Shi Du Yan

RAGE mediates amyloid-beta peptide transport across the blood-brain barrier and accumulation in brain.

Amyloid-β peptide (Aβ) interacts with the vasculature to influence Aβ levels in the brain and cerebral blood flow, providing a means of amplifying the Aβ-induced cellular stress underlying neuronal dysfunction and dementia. Systemic Aβ infusion and studies in genetically manipulated mice show that Aβ interaction with receptor for advanced glycation end products (RAGE)-bearing cells in the vessel wall results in transport of Aβ across the blood-brain barrier (BBB) and expression of proinflammatory cytokines and endothelin-1 (ET-1), the latter mediating Aβ-induced vasoconstriction. Inhibition of RAGE-ligand interaction suppresses accumulation of Aβ in brain parenchyma in a mouse transgenic model. These findings suggest that vascular RAGE is a target for inhibiting pathogenic consequences of Aβ-vascular interactions, including development of cerebral amyloidosis.

Advanced glycation endproducts interacting with their endothelial receptor induce expression of vascular cell adhesion molecule-1 (VCAM-1) in cultured human endothelial cells and in mice. A potential mechanism for the accelerated vasculopathy of diabetes.

Vascular cell adhesion molecule-1 (VCAM-1), an inducible cell-cell recognition protein on the endothelial cell surface (EC), has been associated with early stages of atherosclerosis. In view of the accelerated vascular disease observed in patients with diabetes, and the enhanced expression of VCAM-1 in diabetic rabbits, we examined whether irreversible advanced glycation endproducts (AGEs), could mediate VCAM-1 expression by interacting with their endothelial cell receptor (receptor for AGE, RAGE). Exposure of cultured human ECs to AGEs induced expression of VCAM-1, increased adhesivity of the monolayer for Molt-4 cells, and was associated with increased levels of VCAM-1 transcripts. The inhibitory effect of anti-RAGE IgG, a truncated form of the receptor (soluble RAGE) or N-acetylcysteine on VCAM-1 expression indicated that AGE-RAGE-induced oxidant stress was central to VCAM-1 induction. Electrophoretic mobility shift assays on nuclear extracts from AGE-treated ECs showed induction of specific DNA binding activity for NF-kB in the VCAM-1 promoter, which was blocked by anti-RAGE IgG or N-acetylcysteine. Soluble VCAM-1 antigen was elevated in human diabetic plasma. These data are consistent with the hypothesis that AGE-RAGE interaction induces expression of VCAM-1 which can prime diabetic vasculature for enhanced interaction with circulating monocytes.

Activation of Receptor for Advanced Glycation End Products A Mechanism for Chronic Vascular Dysfunction in Diabetic Vasculopathy and Atherosclerosis

Receptor for advanced glycation end products (RAGE) is a member of the immunoglobulin superfamily of cell surface molecules and engages diverse ligands relevant to distinct pathological processes. One class of RAGE ligands includes glycoxidation products, termed advanced glycation end products, which occur in diabetes, at sites of oxidant stress in tissues, and in renal failure and amyloidoses. RAGE also functions as a signal transduction receptor for amyloid beta peptide, known to accumulate in Alzheimer disease in both affected brain parenchyma and cerebral vasculature. Interaction of RAGE with these ligands enhances receptor expression and initiates a positive feedback loop whereby receptor occupancy triggers increased RAGE expression, thereby perpetuating another wave of cellular activation. Sustained expression of RAGE by critical target cells, including endothelium, smooth muscle cells, mononuclear phagocytes, and neurons, in proximity to these ligands, sets the stage for chronic cellular activation and tissue damage. In a model of accelerated atherosclerosis associated with diabetes in genetically manipulated mice, blockade of cell surface RAGE by infusion of a soluble, truncated form of the receptor completely suppressed enhanced formation of vascular lesions. Amelioration of atherosclerosis in these diabetic/atherosclerotic animals by soluble RAGE occurred in the absence of changes in plasma lipids or glycemia, emphasizing the contribution of a lipid- and glycemia-independent mechanism(s) to atherogenesis, which we postulate to be interaction of RAGE with its ligands. Future studies using mice in which RAGE expression has been genetically manipulated and with selective low molecular weight RAGE inhibitors will be required to definitively assign a critical role for RAGE activation in diabetic vasculopathy. However, sustained receptor expression in a microenvironment with a plethora of ligand makes possible prolonged receptor stimulation, suggesting that interaction of cellular RAGE with its ligands could be a factor contributing to a range of important chronic disorders.

