Knockout Mice Reveal Key Roles for Claudin 18 in Alveolar Barrier Properties and Fluid Homeostasis

Abstract

Claudin proteins are major constituents of epithelial and endothelial tight junctions (TJs) that regulate paracellular permeability to ions and solutes. Claudin 18, a member of the large claudin family, is highly expressed in lung alveolar epithelium. To elucidate the role of claudin 18 in alveolar epithelial barrier function, we generated claudin 18 knockout (C18 KO) mice. C18 KO mice exhibited increased solute permeability and alveolar fluid clearance (AFC) compared with wild-type control mice. Increased AFC in C18 KO mice was associated with increased b-adrenergic receptor signaling together with activation of cystic fibrosis transmembrane conductance regulator, higher epithelial sodium channel, and Na-K-ATPase (Na pump) activity and increased Na-K-ATPase b1 subunit expression. Consistent with in vivo findings, C18 KO alveolar epithelial cell (AEC) monolayers exhibited lower transepithelial electrical resistance and increased solute and ion permeability with unchanged ion selectivity. Claudin 3 and claudin 4 expression was markedly increased in C18 KO mice, whereas claudin 5 expression was unchanged and occludin significantly decreased. Microarray analysis revealed changes in cytoskeletonassociated gene expression in C18 KO mice, consistent with observed F-actin cytoskeletal rearrangement in AEC monolayers. These findings demonstrate a crucial nonredundant role for claudin 18 in the regulation of alveolar epithelial TJ composition and permeability properties. Increased AFC in C18 KO mice identifies a role for claudin 18 in alveolar fluid homeostasis beyond its direct contributions to barrier properties that may, at least in part, compensate for increased permeability.

Claudin-18–mediated YAP activity regulates lung stem and progenitor cell homeostasis and tumorigenesis

“…Deletion of the stomach-specific Cldn18.2 isoform in mice leads to loss of TJ strands and increased paracellular H + leakage in the stomach, resulting in atrophic gastritis and metaplasia but without evidence of tumor formation (23). Recently generated Cldn18 -/-mice with deletion of both isoforms showed increased lung permeability to ions and solutes, consistent with known roles of claudins in regulation of barrier function (24,25).…”

“…We previously generated mice with total knockout of Cldn18 (24). In addition to alterations in epithelial permeability and ion transport, we observed increased lung cellularity from embryonic day 18 (E18) onward ( Figure 1A and Supplemental Figure 1A; supplemental material available online with this article; https://doi.…”

“…-/-mice were prepared as previously described (24). To quantify tumors, H&E-stained lung sections from WT and Cldn18 -/-mice were examined by light microscopy in a blinded fashion.…”

Evolution of the vertebrate claudin gene family: insights from a basal vertebrate, the sea lamprey

“…This was surprising, because Cldn18 has not been reported to be expressed during neural crest development in any other vertebrate. Claudin 18 knockout mice are viable and exhibit changes in cell architecture and differentiation of alveolar epithelium, but no neural crest phenotype, suggesting that it is not involved in neural crest development in mice (LaFemina et al, 2014, Li et al, 2014. In chick, claudin 18 expression has been reported in Hensen's node, epiblast, neural and non-neural ectoderm, pharynx, eye and limb ectoderm, but not in the migrating neural crest (Collins et al, 2013).…”

Studies on cell junctions in an ex vivo human lung infection model

“…Claudin-1 -/-Skin Barrier [207,208] Claudin-2 -/-Kidney, liver Barrier [209,210] Claudin-3 -/--- [211,212] Claudin-4 -/-Kidney Barrier [213] Claudin-5 -/-Blood-brain barrier Barrier [204] Claudin-7 -/-Intestine Adhesion [214] Claudin-10 -/-Kidney Na + adsorption [215] Claudin-11 -/-Inner ear Barrier [216] Claudin-14 -/-Kidney Regulation [217] Claudin-15 -/-Small intestine Mucosal differentiation [218] Claudin-16 -/-Kidney Barrier [219] Claudin-18 -/-Lung, bone Barrier Bone resorption and osteoclast differentiation [220,221] [222]…”