants in comparison with Msx1(-/-) and wild-type embryos. Whereas mandibular molar development arrested at the bud stage and maxillary molar development arrested at the bud-to-cap transition in Runx2(-/-) mutant mice, both mandibular and maxillary molar tooth germs progressed to the early bell stage, with rescued expression of Msx1 and Bmp4 in the dental papilla as well as expression of Bmp4, p21, and Shh in the primary enamel knot in the Osr2(-/-)Runx2(-/-) compound mutants. In contrast to the Msx1(-/-)Osr2(-/-) compound mutants, which exhibit nearly normal first molar morphogenesis, the Osr2(-/-)Runx2(-/-) compound mutant embryos failed to activate the expression of Fgf3 and Fgf10 in the dental papilla and exhibited significant deficit in cell proliferation in both the dental epithelium and mesenchyme in comparison with the control embryos. These data indicate that Runx2 synergizes with Msx1 to drive tooth morphogenesis through the bud-to-cap transition and that Runx2 controls continued tooth growth and morphogenesis beyond the cap stage through activation of Fgf3 and Fgf10 expression in the dental papilla.

A comparative examination of odontogenic gene expression in both toothed and toothless amniotes

A well-known tenet of murine tooth development is that BMP4 and FGF8 antagonistically initiate odontogenesis, but whether this tenet is conserved across amniotes is largely unexplored. Moreover, changes in BMP4-signaling have previously been implicated in evolutionary tooth loss in Aves. Here we demonstrate that Bmp4, Msx1, and Msx2 expression is limited proximally in the red-eared slider turtle (Trachemys scripta) mandible at stages equivalent to those at which odontogenesis is initiated in mice, a similar finding to previously reported results in chicks. To address whether the limited domains in the turtle and the chicken indicate an evolutionary molecular parallelism, or whether the domains simply constitute an ancestral phenotype, we assessed gene expression in a toothed reptile (the American alligator, Alligator mississippiensis) and a toothed non-placental mammal (the gray short-tailed opossum, Monodelphis domestica). We demonstrate that the Bmp4 domain is limited proximally in M. domestica and that the Fgf8 domain is limited distally in A. mississippiensis just preceding odontogenesis. Additionally, we show that Msx1 and Msx2 expression patterns in these species differ from those found in mice. Our data suggest that a limited Bmp4 domain does not necessarily correlate with edentulism, and reveal that the initiation of odontogenesis in non-murine amniotes is more complex than previously imagined. Our data also suggest a partially conserved odontogenic program in T. scripta, as indicated by conserved Pitx2, Pax9, and Barx1 expression patterns and by the presence of a Shh-expressing palatal epithelium, which we hypothesize may represent potential dental rudiments based on the Testudinata fossil record.

Complex analysis of multiple single nucleotide polymorphisms as putative risk factors of tooth agenesis in the Hungarian population

OBJECTIVES

The role was studied of multiple single nucleotide polymorphisms in tooth agenesis in the Hungarian population using a complex approach.

METHODS

Eight SNPs, PAX9 -912 C/T, PAX9 -1031 A/G, MSX1 3755 A/G, FGFR1 T/C rs881301, IRF6 T/C rs764093, AXIN2-8150 A/G, AXIN2-8434 A/G and AXIN2-30224 C/T, were studied in 192 hypodontia and 17 oligodontia cases and in 260 healthy volunteers. Case-control analysis was performed to test both allelic and genotypic associations as well as associations at the level of haplotypes. Multivariate exploratory Bayesian network-based multi-level analysis of relevance (BN-BMLA) as well as logistic regression analysis were performed.

RESULTS

Conventional statistics showed that PAX9 SNP -912 C/T and the MSX1 SNP changed the incidence of hypodontia, although after Bonferroni correction for multiple hypothesis testing, the effects were only borderline tendencies. Using a statistical analysis better suited for handling multiple hypotheses, the BN-BMLA, PAX9 SNPs clearly showed a synergistic effect. This was confirmed by other multivariate analyses and it remained significant after corrections for multiple hypothesis testing (p < 0.0025). The PAX9-1031-A-PAX9-912-T haplotype was the most relevant combination causing hypodontia. Interaction was weaker between PAX9 and MSX1, while other SNPs had no joint effect on hypodontia.

