Claudin-5: gatekeeper of neurological function

Abstract

Tight junction proteins of the blood-brain barrier are vital for maintaining integrity of endothelial cells lining brain blood vessels. The presence of these protein complexes in the space between endothelial cells creates a dynamic, highly regulated and restrictive microenvironment that is vital for neural homeostasis. By limiting paracellular diffusion of material between blood and brain, tight junction proteins provide a protective barrier preventing the passage of unwanted and potentially damaging material. Simultaneously, this protective barrier hinders the therapeutic effectiveness of central nervous system acting drugs with over 95% of small molecule therapeutics unable to bypass the blood-brain barrier. At the blood-brain barrier, claudin-5 is the most enriched tight junction protein and its dysfunction has been implicated in neurodegenerative disorders such as Alzheimer's disease, neuroinflammatory disorders such as multiple sclerosis as well as psychiatric disorders including depression and schizophrenia. By regulating levels of claudin-5, it is possible to abrogate disease symptoms in many of these disorders. This review will give an overview of the blood-brain barrier and the role of tight junction complexes in maintaining blood-brain barrier integrity before focusing on the role of claudin-5 and its regulation in homeostatic and pathological conditions. We will also summarise therapeutic strategies to restore integrity of cerebral vessels by targeting tight junction protein complexes.

Keywords: Blood–brain barrier; Claudin-5; Endothelial cell; Tight junction.

Fig. 1

Inter-endothelial connections. Located closest to the apical membrane are the tight junction proteins consisting of claudin-1, -3, -5, -12; occludin; and lipolysis-stimulated protein (LSR) which limit paracellular diffusion of ions and solutes across the tightly packed monolayer of endothelial cells. Zonula occludens (ZO)-1, -2 and -3 binds to PDZ motifs on intracellular domains of claudins and occludin and bind to the actin cytoskeleton, providing structural integrity to the tight junction. Other junctional complexes contribute to tight junction properties including junctional adhesion molecule (JAM)-A, -B and -C and endothelial cell adhesion molecules (ESAM). Gap junctions such as connexin-37 and -40 form hemichannels between opposing endothelial cell membranes, contributing to intercellular communication. Located closest to the basolateral membrane are the adherens junction proteins including vascular endothelial (VE)-cadherin and platelet endothelial cell adhesion molecule-1 (PECAM-1)

Fig. 2

Structure of claudin-5. Claudin-5 consists of 4 transmembrane domains (TM), a short NH₂ terminus, two extracellular loops (ECL), a short intracellular loop and a longer COOH terminus. ECL1 contains a disulphide bond and ion binding site as well as a highly conserved signature motif. The long COOH terminus contains the PDZ binding motif for interactions with scaffolding proteins including ZO-1, -2 and -3. Additionally, the COOH terminus contains trafficking and phosphorylation residues (Adapted from [44])

Fig. 3

Dynamic tight junction remodelling in disease. Breakdown of the blood–brain barrier (BBB) and loss and mis-localisation of tight junction proteins leads to immune cell entry to the central nervous system (CNS). In multiple sclerosis, this results in neuroinflammation, neurodegeneration and disease progression and transendothelial migration (TEM) of peripheral blood leukocytes. Claudin-5 positive extracellular vesicles (EV) can bind to blood leukocytes to potentially facilitate TEM of leukocytes into the CNS. BBB breakdown also leads to the perivascular accumulation of plasma-derived proteins such as fibrinogen, albumin and immunoglobulin G (IgG) that is found in humans with temporal lobe epilepsy as well as in rodents injected with the seizure-inducing agent kainic acid. In rodents, glutamate released from neurons and astrocytes can bind to N-Methyl-d-aspartate receptors (NMDAR) on the brain endothelium and regulate tight junction proteins claudin-5 and occludin via upregulation of matrix metalloproteinases (MMP). Extravasation of red blood cells (RBC) following traumatic brain injury releases toxic haemoglobin and free iron culminating in generation of reactive oxygen species (ROS). Extravasation of fibrinogen and albumin activates microglia leading to secretion of MMP and basement membrane (BM) degeneration. Dashed boxes display the signalling pathways and molecules that regulate expression of claudin-5 and subsequent disassembly of the tight junction protein complexes that facilitates paracellular BBB permeability of blood-derived molecules