Injury-Activated ERMs Undergo EMT and May Contribute to Periodontal Repair

Once considered vestigial structures, epithelial rests of Malassez (ERMs) have recently been implicated in periodontal repair, largely based on their location in the periodontal ligament space, adjacent to damaged tissues. This study decodes what ERM activation entails and then tests in a variety of periodontal injury models the consequences of injury activation on ERM behavior and function. A Wnt lineage tracer strain Axin2Cre^ERT2/+;R26R^mTmG/+ was employed to map the distribution of Wnt-responsive cells and their descendants during root development and in response to injuries to the periodontium. Injury-activated murine ERMs were compared against developmental ERMs, and both were analyzed by histology and immunohistochemistry (IHC). Additionally, ERMs isolated from human tissues were analyzed by single-cell RNA sequencing and IHC. During root development, ERMs were surrounded by Wnt-responsive cells and their progeny. In response to injury, both the number and size of ERMs significantly increased, and this injury-induced enlargement did not involve cell proliferation. The injury-activated state of ERMs was accompanied by expression of Wnt pathway components. Compared to ERMs in uninjured tissues, injury-activated ERMs exhibited a shift toward expression of molecular markers associated with the epithelial-to-mesenchymal transition (EMT). When ERMs were juxtaposed to an injured junctional epithelium (JE) or damaged cementum, some ERM cells adopted a stellate morphology, exhibited evidence of matrix remodeling, and showed a loss of cell-cell adhesion, contributing to the repair of cementum and the JE. A dynamic state of Wnt responsiveness exists in injury-activated ERMs, and a subset of ERM cells undergo EMT. Together, these findings raise the possibility that cells in and around these activated ERMs may adopt an active role in periodontal repair.