Inactivation of bone morphogenetic protein 1 (Bmp1) and tolloid-like 1 (Tll1) in cells expressing type I collagen leads to dental and periodontal defects in mice

Bone morphogenetic protein 1 (BMP1) and tolloid-like 1 (TLL1) belong to the BMP1/tolloid-like proteinase family, which cleaves secretory proteins. The constitutive deletion of the Bmp1 or Tll1 genes causes perinatal or embryonic lethality in mice. In this study, we first studied the beta-galactosidase activity in mice in which an IRES-lacZ-Neo cassette was inserted in the intron of either the Bmp1 or the Tll1 gene; the beta-galactosidase activities were used to reflect the expression of endogenous Bmp1 and Tll1, respectively. Our X-gal staining results showed that the odontoblasts in the tooth and cells in the periodontal ligament express both Bmp1 and Tll1. We then created Bmp1 (flox/flox) and Tll1 (flox/flox) mice by removing the IRES-lacZ-Neo cassette. By breeding 2.3 kb Col1a1-Cre mice with the Bmp1 (flox/flox) and Tll1 (flox/flox) mice, we further generated Col1a1-Cre;Bmp1 (flox/flox) ;Tll1 (flox/flox) mice in which both Bmp1 and Tll1 were inactivated in the Type I collagen-expressing cells. We employed X-ray radiography, histology and immunohistochemistry approaches to characterize the Col1a1-Cre;Bmp1 (flox/flox) ;Tll1 (flox/flox) mice. Our results showed that the molars of the Col1a1-Cre;Bmp1 (flox/flox) ;Tll1 (flox/flox) mice had wider predentin, thinner dentin and larger pulp chambers than those of the normal controls. The dentinal tubules of the molars in the Col1a1-Cre;Bmp1 (flox/flox) ;Tll1 (flox/flox) mice appeared disorganized. The level of dentin sialophosphoprotein in the molars of the 6-week-old Col1a1-Cre;Bmp1 (flox/flox) ;Tll1 (flox/flox) mice was lower than in the normal controls. The periodontal ligaments of the Col1a1-Cre;Bmp1 (flox/flox) ;Tll1 (flox/flox) mice were disorganized and had less fibrillin-1. Our findings indicate that the proteinases encoded by Bmp1 and Tll1 genes play essential roles in the development and maintenance of mouse dentin and periodontal ligaments.