LPA-RhoA signals regulate junctional complexes for polarity and morphology establishment of maturation stage ameloblasts.

Objectives: Lysophosphatidic acid (LPA) is a potent bioactive phospholipid that exerts various functions upon binding to six known G protein-coupled receptors (LPA); however; its role in a tooth remains unclear. This study aimed to explore the impact of the LPA/LPA receptor 6 (LPA)/RhoA signaling axis on maturation stage ameloblasts (M-ABs), which are responsible for enamel mineralization.

Methods: The expression of LPA and LPA-producing synthetic enzymes during ameloblast differentiation was explored through immunobiological analysis of mouse incisors and molars. To elucidate the role of LPA in ameloblasts, incisors of LPA KO mice were analyzed. In vitro experiments using ameloblast cell lines were performed to validate the function of LPA-LPA-RhoA signaling in ameloblasts.

Results: LPA and LPA-producing enzymes were strongly expressed in M-ABs. In LPA knockout mice, M-ABs exhibited abnormal morphology with the loss of cell polarity, and an abnormal enamel epithelium containing cyst-like structures was formed. Moreover, the expression of E-cadherin and zonula occludens-1 (ZO-1) significantly decreased in M-ABs. In vitro experiments demonstrated that LPA upregulated the expression of E-cadherin, ZO-1, and filamentous actin (F-actin) at the cellular membrane, whereas LPA knockdown decreased their expression and changed cell morphology. Furthermore, we showed that RhoA signaling mediates LPA-LPA-induced junctional complexes.

Conclusions: This study demonstrated that LPA-LPA-RhoA signaling is essential for establishing proper cell morphology and polarity, via cell-cell junction and actin cytoskeleton expression and stability, of M-ABs. These results highlight the biological significance of bioactive lipids in a tooth, providing a novel molecular regulatory mechanism of ameloblasts.