LRP/Amyloid β-Peptide Interaction Mediates Differential Brain Efflux of Aβ Isoforms

Abstract LRP (low-density lipoprotein receptor-related protein) is linked to Alzheimers disease (AD). Here, we report amyloid β-peptide Aβ40 binds to immobilized LRP clusters II and IV with high affinity (K d = 0.6–1.2 nM) compared to Aβ42 and mutant Aβ, and LRP-mediated Aβ brain capillary binding, endocytosis, and transcytosis across the mouse blood-brain barrier are substantially reduced by the high β sheet content in Aβ and deletion of the receptor-associated protein gene. Despite low Aβ production in the brain, transgenic mice expressing low LRP-clearance mutant Aβ develop robust Aβ cerebral accumulations much earlier than Tg-2576 Aβ-overproducing mice. While Aβ does not affect LRP internalization and synthesis, it promotes proteasome-dependent LRP degradation in endothelium at concentrations >1 μM, consistent with reduced brain capillary LRP levels in Aβ-accumulating transgenic mice, AD, and patients with cerebrovascular β-amyloidosis. Thus, low-affinity LRP/Aβ interaction and/or Aβ-induced LRP loss at the BBB mediate brain accumulation of neurotoxic Aβ.

Cyclophilin D deficiency attenuates mitochondrial and neuronal perturbation and ameliorates learning and memory in Alzheimer's disease

Cyclophilin D (CypD, encoded by Ppif) is an integral part of the mitochondrial permeability transition pore, whose opening leads to cell death. Here we show that interaction of CypD with mitochondrial amyloid-β protein (Aβ) potentiates mitochondrial, neuronal and synaptic stress. The CypD-deficient cortical mitochondria are resistant to Aβ- and Ca2+-induced mitochondrial swelling and permeability transition. Additionally, they have an increased calcium buffering capacity and generate fewer mitochondrial reactive oxygen species. Furthermore, the absence of CypD protects neurons from Aβ- and oxidative stress–induced cell death. Notably, CypD deficiency substantially improves learning and memory and synaptic function in an Alzheimers disease mouse model and alleviates Aβ-mediated reduction of long-term potentiation. Thus, the CypD-mediated mitochondrial permeability transition pore is directly linked to the cellular and synaptic perturbations observed in the pathogenesis of Alzheimers disease. Blockade of CypD may be a therapeutic strategy in Alzheimers disease.

Cellular receptors for advanced glycation end products. Implications for induction of oxidant stress and cellular dysfunction in the pathogenesis of vascular lesions.

Advanced glycation end products (AGEs) form by the interaction of aldoses with proteins and the subsequent molecular rearrangements of the covalently linked sugars, eventuating in a diverse group of fluorescent compounds of yellow-brown color. This heterogeneous class of nonenzymatically glycated proteins or lipids is found in the plasma and accumulates in the vessel wall and tissues even in normal aging. As a consequence of hyperglycemia, AGE formation and deposition are much enhanced in diabetes, in which their presence has been linked to secondary complications, especially microvascular disease. This review summarizes the cellular interactions of AGEs and describes the central role of a novel receptor for AGE (RAGE). RAGE, an immunoglobulin superfamily member, mediates the binding of AGEs to endothelial cells and mononuclear phagocytes, interacts with a lactoferrin-like polypeptide that also binds AGEs, and appears to activate intracellular signal transduction mechanisms consequent to its interaction with the glycated ligand. RAGE is expressed by ECs, mononuclear phagocytes, smooth muscle cells, mesangial cells, and neurons, indicating a potential role in the regulation of their properties in homeostasis and/or their dysfunction in the development of diabetic complications. Since AGEs have been shown to generate reactive oxygen intermediates, tethering of AGEs to the cell surface by their receptors focuses oxidant stress on cellular targets, resulting in changes in gene expression and the cellular phenotype. The discovery of RAGE and development of reagents to block its interaction with AGEs should provide insights into the role of this ligand-receptor interaction in the pathogenesis of diabetic complications and, potentially, atherosclerosis.

