Methylation differences in Alzheimer's disease neuropathologic change in the aged human brain

Alzheimer's disease (AD) is the most common cause of dementia with advancing age as its strongest risk factor. AD neuropathologic change (ADNC) is known to be associated with numerous DNA methylation changes in the human brain, but the oldest old (> 90 years) have so far been underrepresented in epigenetic studies of ADNC. Our study participants were individuals aged over 90 years (n = 47) from The 90+ Study. We analyzed DNA methylation from bulk samples in eight precisely dissected regions of the human brain: middle frontal gyrus, cingulate gyrus, entorhinal cortex, dentate gyrus, CA1, substantia nigra, locus coeruleus and cerebellar cortex. We deconvolved our bulk data into cell-type-specific (CTS) signals using computational methods. CTS methylation differences were analyzed across different levels of ADNC. The highest amount of ADNC related methylation differences was found in the dentate gyrus, a region that has so far been underrepresented in large scale multi-omic studies. In neurons of the dentate gyrus, DNA methylation significantly differed with increased burden of amyloid beta (Aβ) plaques at 5897 promoter regions of protein-coding genes. Amongst these, higher Aβ plaque burden was associated with promoter hypomethylation of the Presenilin enhancer 2 (PEN-2) gene, one of the rate limiting genes in the formation of gamma-secretase, a multicomponent complex that is responsible in part for the endoproteolytic cleavage of amyloid precursor protein into Aβ peptides. In addition to novel ADNC related DNA methylation changes, we present the most detailed array-based methylation survey of the old aged human brain to date. Our open-sourced dataset can serve as a brain region reference panel for future studies and help advance research in aging and neurodegenerative diseases.

Visualization of specific γ-secretase complexes using bimolecular fluorescence complementation

γ-Secretase is involved in the regulated intramembrane proteolysis of amyloid-β protein precursor (AβPP) and of many other important physiological substrates. γ-secretase is a multiproteic complex made of four main core components, namely presenilin 1 or 2, APH-1, PEN-2, and Nicastrin. Since APH-1 exists as different variants, combinations of these proteins can theoretically yield distinct γ-secretase complexes. Whether γ-secretase complexes trafficking and targeting to either similar or distinct subcellular compartments depend upon their molecular composition remains unknown. A differential complex-specific distribution may drive a narrow specificity for a subset of substrates that would traffic within the same cellular compartments. Here, we generated bigenic expression vectors to co-express untagged nicastrin or presenilin 1 together with either PEN-2 or distinct variants of APH-1 (aL, aS and b) tagged with complementary fragments of the fluorescent protein Venus. We show that these constructs allow the formation of functional γ-secretase complexes and their visualization with bimolecular fluorescence complementation (BiFC). BiFC can be detected at the plasma membrane as well as in endosomes/lysosomes in addition to the endoplasmic reticulum (ER) of COS-7 cells transfected with the different variants of APH-1. However, the majority of cells co-transfected with APH-1b presented BiFC signal only in the ER, suggesting enhanced retention/retrieval of APH-1b-containing γ-secretase complexes. Therefore, the new tools described here should be helpful to decipher the precise subcellular trafficking of γ-secretase complexes and to delineate the distinct variant-linked pathways in various cellular systems.

General anesthetics and β-amyloid protein

With roughly 234 million people undergoing surgery with anesthesia each year worldwide, it is important to determine whether commonly used anesthetics can induce any neurotoxicity. Alzheimer's disease (AD) is the most common form of age-related dementia, and a rapidly growing health problem. Several studies suggest that anesthesia could be associated with the development of AD. Moreover, studies in cultured cells and animals show that commonly used inhalation anesthetics may induce changes consistent with AD neuropathogenesis, e.g., β-amyloid protein accumulation. Therefore, in this mini review, we focus on the recent research investigating the effects of commonly used anesthetics including isoflurane, sevoflurane, desflurane, nitrous oxide, and propofol, on Aβ accumulation in vitro and in vivo. We further discuss the future direction of the research determining the effects of anesthetics on β-amyloid protein accumulation.

Ferritin light chain interacts with PEN-2 and affects γ-secretase activity

Alzheimer's disease (AD) is primarily caused by overproduction/deposition of β-amyloid (Aβ) in the brain. Dysregulation of iron in the brain also contributes to AD. Although iron affects β-amyloid precursor protein (APP) expression and Aβ deposition, detailed role of iron in AD requires further elucidation. Aβ is produced by sequential proteolytic cleavages of APP by β-secretase and γ-secretase. The γ-secretase complex comprises presenilins (PS1 or PS2), nicastrin, APH-1, and PEN-2. Herein, we find that PEN-2 can interact with ferritin light chain (FTL), an important component of the iron storage protein ferritin. In addition, we show that overexpression of FTL increases the protein levels of PEN-2 and PS1 amino-terminal fragment (NTF) and promotes γ-secretase activity for more production of Aβ and notch intracellular domain (NICD). Furthermore, iron treatments increase the levels of FTL, PEN-2 and PS1 NTF and promote γ-secretase-mediated NICD production. Moreover, downregulation of FTL decreases the levels of PEN-2 and PS1 NTF. Together, our results suggest that iron can increase γ-secretase activity through promoting the level of FTL that interacts with and stabilizes PEN-2, providing a new molecular link between iron, PEN-2/γ-secretase and Aβ generation in AD.