Epithelial Rests of Malassez Form a Wnt-Dependent Mechanoresponsive Network

Epithelial rests of Malassez (ERMs) constitute a 3-dimensional epithelial meshwork extending from the tooth apex to the junctional epithelium (JE). This position, coupled with observations from a variety of injury models, suggests that ERMs may be involved in sensing disruptions to the mechanical integrity of the periodontium. To test this hypothesis, finite element modeling was employed to predict strain states in tissues surrounding ERMs (e.g., the periodontal ligament [PDL], supracrestal connective tissues, and the JE), which, by virtue of their attachment to the tooth surface, experience distortional strains during mastication. An in vivo hyper-loading mouse model was employed to map these distortional strains to location-specific and time-dependent morphological changes in ERMs. Quantitative immunohistochemistry was used to interrogate ERMs for changes in the expression of Piezo1, β4 integrin, plectin, YAP, TRPV4, and phosphorylated FAK. Collagen and oxytalan fiber organization around ERMs was compared between normal- and hyper-loading conditions. Axin2Cre^ERT2;R26R^mTmG/+ mice were used to lineage-trace Wnt-responsive ERMs under normal and hyper-loading conditions, and Wnt signaling from ERMs and the JE was disrupted using K14Cre^ERT2;Wls^fl/fl mice. Collectively, these analyses predicted that hyper-loading produced high distortional strains in the JE and supracrestal connective tissues, which correlated with sites of ERM enlargement. ERMs expressed mechanosensory proteins, whose levels were significantly elevated by hyper-loading. Load-activated ERMs were Wnt-responsive, and if Wnt secretion was blocked, then ERMs no longer expressed mechanosensory proteins and no longer enlarged in response to hyper-loading. A deterioration in the JE attachment and the PDL followed. Thus, the ERM network is positioned to sense distortional strains produced by mastication; they possess the molecular machinery to sense and respond to masticatory forces and, via a Wnt-dependent mechanism, coordinate tissue-level responses to mechanical loading in the PDL, supracrestal connective tissues, and the JE.