Regulation of vascular endothelial barrier function by Epac, a cAMP-activated exchange factor for Rap GTPase.

Endothelial cell-cell junctional proteins and cortical actin are of central importance for regulating vascular permeability. Rap1, a member of the Ras family of GTPases, is enriched at endothelial cell-cell contacts and activated by cyclic AMP (cAMP) through a PKA-independent pathway. Activation of a cAMP-inducible guanine-exchange factor for Rap, Epac, results in markedly enhanced basal endothelial barrier function by increasing cortical actin and subsequent redistribution of adherens and tight junctional molecules to cell-cell contacts.

Scavenger receptors class A-I/II and CD36 are the principal receptors responsible for the uptake of modified low density lipoprotein leading to lipid loading in macrophages.

Modification of low density lipoprotein (LDL) can result in the avid uptake of these lipoproteins via a family of macrophage transmembrane proteins referred to as scavenger receptors (SRs). The genetic inactivation of either of two SR family members, SR-A or CD36, has been shown previously to reduce oxidized LDL uptake in vitro and atherosclerotic lesions in mice. Several other SRs are reported to bind modified LDL, but their contribution to macrophage lipid accumulation is uncertain.

Naturally occurring mutations in the largest extracellular loops of ABCA1 can disrupt its direct interaction with apolipoprotein A-I.

The ABCA1 transporter contains two large domains into which many of the genetic mutations in individuals with Tangier disease fall. To investigate the structural requirements for the cellular cholesterol efflux mediated by ABCA1, we have determined the topology of these two domains and generated transporters harboring five naturally occurring missense mutations in them. These mutants, unlike wild type ABCA1, produced little or no apoA-I-stimulated cholesterol efflux when transfected into 293 cells, establishing their causality in Tangier disease.