Bone Morphogenetic Protein Type I Receptor Inhibition Induces Cleft Palate Associated with Micrognathia and Cleft Lower Lip in Mice
Yongzhen Lai1, Changfu Xie1, Shixian Zhang1, Guowu Gan1, Di Wu1, and Weihui Chen*1,2
Background: Gain-of- and loss-of-function studies have demonstrated that changes in bone morphogenetic protein (BMP) signaling during embryo development cause craniofacial malformations, including cleft palate. It remains uncertain whether BMP signaling could be targeted pharmacologically to affect craniofacial morphogenesis. Methods: Pregnant C57Bl/6J mice were treated with the BMP type I receptor inhibitor LDN- 193189 at the dose of 3, 6, or 9 mg/kg twice a day by intraperitoneal injection from embryonic day 10.5 (E10.5) to E15.5. At E16.5, embryos were investigated by facial measurement analysis and histology to determine the optimal concentration for malformation. Subsequent embryonic phenotypes were analyzed in detail by histology, whole-mount skeletal staining, micro- computed tomography, and palatal organic culture. We further used immunohistochemistry to analyze protein expression of the BMP-mediated canonical and noncanonical signaling components. Results: The optimal concentration of LDN-193189 was determined to be 6 mg/kg. In utero, LDN- 193189 exposures induced partial clefting of the anterior palate or complete cleft palate, which was attributed to a reduced cell proliferation rate in the
secondary palate, and delayed palatal elevation caused by micrognathia. Analysis of signal transduction in palatal shelves at E12.5 and E13.5 identified a significant reduction of BMP/Smad signaling (p-Smad1/5/8) and unchanged BMP noncanonical signaling (p-p38, p-Erk1/2) after treatment with LDN- 193189. Conclusion: The results of this study indicate that LDN-193189 can be used to manipulate BMP signaling by selectively targeting the BMP/Smad signaling pathway to affect palatal morphogenesis and produce phenotypes mimicking those caused by genetic mutations. This work established a novel mouse model for teratogen-induced cleft palate.
Birth Defects Research (Part A) 00:000–000, 2016.
VC 2016 Wiley Periodicals, Inc.
Key words: BMP type I receptor; cleft lower lip; cleft palate; micrognathia; LDN-193189
Orofacial clefts (OFCs), including cleft lip with or without cleft palate (CL/P) and cleft palate only (CPO), are the most common congenital birth defects in humans, with a rate of approximately 1 in 700 births (Vieira, 2008). These defects cause significant dysfunction and require surgical intervention.
Current research to uncover the etiology of CL/P or CPO has largely focused on possible genetic causes. While numerous candidate genes have been identified to cause a cleft phenotype when mutated or removed in mice, the majority of CL/P or CPO conditions are non-Mendelian (Muenke, 2002; Calzolari, et al., 2007). In most cases, affected patients have no positive family history (Lidral et al., 2008; Vieira, 2008; Kling et al., 2014). More convinc-
Supported by the National Natural Science Foundation of China (31070838/ C100102) and Fujian Foundation for Distinguished Young Scientists in China(2012J06017).
1Department of Oral and Maxillofacial Surgery, Union Hospital, Fujian Medical University, Fuzhou, P. R. China
2Stomatological Research Institute, Fujian Medical University, Fuzhou, Fujian Province, P. R. China
*Correspondence to: Weihui Chen, Department of Oral and Maxillofacial Surgery, Union Hospital, Fujian Medical University, Fuzhou 350001, P. R. China. E-mail: [email protected]
Published online 0 Month 2016 in Wiley Online Library (wileyonlinelibrary.com). Doi: 10.1002/bdra.23504
VC 2016 Wiley Periodicals, Inc.
ing evidence from animal models and human studies indi- cates that the etiology of clefting is complex, and genetic and environmental factors are part of the equation (Gra- ham and Shaw, 2005; Vieira, 2008).
