Quantitative Phenotyping of Brain Endothelial Cell-Cell Junctions for Physiological and Pathophysiological Applications

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The integrity of endothelial cell-cell junctions is required for the maintenance of normal physiological processes. The expression of junctional proteins is particularly important in the endothelial cells of the blood-brain barrier (BBB), the cellular unit that protects the brain via regulated transport between the peripheral blood and the central nervous system. Dysfunction of the BBB is linked with decreased junctional protein localization and is implicated in several diseases including Alzheimer’s disease and multiple sclerosis. On the other hand, the tight junctions of the BBB impede the delivery of medications targeting the brain. Therefore, understanding the key players driving junction stability could hold significant promise for therapeutic discovery and drug delivery applications. Despite this, the mechanisms underlying junction disruption aren’t fully understood. While several studies have linked different junction protein patterns with altered barrier function, the quantification of this parameter remains limited due to the lack of efficient measurement techniques. Here, we aimed to investigate the influence of junction phenotype on brain endothelial barrier properties. To accomplish this, we developed the Junction Analyzer Program (JAnaP) to semi-automatically calculate edge-localization protein phenotypes.

Application of the JAnaP to measure the junctional proteins VE-cadherin and ZO-1 in different physiological and pathophysiological conditions revealed that discontinuous junctions contribute more to barrier permeability compared to continuous, linear junctions. Continuous junctions were also increased in endothelial cells with decreased contractility, mediated biochemically or by lowered subendothelial matrix stiffness. Finally, breast cancer cell secreted factors increased immature adherens junctions, likely through VEGF signaling, but minimally affected tight junction presentation. Thus far, the development and application of the JAnaP has revealed insights into the effects of junction patterns on barrier function, the mechanobiology of endothelial cells, and the response of brain endothelial cells to biochemical cues involved in breast cancer metastasis. Understanding the conditions driving altered junction presentation, and the resultant effects on barrier integrity, could lead to the development of therapeutics capable of traversing the BBB for delivery to the brain or for diseases associated with BBB dysfunction. Future use of this program holds significant potential for physiological and pathophysiological study in various endothelial and epithelial cell systems.