CONCLUSION

This complex analysis shows the important role of PAX9 and MSX1 SNPs and of their interactions in tooth agenesis, while IRF6, FGFR1 and AXIN2 SNPs had no detectable role in the Hungarian population. These results also reveal that risk factors in hypodontia need to be identified in various populations, since there is considerable variability among them.

Molecular patterning of the mammalian dentition

Four conserved signaling pathways, including the bone morphogenetic proteins (Bmp), fibroblast growth factors (Fgf), sonic hedgehog (Shh), and wingless-related (Wnt) pathways, are each repeatedly used throughout tooth development. Inactivation of any of these resulted in early tooth developmental arrest in mice. The mutations identified thus far in human patients with tooth agenesis also affect these pathways. Recent studies show that these signaling pathways interact through positive and negative feedback loops to regulate not only morphogenesis of individual teeth but also tooth number, shape, and spatial pattern. Increased activity of each of the Fgf, Shh, and canonical Wnt signaling pathways revitalizes development of the physiologically arrested mouse diastemal tooth germs whereas constitutive activation of canonical Wnt signaling in the dental epithelium is able to induce supernumerary tooth formation even in the absence of Msx1 and Pax9, two transcription factors required for normal tooth development beyond the early bud stage. Bmp4 and Msx1 act in a positive feedback loop to drive sequential tooth formation whereas the Osr2 transcription factor restricts Msx1-mediated expansion of the mesenchymal odontogenic field along both the buccolingual and anteroposterior axes to pattern mouse molar teeth in a single row. Moreover, the ectodermal-specific ectodysplasin (EDA) signaling pathway controls tooth number and tooth shape through regulation of Fgf20 expression in the dental epithelium, whereas Shh suppresses Wnt signaling through a negative feedback loop to regulate spatial patterning of teeth. In this article, we attempt to integrate these exciting findings in the understanding of the molecular networks regulating tooth development and patterning.

Pax9 regulates a molecular network involving Bmp4, Fgf10, Shh signaling and the Osr2 transcription factor to control palate morphogenesis

Cleft palate is one of the most common birth defects in humans. Whereas gene knockout studies in mice have shown that both the Osr2 and Pax9 transcription factors are essential regulators of palatogenesis, little is known about the molecular mechanisms involving these transcription factors in palate development. We report here that Pax9 plays a crucial role in patterning the anterior-posterior axis and outgrowth of the developing palatal shelves. We found that tissue-specific deletion of Pax9 in the palatal mesenchyme affected Shh expression in palatal epithelial cells, indicating that Pax9 plays a crucial role in the mesenchyme-epithelium interactions during palate development. We found that expression of the Bmp4, Fgf10, Msx1 and Osr2 genes is significantly downregulated in the developing palatal mesenchyme in Pax9 mutant embryos. Remarkably, restoration of Osr2 expression in the early palatal mesenchyme through a Pax9(Osr2KI) allele rescued posterior palate morphogenesis in the absence of Pax9 protein function. Our data indicate that Pax9 regulates a molecular network involving the Bmp4, Fgf10, Shh and Osr2 pathways to control palatal shelf patterning and morphogenesis.

Msx1 and Tbx2 antagonistically regulate Bmp4 expression during the bud-to-cap stage transition in tooth development

Bmp4 expression is tightly regulated during embryonic tooth development, with early expression in the dental epithelial placode leading to later expression in the dental mesenchyme. Msx1 is among several transcription factors that are induced by epithelial Bmp4 and that, in turn, are necessary for the induction and maintenance of dental mesenchymal Bmp4 expression. Thus, Msx1(-/-) teeth arrest at early bud stage and show loss of Bmp4 expression in the mesenchyme. Ectopic expression of Bmp4 rescues this bud stage arrest. We have identified Tbx2 expression in the dental mesenchyme at bud stage and show that this can be induced by epithelial Bmp4. We also show that endogenous Tbx2 and Msx1 can physically interact in mouse C3H10T1/2 cells. In order to ascertain a functional relationship between Msx1 and Tbx2 in tooth development, we crossed Tbx2 and Msx1 mutant mice. Our data show that the bud stage tooth arrest in Msx1(-/-) mice is partially rescued in Msx1(-/-);Tbx2(+/-) compound mutants. This rescue is accompanied by formation of the enamel knot (EK) and by restoration of mesenchymal Bmp4 expression. Finally, knockdown of Tbx2 in C3H10T1/2 cells results in an increase in Bmp4 expression. Together, these data identify a novel role for Tbx2 in tooth development and suggest that, following their induction by epithelial Bmp4, Msx1 and Tbx2 in turn antagonistically regulate odontogenic activity that leads to EK formation and to mesenchymal Bmp4 expression at the key bud-to-cap stage transition.