Bone morphogenetic proteins (BMPs), a subfamily of multifunctional growth factors, belong to the transforming growth factor-beta (TGF-b) superfamily. BMP ligands pro- mote the phosphorylation and activation of BMP type I receptors (Alk2, Alk3, Alk6) through BMP type II receptors (BMPRII, ActRIIA, and ActRIIB) to activate the Smad1/5/ 8-dependent pathway (also known as BMP canonical path- way) and Smad-independent pathway (BMP noncanonical pathway), such as MAPK and Akt mediated signaling (Xiao et al., 2007).
BMP type I receptors (Alk2, Alk3, Alk6) play an impor- tant role in determining the signaling specificity. Inactiva- tion of Alk2 in the mouse cranial neural crest lineage resulted in cleft palate and small mandible (Dudas et al., 2004). Craniofacial primordial inactivation of Bmpr1a (Alk3) showed a strong phenotype with bilateral cleft lip and palate in the mouse (Liu et al., 2005b). Meanwhile, neural crest specific inactivation of Bmpr1a led to anterior clefting of the secondary palate, and severe hypoplasia of the mandible (Li et al., 2011). Similar type of clefting in the anterior secondary palate was also observed in the transgenic mouse model in which Bmpr1a was inactivated in the developing palatal mesenchyme (Baek et al., 2011). Thus, signaling mediated by BMP type I receptors plays an
important role in craniofacial development particularly in the palate, mandible, and other regions of the craniofacial skeleton.
However, in contrast to genetic abrogation, interference with BMP type I receptor mediated signaling in vivo by exogenous signaling inhibitors has rarely been studied. In the current study, we chose LDN-193189, a selective chemical inhibitor that was initially identified in an embry- onic zebrafish screening assay (Cuny et al., 2008) to dis- rupt BMP signaling. LDN-193189 disrupted the BMP signaling pathway by binding to the ATP binding site in the kinase domain of the type I receptors (Boergermann et al., 2010) thereby efficiently inhibiting the activity of BMP type I receptors (Alk2, Alk3) but also resulted in desirable pharmacokinetic characteristics in mice (plasma t1/2 5 1.6 hr) (Yu et al., 2008). As the current lead com- pound, it has been widely used in a variety of mouse dis- ease models in which BMP signaling is implicated, such as craniosynostosis, fibrodysplasia ossificans progressive, ath- eroma, and epithelial ovarian cancer (Yu et al., 2008; Der- wall et al., 2012; Komatsu et al., 2013; Ali et al., 2015).
Embryonic cells can integrate various signaling path- ways to trigger precise developmental phenotype. The purpose of this research was to determine if BMP signal- ing could be targeted pharmacologically by LDN-193189 to affect craniofacial morphogenesis and mimic the effects caused by genetic mutations.
In the current study, we analyzed the phenotypic spec- trum resulting from in utero BMP signaling inhibition by LDN-193189 in mice. We also examined the distribution and expression of downstream signaling molecule path- ways, including Smad and non-Smad, in palatal shelves after treatment with the inhibitor.
Materials and Methods
All animal experiments were approved by the Institutional Animal Care and Use Committee of Fujian Medical Univer- sity. Timed pregnancies were established in-house. Two female C57BL/6J mice (SLAC Laboratory Animal) at 12 to
16 weeks of age were mated with one C57BL/6J male mouse overnight (12/12 hr light/dark cycle). The presence of a vaginal plug on the following morning was set as embryonic day 0.5 (E0.5)
To establish an optimal dysmorphogenic concentration, 12 pregnant females at E10.5 were randomly divided into four groups for treatment with LDN-193189 (Selleck Chemicals, Houston, Texas). LDN-193189 was dissolved in sterile endotoxin-free water and injected intraperitoneally twice a day at concentrations of 0 mg/kg, 3 mg/kg, 6 mg/ kg, or 9 mg/kg on E10.5 through E15.5 which is in accord- ance with the initial timing of BMP type I receptor (ALK2, ALK3) inactivation in classical mouse mutant models
(Dudas et al., 2004; Liu et al., 2005b). Embryos were col- lected at E16.5 for morphological assessment.
