Transforming growth factor beta (TGF-β) is a cytokine that induces proliferation, differentiation, chemotaxis, and apoptosis in monocytic and epithelial cells, mesenchymal cells, and nerve cells (Kubiczkova et al., 2012). In the process of enamel formation, TGF-β is involved in the regulation of protein and protease expression in ameloblasts (Cho et al., 2013, Gao et al., 2009, Liu et al., 2019, Song et al., 2018), endocytosis of ameloblast degradation products (Okubo et al., 2019), and apoptosis of ameloblasts (Tsuchiya et al., 2009). In the secretory stage of enamel formation, TGF-β1 (Nagano et al., 2006), which is present in porcine immature enamel, is secreted from ameloblasts in an inactive form. After activation by matrix metalloproteinase 20, it binds to amelogenin, a major enamel protein, and maintains its activity. The water-soluble amelogenin-TGF-β1 complex then exhibits an autocrine signaling mechanism that migrates in the liquid phase and binds to the TGF-β receptor on the surface of ameloblasts to trigger signaling (Kobayashi-Kinoshita et al., 2016). TGF-β has three isoforms in mammals, namely TGF-β1, TGF-β2, and TGF-β3, each of which exhibit similar biological activities (Kingsley, 1994, Massague, 1992). However, during the tooth development stage, their expression differs across isoforms (Sassa Benedete et al., 2008). During enamel formation, different reactivities between TGF-β isoforms in terms of related gene expression, apoptosis, and endocytosis have also been observed (Okubo et al., 2019). Recently, TGF-β signaling has been shown to regulate the differentiation of epithelial cells in adult mouse incisors (Machiya et al., 2020), but it has not been clarified whether there are differences across TGF-β isoforms.

Here, we report the differences across isoforms of TGF-β signaling with respect to enamel epithelial cell differentiation and morphological changes after the secretory stage.