Mesenchymal fibroblast growth factor receptor signaling regulates palatal shelf elevation during secondary palate formation

Cleft Palate in Apert Syndrome

Apert syndrome is a rare genetic disorder characterized by craniosynostosis, midface retrusion, and limb anomalies. Cleft palate occurs in a subset of Apert syndrome patients. Although the genetic causes underlying Apert syndrome have been identified, the downstream signaling pathways and cellular mechanisms responsible for cleft palate are still elusive. To find clues for the pathogenic mechanisms of palatal defects in Apert syndrome, we review the clinical characteristics of the palate in cases of Apert syndrome, the palatal phenotypes in mouse models, and the potential signaling mechanisms involved in palatal defects. In Apert syndrome patients, cleft of the soft palate is more frequent than of the hard palate. The length of the hard palate is decreased. Cleft palate is associated most commonly with the S252W variant of FGFR2. In addition to cleft palate, high-arched palate, lateral palatal swelling, or bifid uvula are common in Apert syndrome patients. Mouse models of Apert syndrome display palatal defects, providing valuable tools to understand the underlying mechanisms. The mutations in FGFR2 causing Apert syndrome may change a signaling network in epithelial–mesenchymal interactions during palatogenesis. Understanding the pathogenic mechanisms of palatal defects in Apert syndrome may shed light on potential novel therapeutic solutions.

New developments in the biology of fibroblast growth factors

The fibroblast growth factor (FGF) family is composed of 18 secreted signaling proteins consisting of canonical FGFs and endocrine FGFs that activate four receptor tyrosine kinases (FGFRs 1-4) and four intracellular proteins (intracellular FGFs or iFGFs) that primarily function to regulate the activity of voltage-gated sodium channels and other molecules. The canonical FGFs, endocrine FGFs, and iFGFs have been reviewed extensively by us and others. In this review, we briefly summarize past reviews and then focus on new developments in the FGF field since our last review in 2015. Some of the highlights in the past 6 years include the use of optogenetic tools, viral vectors, and inducible transgenes to experimentally modulate FGF signaling, the clinical use of small molecule FGFR inhibitors, an expanded understanding of endocrine FGF signaling, functions for FGF signaling in stem cell pluripotency and differentiation, roles for FGF signaling in tissue homeostasis and regeneration, a continuing elaboration of mechanisms of FGF signaling in development, and an expanding appreciation of roles for FGF signaling in neuropsychiatric diseases. This article is categorized under: Cardiovascular Diseases > Molecular and Cellular Physiology Neurological Diseases > Molecular and Cellular Physiology Congenital Diseases > Stem Cells and Development Cancer > Stem Cells and Development.

The Evaluation of FGFR1, FGFR2 and FOXO1 in Orofacial Cleft Tissue

Although cleft lip with or without cleft palate (CL/P) is one of the most common congenital anomalies worldwide, the morphopathogenesis of non-syndromic orofacial clefts is still unclear. Many candidate genes have been proposed to play a causal role; however, only a few have been confirmed, leaving many still to be assessed. Taking into account the significance of FGFR1, FGFR2 and FOXO1 in embryogenesis, the aim of this work was to detect and compare the three candidate genes in cleft-affected lip and palatine tissue. Ten soft tissue samples were taken during cheiloplasty and veloplasty. The signals of the candidate genes were visualized using chromogenic in situ hybridization and analyzed using a semi-quantitative method. No statistically important difference in the distribution of FGFR1, FGFR2 and FOXO1 between neither the patients’ lip and vomer mucosa nor the control group was observed. Statistically significant very strong and strong correlations were found between genes in the lip and palatine tissue. The expression of FGFR1, FGFR2 and FOXO1 in cleft-affected lip and palatine tissue seems to be highly individual. Numerous intercorrelations between the genes do not exclude their role in the possible complex morphopathogenesis of orofacial clefts.

Sites of Cre-recombinase activity in mouse lines targeting skeletal cells

The Cre/Lox system is a powerful tool in the biologist's toolbox, allowing loss-of-function and gain-of-function studies, as well as lineage tracing, through gene recombination in a tissue-specific and inducible manner. Evidence indicates, however, that Cre transgenic lines have a far more nuanced and broader pattern of Cre activity than initially thought, exhibiting "off-target" activity in tissues/cells other than the ones they were originally designed to target. With the goal of facilitating the comparison and selection of optimal Cre lines to be used for the study of gene function, we have summarized in a single manuscript the major sites and timing of Cre activity of the main Cre lines available to target bone mesenchymal stem cells, chondrocytes, osteoblasts, osteocytes, tenocytes, and osteoclasts, along with their reported sites of "off-target" Cre activity. We also discuss characteristics, advantages, and limitations of these Cre lines for users to avoid common risks related to overinterpretation or misinterpretation based on the assumption of strict cell-type specificity or unaccounted effect of the Cre transgene or Cre inducers. © 2021 American Society for Bone and Mineral Research (ASBMR).

