Claudin Loss-of-Function Disrupts Tight Junctions and Impairs Amelogenesis

Abstract

Claudins are a family of proteins that forms paracellular barriers and pores determining tight junctions (TJ) permeability. Claudin-16 and -19 are pore forming TJ proteins allowing calcium and magnesium reabsorption in the thick ascending limb of Henle's loop (TAL). Loss-of-function mutations in the encoding genes, initially identified to cause Familial Hypomagnesemia with Hypercalciuria and Nephrocalcinosis (FHHNC), were recently shown to be also involved in Amelogenesis Imperfecta (AI). In addition, both claudins were expressed in the murine tooth germ and Claudin-16 knockout (KO) mice displayed abnormal enamel formation. Claudin-3, an ubiquitous claudin expressed in epithelia including kidney, acts as a barrier-forming tight junction protein. We determined that, similarly to claudin-16 and claudin-19, claudin-3 was expressed in the tooth germ, more precisely in the TJ located at the apical end of secretory ameloblasts. The observation of Claudin-3 KO teeth revealed enamel defects associated to impaired TJ structure at the secretory ends of ameloblasts and accumulation of matrix proteins in the forming enamel. Thus, claudin-3 protein loss-of-function disturbs amelogenesis similarly to claudin-16 loss-of-function, highlighting the importance of claudin proteins for the TJ structure. These findings unravel that loss-of-function of either pore or barrier-forming TJ proteins leads to enamel defects. Hence, the major structural function of claudin proteins appears essential for amelogenesis.

Novel Aspects of Renal Magnesium Homeostasis

“…A novel recent finding was that the absence of CLDN16 in ameloblasts explains the clinical finding of amelogenesis imperfecta in FHHNC patients and mice with CLDN16 deficiency ( 45 ). A similar finding was reported for patients with mutations in the genes coding for CLDN19 and similar for mice (CLDN3) ( 20 , 46 ).…”

“…Moreover, other Claudins (e.g., Claudin-3) have been demonstrated to be involved, in men and animals in tumor invasiveness and in autoimmune disorders ( 18 , 19 ). Also, several Claudins are vital for teeth development in mice and humans ( 20 ). A great step toward the determination of the role of Claudins has been achieved by the resolution of the crystal structure of the Claudin protein Claudin-15 ( 21 , 22 ) (Figure 3 B).…”

Expression Patterns of Claudin Family Members During Tooth Development and the Role of Claudin-10 (Cldn10) in Cytodifferentiation of Stratum Intermedium

“…Although TJs are broadly distributed in the dental epithelium, only a few claudins have been investigated and proven to play the role in tooth development (Ohazama and Sharpe, 2007;Hagen, 2017;Kim et al, 2020). Mutations in CLDN10, CLDN16, and CLDN19 are associated with amelogenesis imperfecta in humans (Bardet et al, 2016;Yamaguti et al, 2017;Kim et al, 2020) and in mice, deletion of Cldn3 significantly reduces the enamel volume compared to that of wild-type mice (Bardet et al, 2017). To date, 10 members of the claudin family have been reported to be expressed in the developing teeth.…”

Functional Validation of CLDN Variants Identified in a Neural Tube Defect Cohort Demonstrates Their Contribution to Neural Tube Defects

“…Moreover, we did not expect a claudin null allele to cause an NTD given that none of the CLDN null mice have NTDs (Gow et al, 1999;Furuse et al, 2002;Ben-Yosef et al, 2003;Nitta et al, 2003;Kitajiri et al, 2004;Anderson et al, 2008;Tamura et al, 2008;Nakano et al, 2009;Muto et al, 2010;Ding et al, 2012;Fujita et al, 2012;Hayashi et al, 2012; Kage et al, 2014;Kooij et al, 2014;Li et al, 2014;Matsumoto et al, 2014;Ahmad et al, 2017;Bardet et al, 2017). Although a Cldn8 null mouse has not yet been reported, Cldn3 (Kooij et al, 2014;Ahmad et al, 2017;Bardet et al, 2017) and Cldn19 null mice (Miyamoto et al, 2005) have closed neural tubes. This is perhaps not surprising as knockout or knockdown of a single claudin often results in compensatory upregulation of another family member (Li et al, 2014).…”

“…The initial contact between apposing neural folds is mediated by extracellular matrix proteins secreted from the neural and non-neural ectoderm in the chick embryo. It was shown that Cldn3 and -19 are located in tight junctions of ameloblasts in the mouse tooth germ where they play a role in regulating extracellular pH, which is critical for processing and secretion of extracellular matrix proteins (Bardet et al, 2017;Yamaguti et al, 2017). Cldn3 knockout mice exhibit enamel defects caused by excess accumulation of extracellular matrix proteins in the forming enamel and mutations in CLDN19 are associated with Amelogenesis Imperfecta, a genetic disorder characterized by tooth enamel defects (Bardet et al, 2017;Yamaguti et al, 2017).…”