Another 24 pregnant females at E10.5 were randomly divided into eight groups. Four groups were treated with LDN-193189 at the optimal concentration, and the other four groups were treated with sterile water as a control. Embryos of treated and control groups were collected at E12.5, E13.5, E14, and E14.5 for histology, in vitro culture, and immunohistochemistry (IHC) analyses.
EMBRYONIC MORPHOLOGICAL ASSESSMENT
Embryos at E16.5 were collected by laparotomy and fixed in 4% paraformaldehyde after examination under a dis- secting microscope for gross craniofacial malformations. Lateral and frontal view photographs were taken, digitized, and measured using analysis software. Linear measure- ments of snout width (SW), snout length (SL), mandibular retrognathia (MR), and mandible length (ML) were con- ducted as reported previously (Lipinski et al., 2014). The degree of MR was determined using two parallel lines along the tip of the nose and the most prominent point of the chin (Fig. 1). These lines were perpendicular to a ref- erence line from the center of the eye to the inferior mar- gin of the nose (Fig. 1). The ML was measured from the chin point to the chin hair follicles point.
BONE AND CARTILAGE STAINING
At E14.5, four embryos treated with LDN-193189 at 6 mg/kg and vehicle were fixed in 100% ethanol overnight. Samples were stained in Alcian Blue solution containing 8 ml of 100% ethanol and 2 ml of 1% Alcian Blue/3% acetic acid (Sigma). Tissue was cleared in 1% KOH for 3 days, stored in 50% glycerol/0.1% KOH, and imaged.
MICRO-COMPUTED TOMOGRAPHY ANALYSIS
The mandibular samples (n 5 3) from different litters at E16.5 treated with LDN-193189 at 6 mg/kg and vehicle were scanned using a micro-computed tomography (micro-CT) system (Research Center of Stomatology, Guangzhou Medical University, Guangzhou, China) at 12 mm of thickness, 55 kV of energy, and 145 mA of intensity (lCT40) and reconstructed to produce three-dimensional (3D) images (Bouxsein et al., 2010).
HISTOCHEMICAL STAINING AND IHC
Embryos were collected from both control and LDN-treated females at different embryonic time points. Embryos were fixed in 4% paraformaldehyde overnight, paraffin-embedded, and sectioned for histochemical staining and IHC. The pri- mary antibodies used were as follows: anti-p-Smad1/5/8, (1:100, Santa Cruz, CA), anti-p-p38, anti-p-Erk1/2 (1:200 Cell Signaling Technology, Danvers, MA), and anti-pH3 (1:1000 Abcam, Cambridge, MA). Signal detection was carried out using ultrasensitive biotinylated secondary goat anti-rabbit antibodies (Maixin, China). Staining was completed by incu- bation with diaminobenzidine and counterstained with
FIGURE 1. LDN-193189-induced facial dysmorphology of mice at E16.5. From stereomicrographs, frontal and lateral views of facial surfaces are shown for a vehicle-exposed control embryo (A,B) along with three representative embryos with phenotypic spectrum observed in the LDN-193189-exposed 3 mg/kg group (C,D), 6 mg/kg anterior cleft group (E,F), 6 mg/kg complete cleft group (G,H). Snout width (SW), snout length (SL), mandible length (ML), mandible retrognathia (MR) were measured from facial structures. Values represent the mean 6 SEM: *p < 0.05 compared with vehicle-exposed control group, #p < 0.05 compared with 3 mg/kg group. Arrows in E,G point to the mandibular notch.
hematoxylin. p-Smad1/5/8, p-p38, and p-Erk1/2 were deter- mined using a quantitative image analysis system (Image pro plus6.0, Media Cybernetics, Rockville, MD). The palatal shelf was outlined and positive staining identified. Then the per- centage of positively stained area within the outlined area of interest was determined. Data were expressed as relative expression level to the control. The proportion of p-H3- positive cells to total cells in the palatal shelf was determined. All counting was performed on blinded slides. Palate samples (n 5 4) from each of three different litters at E12.5 and E13.5 with LDN-193189 at 6 mg/kg and vehicle treatment were
evaluated for every tested protein. Data were counted from three randomly selected sections (36 sections per region).
