Interplay between autophagy and amyloid beta metabolism in Alzheimer’s disease

Abstract

Alzheimer’s disease (AD), the most common neurodegenerative disease, is characterized by two pathological hallmarks: extracellular amyloid-beta peptide (Aβ) plaque depositions and neurofibrillary tangles (NFTs) composed of intracellular hyperphosphorylated tau aggregations. A cellular degradation system, autophagy is additionally dysfunctional in AD and plays a key role in Aβ and tau metabolisms.

Paper I and II. To investigate the molecular mechanisms of autophagy alterations induced by Aβ amyloidosis, we characterized the autophagy status in amyloid precursor protein (App) knock-in mouse models exhibiting robust Aβ pathology. Interestingly, impaired autophagy was a general phenomenon in the brains which was specifically pronounced in the surrounding regions of Aβ plaques, especially in neurites and pre-synapses in App knock-in mice. The region-specific autophagy impairment was substantiated by electron microscopy imaging showing autophagic vacuole accumulation in dystrophic neurites around Aβ plaques. Time course bulk RNA sequencing from hippocampi of App knock-in mouse brains further revealed alterations of autophagy-associated gene expression.

Paper III. Cerebrospinal fluid (CSF) is an important body fluid source to study brain-derived biomarkers for AD diagnosis. Comparing the CSF proteomes from App knock-in mice and AD human subjects revealed an extracellular matrix protein, decorin, as significantly increased in preclinical AD subjects having abnormal CSF-Aβ42 but normal CSF-total-tau (a+t-) levels and in AppNL-F mice exhibiting mild Aβ pathology. In a+t- preclinical AD subjects, CSF-decorin levels positively correlated with CSF-Aβ42 levels and negatively correlated with CSF phosphorylated and total tau levels. Increase of CSF-decorin could predict an AD subtype having innate immune activation and potential choroid plexus dysfunction in the brain with high sensitivity and specificity. Consistently, increased CSFdecorin in AppNL-F mice correlated with the decorin levels in choroid plexus and Aβ plaque load.

Paper IV. To directly investigate the role of autophagy in Aβ metabolism, we generated autophagy-deficient AD mouse models by crossing App knock-in mice with autophagyrelated gene 7 (Atg7) conditional knockout mice. Loss of autophagy in excitatory neurons lowered Aβ plaque load but raised intracellular Aβ levels. Severe Aβ pathology together with lack of autophagy led to an autistic-like behavior, decreased anxiety and memory deficits which potentially was related to activated programmed cell death, synaptic impairment, and degraded gamma oscillation power. However, a mild Aβ amyloidosis in autophagy-deficient AppNL-F mice ameliorated the autistic-like behavior driven by loss of autophagy. Notably, proteomic analysis of CA1 pyramidal cell layer in hippocampus unveiled that loss of autophagy upregulated cell transport but downregulated protein translation which can be alleviated by a mild Aβ amyloidosis.

Paper V. Limitations of App knock-in mice include a less pronounced tau pathology and lack of neurodegeneration. Generation of an App knock-in rat model, AppNL-G-F, circumvented above mentioned shortages by inducing phosphorylated tau aggregations and neuronal loss. AppNL-G-F rats additionally exhibited enhanced gliosis, impaired spatial learning, and memory deficits including episodic-like memory.