Role of c-Fos in orthodontic tooth movement: an in vivo study using transgenic mice

Abstract

Objectives: The transcription factor c-Fos controls the differentiation of osteoclasts and is expressed in periodontal ligament cells after mechanical stimulation in vitro. However, it is unclear how c-Fos regulates orthodontic tooth movement (OTM) in vivo. The aim of this study was therefore to analyse OTM in transgenic mice with overexpression of c-Fos.

Materials and methods: We employed c-Fos transgenic mice (c-Fos tg) and wild-type littermates (WT) in a model of OTM induced by Nitinol tension springs that were bonded between the left first maxillary molars and the upper incisors. The unstimulated contralateral side served as an internal control. Mice were analysed by contact radiography, micro-computed tomography, decalcified histology and histochemistry.

Results: Our analysis of the unstimulated side revealed that alveolar bone and root morphology were similar between c-Fos tg and control mice. However, we observed more osteoclasts in the alveolar bone of c-Fos tg mice as tartrate-resistant acid phosphatase (TRAP)-positive cells were increased by 40%. After 12 days of OTM, c-Fos tg mice exhibited 62% increased tooth movement as compared with WT mice. Despite the faster tooth movement, c-Fos tg and WT mice displayed the same amount of root resorption. Importantly, we did not observe orthodontically induced tissue necrosis (i.e. hyalinization) in c-Fos tg mice, while this was a common finding in WT mice.

Conclusion: Overexpression of c-Fos accelerates tooth movement without causing more root resorption.

Clinical relevance: Accelerated tooth movement must not result in more root resorption as higher tissue turnover may decrease the amount of mechanically induced tissue necrosis.

Keywords: Bone remodelling; Mechanical stimulation; Orthodontic tooth movement; Root resorption; c-Fos.

Fig. 1

Overexpression of c-Fos accelerates orthodontic tooth movement in mice. a Schematic drawing of the OTM model. The Nitinol spring (blue) was bonded to the incisors and to the left first molar (M1) with a light-curing composite (red circles). b Photographs of the bonding procedure. The first molar (red arrow) was dried (upper left panel) and etched (upper right panel) before the mesial end of the spring was bonded (lower left panel). After activation of the spring with a force of 35 centinewton, the distal end of the spring was bonded to the incisors (lower right panel). c Contact radiography showing the activated nitinol spring

Fig. 2

Micro-CT analysis of alveolar bone, tooth structure and OTM. a Micro-CT scanning of maxillary molars from 12-week-old wild-type (WT) and c-Fos transgenic (c-Fos tg) mice. Alveolar bone loss (highlighted in red) was measured on 3D reconstructions of teeth that were not subjected to OTM (upper panels). Palatal thickness was measured on cross-sections of the palate (lower panels). Scale bars = 3 mm. b, c Quantification of the alveolar bone loss (b) and palatal thickness (c) of 12-week-old WT and c-Fos tg mice. n ≥ 3. *P < 0.05, versus WT. d Cross-sections based on micro-CT scans of untreated (OTM−) and treated (OTM+) maxillary molars of wild-type (WT) and c-Fos transgenic mice (c-Fos tg). The mechanical loading created a gap between the first and second molar (red arrows). Scale bars = 1 mm. e Quantification of the smallest distance between the first and second molar as a surrogate measurement for OTM in 12-week-old wild-type (WT) and c-Fos tg mice. n = 4. *P < 0.05, versus wild-type

Fig. 3

Histological analysis of OTM in c-Fos tg and WT mice. a Schematic drawing of a murine skull (upper panel) and a maxillary first molar (lower panel) illustrating the forces (green arrow) and moments (blue circle) induced by OTM. The direction of the force is mesial and intrusive, which creates different areas of compression (red) and tension (blue). b, c Toluidine-blue stained histological sections of teeth without OTM (b) and with OTM (c) of 12-week-old WT and c-Fos tg mice. Lower panels show magnification of the regions outlined by the red boxes. The intercoronal gaps induced by OTM (black lines) offer a retention for debris (white asterisk) resulting in a mild gingivitis (red arrows). Scale bars = 250 μm. d Schematic drawing of the maxillary first molar. The red boxes indicate areas of OTM-induced tension (upper panel) and pressure (lower panel) around the distal root. e, f Toluidine-blue stained histological sections of the regions indicated in d of teeth without OTM (e) and with OTM (f) of the same mice. Osteoblasts (black arrow) and osteoclasts (white arrows) are clearly evident after OTM. Scale bars = 50 μm

Fig. 4

Immunohistochemical staining for osteoclasts in c-Fos tg and WT mice. a Schematic drawing of the maxillary first molar. The red boxes indicate areas of OTM-induced pressure at the distal (upper panel) and mesial (lower panel) root. b, c TRAP-stained decalcified sections of teeth without OTM (b) and with OTM (c) of 12-week-old WT and c-Fos tg mice. Red arrows indicate TRAP-positive cells. Scale bars = 200 μm. d Schematic drawing of the maxillary first molar. TRAP-positive cells were quantified in the area indicated in red. e Quantification of TRAP-positive cells. n = 4. ***P < 0.001, versus control. ##P < 0.01, versus wild-type

Fig. 5

Histological analysis of OTM-induced root resorption and hyalinization in c-Fos tg and WT mice. a Schematic drawing of the maxillary first molar. The red boxes indicate areas of OTM-induced pressure at the distal (upper panel) and mesial (lower panel) root. b, c Toluidine-blue stained histological sections of teeth without OTM (b) and with OTM (c) of 12-week-old WT and c-Fos tg mice. Scale bars = 100 μm. Whereas lateral resorption at the distal root extends along almost the entire root surface (white dotted line), lateral resorption at the mesial root has a drop-like appearance (white arrow). Areas of hyalinization (black asterisk) were only evident in WT mice. Scale bars = 100 μm. d Schematic drawing of the maxillary first molar. E–G Quantification of the resorbed root surface per root surface (RRS/RS), resorbed area and hyalinized area in the PDL of 12-week-old WT and c-Fos tg mice. n = 4