PALATAL ORGAN CULTURE
A previously reported technique was performed to culture the maxillary arch with paired palatal shelves in a serum- less medium (Shiota et al., 1990). To do this, the mandible and tongue were removed, and the upper cranial tissue was cut off at the eye level from the head of E13.5 embryos. The resulting midfacial region from the control and LDN-193189-treated embryos was then cultured in
FIGURE 2. Cleft palate and abnormal attachment of genioglossus in LDN-exposed embryos at E16.5. Coronal sections of E16.5 vehicle-exposed embryos (A–D) and LDN-193189-treated embryos (6 mg/kg) show anterior clefting (E–H) and complete cleft palate (I-L). Sections (A,E,I) and (C,G,K) were cut through the mid- dle of the maxillary first molar tooth germs. Sections (B,F,J) are from the anterior end of the developing secondary palate. D,H,L: The posterior soft palate region. Asterisks in F,I,J,K,L mark palate clefting. Arrows in E,I point to the genioglossus abnormal attachment. Dashed lines in D,H demarcate bone mineralization. Scale bars 5 10 lm. The scale bar shown in I is applied to A,E,I, and the scale bar shown in L is applied to all the remaining panels.
suspension in DMEM supplemented with 10% fetal bovine serum, 1% penicillin/streptomycin, and 4% bovine serum albumin without further addition of the LDN-193189. Pala- tal tissues were maintained in an incubator at 378C with a gas mixture of 5%CO2/50%O2/N2 for 60 hrs, and the medium was replaced every 12 hrs.
Statistical analysis was performed using SPSS version 19.0 for Windows. IHC assays were performed in triplicate. Data were analyzed using unpaired Student’s t test. One- way analyses of variance were used to determine signifi- cant group differences in linear measurements. p < 0.05 was considered statistically significant.
IN VIVO TERATOGENICITY TRIALS
To determine the optimal teratogenic concentration of LDN-193189 in vivo, various doses of LDN-193189 were injected intraperitoneally into pregnant mice. Facial mor- phology of each embryo was assessed by gross inspection. There was no maternal death in any of the groups and no embryo lethality in the control group. However, embryos exposed to the 9 mg/kg concentration of LDN-193189
showed maximal embryonic toxicity with all fetuses being resorbed. Dams receiving the 3 mg/kg and 6 mg/kg con- centrations showed similar embryonic death rates (16.6% and 20.8%) and average body weights (0.411 g and 0.405 g). However, only at the 6 mg/kg dose of LDN-193189 did the fetuses exhibit craniofacial abnormalities, including complete cleft palate (6/21), partial anterior clefting of the palate (13/21) (Fig. 2), and most interestingly, a notch formation in the mandible (Fig. 1E,G) with full penetrance (21/21) (Table 1).
FACIAL MEASUREMENT ANALYSIS
To further assess the facial morphology, we measured the length of the facial structures (Fig. 1). Compared with vehicle-exposed embryos, in the LDN-exposed fetuses, the SW, SL, and ML were all reduced, and the length of man- dibular retrognathia was increased, with linear correlation with the concentration. To illustrate the relationship between these changes, we compared the ratio of linear measurements of each embryo. The SW/SL ratio was not statistically different between controls, 3 mg/kg, and 6 mg/kg exposure groups. Fetuses exposed to LDN-193189 showed a concentration-dependent reduction in ML/SL ratio and an increase in MR/SL ratio.
TABLE 1. Craniofacial Abnormal of Embryos with Different Concentrations of LDN
Dose, Pregnant females Total fetus Viable embryos Complete cleft palate Anterior cleft palate Mandibular notch Average weight
mg/kg (n) (n) (n) (n) (n) (n) (g)
0 3 21 21 0 0 0 0.504
3 3 24 20 0 0 0 0.411
6 3 26 21 6 13 21 0.405
9 3 23 0 a a a a
aThere were no viable embryos for imaging analysis.
