Claudins are a family of proteins which, along with occludin, are the most important components of the tight junctions (zonulae occludentes). Tight junctions establish the paracellular barrier that controls the flow of molecules in the intercellular space between the cells of an epithelium. They have four transmembrane domains, with the N-terminus and the C-terminus in the cytoplasm. 

Claudins are small (20–27 kilodalton (kDa)) transmembrane proteins which are found in many organisms, ranging from nematodes to human beings, and are very similar in their structure, although this conservation is not observed on the genetic level. Claudins span the cellular membrane 4 times, with the N-terminal end and the C-terminal end both located in the cytoplasm, and two extracellular loops which show the highest degree of conservation. It is suspected that the cysteines of individual or separate claudins form disulfide bonds. All human claudins (with the exception of Claudin 12) have domains that let them bind to PDZ domains of scaffold proteins.

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.

While a 3D structure of a brain associated claudin protein has yet to be published, recently the crystal structure of claudin-15 has been determined which is in the same class of claudin as claudin-5. This model reveals that claudin-5 may contain a β-sheet consisting of the two extracellular domains tethered to the transmembranous 4 helix bundle.

Claudin-5 expression is regulated by several upstream signalling pathways at the transcriptional and post-translational levels. Additionally, regulation of claudin-5 and of tight junction properties in general occurs via physical interactions with cytoplasmic scaffolding proteins and interactions with proteins on the same (cis) or adjacent cells (trans). Together, these processes determine claudin-5 assembly, remodelling and degradation.

Treatment of brain endothelial cells with the GSK3β inhibitor lithium chloride increases barrier properties concomitant with an increase in the amount of claudin-5 and occludin. This process was attributed to reduced protein turnover owing to increased half-life of claudin-5 and occluding.

Regards
Meria Den
Managing Editor
Stroke Research & Therapy