Early mandibular morphological differences in patients with FGFR2 and FGFR3-related syndromic craniosynostoses: A 3D comparative study

Syndromic craniosynostoses are defined by the premature fusion of one or more cranial and facial sutures, leading to skull vault deformation, and midfacial retrusion. More recently, mandibular shape modifications have been described in FGFR-related craniosynostoses, which represent almost 75% of the syndromic craniosynostoses. Here, further characterisation of the mandibular phenotype in FGFR-related craniosynostoses is provided in order to confirm mandibular shape modifications, as this could contribute to a better understanding of the involvement of the FGFR pathway in craniofacial development. The aim of our study was to analyse early mandibular morphology in a cohort of patients with FGFR2- (Crouzon and Apert) and FGFR3- (Muenke and Crouzonodermoskeletal) related syndromic craniosynostoses. We used a comparative geometric morphometric approach based on 3D imaging. Thirty-one anatomical landmarks and eleven curves with sliding semi-landmarks were defined to model the shape of the mandible. In total, 40 patients (12 with Crouzon, 12 with Apert, 12 with Muenke and 4 with Crouzonodermoskeletal syndromes) and 40 age and sex-matched controls were included (mean age: 13.7 months ±11.9). Mandibular shape differed significantly between controls and each patient group based on geometric morphometrics. Mandibular shape in FGFR2-craniosynostoses was characterized by open gonial angle, short ramus height, and high and prominent symphysis. Short ramus height appeared more pronounced in Apert than in Crouzon syndrome. Additionally, narrow inter-condylar and inter-gonial distances were observed in Crouzon syndrome. Mandibular shape in FGFR3-craniosynostoses was characterized by high and prominent symphysis and narrow inter-gonial distance. In addition, narrow condylar processes affected patients with Crouzonodermoskeletal syndrome. Statistical analysis of variance showed significant clustering of Apert and Crouzon, Crouzon and Muenke, and Apert and Muenke patients (p < 0.05). Our results confirm distinct mandibular shapes at early ages in FGFR2- (Crouzon and Apert syndromes) and FGFR3-related syndromic craniosynostoses (Muenke and Crouzonodermoskeletal syndromes) and reinforce the hypothesis of genotype-phenotype correspondence concerning mandibular morphology.

Genetics and signaling mechanisms of orofacial clefts

Craniofacial development involves several complex tissue movements including several fusion processes to form the frontonasal and maxillary structures, including the upper lip and palate. Each of these movements are controlled by many different factors that are tightly regulated by several integral morphogenetic signaling pathways. Subject to both genetic and environmental influences, interruption at nearly any stage can disrupt lip, nasal, or palate fusion and result in a cleft. Here, we discuss many of the genetic risk factors that may contribute to the presentation of orofacial clefts in patients, and several of the key signaling pathways and underlying cellular mechanisms that control lip and palate formation, as identified primarily through investigating equivalent processes in animal models, are examined.

MiR-106a-5p modulates apoptosis and metabonomics changes by TGF-β/Smad signaling pathway in cleft palate

BACKGROUND

The molecular mechanisms of abnormal palatogenesis were investigated in this study. A key regulator, miR-106a-5p, and its target pathway were analyzed.

OBJECTIVES

This research is trying to clarify the underlying mechanism of the modulation of miRNA transcription during the formation of cleft palate by 7T and 9.4T NMR metabolomic platforms.

METHOD

Differentially expressed miRNAs and mRNAs were analyzed by microarray analysis and verified by qRT-PCR. The protein expression in TGFβ signaling pathways were analyzed by Western Blotting. The relationship between miR-106a-5p and TGFβ were analyzed by luciferase reporter assay. Cell apoptosis were analyzed by flow cytometer. And finally, the metabonomics were analyzed by NMR and multivariate data analysis models (MVDA).

