Msx1 and Dlx5 act independently in development of craniofacial skeleton, but converge on the regulation of Bmp signaling in palate formation

Function of Transforming Growth Factor β2 and β3 in Palatogenesis

INTRODUCTION

This study aimed to examine the transforming growth factor (TGF)-β signaling pathway during secondary palate fusion by transfecting single and double small interfering RNA (siRNAs) for TGF-β2 and -β3. This investigation also focused on understanding the phenotype of palatal development.

METHODS

siRNAs targeting TGF-β2 and -β3 were used in an organ culture model of fusion of the secondary palate of 13-day embryonic ICR mice cultured for up to 72 h. The palatal shelves were collected at different times following the initiation of organ culture and were examined for TGF-β2 and -β3 gene expression. Downstream signaling was characterized using Western blotting and PCR.

RESULTS

In the double siRNA-treated palatal shelves, approximately 90% (91% anterior, 89% posterior with phenotype A) showed fusion failure in hematoxylin and eosin staining. Phosphorylation of Smad-dependent and -independent signaling showed a significant reduction in phosphorylation in double knockdown palate organ cultures when compared to single knockdown cultures. Although, the expression of matrix metalloproteinase 13 and TIMP2 were small influenced by siTGF-β2, the extracellular matrix and transcription factor expressions showed to be significantly reduced in double knockdown palate compared to single knockdown palates.

CONCLUSIONS

This study demonstrates that double siRNAs targeting TGF-β2 and -β3 results in phenotypes during secondary palatal fusion and that they could be affected phosphorylation of Smad-dependent and -independent signaling synergistically compared to single knockdown of TGF-β2 and -β3. The results of this study demonstrate important functions during secondary palatal fusion and will contribute to our understanding of the etiology of cleft palate.

Early embryogenesis in CHDFIDD mouse model reveals facial clefts and altered cranial neurogenesis

CDK13-related disorder, also known as congenital heart defects, dysmorphic facial features and intellectual developmental disorder (CHDFIDD) is associated with mutations in the CDK13 gene encoding transcription-regulating cyclin-dependent kinase 13 (CDK13). Here, we focused on the development of craniofacial structures and analyzed early embryonic stages in CHDFIDD mouse models, with one model comprising a hypomorphic mutation in Cdk13 and exhibiting cleft lip/palate, and another model comprising knockout of Cdk13, featuring a stronger phenotype including midfacial cleft. Cdk13 was found to be physiologically expressed at high levels in the mouse embryonic craniofacial structures, namely in the forebrain, nasal epithelium and maxillary mesenchyme. We also uncovered that Cdk13 deficiency leads to development of hypoplastic branches of the trigeminal nerve including the maxillary branch. Additionally, we detected significant changes in the expression levels of genes involved in neurogenesis (Ache, Dcx, Mef2c, Neurog1, Ntn1, Pou4f1) within the developing palatal shelves. These results, together with changes in the expression pattern of other key face-specific genes (Fgf8, Foxd1, Msx1, Meis2 and Shh) at early stages in Cdk13 mutant embryos, demonstrate a key role of CDK13 in the regulation of craniofacial morphogenesis.

Dynamic regulation of gene expression and morphogenesis in the zebrafish embryo test after exposure to all-trans retinoic acid

The zebrafish embryotoxicity test (ZET) is widely used in developmental toxicology. The analysis of gene expression regulation in ZET after chemical exposure provides mechanistic information about the effects of chemicals on morphogenesis in the test. The gene expression response magnitude has been shown to change with exposure duration. The objective of this work is to study the effect of the exposure duration on the magnitude of gene expression changes in the all-trans retinoic acid (ATRA) signaling pathway in the ZET. Retinoic acid regulation is a key driver of morphogenesis and is therefore employed here as an indicator for the regulation of developmental genes. A teratogenic concentration of 7.5 nM of ATRA was given at 3 hrs post fertilization (hpf) for a range of exposure durations until 120 hrs of development. The expression of a selection of genes related to ATRA signaling and downstream developmental genes was determined. The highest magnitudes of gene expression regulation were observed after 2-24 hrs exposure with an optimal response after 4 hrs. Longer exposures showed a decrease in the gene expression response, although continued exposure to 120 hpf caused malformations and lethality. This study shows that assessment of gene expression regulation at early time points after the onset of exposure in the ZET may be optimal for the prediction of developmental toxicity. We believe these results could help optimize sensitivity in future studies with ZET.

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Revisiting the embryogenesis of lip and palate development

Clefts of the lip and palate (CLP), the major causes of congenital facial malformation globally, result from failure of fusion of the facial processes during embryogenesis. With a prevalence of 1 in 500-2500 live births, CLP causes major morbidity throughout life as a result of problems with facial appearance, feeding, speaking, obstructive apnoea, hearing and social adjustment and requires complex, multi-disciplinary care at considerable cost to healthcare systems worldwide. Long-term outcomes for affected individuals include increased mortality compared with their unaffected siblings. The frequent occurrence and major healthcare burden imposed by CLP highlight the importance of dissecting the molecular mechanisms driving facial development. Identification of the genetic mutations underlying syndromic forms of CLP, where CLP occurs in association with non-cleft clinical features, allied to developmental studies using appropriate animal models is central to our understanding of the molecular events underlying development of the lip and palate and, ultimately, how these are disturbed in CLP.

KDM6B interacts with TFDP1 to activate P53 signalling in regulating mouse palatogenesis

Epigenetic regulation plays extensive roles in diseases and development. Disruption of epigenetic regulation not only increases the risk of cancer, but can also cause various developmental defects. However, the question of how epigenetic changes lead to tissue-specific responses during neural crest fate determination and differentiation remains understudied. Using palatogenesis as a model, we reveal the functional significance of Kdm6b, an H3K27me3 demethylase, in regulating mouse embryonic development. Our study shows that Kdm6b plays an essential role in cranial neural crest development, and loss of Kdm6b disturbs P53 pathway-mediated activity, leading to complete cleft palate along with cell proliferation and differentiation defects in mice. Furthermore, activity of H3K27me3 on the promoter of Trp53 is antagonistically controlled by Kdm6b, and Ezh2 in cranial neural crest cells. More importantly, without Kdm6b, the transcription factor TFDP1, which normally binds to the promoter of Trp53, cannot activate Trp53 expression in palatal mesenchymal cells. Furthermore, the function of Kdm6b in activating Trp53 in these cells cannot be compensated for by the closely related histone demethylase Kdm6a. Collectively, our results highlight the important role of the epigenetic regulator KDM6B and how it specifically interacts with TFDP1 to achieve its functional specificity in regulating Trp53 expression, and further provide mechanistic insights into the epigenetic regulatory network during organogenesis.

Essential function and targets of BMP signaling during midbrain neural crest delamination

BMP signaling plays iterative roles during vertebrate neural crest development from induction through craniofacial morphogenesis. However, far less is known about the role of BMP activity in cranial neural crest epithelial-to-mesenchymal transition and delamination. By measuring canonical BMP signaling activity as a function of time from specification through early migration of avian midbrain neural crest cells, we found elevated BMP signaling during delamination stages. Moreover, inhibition of canonical BMP activity via a dominant negative mutant Type I BMP receptor showed that BMP signaling is required for neural crest migration from the midbrain, independent from an effect on EMT and delamination. Transcriptome profiling on control compared to BMP-inhibited cranial neural crest cells identified novel BMP targets during neural crest delamination and early migration including targets of the Notch pathway that are upregulated following BMP inhibition. These results suggest potential crosstalk between the BMP and Notch pathways in early migrating cranial neural crest and provide novel insight into mechanisms regulated by BMP signaling during early craniofacial development.

