Irma Thesleff

Identification of BMP-4 as a signal mediating secondary induction between epithelial and mesenchymal tissues during early tooth development

Growth factor-mediated signaling has been implicated in the regulation of epithelial-mesenchymal interactions during organogenesis. Bone morphogenetic protein 4 (BMP-4), a member of the transforming growth factor beta superfamily, is expressed in the presumptive dental epithelium at the initiation of tooth development. Subsequently, epithelial signaling leads to mesenchymal induction of BMP-4 expression. To address the role of this factor, BMP-4-releasing agarose beads were added to dental mesenchyme in culture. These beads induced a translucent mesenchymal zone similar to that induced by dental epithelium. Moreover, three transcription factors (Msx-1, Msx-2, and Egr-1) whose expression is governed by epithelial signaling were induced in response to BMP-4. In addition, BMP-4 induced its own mesenchymal expression. These findings support the hypothesis that BMP-4 mediates epithelial-mesenchymal interactions during early tooth development.

Reiterative signaling and patterning during mammalian tooth morphogenesis.

Mammalian dentition consists of teeth that develop as discrete organs. From anterior to posterior, the dentition is divided into regions of incisor, canine, premolar and molar tooth types. Particularly teeth in the molar region are very diverse in shape. The development of individual teeth involves epithelial-mesenchymal interactions that are mediated by signals shared with other organs. Parts of the molecular details of signaling networks have been established, particularly in the signal families BMP, FGF, Hh and Wnt, mostly by the analysis of gene expression and signaling responses in knockout mice with arrested tooth development. Recent evidence suggests that largely the same signaling cascade is used reiteratively throughout tooth development. The successional determination of tooth region, tooth type, tooth crown base and individual cusps involves signals that regulate tissue growth and differentiation. Tooth type appears to be determined by epithelial signals and to involve differential activation of homeobox genes in the mesenchyme. This differential signaling could have allowed the evolutionary divergence of tooth shapes among the four tooth types. The advancing tooth morphogenesis is punctuated by transient signaling centers in the epithelium corresponding to the initiation of tooth buds, tooth crowns and individual cusps. The latter two signaling centers, the primary enamel knot and the secondary enamel knot, have been well characterized and are thought to direct the differential growth and subsequent folding of the dental epithelium. Several members of the FGF signal family have been implicated in the control of cell proliferation around the non-dividing enamel knots. Spatiotemporal induction of the secondary enamel knots determines the cusp patterns of individual teeth and is likely to involve repeated activation and inhibition of signaling as suggested for patterning of other epithelial organs.

Signalling networks regulating dental development.

There has been rapid progress recently in the identification of signalling pathways regulating tooth development. It has become apparent that signalling networks involved in Drosophila development and development of mammalian organs such as the limb are also used in tooth development. Teeth are epithelial appendages formed in the oral region of vertebrates and their early developmental anatomy resembles that of other appendages, such as hairs and glands. The neural crest origin of tooth mesenchyme has been confirmed and recent evidence suggests that specific combinations of homeobox genes expressed in the neural crest cells may regulate the types of teeth and their patterning. Signalling molecules in the Shh, FGF, BMP and Wnt families appear to regulate the early steps of tooth morphogenesis and some transcription factors associated with these pathways have been shown to be necessary for tooth development. Several of the conserved signals are also transiently expressed in the enamel knots in the dental epithelium. The enamel knots are associated with the characteristic epithelial folding morphogenesis which is responsible for the development of tooth shape and it is currently believed that the enamel knots function as signalling centres regulating tooth shape development. The developing tooth has proven to be an excellent model in studies of the molecular basis of patterning and morphogenesis of organs and it can be expected that continuing studies will rapidly increase the understanding of these mechanisms.

Epithelial-mesenchymal signalling regulating tooth morphogenesis

Teeth develop as ectodermal appendages in vertebrate embryos, and their early development resembles morphologically as well as molecularly other organs such as hairs and glands. Interactions between the ectoderm and underlying mesenchyme constitute a central mechanism regulating the morphogenesis of

Mutations in AXIN2 Cause Familial Tooth Agenesis and Predispose to Colorectal Cancer

Wnt signaling regulates embryonic pattern formation and morphogenesis of most organs. Aberrations of regulation of Wnt signaling may lead to cancer. Here, we have used positional cloning to identify the causative mutation in a Finnish family in which severe permanent tooth agenesis (oligodontia) and colorectal neoplasia segregate with dominant inheritance. Eleven members of the family lacked at least eight permanent teeth, two of whom developed only three permanent teeth. Colorectal cancer or precancerous lesions of variable types were found in eight of the patients with oligodontia. We show that oligodontia and predisposition to cancer are caused by a nonsense mutation, Arg656Stop, in the Wnt-signaling regulator AXIN2. In addition, we identified a de novo frameshift mutation 1994-1995insG in AXIN2 in an unrelated young patient with severe tooth agenesis. Both mutations are expected to activate Wnt signaling. The results provide the first evidence of the importance of Wnt signaling for the development of dentition in humans and suggest that an intricate control of Wnt-signal activity is necessary for normal tooth development, since both inhibition and stimulation of Wnt signaling may lead to tooth agenesis. Our findings introduce a new gene for hereditary colorectal cancer and suggest that tooth agenesis may be an indicator of cancer susceptibility.

Cloning of a novel bacteria-binding receptor structurally related to scavenger receptors and expressed in a subset of macrophages.

