microRNA-33 Regulates ApoE Lipidation and Amyloid-ß Metabolism in the Brain.

Dysregulation of amyloid-ß (Aß) metabolism is critical for Alzheimer's disease (AD) pathogenesis. Mounting evidence suggests that apolipoprotein E (ApoE) is involved in Aß metabolism. ATP-binding cassette transporter A1 (ABCA1) is a key regulator of ApoE lipidation, which affects Aß levels. Therefore, identifying regulatory mechanisms of ABCA1 expression in the brain may provide new therapeutic targets for AD. Here, we demonstrate that microRNA-33 (miR-33) regulates ABCA1 and Aß levels in the brain. Overexpression of miR-33 impaired cellular cholesterol efflux and dramatically increased extracellular Aß levels by promoting Aß secretion and impairing Aß clearance in neural cells. In contrast, genetic deletion of mir-33 in mice dramatically increased ABCA1 levels and ApoE lipidation, but it decreased endogenous Aß levels in cortex. Most importantly, pharmacological inhibition of miR-33 via antisense oligonucleotide specifically in the brain markedly decreased Aß levels in cortex of APP/PS1 mice, representing a potential therapeutic strategy for AD. SIGNIFICANCE STATEMENT: Brain lipid metabolism, in particular Apolipoprotein E (ApoE) lipidation, is critical to Aß metabolism and Alzheimer's disease (AD). Brain lipid metabolism is largely separated from the periphery due to blood-brain barrier and different repertoire of lipoproteins. Therefore, identifying the novel regulatory mechanism of brain lipid metabolism may provide a new therapeutic strategy for AD. Although there have been studies on brain lipid metabolism, its regulation, in particular by microRNAs, is relatively unknown. Here, we demonstrate that inhibition of microRNA-33 increases lipidation of brain ApoE and reduces Aß levels by inducing ABCA1. We provide a unique approach for AD therapeutics to increase ApoE lipidation and reduce Aß levels via pharmacological inhibition of microRNA in vivo.

Enhancing astrocytic lysosome biogenesis facilitates Aß clearance and attenuates amyloid plaque pathogenesis.

In sporadic Alzheimer's disease (AD), impaired Aß removal contributes to elevated extracellular Aß levels that drive amyloid plaque pathogenesis. Extracellular proteolysis, export across the blood-brain barrier, and cellular uptake facilitate physiologic Aß clearance. Astrocytes can take up and degrade Aß, but it remains unclear whether this function is insufficient in AD or can be enhanced to accelerate Aß removal. Additionally, age-related dysfunction of lysosomes, the major degradative organelles wherein Aß localizes after uptake, has been implicated in amyloid plaque pathogenesis. We tested the hypothesis that enhancing lysosomal function in astrocytes with transcription factor EB (TFEB), a master regulator of lysosome biogenesis, would promote Aß uptake and catabolism and attenuate plaque pathogenesis. Exogenous TFEB localized to the nucleus with transcriptional induction of lysosomal biogenesis and function in vitro. This resulted in significantly accelerated uptake of exogenously applied Aß42, with increased localization to and degradation within lysosomes in C17.2 cells and primary astrocytes, indicating that TFEB is sufficient to coordinately enhance uptake, trafficking, and degradation of Aß. Stereotactic injection of adeno-associated viral particles carrying TFEB driven by a glial fibrillary acidic protein promoter was used to achieve astrocyte-specific expression in the hippocampus of APP/PS1 transgenic mice. Exogenous TFEB localized to astrocyte nuclei and enhanced lysosome function, resulting in reduced Aß levels and shortened half-life in the brain interstitial fluid and reduced amyloid plaque load in the hippocampus compared with control virus-injected mice. Therefore, activation of TFEB in astrocytes is an effective strategy to restore adequate Aß removal and counter amyloid plaque pathogenesis in AD.

In vivo measurement of apolipoprotein E from the brain interstitial fluid using microdialysis.

BACKGROUND: The APOE4 allele variant is the strongest known genetic risk factor for developing late-onset Alzheimer's disease. The link between apolipoprotein E (apoE) and Alzheimer's disease is likely due in large part to the impact of apoE on the metabolism of amyloid ß (Aß) within the brain. Manipulation of apoE levels and lipidation within the brain has been proposed as a therapeutic target for the treatment of Alzheimer's disease. However, we know little about the dynamic regulation of apoE levels and lipidation within the central nervous system. We have developed an assay to measure apoE levels in the brain interstitial fluid of awake and freely moving mice using large molecular weight cut-off microdialysis probes. RESULTS: We were able to recover apoE using microdialysis from human cerebrospinal fluid (CSF) in vitro and mouse brain parenchyma in vivo. Microdialysis probes were inserted into the hippocampus of wild-type mice and interstitial fluid was collected for 36 hours. Levels of apoE within the microdialysis samples were determined by ELISA. The levels of apoE were found to be relatively stable over 36 hours. No apoE was detected in microdialysis samples from apoE KO mice. Administration of the RXR agonist bexarotene increased ISF apoE levels while ISF Aß levels were decreased. Extrapolation to zero-flow analysis allowed us to determine the absolute recoverable concentration of apoE3 in the brain ISF of apoE3 KI mice. Furthermore, analysis of microdialysis samples by non-denaturing gel electrophoresis determined lipidated apoE particles in microdialysis samples were consistent in size with apoE particles from CSF. Finally, we found that the concentration of apoE in the brain ISF was dependent upon apoE isoform in human apoE KI mice, following the pattern apoE2>apoE3>apoE4. CONCLUSIONS: We are able to collect lipidated apoE from the brain of awake and freely moving mice and monitor apoE levels over the course of several hours from a single mouse. Our technique enables assessment of brain apoE dynamics under physiological and pathophysiological conditions and in response to therapeutic interventions designed to affect apoE levels and lipidation within the brain.