Uncovering the secreted signals and transcription factors regulating the development of mammalian middle ear ossicles

The mammalian middle ear comprises a chain of ossicles, the malleus, incus, and stapes that act as an impedance matching device during the transmission of sound from the tympanic membrane to the inner ear. These ossicles are derived from cranial neural crest cells that undergo endochondral ossification and subsequently differentiate into their final functional forms. Defects that occur during middle ear development can result in conductive hearing loss. In this review, we summarize studies describing the crucial roles played by signaling molecules such as sonic hedgehog, bone morphogenetic proteins, fibroblast growth factors, notch ligands, and chemokines during the differentiation of neural crest into the middle ear ossicles. In addition to these cell-extrinsic signals, we also discuss studies on the function of transcription factor genes such as Foxi3, Tbx1, Bapx1, Pou3f4, and Gsc in regulating the development and morphology of the middle ear ossicles.

Relationships between skeletal morphology and patterns of bilateral agenesis of third molars in Japanese orthodontic patients

The aim of this study was to reveal the correlations between bilateral agenesis of third molars (M3s) and skeletal morphology in Japanese male and female orthodontic patients. Sixty patients (30 males, 30 females), with bilateral agenesis of maxillary M3s and without agenesis of mandibular M3s (group U), and 60 patients (30 males, 30 females), with bilateral agenesis of mandibular M3s and without agenesis of maxillary M3s (group L), were selected as agenesis groups. Additionally, 60 patients (30 males, 30 females) with all four M3s were selected as the control group (group C). Patients in these three groups had no agenesis of teeth other than M3s. Lateral cephalograms of each patient were used to evaluate skeletal morphology of the maxilla and mandible. Two-way analysis of variance was used for statistical comparisons. Groups U and L had significantly smaller maxillary length and area than group C. Group U exhibited a significantly smaller lower facial height than group C. Males showed significantly larger maxillary length; total mandibular and mandibular body length; mandibular ramus height; SNB angle; maxillary area; and mandibular symphysis, corpus and ramus areas than females. Females had significantly larger lower facial height, gonial angle and ANB angle than males. Smaller maxillary length and area and lower facial height should be considered in planning orthodontic treatment for patients with bilateral agenesis of maxillary and mandibular M3s.

Alendronate induces postnatal maxillary bone growth by stimulating intramembranous ossification and preventing premature cartilage mineralization in the midpalatal suture of newborn rats

Cleft palate is a common malformation of craniofacial development, and postnatal deficiencies in palate formation may occur. The aim of this study was to determine whether alendronate treatment could induce maxillary mineralization and thus reduce the need for surgical procedures. The effects of alendronate on maxillary bone development, the midpalatal suture, and the levels of transforming growth factor beta-1 (TGF-β1), bone morphogenetic protein 2 (BMP-2), collagen I and II, and V-ATPase were evaluated in newborn rats. Thirty newborn rats were placed in a control group and 30 in a group that received intraperitoneal alendronate (2.5 mg/kg/day). The animals were euthanized on day 7 or 12, and the heads were subjected to histological and immunohistochemical analyses. Specimens from rats that received alendronate presented larger bone matrix deposition in areas of intramembranous ossification of the maxillary bone when compared to controls. Furthermore, higher levels of TGF-β1, BMP-2, and collagen I were observed, whereas osteoclasts showed no V-ATPase. The alendronate group also showed higher levels of TGF-β1 and collagen II in the midpalatal suture, whereas BMP-2 levels were lower than in controls. These results coincided with an expansion of the chondroid. In conclusion, alendronate increased the intramembranous ossification in the maxillary bone in association with increased expression of TGF-β1, BMP-2, and collagen I and decreased V-ATPase. The drug induced an expansion of chondrocytes and a decrease in mineral bone deposition despite the high levels of TGF-β1 in this area. Alendronate may therefore be useful in the treatment of diseases affecting bone growth.

Posterior axis formation requires Dlx5/Dlx6 expression at the neural plate border

Neural tube defects (NTDs), one of the most common birth defects in human, present a multifactorial etiology with a poorly defined genetic component. The Dlx5 and Dlx6 bigenic cluster encodes two evolutionary conserved homeodomain transcription factors, which are necessary for proper vertebrate development. It has been shown that Dlx5/6 genes are essential for anterior neural tube closure, however their role in the formation of the posterior structures has never been described. Here, we show that Dlx5/6 expression is required during vertebrate posterior axis formation. Dlx5 presents a similar expression pattern in neural plate border cells during posterior neurulation of zebrafish and mouse. Dlx5/6-inactivation in the mouse results in a phenotype reminiscent of NTDs characterized by open thoracic and lumbar vertebral arches and failure of epaxial muscle formation at the dorsal midline. The dlx5a/6a zebrafish morphants present posterior NTDs associated with abnormal delamination of neural crest cells showing altered expression of cell adhesion molecules and defects of motoneuronal development. Our findings provide new molecular leads to decipher the mechanisms of vertebrate posterior neurulation and might help to gather a better understanding of human congenital NTDs etiology.

Development of Normal and Cleft Palate: A Central Role for Connective Tissue Growth Factor (CTGF)/CCN2

Development of the palate is the result of an organized series of events that require exquisite spatial and temporal regulation at the cellular level. There are a myriad of growth factors, receptors and signaling pathways that have been shown to play an important role in growth, elevation and/or fusion of the palatal shelves. Altered expression or activation of a number of these factors, receptors and signaling pathways have been shown to cause cleft palate in humans or mice with varying degrees of penetrance. This review will focus on connective tissue growth factor (CTGF) or CCN2, which was recently shown to play an essential role in formation of the secondary palate. Specifically, the absence of CCN2 in KO mice results in defective cellular processes that contribute to failure of palatal shelf growth, elevation and/or fusion. CCN2 is unique in that it has been shown to interact with a number of other factors important for palate development, including bone morphogenetic proteins (BMPs), fibroblast growth factors (FGFs), epidermal growth factor (EGF), Wnt proteins and transforming growth factor-βs (TGF-βs), thereby influencing their ability to bind to their receptors and mediate intracellular signaling. The role that these factors play in palate development and their specific interactions with CCN2 will also be reviewed. Future studies to elucidate the precise mechanisms of action for CCN2 and its interactions with other regulatory proteins during palatogenesis are expected to provide novel information with the potential for development of new pharmacologic or genetic treatment strategies for clinical intervention of cleft palate during development.

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.

Signalling pathways in trophic skeletal development and morphogenesis: Insights from studies on teleost fish

During the development of the vertebrate feeding apparatus, a variety of complicated cellular and molecular processes participate in the formation and integration of individual skeletal elements. The molecular mechanisms regulating the formation of skeletal primordia and their development into specific morphological structures are tightly controlled by a set of interconnected signalling pathways. Some of these pathways, such as Bmp, Hedgehog, Notch and Wnt, are long known for their pivotal roles in craniofacial skeletogenesis. Studies addressing the functional details of their components and downstream targets, the mechanisms of their interactions with other signals as well as their potential roles in adaptive morphological divergence, are currently attracting considerable attention. An increasing number of signalling pathways that had previously been described in different biological contexts have been shown to be important in the regulation of jaw skeletal development and morphogenesis. In this review, I provide an overview of signalling pathways involved in trophic skeletogenesis emphasizing studies of the most species-rich group of vertebrates, the teleost fish, which through their evolutionary history have undergone repeated episodes of spectacular trophic diversification.