Mitochondrial Aβ: a potential focal point for neuronal metabolic dysfunction in Alzheimer’s disease

Although amyloid‐β peptide (Aβ) is the neurotoxic species implicated in the pathogenesis of Alzheimers disease (AD), mechanisms through which intracellular Aβ impairs cellular properties, resulting in neuronal dysfunction, remain to be clarified. Here we demonstrate that intracellular Aβ is present in mitochondria from brains of transgenic mice with targeted neuronal overexpression of mutant human amyloid precursor protein and AD patients. Aβ progressively accumulates in mitochondria and is associated with diminished enzymatic activity of respiratory chain complexes (III and IV) and a reduction in the rate of oxygen consumption. Importantly, mitochondria‐associated Aβ, principally Aβ42, was detected as early as 4 months, before extensive extracellular Aβ deposits. Our studies delineate a new means through which Aβ potentially impairs neuronal energetics, contributing to cellular dysfunction in AD.

Non-enzymatically glycated tau in Alzheimer's disease induces neuronal oxidant stress resulting in cytokine gene expression and release of amyloid |[beta]|-peptide

Paired helical filament (PHF) tau is the principal component of neurofibriliary tangles, a characteristic feature of the neurodegenerative pathology in Alzheimers disease (AD). Post-translational modification of tau, especially phosphorylation, has been considered a major factor in aggregation and diminished microtubule interactions of PHF-tau. Recently, it has been recognized that PHF-tau is also subject to non-enzymatic glycation, with formation of advanced glycation end products (AGEs). We now show that as a consequence of glycation, PHF-tau from AD and AGE-tau generate oxygen free radicals, thereby activating transcription via nuclear factor-κB, increasing amyloid β-protein precursor and release of ∼4 kD amyloid β-peptides. These data provide insight into how PHF-tau disturbs neuronal function, and add to a growing body of evidence that oxidant stress contributes to the pathogenesis of AD.

An intracellular protein that binds amyloid-β peptide and mediates neurotoxicity in Alzheimer's disease

Amyloid-β is a neurotoxic peptide which is implicated in the pathogenesis of Alzheimers disease. It binds an intracellular polypeptide known as ERAB, thought to be a hydroxysteroid dehydrogenase enzyme, which is expressed in normal tissues, but is overexpressed in neurons affected in Alzheimers disease. ERAB immunoprecipitates with amyloid-β, and when cell cultures are exposed to amyloid-β, ERAB inside the cell is rapidly redistributed to the plasma membrane. The toxic effect of amyloid-β on these cells is prevented by blocking ERAB and is enhanced by overexpression of ERAB. By interacting with intracellular amyloid-β, ERAB may therefore contribute to the neuronal dysfunction associated with Alzheimers disease.

Receptor-dependent cell stress and amyloid accumulation in systemic amyloidosis

Accumulation of fibrils composed of amyloid A in tissues resulting in displacement of normal structures and cellular dysfunction is the characteristic feature of systemic amyloidoses. Here we show that RAGE, a multiligand immunoglobulin superfamily cell surface molecule, is a receptor for the amyloidogenic form of serum amyloid A. Interactions between RAGE and amyloid A induced cellular perturbation. In a mouse model, amyloid A accumulation, evidence of cell stress and expression of RAGE were closely linked. Antagonizing RAGE suppressed cell stress and amyloid deposition in mouse spleens. These data indicate that RAGE is a potential target for inhibiting accumulation of amyloid A and for limiting cellular dysfunction induced by amyloid A.