DELAYED PALATAL SHELF ELEVATION
To analyze the pathogenic processes of the cleft palate in LDN-exposed mice, we conducted a detailed histological examination of embryos at different stages. The palatal shelf was completely fused in E16.5 control embryos, while the LDN-exposed embryos displayed a variety of deformed shelf morphologies. The palatal shelves failed to contact each other in the anterior domain, although the middle and posterior portions of the secondary palate had successfully fused in most LDN-exposed embryos (13/21), and in some cases, the palate shelves failed to elevate and remained at the vertical position (6/21) (Fig. 2). At E12.5 and E13.5, LDN-treated palatal shelves did not significantly change in shape and size as compared to controls. How- ever, by E14, while the control palatal shelves had initiated their elevation to the dorsum of the tongue, the LDN- exposed palatal shelves all remained at the vertical posi- tion. At E14.5, the control palatal shelves had already com- pleted their elevation and made contact with each other to form the medial edge epithelial and initial palatal fusion. At the same time, the LDN-exposed palatal shelves remained in the stage of elevation (Fig. 3).
RETARDED GROWTH OF MECKEL’S CARTILAGE AND MANDIBLE
The most prominent external phenotype of LDN-exposed embryos was the formation of a notch in the mandible (Fig. 1E,G). We, therefore, examined the mandibular devel- opment in details. The mandible of LDN-exposed embryos became hypoplastic as early as E14.5 (Fig. 4I,J). The 3D images of mandibles were generated from micro-CT at E16.5 and showed that the bone volume of LDN-exposed mandibles was only half that of control embryos (n 5 3) (Fig. 4A,B). Histological analysis of Meckel’s cartilage in transverse sections at E16.5 revealed that Meckel’s carti- lages failed to fuse in the mental symphysis, and the chon- drocytes in the symphysis area remained immature in controls while becoming apparently hypertrophic in the LDN-treated embryos (Fig. 4C–F). Alcian Blue staining at E14.5 uncovered an undersized Meckel’s cartilage that failed to make contact at the anterior portions in the LDN- exposed embryos (Fig. 4K,L). A rare phenotype of cleft lower lip was also detected in LDN-exposed embryos (Fig. 4G,H).
ABNORMAL GENIOGLOSSUS ATTACHMENT
We also found a rare and interesting phenotype of the tongue, which occurred consistently in all LDN-exposed embryos from E13.5 to E16.5. By histological analysis of both coronal and horizontal views at E16.5 (Figs. 2E,H, 4D), we found that genioglossus of LDN-exposed embryos displayed an abnormal attachment to bilateral sides of the mandible behind the unfused mental symphysis. In con- trast, the genioglossus of control embryos showed attach- ment to the symphysis of Meckel’s cartilages with a thin muscle tendon (Fig. 4C). Embryos accessed at E12.5 revealed that the primordium of the genioglossus already had an abnormal attachment to the unfused ends of ante- rior portion of Meckel’s cartilages (Fig. 3D)
PALATE SHELF FAILS TO CONTACT IN VITRO CULTURE
To demonstrate whether the cleft palate phenotype in LDN-exposed embryos was caused by retarded mandibular growth and abnormal tongue development, we set up a palatal tissue culture in which the processes of shelf eleva- tion can avoid the possible hindrance by the mandible and tongue. There were no differences in the morphology of palatal shelves between both groups at 0 hr and 12 hr of culture (Fig. 5A,D). We found that at 24 hr of culture the vehicle-exposed palatal shelves showed obvious approxi- mation to each other, however, the LDN-exposed palatal shelves showed no sign of approximation (Fig. 5B,E). While shelf elevation and fusion were observed in 75% (n 5 12/16) of the vehicle-exposed palatal shelves at 48 hr none (n 5 0/10) of LDN-exposed palatal shelves showed approximation or fusion (Fig. 5C,F)
EXPRESSION OF SMAD AND NON-SMAD SIGNALING IN THE PALATE SHELVES
To ensure that treatment by LDN-193189 in vivo indeed affects BMP signaling in the palate shelves, we carried out immunochemical staining to examine the expression of p- Smad1/5/8. The results were basically identical between E12.5 and E13.5. At E12.5 and E13.5, p-Smad1/5/8-posi- tive cells were found in the anterior and posterior palatal epithelium and mesenchyme. In contrast, the number of p- Smad1/5/8-positive cells along the anterior and posterior palate were markedly diminished in LDN-exposed embryos
FIGURE 3. Histology of frontal sections of the developing palate and genioglossus from E12.5 (A–D), E13.5 (E–H), E14 (I–L), and E14.5 (M–P) embryos. Vehicle- exposed embryos (A,C,E,G,I,K,M,O) and LDN-193189-exposed embryos (B,D,F.H,J,L,N,P). Sections shown in the Anterior rows (A,B,E,F,I,J,M,N) are from the anterior end of the developing secondary palate. Sections in the Posterior rows (C,D,G,H,K,L,O,P) were cut through the middle of the maxillary first molar tooth germs. Arrows in D,H,L mark the abnormal attachment of the genioglossus. Asterisks in N,P indicate delayed palate elevation. ps 5 palatal shelf; t 5 tongue; m1 5 the first molar; M 5 Meckel’s cartilage; gg 5 genioglossus. Scale bar 5 100 lm. The scale bar shown in P is applied to all panels.