Meta-analysis Reveals Genome-Wide Significance at 15q13 for Nonsyndromic Clefting of Both the Lip and the Palate, and Functional Analyses Implicate GREM1 As a Plausible Causative Gene

Nonsyndromic orofacial clefts are common birth defects with multifactorial etiology. The most common type is cleft lip, which occurs with or without cleft palate (nsCLP and nsCLO, respectively). Although genetic components play an important role in nsCLP, the genetic factors that predispose to palate involvement are largely unknown. In this study, we carried out a meta-analysis on genetic and clinical data from three large cohorts and identified strong association between a region on chromosome 15q13 and nsCLP (P = 8.13×10⁻¹⁴ for rs1258763; relative risk (RR): 1.46, 95% confidence interval (CI): 1.32–1.61)) but not nsCLO (P = 0.27; RR: 1.09 (0.94–1.27)). The 5 kb region of strongest association maps downstream of Gremlin-1 (GREM1), which encodes a secreted antagonist of the BMP4 pathway. We show during mouse embryogenesis, Grem1 is expressed in the developing lip and soft palate but not in the hard palate. This is consistent with genotype-phenotype correlations between rs1258763 and a specific nsCLP subphenotype, since a more than two-fold increase in risk was observed in patients displaying clefts of both the lip and soft palate but who had an intact hard palate (RR: 3.76, CI: 1.47–9.61, P_diff<0.05). While we did not find lip or palate defects in Grem1-deficient mice, wild type embryonic palatal shelves developed divergent shapes when cultured in the presence of ectopic Grem1 protein (P = 0.0014). The present study identified a non-coding region at 15q13 as the second, genome-wide significant locus specific for nsCLP, after 13q31. Moreover, our data suggest that the closely located GREM1 gene contributes to a rare clinical nsCLP entity. This entity specifically involves abnormalities of the lip and soft palate, which develop at different time-points and in separate anatomical regions.

Clefts of the lip and palate are common birth defects, and require long-term multidisciplinary management. Their etiology involves genetic factors and environmental influences and/or a combination of both, however, these interactions are poorly defined. Moreover, although clefts of the lip may or may not involve the palate, the determinants predisposing to specific subphenotypes are largely unknown. Here we demonstrate that variations in the non-coding region near the GREM1 gene show a highly significant association with a particular phenotype in which cleft lip and cleft palate co-occur (nsCLP; P = 8.13×10⁻¹⁴). Our data suggest that the risk is even higher for patients who have a cleft lip and a cleft of the soft palate, but not of the hard palate. Interestingly, this subphenotype corresponds to the expression of the mouse Grem1 gene, which is found in the developing lip and soft palate but not in the hard palate. While Grem1-deficient mice display no lip or palate defects, we demonstrate that ectopic Grem1 protein alters palatal shelve morphogenesis. Together, our results identify a region near GREM1 as the second, genome-wide significant risk locus for nsCLP, and suggest that deregulated GREM1 expression during craniofacial development may contribute to this common birth defect.

Preaxial polydactyly associated with a MSX1 mutation and report of two novel mutations

We report two novel heterozygous missense MSX1 mutations in two Thai families (c.739C>T; p.Pro247Ser and c.607G>A; p.Ala203Thr). The p.Ala203Thr mutation was found in a female patient, her sister, and their father and is associated with unilateral cleft lip and palate, hypodontia, and microdontia. The p.Pro247Ser mutation was found in a three-generation Thai family and was associated with bilateral cleft lip and palate, hypodontia, microdontia, and dens invaginatus. The proband also had preaxial polydactyly of the left hand. The role of Msx1 in limb development in mice is discussed. Intrafamilial variability of the phenotypes is clearly evident. This is the first time that a limb anomaly has been reported to be associated with a mutation in MSX1.

Type III transforming growth factor beta receptor regulates vascular and osteoblast development during palatogenesis

BACKGROUND

Cleft palate occurs in up to 1:1,000 live births and is associated with mutations in multiple genes. Palatogenesis involves a complex choreography of palatal shelf elongation, elevation, and fusion. Transforming growth factor β (TGFβ) and bone morphogenetic protein 2 (BMP2) canonical signaling is required during each stage of palate development. The type III TGFβ receptor (TGFβR3) binds all three TGFβ ligands and BMP2, but its contribution to palatogenesis is unknown.

RESULTS

The role of TGFβR3 during palate formation was found to be during palatal shelf elongation and elevation. Tgfbr3(-) (/) (-) embryos displayed reduced palatal shelf width and height, changes in proliferation and apoptosis, and reduced vascular and osteoblast differentiation. Abnormal vascular plexus organization as well as aberrant expression of arterial (Notch1, Alk1), venous (EphB4), and lymphatic (Lyve1) markers was also observed. Decreased osteoblast differentiation factors (Runx2, alk phos, osteocalcin, col1A1, and col1A2) demonstrated poor mesenchymal cell commitment to the osteoblast lineage within the maxilla and palatal shelves in Tgfbr3(-) (/) (-) embryos. Additionally, in vitro bone mineralization induced by osteogenic medium (OM+BMP2) was insufficient in Tgfbr3(-) (/) (-) palatal mesenchyme, but mineralization was rescued by overexpression of TGFβR3.

CONCLUSIONS

These data reveal a critical, previously unrecognized role for TGFβR3 in vascular and osteoblast development during palatogenesis.

Shape covariation between the craniofacial complex and first molars in humans

The occurrence of mutual genetic loci in morphogenesis of the face and teeth implies shape covariation between these structures. However, teeth finalize their shape at an early age, whereas the face grows and is subjected to environmental influences for a prolonged period; it is therefore conceivable that covariation might modulate with age. Here we investigate the extent of this covariation in humans by measuring the 3D shape of the occlusal surface of the permanent first molars and the shape of the craniofacial complex from lateral radiographs, at two maturations stages. A sample of Greek subjects was divided into two groups (110 adult, 110 prepubertal) with equally distributed gender. The occlusal surfaces of the right first molars were 3D scanned from dental casts; 265 and 274 landmarks (including surface and curve semilandmarks) were digitized on the maxillary and mandibular molars, respectively. The corresponding lateral cephalometric radiographs were digitized with 71 landmarks. Geometric morphometric methods were used to assess shape variation and covariation. The vertical dimension of the craniofacial complex was the main parameter of shape variation, followed by anteroposterior deviations. The male craniofacial complex was larger (4.0-5.7%) and was characterized by a prominent chin and clockwise rotation of the cranial base (adult group only). Allometry was weak and statistically significant only when examined for the sample as a whole (percent variance explained: 2.1%, P = 0.0002). Covariation was statistically significant only between the lower first molar and the craniofacial complex (RV = 14.05%, P = 0.0099, and RV = 12.31%, P = 0.0162, for the prepubertal and adult groups, respectively). Subtle age-related covariation differences were noted, indicating that environmental factors may influence the pattern and strength of covariation. However, the main pattern was similar in both groups: a class III skeletal pattern (relative maxillary retrusion and mandibular protrusion), hyperdivergency, forward rotation of the posterior cranial base and upward rotation of the anterior cranial base were associated with mesiodistal elongation of the lower molars and height reduction of their distal cusps. This pattern mimics phylogeny in humans, where flexion and counterclockwise rotation of the cranial base, considered advantageous to survival, co-occur with tooth reductions that cannot be easily explained in evolutionary terms. The similarity of the phylogenetic and covariation patterns seems to support the pleiotropic gene hypothesis.

