Role of Wnt signaling pathway in adult mouse incisor growth

Abstract

The continuously growing mouse incisor was used as a model in this project to study the role of Wnt signaling in adult mouse incisor growth in vivo. Mouse incisors grow continuously throughout life, which requires continuous cell renewal and compensation for the dental tissue loss due to mastication wear. This is achieved by the availability of the self-renewing MSCs at the proximal end of the mouse incisor. MSCs are slow-dividing cells which is not fast enough for the rapid turnover of the incisor, so they give rise to fast-dividing transit amplifying cells (TACs). However, TACs do not have the ability to self-renew and they terminally differentiate into either odontoblasts or pulp cells. During development, many Wnt ligands are expressed in the developing incisor and have a crucial developmental role. Results obtained from adult mouse incisor RT-qPCR data has shown that their expression continues postnatally. The results obtained from Wnt reporter mouse line Axin2-LacZ and Axin2 lineage tracing in the adult mouse incisor showed that only TACs and odontoblasts are Wnt-responsive cells. To study Wnt signaling role in the adult mouse incisors, inhibition of Wnt ligands secretion by knocking out Wls, a transmembrane protein responsible for Wnt secretion, to deactivate Wnt pathway, and knocking-out Axin2 to continuously activate Wnt/b-catenin pathway were done. Results from these experiments showed that depletion of Wnt signaling in adult mouse incisors leads to an abnormal incisor phenotype characterised by increased dentine thickness, disrupted cervical loop area and reduced growth rate, because of the reduced proliferation rate and increased apoptosis. Conversely, upregulation of Wnt/ β-catenin signaling by knocking out Axin2 enhances adult mouse incisor growth rate with no apparent abnormal phenotype. Both models indicated that Wnt pathway regulates dentine secretion by controlling odontoblast Dspp expression. Utilising different conditional knockout mouse lines to inhibit Wnt secretion from the different dental tissues was helpful to identify the source of Wnt ligands secretion that affect adult mouse incisor growth. Collectively, the obtained results of these knockouts indicated that Wnt ligands that activate Wnt pathway on TACs and odontoblasts are secreted from MSCs, TACs and odontoblasts and showed that epithelial-derived Wnt ligands have no role in the adult mouse incisor growth.