Lipid dys-homeostasis contributes to APOE4-associated AD pathology

The association of the APOE4 (vs. APOE3) isoform with an increased risk of Alzheimer’s disease (AD) is unequivocal, but the underlying mechanisms remain incompletely elucidated. A prevailing hypothesis incriminates the impaired ability of APOE4 to clear neurotoxic amyloid-β peptides (Aβ) from the brain as the main mechanism linking the apolipoprotein isoform to disease etiology. The APOE protein mediates lipid transport both within the brain and from the brain to the periphery, suggesting that lipids may be potential co-factors in APOE4-associated physiopathology. The present study reveals several changes in the pathways of lipid homeostasis in the brains of mice expressing the human APOE4 vs. APOE3 isoform. Carriers of APOE4 had altered cholesterol turnover, an imbalance in the ratio of specific classes of phospholipids, lower levels of phosphatidylethanolamines bearing polyunsaturated fatty acids and an overall elevation in levels of monounsaturated fatty acids. These modifications in lipid homeostasis were related to increased production of Aβ peptides as well as augmented levels of tau and phosphorylated tau in primary neuronal cultures. This suite of APOE4-associated anomalies in lipid homeostasis and neurotoxic protein levels may be related to the accrued risk for AD in APOE4 carriers and provides novel insights into potential strategies for therapeutic intervention

Caractérisation du site de liaison du cholestérol sur l'APP : un modulateur de la sécrétion de peptides amyloïdes

Alzheimer’s disease (AD) is characterized by progressive loss of memory. The amount of cholesterol is higher in the brains of patients with AD. In addition, the existence of a cholesterol binding site (CBS) on APP sequence was demonstrated by nuclear magnetic resonance. Mutations in the CBS of APP completely abolish the interaction between peptides and liposomes loaded with cholesterol. All CBS mutants produce much less Aβ40 and Aβ42 when expressed in HEK293T cells without altering the other APP cleavage products. We identified two mutations: juxtamembrane mutations that increase the secretion of short fragments of Aβ and transmembrane mutations, including the Italian family mutation, which decrease the secretion of short Aβ peptides. These results suggest a shift in the cleavage of mutants by γ-secretase and / or a modification of its processivity leading to the formation of short peptides. Thus, the CBS is a key actor in the secretion of amyloid peptides. At the same time, we have shown that primary cultures of ApoE4-expressing neurons secrete amyloid peptides Aβ38, 40 and 42 in greater quantities compared to neurons expressing ApoE3. ApoE4 neurons express more strongly the tau and phospho-tau proteins, but more weakly the ApoE "full lenght" protein compared to the ApoE3 neurons. The CBS and the ApoE4 genotype therefore contribute to the increase of the amyloid peptides secretion. It would be interesting to know the cellular mechanisms involving the CBS of APP, cholesterol and its carrier ApoE.La maladie d’Alzheimer (MA) se caractérise par une perte mnésique progressive. La quantité de cholestérol est plus élevée dans les cerveaux de patients atteints de la MA. De plus, l’existence d’un site de liaison entre le cholestérol (SLC) et l’Amyloid Precursor Protein (APP) a été démontrée par résonance magnétique nucléaire. Les mutations effectuées dans le SLC de l’APP abolissent totalement l’interaction entre les peptides et des liposomes chargés en cholestérol. Tous les mutants du SLC produisent beaucoup moins d’Aβ40 et Aβ42 sans modifier les produits de clivage de l’APP. Nous avons mis en évidence deux catégories de mutations : les mutations juxtamembranaires qui augmentent la sécrétion de fragments courts d’Aβ et les mutations transmembranaires, incluant la mutation familiale italienne, qui diminuent la sécrétion de peptides courts d’Aβ. Ces résultats suggèrent un décalage du clivage des mutants par la γ-sécrétase et/ou une modification de sa processivité conduisant à la formation de peptides de courtes tailles. Parallèlement, nous avons montré que des cultures primaires de neurones exprimant l’ApoE4 sécrètent en plus grande quantité les peptides amyloïdes Aβ38, 40 et 42 comparé à des neurones exprimant l’ApoE3. Les neurones ApoE4 expriment plus fortement les protéines tau et phospho-tau, mais plus faiblement l’ApoE « full lenght » par rapport aux neurones ApoE3. Le SLC et le génotype ApoE4 contribuent donc à l’augmentation de la sécrétion de peptides amyloïdes. Il serait intéressant de connaître les mécanismes cellulaires impliquant le SLC de l’APP, le cholestérol et son transporteur l’ApoE

Characterization of the cholesterol binding site on the APP : a modulator of the secretion of amyloïd peptides