Inhibition of Wnt/β-catenin pathway by Dickkopf-1 [corrected] affects midfacial morphogenesis in chick embryo

The development of the vertebrate face is regulated by complex interactions among several signaling pathways. Dickkopf-1 (Dkk-1), an inhibitor of the Wnt/β-catenin signaling pathway, can affect midfacial morphogenesis. The downstream target genes of the Wnt/β-catenin signaling pathway in morphogenesis of the developing upper jaw and lip remain unknown. To investigate the functional roles of Wnt/β-catenin signaling in facial development, we performed a loss-of-function experiment using local implantation of beads soaked with Dkk-1 during lip fusion at the maxillary prominence of chick embryos at stage 22(HH22). Antagonism of Wnt/β-catenin signaling by Dkk-1 induced deformities of the premaxilla and jugal bone, which are derived from the maxillary mesenchyme. Real-time and semi-quantitative RT-PCR analysis showed the significant reduction of Lhx8, Msx1 and Msx2 expression levels around the beads in the maxillary mesenchyme at 6 and 24 h after bead implantation. Time course experiments in the HH 22 embryos showed the effect of Dkk-1 on Lhx8, Msx1 and Msx2 expression was not significant after 48 h of the treatment. At HH 26 when the fusion of facial primordial started, Dkk-1 application did not exhibit any significant reduction of those genes. Our findings suggested that Dkk-1 regulates maxillary morphogenesis in chick embryos through Lhx8, Msx1 and Msx2 signals. Wnt/β-catenin signaling is responsible for intrinsic upper jaw development before the lip fusion.

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.

The etiology of cleft palate formation in BMP7-deficient mice

Palatogenesis is a complex process implying growth, elevation and fusion of the two lateral palatal shelves during embryogenesis. This process is tightly controlled by genetic and mechanistic cues that also coordinate the growth of other orofacial structures. Failure at any of these steps can result in cleft palate, which is a frequent craniofacial malformation in humans. To understand the etiology of cleft palate linked to the BMP signaling pathway, we studied palatogenesis in Bmp7-deficient mouse embryos. Bmp7 expression was found in several orofacial structures including the edges of the palatal shelves prior and during their fusion. Bmp7 deletion resulted in a general alteration of oral cavity morphology, unpaired palatal shelf elevation, delayed shelf approximation, and subsequent lack of fusion. Cell proliferation and expression of specific genes involved in palatogenesis were not altered in Bmp7-deficient embryos. Conditional ablation of Bmp7 with Keratin14-Cre or Wnt1-Cre revealed that neither epithelial nor neural crest-specific loss of Bmp7 alone could recapitulate the cleft palate phenotype. Palatal shelves from mutant embryos were able to fuse when cultured in vitro as isolated shelves in proximity, but not when cultured as whole upper jaw explants. Thus, deformations in the oral cavity of Bmp7-deficient embryos such as the shorter and wider mandible were not solely responsible for cleft palate formation. These findings indicate a requirement for Bmp7 for the coordination of both developmental and mechanistic aspects of palatogenesis.

BMP-mediated functional cooperation between Dlx5;Dlx6 and Msx1;Msx2 during mammalian limb development

The Dlx and Msx homeodomain transcription factors play important roles in the control of limb development. The combined disruption of Msx1 and Msx2, as well as that of Dlx5 and Dlx6, lead to limb patterning defects with anomalies in digit number and shape. Msx1;Msx2 double mutants are characterized by the loss of derivatives of the anterior limb mesoderm which is not observed in either of the simple mutants. Dlx5;Dlx6 double mutants exhibit hindlimb ectrodactyly. While the morphogenetic action of Msx genes seems to involve the BMP molecules, the mode of action of Dlx genes still remains elusive. Here, examining the limb phenotypes of combined Dlx and Msx mutants we reveal a new Dlx-Msx regulatory loop directly involving BMPs. In Msx1;Dlx5;Dlx6 triple mutant mice (TKO), beside the expected ectrodactyly, we also observe the hallmark morphological anomalies of Msx1;Msx2 double mutants suggesting an epistatic role of Dlx5 and Dlx6 over Msx2. In Msx2;Dlx5;Dlx6 TKO mice we only observe an aggravation of the ectrodactyly defect without changes in the number of the individual components of the limb. Using a combination of qPCR, ChIP and bioinformatic analyses, we identify two Dlx/Msx regulatory pathways: 1) in the anterior limb mesoderm a non-cell autonomous Msx-Dlx regulatory loop involves BMP molecules through the AER and 2) in AER cells and, at later stages, in the limb mesoderm the regulation of Msx2 by Dlx5 and Dlx6 occurs also cell autonomously. These data bring new elements to decipher the complex AER-mesoderm dialogue that takes place during limb development and provide clues to understanding the etiology of congenital limb malformations.

The zinc finger transcription factor Ovol2 acts downstream of the bone morphogenetic protein pathway to regulate the cell fate decision between neuroectoderm and mesendoderm

During early embryonic development, bone morphogenetic protein (BMP) signaling is essential for neural/non-neural cell fate decisions. BMP signaling inhibits precocious neural differentiation and allows for proper differentiation of mesoderm, endoderm, and epidermis. However, the mechanisms underlying the BMP pathway-mediated cell fate decision remain largely unknown. Here, we show that the expression of Ovol2, which encodes an evolutionarily conserved zinc finger transcription factor, is down-regulated during neural differentiation of mouse embryonic stem cells. Knockdown of Ovol2 in embryonic stem cells facilitates neural conversion and inhibits mesendodermal differentiation, whereas Ovol2 overexpression gives rise to the opposite phenotype. Moreover, Ovol2 knockdown partially rescues the neural inhibition and mesendodermal induction by BMP4. Mechanistic studies further show that BMP4 directly regulates Ovol2 expression through the binding of Smad1/5/8 to the second intron of the Ovol2 gene. In the chick embryo, cOvol2 expression is specifically excluded from neural territory and is up-regulated by BMP4. In addition, ectopic expression of cOvol2 in the prospective neural plate represses the expression of the definitive neural plate marker cSox2. Taken together, these results indicate that Ovol2 acts downstream of the BMP pathway in the cell fate decision between neuroectoderm and mesendoderm to ensure proper germ layer development.

Molecular signaling along the anterior-posterior axis of early palate development

Cleft palate is a common congenital birth defect in humans. In mammals, the palatal tissue can be distinguished into anterior bony hard palate and posterior muscular soft palate that have specialized functions in occlusion, speech or swallowing. Regulation of palate development appears to be the result of distinct signaling and genetic networks in the anterior and posterior regions of the palate. Development and maintenance of expression of these region-specific genes is crucial for normal palate development. Numerous transcription factors and signaling pathways are now recognized as either anterior- (e.g., Msx1, Bmp4, Bmp2, Shh, Spry2, Fgf10, Fgf7, and Shox2) or posterior-specific (e.g., Meox2, Tbx22, and Barx1). Localized expression and function clearly highlight the importance of regional patterning and differentiation within the palate at the molecular level. Here, we review how these molecular pathways and networks regulate the anterior-posterior patterning and development of secondary palate. We hypothesize that the anterior palate acts as a signaling center in setting up development of the secondary palate.

Seven diverse human embryonic stem cell-derived chondrogenic clonal embryonic progenitor cell lines display site-specific cell fates

AIM

The transcriptomes of seven diverse clonal human embryonic progenitor cell lines with chondrogenic potential were compared with that of bone marrow-derived mesenchymal stem cells (MSCs).

MATERIALS & METHODS

The cell lines 4D20.8, 7PEND24, 7SMOO32, E15, MEL2, SK11 and SM30 were compared with MSCs using immunohistochemical methods, gene expression microarrays and quantitative real-time PCR.

RESULTS

In the undifferentiated progenitor state, each line displayed unique combinations of site-specific markers, including AJAP1, ALDH1A2, BMP5, BARX1, HAND2, HOXB2, LHX1, LHX8, PITX1, TBX15 and ZIC2, but none of the lines expressed the MSC marker CD74. The lines showed diverse responses when differentiated in the presence of combinations of TGF-β3, BMP2, 4, 6 and 7 and GDF5, with the lines 4D20.8, SK11, SM30 and MEL2 showing osteogenic markers in some differentiation conditions. The line 7PEND24 showed evidence of regenerating articular cartilage and, in some conditions, markers of tendon differentiation.

CONCLUSION

The scalability of site-specific clonal human embryonic stem cell-derived embryonic progenitor cell lines may provide novel models for the study of differentiation and methods for preparing purified and identified cells types for use in therapy.