(Fig. 6A–D,M). To determine whether LDN-193189 expo- sure could change BMP non-Smad signaling pathways we further examined the expression of p-p38 and p-Erk1/2. The p-p38 signaling was rarely detected in the anterior palate, while it was expressed in the future oral side epi- thelium and mesenchyme of the posterior palate (Fig. 6E– H). The p-Erk1/2 signaling was mainly detected in the epi- thelium of both the anterior and posterior domains of the palate (Fig. 6I–L). We did not find obvious changes in the expression of this non-Smad signaling in general (Fig. 6N,O).
REDUCED CELL PROLIFERATION IN THE PALATAL SHELVES
Histone H3, the core protein of the nucleosome, becomes phosphorylated at the end of prophase. To investigate if an altered cell proliferation rate could contribute to cleft pal- ate formation in LDN-193189-exposed embryos, we car- ried out p-H3 immunochemical staining to detect mitotic cells, which represent the level of cell proliferation in the
palatal shelve. We found a markedly diminished level of mitotic cells (p-H3-positive cell) along the anterior and posterior palate shelves in the LDN-exposed embryo at E12.5 and E13.5, when compared to the vehicle-exposed embryos (Fig. 7).
In this study, we attempted for the first time to demon- strate that LDN-193189, a small molecular inhibitor of BMP type I receptor, caused multiple craniofacial defects in the developing embryos, including cleft palate, micro- gnathia, and cleft lower lip, recapitulating the phenotype of loss of BMP type I receptor (ALK2, ALK3) (Dudas et al., 2004; Li et al., 2011), and highlighting cleft palate and micrognathia as a manifestation of exogenous interference of BMP pathway pharmacologically. As a complementary strategy to genetic approaches, LDN-193189 used to
FIGURE 4. Abnormal development of Meckel’s cartilage and mandible in LDN-exposed embryos. The 3D bone images were generated from micro-CT scans of E16.5 mandibles (A,B). Histological analysis of Meckel’s cartilage (C–F) and lower lip (G,H) was on transversal sections at E16.5. Lateral view of vehicle-exposed
(I) and LDN-193189-exposed (J) embryonic heads at E14.5. Ventral view of Meckel’s cartilage (stained with Alcian Blue) at E14.5 (K,L). Arrows in C,D point to the attachment sites of the genioglossus. Asterisks in F,H mark the unfused anterior region of Meckel’s cartilage and lower lip clefting, respectively. The double- headed arrows in I,J mark the length of the mandible. Dashed lines in K,L demarcate the lower lip. The arrowhead in L marks the unfused anterior region of Meckel’s cartilage. gg 5 genioglossus; M 5 Meckel’s cartilage; LL5 lower lip, Scale bar 5 10 lm. The scale bar in D applies to C, and the scale bar in H applies to E,F,G as well.
modulate BMP signaling in vivo may have greater flexibil- ity and lower cost (Hong and Yu, 2009).