RESULTS

The expression of miR-106a-5p increased in cleft palatal tissue and negatively correlated with the protein level of Tgfbr2. The luciferase assay further proved that the tgfbr2 was a direct target of miR-106a-5p. In another aspect, miR-106a-5p increased apoptosis level in palatal mesenchymal cells, possibly because its inhibition of TGFβ signaling pathway. Moreover, low cholesterol and choline levels with high citric acid and lipid levels were observed by 7T and 9.4T NMR metabonomic analysis, which inferred the disorder of cell membrane synthesis in cleft palate formation. Furthermore, transformation from choline to phosphatidylcholine regulated by miR-106a-5p was also disrupted, resulting in phosphatidic choline synthesis disorder and reduced cell membrane synthesis.

CONCLUSIONS

The regulatory mechanism of cleft palate was studied at transcriptional and metabolomics levels, which may provide important information in understanding the primary cause of this abnormality.

Dynamic activation of Wnt, Fgf, and Hh signaling during soft palate development

The soft palate is a key component of the oropharyngeal complex that is critical for swallowing, breathing, hearing and speech. However, complete functional restoration in patients with cleft soft palate remains a challenging task. New insights into the molecular signaling network governing the development of soft palate will help to overcome these clinical challenges. In this study, we investigated whether key signaling pathways required for hard palate development are also involved in soft palate development in mice. We described the dynamic expression patterns of signaling molecules from well-known pathways, such as Wnt, Hh, and Fgf, during the development of the soft palate. We found that Wnt signaling is active throughout the development of soft palate myogenic sites, predominantly in cells of cranial neural crest (CNC) origin neighboring the myogenic cells, suggesting that Wnt signaling may play a significant role in CNC-myogenic cell-cell communication during myogenic differentiation in the soft palate. Hh signaling is abundantly active in early palatal epithelium, some myogenic cells, and the CNC-derived cells adjacent to the myogenic cells. Hh signaling gradually diminishes during the later stages of soft palate development, indicating its involvement mainly in early embryonic soft palate development. Fgf signaling is expressed most prominently in CNC-derived cells in the myogenic sites and persists until later stages of embryonic soft palate development. Collectively, our results highlight a network of Wnt, Hh, and Fgf signaling that may be involved in the development of the soft palate, particularly soft palate myogenesis. These findings provide a foundation for future studies on the functional significance of these signaling pathways individually and collectively in regulating soft palate development.

Hyaluronic acid is required for palatal shelf movement and its interaction with the tongue during palatal shelf elevation

Palatal shelf elevation is an essential morphogenetic process that results from palatal shelf movement caused by an intrinsic elevating force. The nature of the elevating force remains unclear, but the accumulation of hyaluronic acid (HA) in the extracellular matrix (ECM) of the palatal shelves may play a pivotal role in developing the elevating force. In mammals, HA is synthesized by hyaluronic acid synthases (HAS) that are encoded by three genes (Has1-3). Here, we used the Wnt1-Cre driver to conditionally disrupt hyaluronic acid synthase 2 (Has2) in cranial neural crest cell lineages. All Has2 conditional knockout (cko) mice had cleft palate due to failed shelf elevation during palate development. The HA content was significantly reduced in the craniofacial mesenchyme of Has2 cko mutants. Reduced HA content affected the ECM space and shelf expansion to result in a reduced shelf area and an increased mesenchymal cell density in the palatal shelves of Has2 cko mutants. We examined palatal shelf movement by removal of the tongue and mandible from unfixed E13.5 and early E14.5 embryonic heads. Reduced shelf expansion in Has2 cko mutants altered palatal shelf movement in the medial direction resulting in a larger gap between the palatal shelves than that of littermate controls. We further examined palatal shelf movement in the intact oral cavity by culturing explants containing the maxilla, palate, mandible and tongue (MPMT explants). The palatal shelves elevated alongside morphological changes in the tongue after 24-h culture in MPMT explants of early E14.5 wild type embryos. On the contrary, shelf elevation failed to occur in MPMT explants of age-matched Has2 cko mutants because the tongue obstructs palatal shelf movement, suggesting that reduced shelf expansion could be essential for the palatal shelves to interact with the tongue and overcome tongue obstruction during shelf elevation. Has2 cko mutants also showed micrognathia due to reduced HA content in the mandibular mesenchyme including Meckel's cartilage. Through 3D imaging and morphometric analysis, we demonstrate that mandibular growth results in a significant increase in the vertical dimension of the common oral-nasal cavity that facilitates palatal shelf movement and its interaction with the tongue during shelf elevation.

