Claudin-18 Loss Alters Transcellular Chloride Flux but not Tight Junction Ion Selectivity in Gastric Epithelial Cells.

BACKGROUND & AIMS: Tight junctions form a barrier to the paracellular passage of luminal antigens. Although most tight junction proteins reside within the apical tight junction complex, claudin-18 localizes mainly to the basolateral membrane where its contribution to paracellular ion transport is undefined. Claudin-18 loss in mice results in gastric neoplasia development and tumorigenesis that may or may not be due to tight junction dysfunction. The aim here was to investigate paracellular permeability defects in stomach mucosa from claudin-18 knockout (Cldn18-KO) mice. METHODS: Stomach tissue from wild-type, heterozygous, or Cldn18-KO mice were stripped of the external muscle layer and mounted in Ussing chambers. Transepithelial resistance, dextran 4 kDa flux, and potential difference (PD) were calculated from the chambered tissues after identifying differences in tissue histopathology that were used to normalize these measurements. Marker expression for claudins and ion transporters were investigated by transcriptomic and immunostaining analysis. RESULTS: No paracellular permeability defects were evident in stomach mucosa from Cldn18-KO mice. RNAseq identified changes in 4 claudins from Cldn18-KO mice, particularly the up-regulation of claudin-2. Although claudin-2 localized to tight junctions in cells at the base of gastric glands, its presence did not contribute overall to mucosal permeability. Stomach tissue from Cldn18-KO mice also had no PD versus a lumen-negative PD in tissues from wild-type mice. This difference resulted from changes in transcellular Cl- permeability with the down-regulation of Cl- loading and Cl- secreting anion transporters. CONCLUSIONS: Our findings suggest that Cldn18-KO has no effect on tight junction permeability in the stomach from adult mice but rather affects anion permeability. The phenotype in these mice may thus be secondary to transcellular anion transporter expression/function in the absence of claudin-18.

Thioesterase superfamily member 1 undergoes stimulus-coupled conformational reorganization to regulate metabolism in mice.

In brown adipose tissue, thermogenesis is suppressed by thioesterase superfamily member 1 (Them1), a long chain fatty acyl-CoA thioesterase. Them1 is highly upregulated by cold ambient temperature, where it reduces fatty acid availability and limits thermogenesis. Here, we show that Them1 regulates metabolism by undergoing conformational changes in response to ß-adrenergic stimulation that alter Them1 intracellular distribution. Them1 forms metabolically active puncta near lipid droplets and mitochondria. Upon stimulation, Them1 is phosphorylated at the N-terminus, inhibiting puncta formation and activity and resulting in a diffuse intracellular localization. We show by correlative light and electron microscopy that Them1 puncta are biomolecular condensates that are inhibited by phosphorylation. Thus, Them1 forms intracellular biomolecular condensates that limit fatty acid oxidation and suppress thermogenesis. During a period of energy demand, the condensates are disrupted by phosphorylation to allow for maximal thermogenesis. The stimulus-coupled reorganization of Them1 provides fine-tuning of thermogenesis and energy expenditure.