Because the optimal teratogenic concentration of LDN- 193189 in mice remained unknown, we tested the dosages of 3 mg/kg, 6 mg/kg, and 9 mg/kg by in vivo injection twice per day, as was reported previously (Lee et al., 2011; Peterson et al., 2014; Tsugawa et al., 2014). Consid- ering the fact that embryos exposed to the dose of 6 mg/ kg had the highest rate of visible facial defects, tolerable rate of early embryonic lethality with no obvious embryo toxicity, we chose this dose for our studies.
From the results of facial measurement analysis, we found that LDN-exposed embryos exhibited relatively nor- mal midface width (SW/SL), but showed mandibular hypo- plasia (ML/SL), and mandibular retrognathia (MR/SL). We observed that these facial defects are also associated with abnormal palate development, which resembles human Pierre Robin sequence (PRS) that has typical clinical fea- tures including mandibular hypoplasia, cleft secondary pal- ate, and glossoptosis (Tan and Farlie, 2013).
In the current study, we focused on the etiology of the cleft palate resulting from pharmacological inhibition of BMP signaling. No apparent difference in the shape and size of palatal shelves at E12.5 and E13.5 between vehicle- and LDN-exposed embryos was observed. However, by E14 and E14.5, palatal shelf elevation appeared delayed resulting in complete or partial clefting of the secondary palate in LDN-treated embryos at E16.5.
Palatal shelf elevation is a complex process triggered by intrinsic forces of the palatal shelves (Chou et al., 2004; Jin et al., 2010). It is well known that shelf elevations can be influenced by coordinated growth and movement of other craniofacial structures including sagittal outgrowth of the mandible following tongue decent (Ferguson, 1988; Tsunekawa et al., 2005; Iseki et al., 2007; Meng et al., 2009).
Cleft palate associated with micrognathia has been observed in transgenic mouse models deficient of many BMP genes, such as Alk2, Bmp4, Bmp7, Msx2 (Winograd et al., 1997; Dudas et al., 2004; Kouskoura et al., 2013; He
FIGURE 5. In vitro cultures of palatal explants. Ventral views of upper jaw explants from E13.5 vehicle-exposed and LDN-193189-exposed embryos isolated by removing brain, tongue, and mandible, after 12- (A,D), 24- (B,E), and 48-hr (C,F) in culture. Dashed lines demarcate the edge of the secondary palatal shelves.
et al., 2014) as well as a variety of other related genes such as Sox9, Tak1, Pdrm16, and Col11a1 (Lavrin et al., 2001; Bjork et al., 2010; Lee and Saint-Jeannet, 2011; Song et al., 2013). A recent epidemiological report in a large clinical sample size had shown the most commonly noted associated anomalies with CL/P or CPO affected the mandible (Kling et al., 2014).
BMP signaling plays an important role in regulating mandibular morphogenesis (Minoux and Rijli, 2010). Change in BMP signaling activity resulted in abnormal sig- naling between mandibular epithelium and mesenchyme, leading to hypoplasia of the first branchial arch and subse- quently defects in mandible and Meckel’s cartilage (Ishii et al., 2005; Liu et al., 2005a, 2005b; Bonilla-Claudio et al., 2012). In our study, inhibition of BMP signaling caused an undersized Meckel’s cartilage and resulted in a failure to form the rostral process associated with cleft lower lip, both of which may result from defects of the distal area of the first branchial arch, which warrants future investiga- tion. Consequently, we observed abnormal attachment of the genioglossus to the unfused ends of the anterior por- tion of Meckel’s cartilages. Thus, defects in growth of Meckel’s cartilage may cause delayed descent of the tongue at crucial stages for palatal shelf elevation. Of interest, mice with cranial neural crest deletion of Alk2 and Bmp7 also exhibit unfused Meckel’s cartilage, cleft lower lip, and cleft palate (Dudas et al., 2004; Kouskoura et al., 2013), highlighting the relationship between cleft
palate and micrognathia and the function of BMP signaling among these development processes as well.
