The role of claudin-10 in amelogenesis

Overview & Synopsis

Claudins are a family of 27 proteins that are important components of a membrane complex known as the tight junction (TJ). As TJs are located mainly between two epithelial cells and regulate paracellular permeability by creating a selective barrier for the passage of ions. Enamel is synthesized by ameloblasts (polarized epithelial cells of ectodermal origin), which possess two ion transport pathways: transcellular and paracellular. The association between claudins and amelogenesis has been demonstrated in the Familial Hypomagnesemia, Hypercalciuria and Nephrocalcinosis Syndrome (FHHNC), where patients with a CLDN16 or CLDN19 mutation present structural enamel anomalies (imperfect amelogenesis). It has recently been reported that patients with HELIX syndrome (Hypohidrosis, Electrolyte imbalance, hypoLacrymia, Ichthyosis, Xerostomia) due to CLDN10 mutations, all present early and severe enamel wear. In HELIX patients, enamel loss may have a multifactorial origin: xerostomia, imbalance of mineral homeostasis which could impact enamel mineral structure, and disruption of amelogenesis due to claudin-10 deficiency in the enamel organ. The main aim of our work was to clarify the role of claudin-10 in amelogenesis, but also to determine the origin of enamel wear associated with HELIX syndrome. We studied four mouse models: Cldn10 knock-out (KO) mice, Cldn10 KO mice conditional with the Keratin14 promoter, Cldn10 KO dental germs implanted under the kidney capsule of wild-type (WT) mice, tooth from HELIX patients. Firstly, we studied the expression of claudin-10 in the enamel organ of wild-type mice and showed that the labelling was predominantly found in the stratum intermedium (SI), at the membrane but not at the TJs. In adult conditional KO male mice induced by tamoxifen, we found evidence of delayed enamel mineralization and a more basic pH at the beginning of the maturation stage on the continuously growing incisor, but overall correct enamel structure. This correct structure was confirmed by examining the germs of molars implanted under the kidney capsule and by studying a HELIX tooth collected before eruption. These data suggest that severe wear in HELIX patients is mainly due to the salivary disruption. However, Tof-sims analysis revealed abundant aluminum in both transplanted Cldn10 KO germ and HELIX enamel. This finding indicates a loss of enamel organ integrity in the absence of claudin-10. In parallel, we studied the impact of disturbed mineral homeostasis on amelogenesis through a genetic abnormality of blood calcium levels, familial hypercalcemia hypocalciuria (FHH).