Lipoprotein receptors--an evolutionarily ancient multifunctional receptor family.
ABSTRACT The evolutionarily ancient low-density lipoprotein (LDL) receptor gene family represents a class of widely expressed cell surface receptors. Since the dawn of the first primitive multicellular organisms, several structurally and functionally distinct families of lipoprotein receptors have evolved. In accordance with the now obsolete 'one-gene-one-function' hypothesis, these cell surface receptors were originally perceived as mere transporters of lipoproteins, lipids, and nutrients or as scavenger receptors, which remove other kinds of macromolecules, such as proteases and protease inhibitors from the extracellular environment and the cell surface. This picture has since undergone a fundamental change. Experimental evidence has replaced the perception that these receptors serve merely as cargo transporters. Instead it is now clear that the transport of macromolecules is inseparably intertwined with the molecular machinery by which cells communicate with each other. Lipoprotein receptors are essentially sensors of the extracellular environment that participate in a wide range of physiological processes by physically interacting and coevolving with primary signal transducers as co-regulators. Furthermore, lipoprotein receptors modulate cellular trafficking and localization of the amyloid precursor protein (APP) and the β-amyloid peptide (Aβ), suggesting a role in the pathogenesis of Alzheimer's disease. Moreover, compelling evidence shows that LDL receptor family members are involved in tumor development and progression.
SourceAvailable from: Uwe Beffert[Show abstract] [Hide abstract]
ABSTRACT: Apoer2 is an essential receptor in the central nervous system that binds to the apolipoprotein ApoE. Various splice variants of Apoer2 are produced. We showed that Apoer2 lacking exon 16, which encodes the O-linked sugar (OLS) domain, altered the proteolytic processing and abundance of Apoer2 in cells and synapse number and function in mice. In cultured cells expressing this splice variant, extracellular cleavage of OLS-deficient Apoer2 was reduced, consequently preventing γ-secretase-dependent release of the intracellular domain of Apoer2. Mice expressing Apoer2 lacking the OLS domain had increased Apoer2 abundance in the brain, hippocampal spine density, and glutamate receptor abundance, but decreased synaptic efficacy. Mice expressing a form of Apoer2 lacking the OLS domain and containing an alternatively spliced cytoplasmic tail region that promotes glutamate receptor signaling showed enhanced hippocampal long-term potentiation (LTP), a phenomenon associated with learning and memory. However, these mice did not display enhanced spatial learning in the Morris water maze, and cued fear conditioning was reduced. Reducing the expression of the mutant Apoer2 allele so that the abundance of the protein was similar to that of Apoer2 in wild-type mice normalized spine density, hippocampal LTP, and cued fear learning. These findings demonstrated a role for ApoE receptors as regulators of synaptic glutamate receptor activity and established differential receptor glycosylation as a potential regulator of synaptic function and memory. Copyright © 2014, American Association for the Advancement of Science.Science Signaling 11/2014; 7(353):ra113. DOI:10.1126/scisignal.2005438 · 7.65 Impact Factor
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ABSTRACT: The endothelium represents not only a simple cellular monolayer that lines the vascular tree in humans and other vertebrates. Depending on the location, the endothelium shows significant morphological and functional heterogeneity through differentiated expression of pro- and anticoagulant factors, presence and frequency of intercellular contacts, variable contractility, cell shape, and volume. Altogether, these properties are crucial for adjustment of the endothelial function and further maintenance of the adequate homeostasis in response in local microenvironmental changes. Endothelial cells (ECs) play a critical role in coordinated regulation of blood flow. This is achieved due to the capacity of ECs to create the active anti-thrombotic surface that supports blood fluidity and transfer of blood cells and biomolecules. However, in certain vascular regions that can occur in inflamed sites or in sites with high hydrodynamic shear stress, ECs could lost their anti-thrombotic properties and switch their normal quiescent phenotype towards the prothrombotic, proadhesion, and proinflammatory state. In such an athero-prone site, the proper endothelial function is impaired that increases risk for formation of the atherosclerotic plaque. The endothelial dysfunction not only precedes atherosclerosis but greatly contributes to atherogenesis in all disease stages. Healthy lifestyle and regular intake of correct antioxidant-rich diet such as fresh fruits, vegetables, olive oil, red wine, and tea have beneficial effects on endothelial function and could therefore reduce the cardiovascular risk.Mini Reviews in Medicinal Chemistry 02/2015; 15(4). · 3.19 Impact Factor
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ABSTRACT: The low-density lipoprotein receptor (LDLR) is crucial for cholesterol homeostasis and deficiency in LDLR functions cause hypercholesterolemia. LDLR is a type I transmembrane protein that requires O-glycosylation for stable expression at the cell surface. It has previously been suggested that LDLR O-glycosylation is found N-terminal to the juxtamembrane region. Recently we identified O-glycosylation sites in the linker regions between the characteristic LDLR class A repeats in several LDLR-related receptors using the 'SimpleCell' O-glycoproteome shotgun strategy. Herein, we have systematically characterized O-glycosylation sites on recombinant LDLR shed from HEK293 SimpleCells and CHO wild-type cells. We find that the short linker regions between LDLR class A repeats contain an evolutionarily conserved O-glycosylation site at position -1 of the first cysteine residue of most repeats, which in wild-type CHO cells is glycosylated with the typical sialylated core 1 structure. The glycosites in linker regions of LDLR class A repeats are conserved in LDLR from man to Xenopus and found in other homologous receptors. O-glycosylation is controlled by a large family of polypeptide GalNAc-transferases. Probing into which isoform(s) contributed to glycosylation of the linker regions of the LDLR class A repeats by in vitro enzyme assays suggested a major role of GalNAc-T11. This was supported by expression of LDLR in HEK293 cells, where knockout of the GalNAc-T11 isoform resulted in the loss of glycosylation of three out of four linker regions.Journal of Biological Chemistry 05/2014; 289(25). DOI:10.1074/jbc.M113.545053 · 4.60 Impact Factor