Claudin-5 is a transmembrane cell junction protein that is a component of tight junctions in endothelial cell layers. We have previously shown that claudin-5 also localizes to lateral membranes of murine cardiomyocytes at their junction with the extracellular matrix. Claudin-5 levels are specifically reduced in myocytes from a mouse model of muscular dystrophy with cardiomyopathy. To establish whether claudin-5 is similarly specifically reduced in human cardiomyopathy, we compared the levels of claudin-5 with other cell junction proteins in 62 cardiomyopathic end-stage explant samples. We show that claudin-5 levels are reduced in at least 60% of patient samples compared with non-failing controls. Importantly, claudin-5 reductions can be independent of connexin-43, a gap junction protein previously reported to be reduced in failing heart samples. Other cell junction proteins including α-catenin, β-catenin, γ-catenin, desmoplakin, and N-cadherin are reduced in only a small number of failing samples and only in combination with reduced claudin-5 or connexin-43 levels. We also show that reduced claudin-5 levels can be present independently from dystrophin alterations, which are known to be capable of causing and resulting from cardiomyopathy. These data are the first to show alterations of a tight junction protein in human cardiomyopathy samples and suggest that claudin-5 may participate in novel mechanisms in the pathway to end-stage heart failure.

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Introduction: Sepsis impaired vascular integrity results in multiple organ failure. Circulating lactate level is positively correlated with sepsis-induced mortality. We investigated whether lactate plays a role in causing endothelial barrier dysfunction in sepsis. Methods: Polymicrobial sepsis was induced in mice by cecal ligation and puncture (CLP). Lactic acid was injected i.p. (pH 6.8, 0.5 g/kg body weight) 6 h after CLP or sham surgery. To elucidate the role of heat shock protein A12B (HSPA12B), wild-type, HSPA12B-transgenic, and endothelial HSPA12B-deficient mice were subjected to CLP or sham surgery. To suppress lactate signaling, 3OBA (120 μM) was injected i.p. 3 h before surgery. Vascular permeability was evaluated with the Evans blue dye penetration assay. Results: We found that administration of lactate elevated CLP-induced vascular permeability. Vascular endothelial cadherin (VE-cadherin), claudin 5, and zonula occluden 1 (ZO-1) play a crucial role in the maintenance of endothelial cell junction and vascular integrity. Lactate administration significantly decreased VE-cadherin, claudin 5, and ZO-1 expression in the heart of septic mice. Our in vitro data showed that lactate (10 mM) treatment disrupted VE-cadherin, claudin 5, and ZO-1 in endothelial cells. Mechanistically, we observed that lactate promoted VE-cadherin endocytosis by reducing the expression of HSPA12B. Overexpression of HSPA12B prevented lactate-induced VE-cadherin disorganization. G protein-coupled receptor 81 (GPR81) is a specific receptor for lactate. Inhibition of GPR81 with its antagonist 3OBA attenuated vascular permeability and reversed HSPA12B expression in septic mice. Conclusions: The present study demonstrated a novel role of lactate in promoting vascular permeability by decreasing VE-cadherin junctions and tight junctions in endothelial cells. The deleterious effects of lactate in vascular hyperpermeability are mediated via HSPA12B- and GPR81-dependent signaling.