Tight junctions at the blood brain barrier: physiological architecture and disease-associated dysregulation

Abstract

The Blood-brain barrier (BBB), present at the level of the endothelium of cerebral blood vessels, selectively restricts the blood-to-brain paracellular diffusion of compounds; it is mandatory for cerebral homeostasis and proper neuronal function. The barrier properties of these specialized endothelial cells notably depend on tight junctions (TJs) between adjacent cells: TJs are dynamic structures consisting of a number of transmembrane and membrane-associated cytoplasmic proteins, which are assembled in a multimolecular complex and acting as a platform for intracellular signaling. Although the structural composition of these complexes has been well described in the recent years, our knowledge about their functional regulation still remains fragmentary. Importantly, pericytes, embedded in the vascular basement membrane, and perivascular microglial cells, astrocytes and neurons contribute to the regulation of endothelial TJs and BBB function, altogether constituting the so-called neurovascular unit.The present review summarizes our current understanding of the structure and functional regulation of endothelial TJs at the BBB. Accumulating evidence points to a correlation between BBB dysfunction, alteration of TJ complexes and progression of a variety of CNS diseases, such as stroke, multiple sclerosis and brain tumors, as well as neurodegenerative diseases like Parkinson's and Alzheimer's diseases. Understanding how TJ integrity is controlled may thus help improve drug delivery across the BBB and the design of therapeutic strategies for neurological disorders.

Figure 1

Schematic representation of TJ modulation by the NVU. (a) The basal lamina protein agrin increases claudin-5 (Cld5) and occludin expression [92]; (b) Aquaporin-4 density, regulated by agrin, stabilizes TJ complexes through ZO-1 expression [93]; (c) β1-integrin engagement stabilizes Cld5 localization at the TJ [94]; (d) astrocyte/pericyte-secreted TGF-β induces Cld5 transcription through activation of Smad transcription factor [91]; (e) Shh enhances expression of TJ proteins via its membrane receptor Ptch1/Smo and the transcription factor Gli-1 [95]; (f) Endothelial PDGF-β recruits pericytes which stabilize BBB phenotype [90]; (g) Wnt 7a/7b proteins, via their membrane receptors Frizzled-4 associated to LRP5/6, induce Cld3 transcription through stabilization of β-catenin [96]; (hij) Angiopoietin-1 (Ang1), via its membrane receptor Tie2, enhances VE-Cadherin clustering and Cld5 transcription through inhibition of FoxO1 activity by PI3K and β-catenin sequestration [83,97] ; (k) VE-Cadherin engagement recruits CCM1 and the polarity complex (PCP) leading to TJ stabilization [98] .