The Role of Tricellular Junctions in the Transport of Macromolecules Across Endothelium

Mean Ghim,Yumnah Mohamied,Peter D Weinberg

doi:10.1007/s13239-020-00483-x

Abstract

PurposeTransport of water and solutes across vascular endothelium is important in normal physiology and critical in the development of various diseases, including atherosclerosis. However, there is debate about the routes for such transport. We recently showed that an albumin-sized tracer crossed endothelium at bicellular and tricellular junctions, a tracer having the size of high density lipoprotein crossed only through tricellular junctions, and a tracer with the size of low density lipoprotein was unable to cross by either route and instead traversed the cells themselves. Here we review previous work on the structure and function of tricellular junctions. We then describe a study in which we assessed the role of such junctions in the transport of an albumin-sized tracer.MethodsWe examined normal endothelial monolayers, the effect of agonists that modify their permeability, and the influence of different patterns of shear stress.ResultsUnder normal conditions, approximately 85% of transendothelial transport occurred through tricellular junctions. This fraction was unchanged when permeability was reduced by sphingosine-1-phosphate or increased by thrombin, and also did not differ between endothelium exposed to multidirectional as opposed to uniaxial shear stress despite a > 50% difference in permeability.ConclusionThese data show that tricellular junctions dominate normal transport of this tracer and largely determine influences of agonists and shear. The effects were attributable to changes in both the number and conductivity of the junctions. Further investigation of these structures will lead to increased understanding of endothelial barrier function and may suggest new therapeutic strategies in disease.

Highlights

The vascular endothelium functions as a selective barrier between blood and surrounding tissue
We demonstrated that a larger tracer, NeutrAvidin labelled with R-phycoerythrin, could cross the monolayer at these intersections even though it could not pass through bicellular junctions.[16]
In this study we investigated whether endothelial tricellular junctions play a critical role in paracellular permeability and its regulation by agonists and biomechanical forces, using a recently developed, spatially resolved permeability assay

Summary

Introduction

The vascular endothelium functions as a selective barrier between blood and surrounding tissue. We consider whether tricellular junctions are an important route for the transport of macromolecules across endothelium, and whether they are involved in the physiological and pathological alteration of such transport by agonists and biomechanical forces. This topic has previously received little attention. Shear stress is known to alter endothelial permeability[21,40,46]; it has recently been suggested that increased transport occurs when the endothelium is exposed to multidirectional flow.[16,31] This behaviour may be important in the development of atherosclerosis, which is dependent on the transendothelial transport of blood-borne, cholesterol-carrying macromolecules into and out of the arterial wall and, it has been suggested, on multidirectional flow.[31]. We investigate the permeability of tricellular junctions, and of the endothelium as a whole, under both uniaxial (putatively atheroprotective) and multidirectional (putatively atherogenic) shear

Methods

Results

Conclusion