Date Published: April 3, 2012
Publisher: Public Library of Science
Author(s): Cyrill Géraud, Konstantin Evdokimov, Beate K. Straub, Wiebke K. Peitsch, Alexandra Demory, Yvette Dörflinger, Kai Schledzewski, Astrid Schmieder, Peter Schemmer, Hellmut G. Augustin, Peter Schirmacher, Sergij Goerdt, Aernout Luttun. http://doi.org/10.1371/journal.pone.0034206
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.
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 , , . Leukocyte recruitment upon liver injury , ,  as well as liver colonization by metastatic tumor cells ,  are actively influenced by LSECs. The unique morphology as well as the microenvironment-dependent molecular differentiation of LSECs  define the organ-specific features of this transendothelial barrier. Despite recent advances in understanding extravasation of inflammatory and tumor cells in liver sinusoids , the intercellular junctions between LSECs that considerably contribute to regulating hepatic transmigration have not yet been sufficiently characterized in molecular terms.
In the present study, we performed a comprehensive analysis of the molecular composition of the intercellular junctional complexes of liver sinusoidal endothelium in vivo and in vitro in two different species, i.e. human and rat. Our results unequivocally demonstrate that LSECs assemble the molecular complexes typical of endothelial AJ clearly disproving the notion that LSECs lack interendothelial junctions . Similar to vascular endothelial cells in general, LSECs express the main transmembrane component of endothelial AJ, i.e. VE-cadherin, on mRNA and protein level. In freshly isolated rat LSECs, VE-cadherin is found at the intercellular borders in similar localization patterns as in other ECs in vitro. Co-localization of VE-cadherin with α-, β-, and p120-catenins, as well as with plakoglobin indicates that LSECs form functional AJ complexes supporting the structural integrity of the sinusoidal vessel wall.