Fibroblast Growth Factor Receptors Function Redundantly During Zebrafish Embryonic Development

Fibroblast growth factor (Fgf) signaling regulates many processes during development. In most cases, one tissue layer secretes an Fgf ligand that binds and activates an Fgf receptor (Fgfr) expressed by a neighboring tissue. Although studies have identified the roles of specific Fgf ligands during development, less is known about the requirements for the receptors. We have generated null mutations in each of the five fgfr genes in zebrafish. Considering the diverse requirements for Fgf signaling throughout development, and that null mutations in the mouse Fgfr1 and Fgfr2 genes are embryonic lethal, it was surprising that all zebrafish homozygous mutants are viable and fertile, with no discernable embryonic defect. Instead, we find that multiple receptors are involved in coordinating most Fgf-dependent developmental processes. For example, mutations in the ligand fgf8a cause loss of the midbrain-hindbrain boundary, whereas, in the fgfr mutants, this phenotype is seen only in embryos that are triple mutant for fgfr1a;fgfr1b;fgfr2, but not in any single or double mutant combinations. We show that this apparent fgfr redundancy is also seen during the development of several other tissues, including posterior mesoderm, pectoral fins, viscerocranium, and neurocranium. These data are an essential step toward defining the specific Fgfrs that function with particular Fgf ligands to regulate important developmental processes in zebrafish.

In Vitro Analysis of Palatal Shelf Elevation During Secondary Palate Formation

Palatal shelf elevation is an essential morphogenetic process during secondary palate formation. It has been proposed that shelf elevation results from an intrinsic elevating force and is regulated by extrinsic factors that are associated with development of other orofacial structures. Although dynamic palate culture is a common in vitro approach for studying shelf elevation, it requires the tongue or the tongue and mandible to be removed before culture, which prevents any determination of the role of the extrinsic factors in regulating shelf elevation. We showed that ex vivo removal of the tongue and mandible from unfixed embryonic heads led to spontaneous shelf movements that were more pronounced at late E13.5 and early E14.5 than those of E12.5 and early E13.5, suggesting that the strength of the elevating force increases over time during palate development. We further used a suspension culture technique to analyze palatal shelf movement in an intact oral cavity by culturing the orofacial portion of embryonic heads that include the maxilla, palatal shelves, mandible, and tongue (MPMT). MPMT explants were cultured in the serum-free medium with slow rotation for 24-48 hr. The palatal shelves successfully elevated during culture and displayed intermediate morphologies that closely resemble those of in vivo shelf elevation. We demonstrate that the tongue and mandible facilitate shelf medial movement/growth during shelf elevation and further suggest that the interaction of the palatal shelves and tongue could be one of the extrinsic factors that regulate the elevation process. Anat Rec, 302:1594-1604, 2019. © 2019 American Association for Anatomy.

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.

The ontogeny of Robin sequence

The triad of micrognathia, glossoptosis, and concomitant airway obstruction defined as "Robin sequence" (RS) is caused by oropharyngeal developmental events constrained by a reduced stomadeal space. This sequence of abnormal embryonic development also results in an anatomical configuration that might predispose the fetus to a cleft palate. RS is heterogeneous and many different etiologies have been described including syndromic, RS-plus, and isolated forms. For an optimal diagnosis, subsequent treatment and prognosis, a thorough understanding of the embryology and pathogenesis is necessary. This manuscript provides an update about our current understanding of the development of the mandible, tongue, and palate and possible mechanisms involved in the development of RS. Additionally, we provide the reader with an up-to-date summary of the different etiologies of this phenotype and link this to the embryologic, developmental, and genetic mechanisms.

Inactivation of Fgfr2 gene in mouse secondary palate mesenchymal cells leads to cleft palate

Numerous studies have been conducted to understand the molecular mechanisms controlling mammalian secondary palate development such as growth, reorientation and fusion. However, little is known about the signaling factors regulating palate initiation. Mouse fibroblast growth factor (FGF) receptor 2 gene (Fgfr2) is expressed on E11.5 in the palate outgrowth within the maxillary process, in a region that is responsible for palate cell specification and shelf initiation. Fgfr2 continues to express in palate on E12.5 and E13.5 in both epithelial and mesenchymal cells, and inactivation of Fgfr2 expression in mesenchymal cells using floxed Fgfr2 allele and Osr2-Cre leads to cleft palate at various stages including reorientation, horizontal growth and fusion. Notably, some mutant embryos displayed no sign of palate shelf formation suggesting that FGF receptor 2 mediated FGF signaling may play an important role in palate initiation.

