Deficient Cell Proliferation in Palatal Shelf Mesenchyme of CL/Fr Mouse Embryos

Mesenchymal β-catenin signaling affects palatogenesis by regulating α-actinin-4 and F-actin

OBJECTIVE

Our previous research have found that mesenchymal β-catenin may be involved in palatal shelf (PS) elevation by regulating F-actin. Here, we further investigated the exact mechanism of β-catenin/F-actin in the PS mesenchyme to regulate palatal reorientation.

MATERIALS AND METHODS

(1) Firstly, Ctnnb1ex3f (β-catenin) mice were conditionally overexpressed in the palatal mesenchyme by crossing with the Sox9-creERT2 mice (induced by Tamoxifen injections); (2) Subsequently, histology and immunohistochemistry were used to characterize the variations of PS morphology and expression of key molecules associated with developmental process; (3) Finally, experiments in vivo and ex vivo were employed to identify the critical mechanisms in β-catenin silenced and overexpressed models.

RESULTS

We found that the Sox9CreER; Ctnnb1ex3f mice exhibited failed palatal elevation and visible cleft palate, and overexpression of β-catenin disturbed the F-actin responsible for cytoskeletal remodeling in palatal mesenchymal cells. qRT-PCR results showed mRNA levels of α-actinin4, a gene involved in F-actin cross-linking, were associated with knockdown or overexpression of β-catenin in ex vivo, respectively. Experiments in vivo revealed that mesenchymal specific inactivation or overexpression of β-catenin exhibited decreased or increased α-actinin-4 expression.

CONCLUSIONS

Mesenchymal β-catenin/F-actin plays an essential role in PS reorientation, which mediate α-actinin-4 to regulate F-actin cytoskeleton reorganization.

Mesenchymal Remodeling during Palatal Shelf Elevation Revealed by Extracellular Matrix and F-Actin Expression Patterns

During formation of the secondary palate in mammalian embryos, two vertically oriented palatal shelves rapidly elevate into a horizontal position above the tongue, meet at the midline, and fuse to form a single entity. Previous observations suggested that elevation occurs by a simple 90° rotation of the palatal shelves. More recent findings showed that the presumptive midline epithelial cells are not located at the tips of palatal shelves before elevation, but mostly toward their medial/lingual part. This implied extensive tissue remodeling during shelf elevation. Nevertheless, it is still not known how the shelf mesenchyme reorganizes during this process, and what mechanism drives it. To address this question, we mapped the distinct and restricted expression domains of certain extracellular matrix components within the developing palatal shelves. This procedure allowed to monitor movements of entire mesenchymal regions relative to each other during shelf elevation. Consistent with previous notions, our results confirm a flipping movement of the palatal shelves anteriorly, whereas extensive mesenchymal reorganization is observed more posteriorly. There, the entire lingual portion of the vertical shelves moves close to the midline after elevation, whereas the mesenchyme at the original tip of the shelves ends up ventrolaterally. Moreover, we observed that the mesenchymal cells of elevating palatal shelves substantially align their actin cytoskeleton, their extracellular matrix, and their nuclei in a ventral to medial direction. This indicates that, like in other morphogenetic processes, actin-dependent cell contractility is a major driving force for mesenchymal tissue remodeling during palatogenesis.

Molecular contribution to cleft palate production in cleft lip mice

Cleft palate following cleft lip may include a developmental disorder during palatogenesis. CL/Fr mice fetuses, which develop cleft lip and palate spontaneously, have less capability for in vivo cell proliferation in palatal mesenchyme compared with CL/Fr normal fetuses. In order to know the changes of signaling molecules contributing to cleft palate morphogenesis following cleft lip, the mRNA expression profiles were compared in palatal shelves oriented vertically (before elevation) in CL/Fr fetuses with or without cleft lip. The changes in mRNA profile of cleft palate morphogenesis were presented in a microarray analysis, and genes were restricted to lists contributing to cleft palate development in CL/Fr fetuses with cleft lip. Four candidate genes (Ywhab, Nek2, Tacc1 and Frk) were linked in a gene network that associates with cell proliferation (cell cycle, MAPK, Wnt and Tgf beta pathways). Quantitative real-time RT-PCR highlighted the candidate genes that significantly changed in CL/Fr fetuses with cleft lip (Ywhab, Nek2 and Tacc1). The results of these molecular contributions will provide useful information for a better understanding of palatogenesis in cleft palate following cleft lip. Our data indicated the genetic contribution to cleft palate morphogenesis following cleft lip.

Expression of Wnts in the developing murine secondary palate

Morphogenesis of the mammalian secondary palate requires coordination of cell migration, proliferation, differentiation, apoptosis and synthesis of extracellular matrix molecules by numerous signal transduction pathways. Recent evidence suggests a role for members of the Wnt family of secreted cytokines in orofacial development. However, no study has systematically or comprehensively examined the expression of Wnts in embryonic orofacial tissue. We thus conducted a survey of the expression of all known Wnt genes in the developing murine secondary palate. Using an RT-PCR strategy to assay gene expression, 12 of the 19 known members of the Wnt family were found to be expressed in embryonic palatal tissue during key phases of its development. The expression of 5 Wnt family members was found to be temporally regulated. Moreover, these Wnts had unique spatio-temporal patterns of expression which suggested possible roles in palatal ontogeny.