Arl4c is involved in tooth germ development through osteoblastic differentiation
Murine tooth germ development proceeds in continuous and sequential steps with reciprocal interactions between the odontogenic epithelium and the adjacent mesenchyme, and several growth factor signaling and their activation are required for tooth germ development. The expression of ADP-ribosylation factor (Arf)-like 4c (Arl4c) has been shown to induce cell proliferation, thereby involving in epithelial morphogenesis and tumorigenesis. However, other functions of Arl4c except for cellular growth are largely unknown. Although we recently demonstrated the involvement of the upregulated expression of Arl4c in cell proliferation of ameloblastoma, which has the same origin as odontogenic epithelium, its effect on tooth germ development remains unclear. Herein, single-cell RNA sequencing (scRNA-seq) analysis revealed that the expression of Arl4c, among 17 members of the Arf-family members, was specifically detected in odontogenic epithelial cells, such as stratum intermedium, stella reticulum and outer enamel epithelium, of postnatal day 1 (P1) mouse molar. In addition, scRNA-seq analysis showed higher expression of Arl4c in non-ameloblast and inner enamel epithelium, which include immature cells. In the mouse tooth germ rudiments culture, treatment with SecinH3 (an inhibitor of ARNO/Arf6 pathway) reduced the size, width and cusp height and the thickness of eosinophilic layer, which would involve the synthesis of dentin and enamel matrix organization. In addition, loss-of-function experiments using siRNAs and shRNA revealed that Arl4c expression was involved in cell proliferation and osteoblastic cytodifferentiation in odontogenic epithelial cells. Finally, RNA-seq analysis with gene set enrichment analyses (GSEA) and Gene Ontology (GO) analysis showed that osteoblastic differentiation-related gene set and/or GO terms were downregulated in shArl4c expressing odontogenic epithelial cells. These results suggest that the Arl4c-ARNO/Arf6 pathway axis contributes to tooth germ development through osteoblastic differentiation. Overall design: RNA-seq was performed using RNAs extracted from mDE6 expressing shControl or shArl4c.