An Integrated Gene Regulatory Network Controls Stem Cell Proliferation in Teeth

et al. (2007) An integrated gene regulatory network controls stem cell proliferation in teeth. PLoS Biol 5(6): e159. doi:10.1371/journal.pbio.0050159 An Integrated Gene Regulatory Network Controls Stem Cell Proliferation in Teeth Xiu-Ping Wang 0 1 Marika Suomalainen 0 1 Szabolcs Felszeghy 0 1 Laura C. Zelarayan 0 1 Maria T. Alonso 0 1 Maksim V. Plikus 0 1 Richard L. Maas 0 1 Cheng-Ming Chuong 0 1 Thomas Schimmang 0 1 Irma Thesleff 0 1 0 Academic Editor: Brigid L. M. Hogan, Duke University Medical Center , United States of America 1 1 Developmental Biology Programme, Institute of Biotechnology, Viikki Biocenter, University of Helsinki, Finland, 2 Division of Genetics, Department of Medicine, Brigham and Women's Hospital and Harvard Medical School , Boston , Massachusetts, United States of America, 3 Center for Molecular Neurobiology Hamburg, University of Hamburg , Hamburg, Germany , 4 Institute for Biology and Molecular Genetics, Superior Research Council and University of Valladolid , Valladolid , Spain , 5 Department of Pathology, Keck School of Medicine, University of Southern California , Los Angeles, California , United States of America Epithelial stem cells reside in specific niches that regulate their self-renewal and differentiation, and are responsible for the continuous regeneration of tissues such as hair, skin, and gut. Although the regenerative potential of mammalian teeth is limited, mouse incisors grow continuously throughout life and contain stem cells at their proximal ends in the cervical loops. In the labial cervical loop, the epithelial stem cells proliferate and migrate along the labial surface, differentiating into enamel-forming ameloblasts. In contrast, the lingual cervical loop contains fewer proliferating stem cells, and the lingual incisor surface lacks ameloblasts and enamel. Here we have used a combination of mouse mutant analyses, organ culture experiments, and expression studies to identify the key signaling molecules that regulate stem cell proliferation in the rodent incisor stem cell niche, and to elucidate their role in the generation of the intrinsic asymmetry of the incisors. We show that epithelial stem cell proliferation in the cervical loops is controlled by an integrated gene regulatory network consisting of Activin, bone morphogenetic protein (BMP), fibroblast growth factor (FGF), and Follistatin within the incisor stem cell niche. Mesenchymal FGF3 stimulates epithelial stem cell proliferation, and BMP4 represses Fgf3 expression. In turn, Activin, which is strongly expressed in labial mesenchyme, inhibits the repressive effect of BMP4 and restricts Fgf3 expression to labial dental mesenchyme, resulting in increased stem cell proliferation and a large, labial stem cell niche. Follistatin limits the number of lingual stem cells, further contributing to the characteristic asymmetry of mouse incisors, and on the basis of our findings, we suggest a model in which Follistatin antagonizes the activity of Activin. These results show how the spatially restricted and balanced effects of specific components of a signaling network can regulate stem cell proliferation in the niche and account for asymmetric organogenesis. Subtle variations in this or related regulatory networks may explain the different regenerative capacities of various organs and animal species. - Stem cells reside in specific niches that regulate their selfrenewal and differentiation. In general, these niches consist of various types of neighboring differentiated cells, which provide a milieu of extracellular matrix and signaling molecules that regulate the unique behavior of stem cells. Epithelial stem cells have been identified in several adult tissues that undergo continuous turnover, such as hair, skin, feather, and gut [16]. Although mammals have lost the capacity for recurrent tooth renewal, some mammals, such as rodents, have incisor teeth that grow continuously. On the basis of cell cycle kinetics, DiI tracing, and the location of cells that retain bromodeoxyuridine (BrdU) label long term, it is thought that the epithelial stem cells of rodent incisors reside in the stellate reticulum core of the cervical loops (Figure 1A and 1B; [1,7]). An interesting feature of the mouse incisor is that the cervical loop on the labial side is much thicker than that on the lingual side. It contains abundant stellate reticulum cells in its core, whereas the lingual cervical loop is very thin and only contains a few stellate reticulum cells. However, the molecular mechanisms underlying the asymmetric growth and size of these cervical loops are unknown. Most dental epithelial stem cells give rise to enamelsecreting ameloblasts. The enamel layer covers the dentin layer produced by mesenchymal odontoblasts. Besides the asymmetric growth and morphology of the cervical loops, enamel also exhibits asymmetric distribution in the mouse incisors. Enamel-producing ameloblasts differentiate only along the labial aspect of the mouse incisor, so enamel covers only the labial surface of the tooth, whereas the lingual surface is enamel-free and covered only by dentin (Figure 1A). This, together with the continuous growth and wear of the mouse incisor, maintains its characteristic sharpness, which is crucial to its function for gnawing. We showed Stem cells reside in specific niches that regulate their self-renewal and differentiation, and are responsible for the continuous regeneration of tissues. Although the regenerative potential of mammalian teeth is limited, mouse incisors grow continuously throughout life and contain stem cells at their proximal ends in the so-called cervical loops. We have used a combination of mouse mutant analyses, organ culture experiments, and gene expression studies to identify the key signaling molecules that regulate epithelial stem cell proliferation in the cervical loop stem cell niche. We show that signals from the adjacent mesenchymal tissue regulate epithelial stem cells and form a complex regulatory network with epithelial signals. Stem cell proliferation is stimulated by fibroblast growth factor 3 (FGF3), and bone morphogenetic protein 4 (BMP4) represses Fgf3 expression. In turn, Activin inhibits the repressive effect of BMP4 and Follistatin antagonizes the activity of Activin. We also show that spatial differences in the levels of Activin and Follistatin expression contribute to the characteristic asymmetry of rodent incisors, which are covered by enamel only on their labial (front) side. We suggest that subtle variations in this or related regulatory networks may explain the different regenerative capacities and asymmetric development of various organs and animal species. previously that ameloblast differentiation on the labial side of the incisor is induced by bone morphogenetic protein 4 (BMP4) from mesenchymal odontoblasts, and that Follistatin inhibits BMP function in lingual-side dental epithelium, preventing enamel formation there [8]. Consis (...truncated)