Genetic tools for identifying and manipulating fibroblasts in the mouse

The use of mouse genetic tools to track and manipulate fibroblasts has provided invaluable in vivo information regarding the activities of these cells. Recently, many new mouse strains have been described for the specific purpose of studying fibroblast behavior. Colorimetric reporter mice and lines expressing Cre are available for the study of fibroblasts in the organs prone to fibrosis, including heart, kidney, liver, lung, and skeletal muscle. In this review we summarize the current state of the models that have been used to define tissue resident fibroblast populations. While these complex genetic reagents provide unique insights into the process of fibrosis, they also require a thorough understanding of the caveats and limitations. Here, we discuss the specificity and efficiency of the available genetic models and briefly describe how they have been used to document the mechanisms of fibrosis.

Meckel's and condylar cartilages anomalies in achondroplasia result in defective development and growth of the mandible

Activating FGFR3 mutations in human result in achondroplasia (ACH), the most frequent form of dwarfism, where cartilages are severely disturbed causing long bones, cranial base and vertebrae defects. Because mandibular development and growth rely on cartilages that guide or directly participate to the ossification process, we investigated the impact of FGFR3 mutations on mandibular shape, size and position. By using CT scan imaging of ACH children and by analyzing Fgfr3^Y367C/+ mice, a model of ACH, we show that FGFR3 gain-of-function mutations lead to structural anomalies of primary (Meckel’s) and secondary (condylar) cartilages of the mandible, resulting in mandibular hypoplasia and dysmorphogenesis. These defects are likely related to a defective chondrocyte proliferation and differentiation and pan-FGFR tyrosine kinase inhibitor NVP-BGJ398 corrects Meckel’s and condylar cartilages defects ex vivo. Moreover, we show that low dose of NVP-BGJ398 improves in vivo condyle growth and corrects dysmorphologies in Fgfr3^Y367C/+ mice, suggesting that postnatal treatment with NVP-BGJ398 mice might offer a new therapeutic strategy to improve mandible anomalies in ACH and others FGFR3-related disorders.

FGF signaling in the osteoprogenitor lineage non-autonomously regulates postnatal chondrocyte proliferation and skeletal growth

Fibroblast growth factor (FGF) signaling is important for skeletal development; however, cell-specific functions, redundancy and feedback mechanisms regulating bone growth are poorly understood. FGF receptors 1 and 2 (Fgfr1 and Fgfr2) are both expressed in the osteoprogenitor lineage. Double conditional knockout mice, in which both receptors were inactivated using an osteoprogenitor-specific Cre driver, appeared normal at birth; however, these mice showed severe postnatal growth defects that include an ∼50% reduction in body weight and bone mass, and impaired longitudinal bone growth. Histological analysis showed reduced cortical and trabecular bone, suggesting cell-autonomous functions of FGF signaling during postnatal bone formation. Surprisingly, the double conditional knockout mice also showed growth plate defects and an arrest in chondrocyte proliferation. We provide genetic evidence of a non-cell-autonomous feedback pathway regulating Fgf9, Fgf18 and Pthlh expression, which led to increased expression and signaling of Fgfr3 in growth plate chondrocytes and suppression of chondrocyte proliferation. These observations show that FGF signaling in the osteoprogenitor lineage is obligately coupled to chondrocyte proliferation and the regulation of longitudinal bone growth.

Ablation of the Sox11 Gene Results in Clefting of the Secondary Palate Resembling the Pierre Robin Sequence

Mouse gene inactivation has shown that the transcription factor Sox11 is required for mouse palatogenesis. However, whether Sox11 is primarily involved in the regulation of palatogenesis still remains elusive. In this study, we explored the role ofSox11in palatogenesis by analyzing the developmental mechanism in cleft palate formation in mutants deficient in Sox11. Sox11 is expressed both in the developing palatal shelf and in the surrounding structures, including the mandible. We found that cleft palate occurs only in the mutant in which Sox11is directly deleted. As in the wild type, the palatal shelves in the Sox11 mutant undergo outgrowth in a downward direction and exhibit potential for fusion and elevation. However, mutant palatal shelves encounter clefting, which is associated with a malpositioned tongue that results in physical obstruction of palatal shelf elevation at embryonic day 14.5 (E14.5). We found that loss of Sox11led to reduced cell proliferation in the developing mandibular mesenchyme via Cyclin D1, leading to mandibular hypoplasia, which blocks tongue descent. Extensive analyses of gene expression inSox11 deficiency identified FGF9 as a potential candidate target of Sox11 in the modulation of cell proliferation both in the mandible and the palatal shelf between E12.5 and E13.5. Finally we show, using in vitro assays, that Sox11 directly regulates the expression of Fgf9 and that application of FGF9 protein to Sox11-deficient palatal shelves restores the rate of BrdU incorporation. Taken together, the palate defects presented in the Sox11 loss mutant mimic the clefting in the Pierre Robin sequence in humans.