A novel murine plasma membrane protein has been identified in subpopulations of macrophages. It has an intracellular N-terminal domain, a transmembrane domain, and an extracellular region with a short spacer, an 89 Gly-Xaa-Yaa repeat-containing collagenous domain, and a C-terminal cysteine-rich domain. In situ hybridization and immunohistochemical staining have localized the protein to a subset of macrophages in the marginal zone of the spleen and the medullary cord of lymph nodes. No expression was observed in macrophages of liver or lung. Transfected COS cells synthesized a native trimeric plasma membrane protein that bound labeled bacteria and acetylated LDL, but not yeast or Ficoll. The results suggest that the novel protein is a macrophage-specific membrane receptor with a role in host defense, as it shows postnatal expression in macrophages, which are considered responsible for the binding of bacterial antigens and phagocytosis.

Mechanisms of ectodermal organogenesis

All ectodermal organs, e.g. hair, teeth, and many exocrine glands, originate from two adjacent tissue layers: the epithelium and the mesenchyme. Similar sequential and reciprocal interactions between the epithelium and mesenchyme regulate the early steps of development in all ectodermal organs. Generally, the mesenchyme provides the first instructive signal, which is followed by the formation of the epithelial placode, an early signaling center. The placode buds into or out of the mesenchyme, and subsequent proliferation, cell movements, and differentiation of the epithelium and mesenchyme contribute to morphogenesis. The molecular signals regulating organogenesis, such as molecules in the FGF, TGFbeta, Wnt, and hedgehog families, regulate the development of all ectodermal appendages repeatedly during advancing morphogenesis and differentiation. In addition, signaling by ectodysplasin, a recently identified member of the TNF family, and its receptor Edar is required for ectodermal organ development across vertebrate species. Here the current knowledge on the molecular regulation of the initiation, placode formation, and morphogenesis of ectodermal organs is discussed with emphasis on feathers, hair, and teeth.

Expression patterns of bone morphogenetic proteins (Bmps) in the developing mouse tooth suggest roles in morphogenesis and cell differentiation

Bone morphogenetic proteins (BMP) are secretory signal molecules which have a variety of regulatory functions during morphogenesis and cell differentiation. Teeth are typical examples of vertebrate organs in which development is controlled by sequential and reciprocal signaling between the epithelium and mesenchyme. In addition, tooth development is characterized by formation of mineralized tissues: the bone‐like dentin and cementum as well as epithelially derived enamel. We have performed a comparative in situ hybridization analysis of the expression of six different Bmps (Bmp‐2 to Bmp‐7) starting from initiation of tooth development to completion of crown morphogenesis when dentine and enamel matrices are being deposited. Bmps‐2, ‐4, and ‐7 were frequently codistributed and showed marked associations with epithelial‐mesenchymal interactions. Their expression shifted between the epithelium and mesenchyme starting from the stage of tooth initiation. They were subsequently expressed in the enamel knot, the putative signaling center regulating tooth shape. Their expression domains prior to and during the differentiation of the dentine‐forming odontoblasts and enamel‐forming ameloblasts was in line with functions in regulation of cell differentiation and/or secretory activities of the cells. The expression of Bmp‐3 was confined to mesenchymal cells, in particular to the dental follicle cells which give rise to the cementoblasts, forming the hard tissue covering the roots of teeth. Bmp‐5 was expressed only in the epithelial ameloblasts. It was upregulated as the cells started to polarize and intense expression continued in the secretory ameloblasts. Bmp‐6 was expressed only weakly in the dental mesenchyme during bud and cap stages. Our results are in line with regulatory functions of Bmps at all stages of tooth morphogenesis. Bmps‐2, ‐4, and ‐7 are conceivably parts of signaling networks regulating tooth initiation and shape development. They as well as Bmp‐5 may be involved in the induction and formation of dentine and enamel, and Bmp‐3 in the development of cementum. The remarkable overlaps in the expression domains of different Bmp genes may implicate functional redundancy and/or formation of active heterodimers between different BMPs. Dev. Dyn. 1997;210:383–396.

The enamel knot as a signaling center in the developing mouse tooth

Mammalian tooth forms are produced during development by folding of the enamel epithelium but the molecular mechanisms involved in the formation and patterning of tooth cusps are not understood. We now report that several key signaling molecules found in well-known vertebrate signaling tissues such as the node, the notochord, the apical ectodermal ridge, and the zone of polarizing activity in the limb bud are specifically expressed in cells of the enamel knot, which is a transient cluster of dental epithelial cells. By comparing three-dimensional reconstructions of serial sections following in situ hybridization we localized Sonic hedgehog, Bone morphogenetic proteins-2, -4 and -7, as well as Fibroblast growth factor-4 in nested domains within the enamel knot. We suggest that the enamel knot acts as a signaling or organizing center, which provides positional information for tooth morphogenesis and regulates the growth of tooth cusps.

Continuous tooth generation in mouse is induced by activated epithelial Wnt/β-catenin signaling

The single replacement from milk teeth to permanent teeth makes mammalian teeth different from teeth of most nonmammalian vertebrates and other epithelial organs such as hair and feathers, whose continuous replacement has been linked to Wnt signaling. Here we show that mouse tooth buds expressing stabilized β-catenin in epithelium give rise to dozens of teeth. The molar crowns, however, are typically simplified unicusped cones. We demonstrate that the supernumerary teeth develop by a renewal process where new signaling centers, the enamel knots, bud off from the existing dental epithelium. The basic aspects of the unlocked tooth renewal can be reproduced with a computer model on tooth development by increasing the intrinsic level of activator production, supporting the role of β-catenin pathway as an upstream activator of enamel knot formation. These results may implicate Wnt signaling in tooth renewal, a capacity that was all but lost when mammals evolved progressively more complicated tooth shapes.