LPA6-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(1-6)); however; its role in a tooth remains unclear. This study aimed to explore the impact of the LPA/LPA receptor 6 (LPA(6))/RhoA signaling axis on maturation stage ameloblasts (M-ABs), which are responsible for enamel mineralization. Methods: The expression of LPA(6) and LPA-producing synthetic enzymes during ameloblast differentiation was explored through immunobiological analysis of mouse incisors and molars. To elucidate the role of LPA(6) in ameloblasts, incisors of LPA(6) KO mice were analyzed. In vitro experiments using ameloblast cell lines were performed to validate the function of LPA-LPA(6)-RhoA signaling in ameloblasts. Results: LPA(6) and LPA-producing enzymes were strongly expressed in M-ABs. In LPA(6) 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(6) knockdown decreased their expression and changed cell morphology. Furthermore, we showed that RhoA signaling mediates LPA-LPA(6)-induced junctional complexes. Conclusions: This study demonstrated that LPA-LPA(6)-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. (C) 2022 Japanese Association for Oral Biology. Published by Elsevier B.V. All rights reserved.

Automated summary

This study investigated the impact of the LPA-LPA(6)-RhoA signaling axis on maturation stage ameloblasts. The results showed that LPA and LPA-producing enzymes were highly expressed in ameloblasts. In vitro experiments demonstrated the essential role of LPA-LPA(6)-RhoA signaling in establishing proper cell morphology and polarity, as well as regulating cell-cell junctions and actin cytoskeleton expression and stability.