Unique Cell Type-Specific Junctional Complexes in Vascular Endothelium of Human and Rat Liver Sinusoids

Cyrill Géraud,Konstantin Evdokimov,Beate K Straub,Hellmut G Augustin,Alexandra Demory,Sergij Goerdt,Astrid Schmieder,Peter Schirmacher,Wiebke K Peitsch,Yvette Dörflinger,Peter Schemmer,Kai Schledzewski,Aernout Luttun

doi:10.1371/journal.pone.0034206

Abstract

Liver sinusoidal endothelium is strategically positioned to control access of fluids, macromolecules and cells to the liver parenchyma and to serve clearance functions upstream of the hepatocytes. While clearance of macromolecular debris from the peripheral blood is performed by liver sinusoidal endothelial cells (LSECs) using a delicate endocytic receptor system featuring stabilin-1 and -2, the mannose receptor and CD32b, vascular permeability and cell trafficking are controlled by transcellular pores, i.e. the fenestrae, and by intercellular junctional complexes. In contrast to blood vascular and lymphatic endothelial cells in other organs, the junctional complexes of LSECs have not yet been consistently characterized in molecular terms. In a comprehensive analysis, we here show that LSECs express the typical proteins found in endothelial adherens junctions (AJ), i.e. VE-cadherin as well as α-, β-, p120-catenin and plakoglobin. Tight junction (TJ) transmembrane proteins typical of endothelial cells, i.e. claudin-5 and occludin, were not expressed by rat LSECs while heterogenous immunreactivity for claudin-5 was detected in human LSECs. In contrast, junctional molecules preferentially associating with TJ such as JAM-A, B and C and zonula occludens proteins ZO-1 and ZO-2 were readily detected in LSECs. Remarkably, among the JAMs JAM-C was considerably over-expressed in LSECs as compared to lung microvascular endothelial cells. In conclusion, we show here that LSECs form a special kind of mixed-type intercellular junctions characterized by co-occurrence of endothelial AJ proteins, and of ZO-1 and -2, and JAMs. The distinct molecular architecture of the intercellular junctional complexes of LSECs corroborates previous ultrastructural findings and provides the molecular basis for further analyses of the endothelial barrier function of liver sinusoids under pathologic conditions ranging from hepatic inflammation to formation of liver metastasis.

Highlights

Liver sinusoidal endothelial cells (LSECs) form a fenestrated monolayer at the inner side of the liver sinusoids constituting a barrier between blood flow and hepatocytes facing the perisinusoidal space of Disse [1,2,3]
To further confirm expression of VE-cadherin in LSECs, complementary DNA (cDNA) of freshly isolated highly pure rat LSECs was analyzed by reverse transcriptase PCR (RT-PCR) with two independent sets of primers specific for VEcadherin
Both sets of primers showed a similar level of expression of VE-cadherin mRNA in LSECs as compared to lung microvascular endothelial cells (LMECs)

Summary

Introduction

Liver sinusoidal endothelial cells (LSECs) form a fenestrated monolayer at the inner side of the liver sinusoids constituting a barrier between blood flow and hepatocytes facing the perisinusoidal space of Disse [1,2,3]. Leukocyte recruitment upon liver injury [4,5,6] as well as liver colonization by metastatic tumor cells [7,8] are actively influenced by LSECs. The unique morphology as well as the microenvironment-dependent molecular differentiation of LSECs [19] define the organ-specific features of this transendothelial barrier. Besides diffusion through the fenestrae, LSECs actively support uptake and degradation as well as transendothelial transfer of macromolecules by their high endocytic capacity. The hepatic clearance function of LSECs is highly important for the homeostasis of the whole organism protecting distant organs such as the kidney from noxious blood factors [14]

Methods

Results

Conclusion