A Becn1 mutation mediates hyperactive autophagic sequestration of amyloid oligomers and improved cognition in Alzheimer's disease

Abstract

Impairment of the autophagy pathway has been observed during the pathogenesis of Alzheimer’s disease (AD), a neurodegenerative disorder characterized by abnormal deposition of extracellular and intracellular amyloid β (Aβ) peptides. Yet the role of autophagy in Aβ production and AD progression is complex. To study whether increased basal autophagy plays a beneficial role in Aβ clearance and cognitive improvement, we developed a novel genetic model to hyperactivate autophagy in vivo. We found that knock-in of a point mutation F121A in the essential autophagy gene Beclin 1/ Becn1 in mice significantly reduces the interaction of BECN1 with its inhibitor BCL2, and thus leads to constitutively active autophagy even under non-autophagy-inducing conditions in multiple tissues, including brain. Becn1 ^F121A-mediated autophagy hyperactivation significantly decreases amyloid accumulation, prevents cognitive decline, and restores survival in AD mouse models. Using an immunoisolation method, we found biochemically that Aβ oligomers are autophagic substrates and sequestered inside autophagosomes in the brain of autophagy-hyperactive AD mice. In addition to genetic activation of autophagy by Becn1 gain-of-function, we also found that ML246, a small-molecule autophagy inducer, as well as voluntary exercise, a physiological autophagy inducer, exert similar Becn1-dependent protective effects on Aβ removal and memory in AD mice. Taken together, these results demonstrate that genetically disrupting BECN1-BCL2 binding hyperactivates autophagy in vivo, which sequestrates amyloid oligomers and prevents AD progression. The study establishes new approaches to activate autophagy in the brain, and reveals the important function of Becn1-mediated autophagy hyperactivation in the prevention of AD.

Author summary

Accumulation of amyloid β peptides (Aβ) is a major cause of Alzheimer’s disease (AD). Although many efforts have been made, no effective therapies are available to cure AD. Autophagy is a stress-induced pathway nerve cells use to dispose damaged structures, and may be a strategy to eliminate Aβ aggregation. However, direct evidence of autophagic disposal of Aβ is lacking. Here we described a new AD mouse model that shows high autophagy activity even under non-autophagy-inducing conditions. In these mice, we engineered a single mutation into a key autophagy gene Becn1, which disrupts an inhibitory binding and leads to constitutively active autophagy in brain. Compared to regular AD mice, the autophagy-hyperactive AD mice are protected from amyloid accumulation, memory deficits and high mortality. We also isolated autophagic vesicle from these mice by an antibody, and found that Aβ oligomers are incorporated inside the vesicles. Thus, we obtained direct evidence for the first time that oligomerized amyloids are substrates of autophagy. These findings are important, as we demonstrated the reversal of AD progression by modulating the activity of a single autophagy gene. Our findings also revealed the therapeutic potential of brain-permeable autophagy-inducing chemicals in the prevention of AD by reducing intracellular amyloids.