In our model, there is not a direct causality between cleft palate and micrognathia because Bmpr1a is also active in the palatal shelves during outgrowth (Li et al., 2011) and inactivation of Bmpr1a in the developing palatal mesenchyme also led to a delayed palatal elevation with- out obvious mandibular defect (Baek et al., 2011). We, therefore, dissected palate shelves of embryos at E13.5 and carried out palatal organ culture, which avoids the potential interference of the tongue and mandible. Palatal shelf elevation was normal in LDN-exposed embryos after culture for 24 hr however the contact and fusion of LDN- exposed palate shelves was never attained after culture for 48 hr. This result demonstrated that the small mandi- ble and abnormal tongue attachment play a role in delay- ing palate shelf elevation, but was not solely responsible for cleft palate. The same culture environment without LDN-193189 exposure suggested that palate shelves in LDN-exposed embryos have an intrinsic defect before E13.5, which likely contributes to the cleft palate formation.
We examined the BMP/Smad and BMP/non-Smad sig- naling in the palatal shelves at E12.5 and E13.5. We found that the level of p-smad1/5/8 was significantly reduced and the levels of p-p38 and p-Erk1/2 were not changed in the growing palate after pharmacological inhibition of BMP signaling. We also found a correlation between the
FIGURE 6. Altered BMP/Smad and BMP/non-Smad signaling in LDN-193189-exposed palatal shelves. Immunohistochemical staining shows p-Smad1/5/8 signals in the palatal shelf of E12.5 vehicle-exposed (A,C) and LDN-193189-exposed embryos (B,D). Immunohistochemical staining shows expression of pp38 (E–H) and p-Erk1/2 (I–L) in E12.5 vehicle-exposed (E,G,I,K) and LDN-193189-exposed embryos (F,H,J,L). Comparison of relative expression level of p-Smad1/5/8 (M), p- p38(N), p-Erk1/2 (O) in the palatal shelves in the control and LDN-exposed embryos at E12.5 and E13.5. Red dash lines demarcate the palatal region for immu- nostaining counting. Standard deviation values were presented as error bars, and asterisks indicate p < 0.05. Scale bar 5 2 lm. The scale bar shown in L
applies to all panels.
FIGURE 7. Reduced cell proliferation rate in the anterior and posterior palatal shelf of the LDN-exposed embryos. A–H: Coronal sections show p-H3-positive cells in the palatal shelves of E12.5 and E13.5 in vehicle-exposed (A,C,E,G) and LDN-exposed embryos (B,D,F,H). I: Comparison of percentage of p-H3- positive cells in the palatal shelves in the control and LDN-exposed embryos. Red dash lines demarcate the palatal region for immunostaining counting. Standard deviation values were presented as error bars, *p < 0.05. Scale bar 5 2 lm. The scale bar shown in H applies to all panels.
decrease of cell proliferation and reduction of p-Smad1/5/ 8 in palatal shelves of LDN-193189-exposed embryos, sug- gesting that BMP/Smad signaling plays a positive role in the regulation of cell proliferation. This observation is con- sistent with previous findings for augmentation or reduc- tion of BMP/Smad signaling in the palate shelves by genetic approaches that lead to alteration in cell prolifera- tion (Li et al., 2011, 2013). Thus, the reduced cell prolifer- ation rate in the palate shelves represents a major cellular mechanism defect which may be responsible for the cleft palate formation in LDN-exposed embryos.
Development of craniofacial structure is a complex pro- cess with many key stages. We expected that differences in timing of exposure to the LDN-193189 would induce distinct phenotypic forms and provide significant insight into the dynamic role of BMP signaling in craniofacial development as well as cleft palate, and will be the focus of future studies. Additionally, with the development of potent and biologically available selective inhibitors of BMP type I receptor (Hao et al., 2010; Mohedas et al., 2013; Sanvitale et al., 2013), new insights and possible prevention strategies to common birth defects might be further achieved.
The authors are grateful to Dr. YiPing Chen of Tulane Univer- sity, New Orleans, Louisiana, for his advice during the study and for his commenting and editing of the manuscript.
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