Molecular factors resulting in tooth agenesis and contemporary approaches for regeneration: a review

AIM

This review discusses the complex epithelial-mesenchymal interactions that occur during tooth development and systemic anomalies that may result in hypodontia. Emphasis is placed on four interacting signaling families (Shh, FGF, BMP, and Wnt) that have been identified for their integral role in complete tooth development and on several genetic mutations in the MSX1, PAX9, EDA, and AXIN2 genes that arrest tooth development. Proposed treatment options are presented, including signaling factor supplementation and stem cell isolation for bioengineering new teeth.

Roles of Bmp4 during tooth morphogenesis and sequential tooth formation

Previous studies have suggested that Bmp4 is a key Msx1-dependent mesenchymal odontogenic signal for driving tooth morphogenesis through the bud-to-cap transition. Whereas all tooth germs were arrested at the bud stage in Msx1(-/-) mice, we show that depleting functional Bmp4 mRNAs in the tooth mesenchyme, through neural crest-specific gene inactivation in Bmp4(f/f);Wnt1Cre mice, caused mandibular molar developmental arrest at the bud stage but allowed maxillary molars and incisors to develop to mineralized teeth. We found that expression of Osr2, which encodes a zinc finger protein that antagonizes Msx1-mediated activation of odontogenic mesenchyme, was significantly upregulated in the molar tooth mesenchyme in Bmp4(f/f);Wnt1Cre embryos. Msx1 heterozygosity enhanced maxillary molar developmental defects whereas Osr2 heterozygosity partially rescued mandibular first molar morphogenesis in Bmp4(f/f);Wnt1Cre mice. Moreover, in contrast to complete lack of supernumerary tooth initiation in Msx1(-/-)Osr2(-/-) mice, Osr2(-/-)Bmp4(f/f);Wnt1Cre compound mutant mice exhibited formation and subsequent arrest of supernumerary tooth germs that correlated with downregulation of Msx1 expression in the tooth mesenchyme. In addition, we found that the Wnt inhibitors Dkk2 and Wif1 were much more abundantly expressed in the mandibular than maxillary molar mesenchyme in wild-type embryos and that Dkk2 expression was significantly upregulated in the molar mesenchyme in Bmp4(f/f);Wnt1Cre embryos, which correlated with the dramatic differences in maxillary and mandibular molar phenotypes in Bmp4(f/f);Wnt1Cre mice. Together, these data indicate that Bmp4 signaling suppresses tooth developmental inhibitors in the tooth mesenchyme, including Dkk2 and Osr2, and synergizes with Msx1 to activate mesenchymal odontogenic potential for tooth morphogenesis and sequential tooth formation.

Developmental basis of toothlessness in turtles: insight into convergent evolution of vertebrate morphology

The tooth is a major component of the vertebrate feeding apparatus and plays a crucial role in species survival, thus subjecting tooth developmental programs to strong selective constraints. However, irrespective of their functional importance, teeth have been lost in multiple lineages of tetrapod vertebrates independently. To understand both the generality and the diversity of developmental mechanisms that cause tooth agenesis in tetrapods, we investigated expression patterns of a series of tooth developmental genes in the lower jaw of toothless turtles and compared them to that of toothed crocodiles and the chicken as a representative of toothless modern birds. In turtle embryos, we found impairment of Shh signaling in the oral epithelium and early-stage arrest of odontoblast development caused by termination of Msx2 expression in the dental mesenchyme. Our data indicate that such changes underlie tooth agenesis in turtles and suggest that the mechanism that leads to early-stage odontogenic arrest differs between birds and turtles. Our results demonstrate that the cellular and molecular mechanisms that regulate early-stage arrest of tooth development are diverse in tetrapod lineages, and odontogenic developmental programs may respond to changes in upstream molecules similarly thereby evolving convergently with feeding morphology.