Wnt/β-catenin signaling and Msx1 promote outgrowth of the maxillary prominences

Facial morphogenesis requires a series of precisely orchestrated molecular events to promote the growth and fusion of the facial prominences. Cleft palate (CP) results from perturbations in this process. The transcriptional repressor Msx1 is a key participant in these molecular events, as demonstrated by the palatal clefting phenotype observed in Msx1^−/− embryos. Here, we exploited the high degree of conservation that exists in the gene regulatory networks that shape the faces of birds and mice, to gain a deeper understanding of Msx1 function in CP. Histomorphometric analyses indicated that facial development was disrupted as early as E12.5 in Msx1^−/− embryos, long before the palatal shelves have formed. By mapping the expression domain of Msx1 in E11.5 and E12.5 embryos, we found the structures most affected by loss of Msx1 function were the maxillary prominences. Maxillary growth retardation was accompanied by perturbations in angiogenesis that preceded the CP phenotype. Experimental chick manipulations and in vitro assays showed that the regulation of Msx1 expression by the Wnt/β-catenin pathway is highly specific. Our data in mice and chicks indicate a conserved role for Msx1 in regulating the outgrowth of the maxillary prominences, and underscore how imbalances in Msx1 function can lead of growth disruptions that manifest as CP.

Laser ablation of the sonic hedgehog-a-expressing cells during fin regeneration affects ray branching morphogenesis

The zebrafish fin is an excellent system to study the mechanisms of dermal bone patterning. Fin rays are segmented structures that form successive bifurcations both during ontogenesis and regeneration. Previous studies showed that sonic hedgehog (shha) may regulate regenerative bone patterning based on its expression pattern and functional analysis. The present study investigates the role of the shha-expressing cells in the patterning of fin ray branches. The shha expression domain in the basal epidermis of each fin ray splits into two prior to ray bifurcation. In addition, the osteoblast proliferation profile follows the dynamic expression pattern of shha. A zebrafish transgenic line, 2.4shh:gfpABC#15, in which GFP expression recapitulates the endogenous expression of shha, was used to specifically ablate shha-expressing cells with a laser beam. Such ablations lead to a delay in the sequence of events leading to ray bifurcation without affecting the overall growth of the fin ray. These results suggest that shha-expressing cells direct localized osteoblast proliferation and thus regulate branching morphogenesis. This study reveals the fin ray as a new accessible system to investigate epithelial-mesenchymal interactions leading to organ branching.

Morphological integration of soft-tissue facial morphology in Down Syndrome and siblings

Down syndrome (DS), resulting from trisomy of chromosome 21, is the most common live-born human aneuploidy. The phenotypic expression of trisomy 21 produces variable, though characteristic, facial morphology. Although certain facial features have been documented quantitatively and qualitatively as characteristic of DS (e.g., epicanthic folds, macroglossia, and hypertelorism), all of these traits occur in other craniofacial conditions with an underlying genetic cause. We hypothesize that the typical DS face is integrated differently than the face of non-DS siblings, and that the pattern of morphological integration unique to individuals with DS will yield information about underlying developmental associations between facial regions. We statistically compared morphological integration patterns of immature DS faces (N = 53) with those of non-DS siblings (N = 54), aged 6-12 years using 31 distances estimated from 3D coordinate data representing 17 anthropometric landmarks recorded on 3D digital photographic images. Facial features are affected differentially in DS, as evidenced by statistically significant differences in integration both within and between facial regions. Our results suggest a differential affect of trisomy on facial prominences during craniofacial development.

Dysregulation of Semaphorin7A/β1-integrin signaling leads to defective GnRH-1 cell migration, abnormal gonadal development and altered fertility

Reproduction in mammals is dependent on the function of specific neurons that secrete gonadotropin-releasing hormone-1 (GnRH-1). These neurons originate prenatally in the nasal placode and migrate into the forebrain along the olfactory-vomeronasal nerves. Alterations in this migratory process lead to defective GnRH-1 secretion, resulting in heterogeneous genetic disorders such as idiopathic hypogonadotropic hypogonadism (IHH), and other reproductive diseases characterized by the reduction or failure of sexual competence. Combining mouse genetics with in vitro models, we demonstrate that Semaphorin 7A (Sema7A) is essential for the development of the GnRH-1 neuronal system. Loss of Sema7A signaling alters the migration of GnRH-1 neurons, resulting in significantly reduced numbers of these neurons in the adult brain as well as in reduced gonadal size and subfertility. We also show that GnRH-1 cells differentially express the Sema7 receptors β1-integrin and Plexin C1 as a function of their migratory stage, whereas the ligand is robustly expressed along developing olfactory/vomeronasal fibers. Disruption of Sema7A function in vitro inhibits β1-integrin-mediated migration. Analysis of Plexin C1(-/-) mice did not reveal any difference in the migratory process of GnRH-1 neurons, indicating that Sema7A mainly signals through β1-integrin to regulate GnRH-1 cell motility. In conclusion, we have identified Sema7A as a gene implicated in the normal development of the GnRH-1 system in mice and as a genetic marker for the elucidation of some forms of GnRH-1 deficiency in humans.

Identification of direct downstream targets of Dlx5 during early inner ear development

Dlx5, a homeobox transcription factor, plays a key role in the development of many organ systems. It is a candidate gene for human split-hand/split-foot type 1 malformation associated with sensorineural hearing loss. A deletion of one of its enhancers has been implicated in human craniofacial defects/hearing loss and it has also been associated with autism. However, little is known of how Dlx5 exerts its regulatory effects. We identified direct targets of Dlx5 in the mouse inner ear by gene expression profiling wild-type and Dlx5 null otic vesicles from embryonic stages E10 and E10.5. Four hundred genes were differentially expressed. We examined the genomic DNA sequences in the promoter regions of these genes for (i) previously described Dlx5 binding sites, (ii) novel 12 bp long motifs with a canonical homeodomain element shared by two or more genes and (iii) 100% conservation of these motifs in promoters of human orthologs. Forty genes passed these filters, 12 of which are expressed in the otic vesicle in domains that overlap with Dlx5. Chromatin immunoprecipitation using a Dlx5 antibody confirmed direct binding of Dlx5 to promoters of seven of these (Atbf1, Bmper, Large, Lrrtm1, Msx1, Ebf1 and Lhx1) in a cell line over-expressing Dlx5. Bmper and Lrrtm1 were up-regulated in this cell line, further supporting their identification as targets of Dlx5 in the inner ear and potentially in other organs. These direct targets support a model in which Bmp signaling is downstream of Dlx5 in the early inner ear and provide new insights into how the Dlx5 regulatory cascade is initiated.

Can you hear me now? Understanding vertebrate middle ear development

The middle ear is a composite organ formed from all three germ layers and the neural crest. It provides the link between the outside world and the inner ear, where sound is transduced and routed to the brain for processing. Extensive classical and modern studies have described the complex morphology and origin of the middle ear. Non-mammalian vertebrates have a single ossicle, the columella. Mammals have three functionally equivalent ossicles, designated the malleus, incus and stapes. In this review, I focus on the role of genes known to function in the middle ear. Genetic studies are beginning to unravel the induction and patterning of the multiple middle ear elements including the tympanum, skeletal elements, the air-filled cavity, and the insertion point into the inner ear oval window. Future studies that elucidate the integrated spatio-temporal signaling mechanisms required to pattern the middle ear organ system are needed. The longer-term translational benefits of understanding normal and abnormal ear development will have a direct impact on human health outcomes.

