An Explanation for How FGFs Predict Species-Specific Tooth Cusp Patterns

Abstract

Species-specific cusp patterns result from the iterative formation of enamel knots, the epithelial signaling centers, at the future cusp positions. The expressions of fibroblast growth factors (FGFs), especially Fgf4, in the secondary enamel knots in the areas of the future cusp tips are generally used to manifest the appearance of species-specific tooth shapes. However, the mechanism underlying the predictive role of FGFs in species-specific cusp patterns remains obscure. Here, we demonstrated that gerbils, which have a lophodont pattern, exhibit a striped expression pattern of Fgf4, whereas mice, which have a bunodont pattern, have a spotted expression pattern, and these observations verify the predictive role of Fgf4 in species-specific cusp patterns. By manipulating FGFs' signaling in the inner dental epithelium of gerbils, we provide evidence for the intracellular participation of FGF signaling, specifically FGF4 and FGF20, in Rac1- and RhoA-regulated cellular geometry remolding during the determination of different cusp patterns. Our study presents a novel explanation of how different FGF expression patterns produce different cusp patterns and implies that a conserved intracellular FGF-GTPase signaling module might represent an underlying developmental basis for evolutionary changes in cusp patterns.

Developmental Roles of FUSE Binding Protein 1 (Fubp1) in Tooth Morphogenesis

“…We observed a continuous expression pattern of Fgf4 along the enamel rope in the Fubp1 knock-down tooth (Figure 4b,c). Consistent with this result, it has been reported that its expression is restricted along the enamel knot during the bell stage of mice molar development [21,35,37]. Copious studies have reported that various paracrine factors are involved in tooth morphogenesis, including Bmps, Fgfs, Shh, and Wnts [3,[6][7][8][9].…”

“…Knock-down of Fubp1 led to an observed decrease in the expression of Bmp4 mRNA, indicating its role in Bmp signalling (Figure 4d,e). At the bell stage of tooth development, inner enamel epithelial cells start to differentiate into ameloblasts and DP cells differentiate into odontoblasts [31] and the expression pattern of Fgf4 plays a crucial role in future cusp patterning [35,36]. We observed a continuous expression pattern of Fgf4 along the enamel rope in the Fubp1 knock-down tooth (Figure 4b,c).…”

“…The knock-down effect of Fubp1 up-regulated Fgf4 and altered the crown height (Figure 4, Supplementary Figure S4). However, a similar expression pattern of Fgf4 resulted in a long cusp height in gerbils [35] suggesting that Fgf signalling not only to cuspal height morphogenesis but also with crown height. Our study showed down-regulation of Bmp2 and Bmp4 after Fubp1 knock-down during the bell stage.…”

“…Sections were prepared by using a diamond saw (Accutom 50, Ballerup, Denmark), ground (Rotopol-35, Ballerup, Denmark), then mounted on slides in historesin mounting media (Leica, Hanau, Germany). SEM (SEM; JEOL JSM-6700 F Field emission SEM, Tokyo, Japan) was performed, as detailed by [35]. Funding: This study was supported by the National Research Foundation of Korea (grant NRF-2018R1A2A3075600, 2017R1A5A2015391 and 2015R1A2A2A01008180) funded by the Ministry of Education, Science and Technology, Republic of Korea.…”

Cuspal Shape Alterations by Bmp4 Directing Cell Proliferation and Apoptosis

“…Wnt, BMP, Hedgehog, and FGF signaling in EKs is related to tooth morphogenesis (Jernvall et al 1994; Zhang et al 2000; Liu et al 2008; Chen et al 2009). Our previous studies demonstrated that Fgf4 regulates IDE invagination through Rac1 and RhoA (Li et al 2016; Li et al 2018). This study is novel in that only EK regions of mouse and gerbil tooth germs at the cap stage were isolated, and the differences in EKs were analyzed between 2 species.…”