The CLDN5 gene at the blood-brain barrier in health and disease

Abstract

The CLDN5 gene encodes claudin-5 (CLDN-5) that is expressed in endothelial cells and forms tight junctions which limit the passive diffusions of ions and solutes. The blood-brain barrier (BBB), composed of brain microvascular endothelial cells and associated pericytes and end-feet of astrocytes, is a physical and biological barrier to maintain the brain microenvironment. The expression of CLDN-5 is tightly regulated in the BBB by other junctional proteins in endothelial cells and by supports from pericytes and astrocytes. The most recent literature clearly shows a compromised BBB with a decline in CLDN-5 expression increasing the risks of developing neuropsychiatric disorders, epilepsy, brain calcification and dementia. The purpose of this review is to summarize the known diseases associated with CLDN-5 expression and function. In the first part of this review, we highlight the recent understanding of how other junctional proteins as well as pericytes and astrocytes maintain CLDN-5 expression in brain endothelial cells. We detail some drugs that can enhance these supports and are being developed or currently in use to treat diseases associated with CLDN-5 decline. We then summarise mutagenesis-based studies which have facilitated a better understanding of the physiological role of the CLDN-5 protein at the BBB and have demonstrated the functional consequences of a recently identified pathogenic CLDN-5 missense mutation from patients with alternating hemiplegia of childhood. This mutation is the first gain-of-function mutation identified in the CLDN gene family with all others representing loss-of-function mutations resulting in mis-localization of CLDN protein and/or attenuated barrier function. Finally, we summarize recent reports about the dosage-dependent effect of CLDN-5 expression on the development of neurological diseases in mice and discuss what cellular supports for CLDN-5 regulation are compromised in the BBB in human diseases.

Keywords: Blood–brain barrier; Claudin-5; Psychiatric diseases; Tight junction; Vascular permeability.

Fig. 1

The schematic illustration of the BBB. (a) The BBB is composed of microvascular endothelial cells, pericytes and surrounded end-feet of astrocytes. One of the key features in brain microvascular endothelial cells is its well-developed tight junctions in their cellular clefts. Tight junctions can be observed as the mesh-like strands composed of intramembrane particles. At the membrane particles, the two plasma membranes are almost fused (also called kissing-points). (b) The junctional proteins in brain endothelial cells. VE-cadherin is a component of adherens junctions, but the subcellular localization of tight junctions and adherens junctions are almost same in brain endothelial cells. These proteins are interacted with ZO-1/-2 using different binding domain of ZO-1/-2. ZO-1 and -2 are also oligomerized by themselves. Paracingulin is a recruiter of guanidine exchange factors (GEFs) to junctional areas and GEFs are necessary to activate small GTPases Rac1 or RhoA. Rac1 strengthens the tight junctions while RhoA destabilizes the tight junctions and they inhibit each other

Fig. 2

The basic information of CLDN5 gene The key characteristics of transcriptional variants of CLDN5. The open-reading frames are highlighted by black bars

Fig. 3

The TJ proteins mediated CLDN5 regulation in the brain endothelial cells (a) VE-cadherin-mediated CLDN5 regulation via PI3K/Akt signaling. The trans-interaction of VE-cadherin activates PI3K/Akt signaling and leads Rac1-mediated junctional stabilization and inhibits β-catenin/FoxO1-mediated CLDN5 suppression. (b) JAM-A-mediated CLDN5 regulation via cAMP signaling. The trans-interactions of JAM-A enhances cAMP level and leads protein kinase A (PKA)-dependent barrier stabilization via Rac1 activation and PKA-independent CLDN5 up-regulation via C-EBP-α. (c) ZO-1-mediated CLDN5 regulation by preventing non-junctional RhoA activation. Well-stabilized ZO-1 prevents the translocalization of ZO-1-associated nucleic acid binding protein (ZONAB) to the nucleus by direct interaction and by inhibiting non-junctional GEF-H1 activation via paracingulin. RhoA activation induces cell contraction by accumulated phosphorylated myosin light chain (MLC). Nitric oxidase (NO) and cAMP signaling inhibits cell contraction, but calcium signaling promotes it. CaM, calmodulin; CaMK, calmodulin kinase; C/EBP-α, CCAAT/enhancer-binding protein-α; eNOS, endothelial nitric oxide synthase; EPAC, exchange proteins directly activated by cAMP; GSK-3β, glycogen synthase kinase-3β; MLC, myosin light chain; MLCK, myosin light chain kinase; MLCP, myosin light chain phosphatase; PI3K, phosphoinositide 3-kinases; PKA, protein kinase A; PKG, protein kinase G

Fig. 4

The basic information of CLDN-5 protein (a) The homology-based model of CLDN-5 based on the crystal structure of mouse CLDN-15 (PDB; 4P79) and (b) schematic illustration of CLDN-5 highlighting with key domains for forming CLDN dimers/oligomers and amino acids for post-translational modifications. (c) The sequence alignment of CLDN-5 with crystal structure confirmed CLDNs (CLDN-3, -4, and -15). The classic CLDN signature is highlighted by yellow and conserved cis-interaction sites are highlighted by cyan. (d) The similarity of a putative ion pathway between mouse CLDN-15 and CLDN-5 G60R is shown. The charged amino acids are highlighted by different colors