Msx1 and Dlx5 function synergistically to regulate frontal bone development

The Msx and Dlx families of homeobox proteins are important regulators for embryogenesis. Loss of Msx1 in mice results in multiple developmental defects including craniofacial malformations. Although Dlx5 is widely expressed during embryonic development, targeted null mutation of Dlx5 mainly affects the development of craniofacial bones. Msx1 and Dlx5 show overlapping expression patterns during frontal bone development. To investigate the functional significance of Msx1/Dlx5 interaction in regulating frontal bone development, we generated Msx1 and Dlx5 double null mutant mice. In Msx1(-/-) ;Dlx5(-/-) mice, the frontal bones defect was more severe than that of either Msx1(-/-) or Dlx5(-/-) mice. This aggravated frontal bone defect suggests that Msx1 and Dlx5 function synergistically to regulate osteogenesis. This synergistic effect of Msx1 and Dlx5 on the frontal bone represents a tissue specific mode of interaction of the Msx and Dlx genes. Furthermore, Dlx5 requires Msx1 for its expression in the context of frontal bone development. Our study shows that Msx1/Dlx5 interaction is crucial for osteogenic induction during frontal bone development.

Modulation of BMP signaling by Noggin is required for the maintenance of palatal epithelial integrity during palatogenesis

BMP signaling plays many important roles during organ development, including palatogenesis. Loss of BMP signaling leads to cleft palate formation. During development, BMP activities are finely tuned by a number of modulators at the extracellular and intracellular levels. Among the extracellular BMP antagonists is Noggin, which preferentialy binds to BMP2, BMP4 and BMP7, all of which are expressed in the developing palatal shelves. Here we use targeted Noggin mutant mice as a model for gain of BMP signaling function to investigate the role of BMP signaling in palate development. We find prominent Noggin expression in the palatal epithelium along the anterior-posterior axis during early palate development. Loss of Noggin function leads to overactive BMP signaling, particularly in the palatal epithelium. This results in disregulation of cell proliferation, excessive cell death, and changes in gene expression, leading to formation of complete palatal cleft. The excessive cell death in the epithelium disrupts the palatal epithelium integrity, which in turn leads to an abnormal palate-mandible fusion and prevents palatal shelf elevation. This phenotype is recapitulated by ectopic expression of a constitutively active form of BMPR-IA but not BMPR-IB in the epithelium of the developing palate; this suggests a role for BMPR-IA in mediating overactive BMP signaling in the absence of Noggin. Together with the evidence that overexpression of Noggin in the palatal epithelium does not cause a cleft palate defect, we conclude from our results that Noggin mediated modulation of BMP signaling is essential for palatal epithelium integrity and for normal palate development.

BmprIa is required in mesenchymal tissue and has limited redundant function with BmprIb in tooth and palate development

The BMP signaling plays a pivotal role in the development of craniofacial organs, including the tooth and palate. BmprIa and BmprIb encode two type I BMP receptors that are primarily responsible for BMP signaling transduction. We investigated mesenchymal tissue-specific requirement of BmprIa and its functional redundancy with BmprIb during the development of mouse tooth and palate. BmprIa and BmprIb exhibit partially overlapping and distinct expression patterns in the developing tooth and palatal shelf. Neural crest-specific inactivation of BmprIa leads to formation of an unusual type of anterior clefting of the secondary palate, an arrest of tooth development at the bud/early cap stages, and severe hypoplasia of the mandible. Defective tooth and palate development is accompanied by the down-regulation of BMP-responsive genes and reduced cell proliferation levels in the palatal and dental mesenchyme. To determine if BmprIb could substitute for BmprIa during tooth and palate development, we expressed a constitutively active form of BmprIb (caBmprIb) in the neural crest cells in which BmprIa was simultaneously inactivated. We found that substitution of BmprIa by caBmprIb in neural rest cells rescues the development of molars and maxillary incisor, but the rescued teeth exhibit a delayed odontoblast and ameloblast differentiation. In contrast, caBmprIb fails to rescue the palatal and mandibular defects including the lack of lower incisors. Our results demonstrate an essential role for BmprIa in the mesenchymal component and a limited functional redundancy between BmprIa and BmprIb in a tissue-specific manner during tooth and palate development.

Palate morphogenesis: current understanding and future directions

In the past, most scientists conducted their inquiries of nature via inductivism, the patient accumulation of "pieces of information" in the pious hope that the sum of the parts would clarify the whole. Increasingly, modern biology employs the tools of bioinformatics and systems biology in attempts to reveal the "big picture." Most successful laboratories engaged in the pursuit of the secrets of embryonic development, particularly those whose research focus is craniofacial development, pursue a middle road where research efforts embrace, rather than abandon, what some have called the "pedestrian" qualities of inductivism, while increasingly employing modern data mining technologies. The secondary palate has provided an excellent paradigm that has enabled examination of a wide variety of developmental processes. Examination of cellular signal transduction, as it directs embryogenesis, has proven exceptionally revealing with regard to clarification of the "facts" of palatal ontogeny-at least the facts as we currently understand them. Herein, we review the most basic fundamentals of orofacial embryology and discuss how functioning of TGFbeta, BMP, Shh, and Wnt signal transduction pathways contributes to palatal morphogenesis. Our current understanding of palate medial edge epithelial differentiation is also examined. We conclude with a discussion of how the rapidly expanding field of epigenetics, particularly regulation of gene expression by miRNAs and DNA methylation, is critical to control of cell and tissue differentiation, and how examination of these epigenetic processes has already begun to provide a better understanding of, and greater appreciation for, the complexities of palatal morphogenesis.

A regulatory feedback loop involving p63 and IRF6 links the pathogenesis of 2 genetically different human ectodermal dysplasias

The human congenital syndromes ectrodactyly ectodermal dysplasia-cleft lip/palate syndrome, ankyloblepharon ectodermal dysplasia clefting, and split-hand/foot malformation are all characterized by ectodermal dysplasia, limb malformations, and cleft lip/palate. These phenotypic features are a result of an imbalance between the proliferation and differentiation of precursor cells during development of ectoderm-derived structures. Mutations in the p63 and interferon regulatory factor 6 (IRF6) genes have been found in human patients with these syndromes, consistent with phenotypes. Here, we used human and mouse primary keratinocytes and mouse models to investigate the role of p63 and IRF6 in proliferation and differentiation. We report that the DeltaNp63 isoform of p63 activated transcription of IRF6, and this, in turn, induced proteasome-mediated DeltaNp63 degradation. This feedback regulatory loop allowed keratinocytes to exit the cell cycle, thereby limiting their ability to proliferate. Importantly, mutations in either p63 or IRF6 resulted in disruption of this regulatory loop: p63 mutations causing ectodermal dysplasias were unable to activate IRF6 transcription, and mice with mutated or null p63 showed reduced Irf6 expression in their palate and ectoderm. These results identify what we believe to be a novel mechanism that regulates the proliferation-differentiation balance of keratinocytes essential for palate fusion and skin differentiation and links the pathogenesis of 2 genetically different groups of ectodermal dysplasia syndromes into a common molecular pathway.

Dlx homeobox gene family expression in osteoclasts

Skeletal growth and homeostasis require the finely orchestrated secretion of mineralized tissue matrices by highly specialized cells, balanced with their degradation by osteoclasts. Time- and site-specific expression of Dlx and Msx homeobox genes in the cells secreting these matrices have been identified as important elements in the regulation of skeletal morphology. Such specific expression patterns have also been reported in osteoclasts for Msx genes. The aim of the present study was to establish the expression patterns of Dlx genes in osteoclasts and identify their function in regulating skeletal morphology. The expression patterns of all Dlx genes were examined during the whole osteoclastogenesis using different in vitro models. The results revealed that Dlx1 and Dlx2 are the only Dlx family members with a possible function in osteoclastogenesis as well as in mature osteoclasts. Dlx5 and Dlx6 were detected in the cultures but appear to be markers of monocytes and their derivatives. In vivo, Dlx2 expression in osteoclasts was examined using a Dlx2/LacZ transgenic mouse. Dlx2 is expressed in a subpopulation of osteoclasts in association with tooth, brain, nerve, and bone marrow volumetric growths. Altogether the present data suggest a role for Dlx2 in regulation of skeletal morphogenesis via functions within osteoclasts.

