The Prx1 homeobox gene is critical for molar tooth morphogenesis

Abstract

The paired-related homeobox genes, Prx1 and Prx2, encode transcription factors critical for orofacial development. Prx1(-/-)/Prx2(-/-) neonates have mandibular hypoplasia and malformed mandibular incisors. Although the mandibular incisor phenotype has been briefly described (ten Berge et al., 1998, 2001; Lu et al., 1999), very little is known about the role of Prx proteins during tooth morphogenesis. Since the posterior mandibular region was relatively normal, we examined molar tooth development in Prx1(-/-)/Prx2(-/-) embryos to determine whether the tooth malformation is primary to the loss of Prx protein or secondary to defects in surrounding tissues. Three-dimensional (3D) morphological reconstructions demonstrated that Prx1(-/-)/Prx2(-/-) embryos had molar malformations, including cuspal changes and ectopic epithelial projections. Although we demonstrate that Prx1 protein is expressed only mesenchymally, 3D reconstructions showed important morphological defects in epithelial tissues at the cap and bell stages. Analysis of these data suggests that the Prx homeoproteins are critical for mesenchymal-epithelial signaling during tooth morphogenesis.

Figure 1

Expression of Prx1 protein during molar and incisor tooth development in WT mice. (A color version of this Fig. is included in the APPENDIX.) (A) Western analysis of Prx1 expression in tissue lysates derived from the anterior and posterior halves of the mandible, containing incisor and molar primordia, respectively. The data illustrate temporal changes in absolute levels of the 2 Prx1 isoforms (Prx1a and Prx1b) and a change in their ratio. (B-D) Immunohistochemistry of Prx1 protein expression (green) in frontal sections during molar and incisor tooth development (the 2 Prx1 isoforms cannot be distinguished by this method). Tissues were counterstained with propidium iodide (red). (B) At E13.5, high levels of Prx1 protein expression are present in mesenchyme surrounding maxillary (M¹) and mandibular (M₁) molar tooth buds; Prx1 protein is also localized in condensed mesenchyme adjacent to Meckel's cartilage (data not shown). There is no expression in oral or dental epithelium. (C) By E14.5, Prx1 expression is more refined and is strong in condensed mesenchyme surrounding the maxillary and mandibular molars. Prx1 expression is also highly localized in the mesenchyme of the developing oral vestibule. (D) At E15.5, there is reduced Prx1 protein expression in the dental papilla, but expression remains constant in mesenchymal tissues lateral to M¹ and M₁. (E) In a frontal section at E14.5, Prx1 protein is detected in the dental papilla of the developing maxillary incisor tooth germ (I¹). Key: oral cavity (oc), tongue (t).

Figure 2

Prx1^-/-/Prx2^-/- molars show patterning defects at E16. (A color version of this Fig. is included in the APPENDIX.) WT (n = 3) and MUT (n = 3) 3D reconstructions (A-H) are presented from various viewpoints: lateral (A,E), superior (B,F), inferior (C,G), and frontal (D,H). Histological images of frontal sections are displayed for WT mice (I,J) and MUT (K,L). Morphological differences between WT and MUT include hypoplastic cervical loops (cl) (arrowheads), and dental papilla (dp) in the MUT-M¹ and MUT-M₁ are hypoplastic. In addition, MUT-M₁ epithelium is more malformed and larger than the corresponding M¹. WT-M¹ is still inclined medially, while MUT-M¹ lacks this positional displacement. WT maxillary (M²) and mandibular (M₂) second molars are developing posterior to bell-stage WT-M¹ and WT-M₁, while MUT-M² is hypoplastic (#) and MUT-M₂ is absent (##). Key: oral cavity (oc), is anterior (ant), posterior (pos), medial (me), lateral (la).

Figure 3

Altered morphology in MUT-M² and MUT-M₂ was even more evident at E18 (A-H). (A color version of this Fig. is included in the APPENDIX.) Histological images of frontal sections are also displayed for MUT-M¹ (I) and MUT-M₁ (J). By E18, WT epithelia of first (M¹ and M₁) and second (M² and M₂) molars had undergone morphological refinement to form the bell and early cap stages, respectively. Compared with preceding timepoints, the cervical loops (cl) of MUT-M₁ were more developed, resulting in a more substantive region of dental papilla (dp), yet lateral cervical loops were underdeveloped (arrowhead) compared with WT (G). Epithelium posterior to MUT-M₁ that normally gives rise to M₂ was severely hypoplastic (##), and a distinct MUT-M₂ was not readily apparent (G). MUT-M₁ ectopic epithelia (double stars), first seen at E14, was larger and extended along the entire lateral surface (E,H,J). As demonstrated at E15 and E16, MUT-M₁ was enlarged in every dimension compared with its corresponding M¹. Although MUT-M¹ and MUT-M² epithelia had the general outline of WT, these tissues were severely hypoplastic, including the cervical loops (arrowhead) and the epithelial connection between MUT-M¹ and MUT-M². Key: oral cavity (oc), anterior (ant), posterior (pos), medial (me), lateral (la).

Figure 4

Cuspal patterning is altered in MUT-M¹ at E18. (A color version of this Fig. is included in the APPENDIX.) M¹ cuspal surfaces for WT (A) and MUT (B) are displayed from an elevated, anterior viewpoint; representative frontal sections through the widest portion of the same molar models are also depicted (C,D). 3D reconstructions of M¹ were generated with 80 and 110 serial frontal sections for MUT and WT, respectively. Although MUT-M¹ was overall hypoplastic and had hypoplastic cervical loops compared with WT, it also had altered morphology. Eight cuspal regions lie along the aboral surface of WT-M¹ epithelium; 3 primary cuspal regions are labeled in panel A. In contrast, the MUT-M¹ epithelial surface was very irregular, with the primary cuspal regions having a very different shape compared with WT, as well as containing unique and prominent mounds (stars), which together dramatically altered cuspal patterning (blue arrowhead). Key: oral epithelium (oe), anterior (ant), posterior (pos), medial (me), lateral (la).