La maladie d’Alzheimer (MA) se caractérise par une perte mnésique progressive. La quantité de cholestérol est plus élevée dans les cerveaux de patients atteints de la MA. De plus, l’existence d’un site de liaison entre le cholestérol (SLC) et l’Amyloid Precursor Protein (APP) a été démontrée par résonance magnétique nucléaire. Les mutations effectuées dans le SLC de l’APP abolissent totalement l’interaction entre les peptides et des liposomes chargés en cholestérol. Tous les mutants du SLC produisent beaucoup moins d’Aβ40 et Aβ42 sans modifier les produits de clivage de l’APP. Nous avons mis en évidence deux catégories de mutations : les mutations juxtamembranaires qui augmentent la sécrétion de fragments courts d’Aβ et les mutations transmembranaires, incluant la mutation familiale italienne, qui diminuent la sécrétion de peptides courts d’Aβ. Ces résultats suggèrent un décalage du clivage des mutants par la γ-sécrétase et/ou une modification de sa processivité conduisant à la formation de peptides de courtes tailles. Parallèlement, nous avons montré que des cultures primaires de neurones exprimant l’ApoE4 sécrètent en plus grande quantité les peptides amyloïdes Aβ38, 40 et 42 comparé à des neurones exprimant l’ApoE3. Les neurones ApoE4 expriment plus fortement les protéines tau et phospho-tau, mais plus faiblement l’ApoE « full lenght » par rapport aux neurones ApoE3. Le SLC et le génotype ApoE4 contribuent donc à l’augmentation de la sécrétion de peptides amyloïdes. Il serait intéressant de connaître les mécanismes cellulaires impliquant le SLC de l’APP, le cholestérol et son transporteur l’ApoE.Alzheimer’s disease (AD) is characterized by progressive loss of memory. The amount of cholesterol is higher in the brains of patients with AD. In addition, the existence of a cholesterol binding site (CBS) on APP sequence was demonstrated by nuclear magnetic resonance. Mutations in the CBS of APP completely abolish the interaction between peptides and liposomes loaded with cholesterol. All CBS mutants produce much less Aβ40 and Aβ42 when expressed in HEK293T cells without altering the other APP cleavage products. We identified two mutations: juxtamembrane mutations that increase the secretion of short fragments of Aβ and transmembrane mutations, including the Italian family mutation, which decrease the secretion of short Aβ peptides. These results suggest a shift in the cleavage of mutants by γ-secretase and / or a modification of its processivity leading to the formation of short peptides. Thus, the CBS is a key actor in the secretion of amyloid peptides. At the same time, we have shown that primary cultures of ApoE4-expressing neurons secrete amyloid peptides Aβ38, 40 and 42 in greater quantities compared to neurons expressing ApoE3. ApoE4 neurons express more strongly the tau and phospho-tau proteins, but more weakly the ApoE "full lenght" protein compared to the ApoE3 neurons. The CBS and the ApoE4 genotype therefore contribute to the increase of the amyloid peptides secretion. It would be interesting to know the cellular mechanisms involving the CBS of APP, cholesterol and its carrier ApoE

Cholesterol and ApoE in Alzheimer’s disease

Genetic, neuropathological and biochemical studies suggest strong links between cholesterol, the apolipoprotein E (APOE) and Alzheimer’s disease (AD), both in humans and in animal models of the disease. From the literature and our work, we can predict that transient increase of the levels of cholesterol at the membrane of neurons would profoundly affect the processing of the transmembrane Amyloid Precursor Protein (APP) by triggering its clathrin dependent endocytosis and the resulting production of amyloid-β (Aβ) peptides. Here, we will review these data together with structural and molecular dynamic studies that characterized the role of cholesterol on APP conformation and positioning at the membrane. Specifically decreasing brain cholesterol or replacing it with plant sterols crossing the blood brain barrier appear like promising strategies to either delay or counteract the development of sporadic AD

Cholesterol and ApoE in Alzheimer’s disease

Genetic, neuropathological and biochemical studies suggest strong links between cholesterol, the apolipoprotein E (APOE) and Alzheimer’s disease (AD), both in humans and in animal models of the disease. From the literature and our work, we can predict that transient increase of the levels of cholesterol at the membrane of neurons would profoundly affect the processing of the transmembrane Amyloid Precursor Protein (APP) by triggering its clathrin dependent endocytosis and the resulting production of amyloid-β (Aβ) peptides. Here, we will review these data together with structural and molecular dynamic studies that characterized the role of cholesterol on APP conformation and positioning at the membrane. Specifically decreasing brain cholesterol or replacing it with plant sterols crossing the blood brain barrier appear like promising strategies to either delay or counteract the development of sporadic AD

Mutating the cholesterol binding site on amyloid precursor protein alters the processing and production of Aβ-amyloid peptides (Oral)

International audienc

Specific Mutations in the Cholesterol-Binding Site of APP Alter Its Processing and Favor the Production of Shorter, Less Toxic Aβ Peptides

International audienceAbstract Excess brain cholesterol is strongly implicated in the pathogenesis of Alzheimer’s disease (AD). Here we evaluated how the presence of a cholesterol-binding site (CBS) in the transmembrane and juxtamembrane regions of the amyloid precursor protein (APP) regulates its processing. We generated nine point mutations in the APP gene, changing the charge and/or hydrophobicity of the amino-acids which were previously shown as part of the CBS. Most mutations triggered a reduction of amyloid-β peptides Aβ40 and Aβ42 secretion from transiently transfected HEK293T cells. Only the mutations at position 28 of Aβ in the APP sequence resulted in a concomitant significant increase in the production of shorter Aβ peptides. Mass spectrometry (MS) confirmed the predominance of Aβx-33 and Aβx-34 with the APP K28A mutant. The enzymatic activity of α-, β-, and γ-secretases remained unchanged in cells expressing all mutants. Similarly, subcellular localization of the mutants in early endosomes did not differ from the APP WT protein. A transient increase of plasma membrane cholesterol enhanced the production of Aβ40 and Aβ42 by APP WT , an effect absent in APP K28A mutant. Finally, WT but not CBS mutant Aβ derived peptides bound to cholesterol-rich exosomes. Collectively, the present data revealed a major role of juxtamembrane amino acids of the APP CBS in modulating the production of toxic Aβ species. More generally, they underpin the role of cholesterol in the pathophysiology of AD

Increasing membrane cholesterol of neurons in culture recapitulates Alzheimer’s disease early phenotypes

It is suspected that excess of brain cholesterol plays a role in Alzheimer's disease (AD). Membrane-associated cholesterol was shown to be increased in the brain of individuals with sporadic AD and to correlate with the severity of the disease. We hypothesized that an increase of membrane cholesterol could trigger sporadic AD early phenotypes