Epithelial‐specific knockout of the Rac1 gene leads to enamel defects

Abstract

Rac1 encodes a 21kDa GTP-binding protein belonging to the RAS superfamily. RAS members play important roles in controlling focal adhesion complex formation and cytoskeleton contraction; activities with consequences to cell growth, adhesion, migration, and differentiation. To examine the role(s) played by Rac1 protein in cell-to-matrix interaction and in enamel matrix biomineralization we used the Cre/loxP binary recombination system to characterize enamel matrix proteins expression and enamel formation in Rac1 knockout mice. Mating between mice bearing the floxed Rac1 allele with mice bearing a keratin14-Cre transgene generate animals in which Rac1 is absent from epithelial organs. The enamel of Rac1 conditional knockout mouse was characterized by computerized tomography (microCT), light microscopy, histochemistry, and back-scatter electron microscopy. Enamel matrix protein expression was analyzed by Western blotting. Major findings showed that the Tomes’ processes of Rac1−/− ameloblasts loose contact with the forming enamel matrix in un-erupted teeth. The abundance of amelogenin and ameloblastin was reduced in the Rac1−/− ameloblasts. After eruption, the enamel from the Rac1−/− mice displayed severe structural defects with the complete loss of enamel. These results support an essential role for Rac1 function in the dental epithelium involving cell-matrix interaction and matrix biomineralization.

Expression and roles of syndecan-4 in dental epithelial cell differentiation

“…Proteolytic cleavage and shedding of its extracellular domain can spread FGF signaling to adjacent cells, thus regulating the local reception of FGF signaling activity (7). Its cytoplasmic domains can also initiate FGF-induced signaling independently of FGFRs through the activation of Rho GTPases, such as Rac1, which plays an essential role in the dental epithelium, involving cell-matrix interactions and matrix biomineralization (8,9).…”

“…Proteolytic cleavage and shedding of its extracellular domain can spread FGF signaling to adjacent cells, thus regulating the local reception of FGF signaling activity (7). Its cytoplasmic domains can also initiate FGF-induced signaling independently of FGFRs through the activation of Rho GTPases, such as Rac1, which plays an essential role in the dental epithelium, involving cell-matrix interactions and matrix biomineralization (8,9).FGF signaling plays critical roles in tooth development (10). In molar tooth development, FGF4, 8 and 9 act as epithelial signals, mediating inductive interactions between the dental epithelium and the mesenchyme during the initiation of tooth development and the regulation of tooth shape (11).…”

Rho GTPases in ameloblast differentiation

“…In the Tg mice, the Tomes’ processes lose contact with the forming enamel matrix in unerupted incisors, and the quantity of amelogenin and ameloblastin is reduced in the ameloblasts. After eruption, the enamel of the Tg mice shows severe structural defects, with complete loss of enamel [23]. These results suggest the involvement of Rac1 in cell–matrix interaction and matrix biomineralization.…”

“…The role of Rac1 in cell–matrix interaction, and subsequent matrix biomineralization, during enamel formation has been determined using Rac1 conditional knockout mice in which the cytokeratin 14 (K14) promoters bring about Cre expression in dental epithelial cells [23]. In the Tg mice, the Tomes’ processes lose contact with the forming enamel matrix in unerupted incisors, and the quantity of amelogenin and ameloblastin is reduced in the ameloblasts.…”

“…Further, they are involved in osteoclastogenesis as well as in hematopoiesis and hemopathies [12], [13]. Recently, evidence has emerged showing the involvement of Rho signaling in tooth development [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]. In this article, we summarize some interesting recent findings that provide molecular insights into how signaling by Rho GTPases results in tooth development, focusing on ameloblast differentiation in particular.…”