Indirect modulation of Shh signaling by Dlx5 affects the oral-nasal patterning of palate and rescues cleft palate in Msx1-null mice

Cleft palate represents one of the most common congenital birth defects in human. During embryonic development, palatal shelves display oronasal (O-N) and anteroposterior polarity before the onset of fusion, but how the O-N pattern is established and how it relates to the expansion and fusion of the palatal shelves are unknown. Here we address these questions and show that O-N patterning is associated with the expansion and fusion of the palatal shelves and that Dlx5 is required for the O-N patterning of palatal mesenchyme. Loss of Dlx5 results in downregulation of Fgf7 and expanded Shh expression from the oral to the nasal side of the palatal shelf. This expanded Shh signaling is sufficient to restore palatal expansion and fusion in mice with compromised palatal mesenchymal cell proliferation, such as Msx1-null mutants. Exogenous Fgf7 inhibits Shh signaling and reverses the cranial neural crest (CNC) cell proliferation rescue in the Msx1/Dlx5 double knockout palatal mesenchyme. Thus, Dlx5-regulated Fgf7 signaling inhibits the expression of Shh, which in turn controls the fate of CNC cells through tissue-tissue interaction and plays a crucial role during palatogenesis. Our study shows that modulation of Shh signaling may be useful as a potential therapeutic approach for rescuing cleft palate.

Role for TGF-beta superfamily signaling in telencephalic GABAergic neuron development

Signaling mechanisms mediated by the Transforming Growth Factor-β (TGF-β) superfamily regulate a variety of developmental processes. Here we show that components of both bone morphogenetic protein/growth differentiation factor and TGF-β/activin/Nodal branches of TGF-β superfamily signaling are expressed in the developing subpallium. Furthermore, Smad proteins, transcriptional effectors of TGF-β signaling, are co-expressed and physically interact in the basal ganglia with Dlx homeodomain transcription factors, which are critical regulators of the differentiation, migration and survival of telencephalic GABAergic neurons. We also show that Dlx and Smad proteins localize to promoters/enhancers of a number of common telencephalic genes in vivo and that Smad proteins co-activate transcription with Dlx family members, except with certain mutated human DLX proteins identified in autistic individuals. In agreement with these observations, expression of dominant-negative Smads in the developing basal ganglia phenocopies the cell migration defects observed in Dlx1/2-deficient mice. Together, these results suggest that TGF-β superfamily signaling plays a role in telencephalic GABAergic neuron development through functional interactions with Dlx transcription factors.

Concerted stimuli regulating osteo-chondral differentiation from stem cells: phenotype acquisition regulated by microRNAs

Bone and cartilage are being generated de novo through concerted actions of a plethora of signals. These act on stem cells (SCs) recruited for lineage-specific differentiation, with cellular phenotypes representing various functions throughout their life span. The signals are rendered by hormones and growth factors (GFs) and mechanical forces ensuring proper modelling and remodelling of bone and cartilage, due to indigenous and programmed metabolism in SCs, osteoblasts, chondrocytes, as well as osteoclasts and other cell types (eg T helper cells).This review focuses on the concerted action of such signals, as well as the regulatory and/or stabilizing control circuits rendered by a class of small RNAs, designated microRNAs. The impact on cell functions evoked by transcription factors (TFs) via various signalling molecules, also encompassing mechanical stimulation, will be discussed featuring microRNAs as important members of an integrative system. The present approach to cell differentiation in vitro may vastly influence cell engineering for in vivo tissue repair.

Mechanotransduction in osteoblast regulation and bone disease

Osteoblasts are key components of the bone multicellular unit and have a seminal role in bone remodeling, which is an essential function for the maintenance of the structural integrity and metabolic capacity of the skeleton. The coordinated function of skeletal cells is regulated by several hormones, growth factors and mechanical cues that act via interconnected signaling networks, resulting in the activation of specific transcription factors and, in turn, their target genes. Bone cells are responsive to mechanical stimuli and this is of pivotal importance in developing biomechanical strategies for the treatment of osteodegenerative diseases. Here, we review the molecular pathways and players activated by mechanical stimulation during osteoblastic growth, differentiation and activity in health, and consider the role of mechanostimulatory approaches in treating various bone pathophysiologies.

Dento-craniofacial phenotypes and underlying molecular mechanisms in hypohidrotic ectodermal dysplasia (HED): a review

The hypohidrotic ectodermal dysplasias (HED) belong to a large and heterogeneous nosological group of polymalfomative syndromes characterized by dystrophy or agenesis of ectodermal derivatives. Molecular etiologies of HED consist of mutations of the genes involved in the Ectodysplasin (EDA)-NF-kappaB pathway. Besides the classic ectodermal signs, craniofacial and bone manifestations are associated with the phenotypic spectrum of HED. The dental phenotype of HED consists of various degrees of oligodontia with other dental abnormalities, and these are important in the early diagnosis and identification of persons with HED. Phenotypic dental markers of heterozygous females for EDA gene mutation-moderate oligodontia, conical incisors, and delayed dental eruption-are important for individuals giving reliable genetic counseling. Some dental ageneses observed in HED are also encountered in non-syndromic oligodontia. These clinical similarities may reflect possible interactions between homeobox genes implicated in early steps of odontogenesis and the Ectodysplasin (EDA)-NF-kappaB pathway. Craniofacial dysmorphologies and bone structural anomalies are also associated with the phenotypic spectrum of persons with HED patients. The corresponding molecular mechanisms involve altered interactions between the EDA-NF-kappaB pathway and signaling molecules essential in skeletogenic neural crest cell differentiation, migration, and osteoclastic differentiation. Regarding oral treatment of persons with HED, implant-supported prostheses are used with a relatively high implant survival rate. Recently, groundbreaking experimental approaches with recombinant EDA or transgenesis of EDA-A1 were developed from the perspective of systemic treatment and appear very promising. All these clinical observations and molecular data allow for the specification of the craniofacial phenotypic spectrum in HED and provide a better understanding of the mechanisms involved in the pathogenesis of this syndrome.

Claudin-1 is a p63 target gene with a crucial role in epithelial development

The epidermis of the skin is a self-renewing, stratified epithelium that functions as the interface between the human body and the outer environment, and acts as a barrier to water loss. Components of intercellular junctions, such as Claudins, are critical to maintain tissue integrity and water retention. p63 is a transcription factor essential for proliferation of stem cells and for stratification in epithelia, mutated in human hereditary syndromes characterized by ectodermal dysplasia. Both p63 and Claudin-1 null mice die within few hours from birth due to dehydration from severe skin abnormalities. These observations suggested the possibility that these two genes might be linked in one regulatory pathway with p63 possibly regulating Claudin-1 expression. Here we show that silencing of ΔNp63 in primary mouse keratinocytes results in a marked down-regulation of Claudin-1 expression (−80%). ΔNp63α binds in vivo to the Claudin-1 promoter and activates both the endogenous Claudin-1 gene and a reporter vector containing a –1.4 Kb promoter fragment of the Claudin-1 gene. Accordingly, Claudin-1 expression was absent in the skin of E15.5 p63 null mice and natural p63 mutant proteins, specifically those found in Ankyloblepharon–Ectodermal dysplasia–Clefting (AEC) patients, were indeed altered in their capacity to regulate Claudin-1 transcription. This correlates with deficient Claudin-1 expression in the epidermis of an AEC patient carrying the I537T p63 mutation. Notably, AEC patients display skin fragility similar to what observed in the epidermis of Claudin-1 and p63 null mice. These findings reinforce the hypothesis that these two genes might be linked in a common regulatory pathway and that Claudin-1 may is an important p63 target gene involved in the pathogenesis of ectodermal dysplasias.

Retinoic acid-induced inner ear teratogenesis caused by defective Fgf3/Fgf10-dependent Dlx5 signaling

BACKGROUND

Retinoic acid (RA) is essential for inner ear development. However, exposure to excess RA at a critical period leads to inner ear defects. These defects are associated with disruption in epithelial-mesenchymal interactions.

METHODS

This study investigates the role of Dlx5 in the epithelial-mesenchymal interactions that guide otic capsule chondrogenesis, as well as the effect of excess in utero RA exposure on Dlx5 expression in the developing mouse inner ear. Control of Dlx5 by Fgf3 and Fgf10 under excess RA conditions is investigated by examining the developmental window during which Fgf3 and Fgf10 are altered by in utero RA exposure and by testing the ability of Fgf3 and Fgf10 to mitigate the reduction in chondrogenesis and Dlx5 expression mediated by RA in high-density cultures of periotic mesenchyme containing otic epithelium, a model of epithelial-mesenchymal interactions in which chondrogenic differentiation of periotic mesenchyme ensues in response to induction by otic epithelium.

RESULTS

Dlx5 deletion alters expression of TGFbeta(1), important for otic capsule chondrogenesis, in the developing inner ear and compromises the ability of cultured periotic mesenchyme containing otic epithelium, harvested from Dlx5 null embryos, to differentiate into cartilage when compared with control cultures. Downregulation in Dlx5 ensues as a consequence of in utero RA exposure in association with inner ear dysmorphogenesis. This change in Dlx5 is noted at embryonic day 10.5 (E10.5), but not at E9.5, suggesting that Dlx5 is not a direct RA target. Before Dlx5 downregulation, Fgf3 and Fgf10 expression is modified in the inner ear by excess RA, with the ability of exogenous Fgf3 and Fgf10 to rescue chondrogenesis and Dlx5 expression in RA-treated cultures of periotic mesenchyme containing otic epithelium supporting these fibroblast growth factors (FGFs) as intermediary genes by which RA mediates its effects.

CONCLUSIONS

Disruption in an Fgf3, -10/Dlx5 signaling cascade is operant in molecular mechanisms of inner ear teratogenesis by excess RA.

Regulation of Dlx5 and Dlx6 gene expression by p63 is involved in EEC and SHFM congenital limb defects

The congenital malformation Split Hand-Foot Malformation (SHFM, or ectrodactyly) is characterized by a medial cleft of hands and feet, and missing central fingers. Five genetically distinct forms are known in humans; the most common (type-I) is linked to deletions of DSS1 and the distalless-related homeogenes DLX5 and DLX6. As Dlx5;Dlx6 double-knockout mice show a SHFM-like phenotype, the human orthologs are believed to be the disease genes. SHFM-IV and Ectrodactyly-Ectodermal dysplasia-Cleft lip (EEC) are caused by mutations in p63, an ectoderm-specific p53-related transcription factor. The similarity in the limb phenotype of different forms of SHFM may underlie the existence of a regulatory cascade involving the disease genes. Here, we show that p63 and Dlx proteins colocalize in the nuclei of the apical ectodermal ridge (AER). In homozygous p63- (null) and p63EEC (R279H) mutant limbs, the AER fails to stratify and the expression of four Dlx genes is strongly reduced; interestingly, the p63+/EEC and p63+/- hindlimbs, which develop normally and have a normally stratified AER, show reduced Dlx gene expression. The p63+/EEC mutation combined with an incomplete loss of Dlx5 and Dlx6 alleles leads to severe limb phenotypes, which are not observed in mice with either mutation alone. In vitro, DeltaNp63alpha induces transcription from the Dlx5 and Dlx6 promoters, an activity abolished by EEC and SHFM-IV mutations, but not by Ankyloblepharon-Ectodermal defects-Cleft lip/palate (AEC) mutations. ChIP analysis shows that p63 is directly associated with the Dlx5 and Dlx6 promoters. Thus, our data strongly implicate p63 and the Dlx5-Dlx6 locus in a pathway relevant in the aetio-pathogenesis of SHFM.

Transcription factors controlling osteoblastogenesis

The recent development of molecular biology and mouse genetics and the analysis of the skeletal phenotype induced by genetic mutations in humans led to a better understanding of the role of transcription factors that govern bone formation. This review summarizes the role of transcription factors in osteoblastogenesis and provides an integrated perspective on how the activities of multiple classes of factors are coordinated for the complex process of developing the osteoblast phenotype. The roles of Runx2, the principal transcriptional regulator of osteoblast differentiation, Osterix, beta-Catenin and ATF which act downstream of Runx2, and other transcription factors that contribute to the control of osteoblastogenesis including the AP1, C/EBPs, PPARgamma and homeodomain, helix-loop-helix proteins are discussed. This review also updates the regulation of transcription factor expression by signaling factors and hormones that control osteoblastogenesis.

Follistatin antagonizes transforming growth factor-beta3-induced epithelial-mesenchymal transition in vitro: implications for murine palatal development supported by microarray analysis

Epithelial-mesenchymal transition (EMT) is involved in normal embryonic development as well as in tumor progression and invasiveness. This process is also known to be a crucial step in palatogenesis during fusion of the bi-lateral palatal processes. Disruption of this step results in a cleft palate, which is among the most frequent birth defects in humans. A number of genes and encoded proteins have been shown to play a role in this developmental stage. The central role is attributed to the cytokine transforming growth factor-beta3 (TGF-beta3), which is expressed in the medial edge epithelium (MEE) already before the fusion process. The MEE covers the tips of the growing palatal shelves and eventually undergoes EMT or programmed cell death (apoptosis). TGF-beta3 is described to induce EMT in embryonic palates. With regard to the early expression of this molecule before the fusion process, it is not well understood which mechanisms prevent the TGF-beta3 producing epithelial cells from undergoing differentiation precociously. We used the murine palatal fusion to study the regulation of EMT. Specifically, we analyzed the MEE for the expression of known antagonists of TGF-beta molecules using in situ hybridization and detected the gene coding for Follistatin to be co-expressed with TGF-beta3. Further, we could show that Follistatin directly binds to TGF-beta3 and that it completely blocks TGF-beta3-induced EMT of the normal murine mammary gland (NMuMG) epithelial cell line in vitro. In addition, we analyzed the gene expression profile of NMuMG cells during TGF-beta3-induced EMT by microarray hybridization, detecting strong changes in the expression of apoptosis-regulating genes.

Wnt signaling mediates regional specification in the vertebrate face

At early stages of development, the faces of vertebrate embryos look remarkably similar, yet within a very short timeframe they adopt species-specific facial characteristics. What are the mechanisms underlying this regional specification of the vertebrate face? Using transgenic Wnt reporter embryos we found a highly conserved pattern of Wnt responsiveness in the developing mouse face that later corresponded to derivatives of the frontonasal and maxillary prominences. We explored the consequences of disrupting Wnt signaling, first using a genetic approach. Mice carrying compound null mutations in the nuclear mediators Lef1 and Tcf4 exhibited radically altered facial features that culminated in a hyperteloric appearance and a foreshortened midface. We also used a biochemical approach to perturb Wnt signaling and found that in utero delivery of a Wnt antagonist, Dkk1,produced similar midfacial malformations. We tested the hypothesis that Wnt signaling is an evolutionarily conserved mechanism controlling facial morphogenesis by determining the pattern of Wnt responsiveness in avian faces,and then by evaluating the consequences of Wnt inhibition in the chick face. Collectively, these data elucidate a new role for Wnt signaling in regional specification of the vertebrate face, and suggest possible mechanisms whereby species-specific facial features are generated.