Neuronal Rubicon Represses Extracellular APP/Amyloid β Deposition in Alzheimer’s Disease

Abstract

Alzheimer’s disease (AD) is the most prevalent age-associated neurodegenerative disease. A decrease in autophagy during aging contributes to brain disorders by accumulating potentially toxic substrates in neurons. Rubicon is a well-established inhibitor of autophagy in all cells. However, Rubicon participates in different pathways depending on cell type, and little information is currently available on neuronal Rubicon’s role in the AD context. Here, we investigated the cell-specific expression of Rubicon in postmortem brain samples from AD patients and 5xFAD mice and its impact on amyloid β burden in vivo and neuroblastoma cells. Further, we assessed Rubicon levels in human-induced pluripotent stem cells (hiPSCs), derived from early-to-moderate AD and in postmortem samples from severe AD patients. We found increased Rubicon levels in AD-hiPSCs and postmortem samples and a notable Rubicon localization in neurons. In AD transgenic mice lacking Rubicon, we observed intensified amyloid β burden in the hippocampus and decreased Pacer and p62 levels. In APP-expressing neuroblastoma cells, increased APP/amyloid β secretion in the medium was found when Rubicon was absent, which was not observed in cells depleted of Atg5, essential for autophagy, or Rab27a, required for exosome secretion. Our results propose an uncharacterized role of Rubicon on APP/amyloid β homeostasis, in which neuronal Rubicon is a repressor of APP/amyloid β secretion, defining a new way to target AD and other similar diseases therapeutically.

The autophagy protein Def8 is altered in Alzheimer's disease and Aβ42-expressing Drosophila brains

“…On the other hand, Rubicon loss of function in neurons improved lifespan in female flies 48 , which confirms the importance of autophagy activity to CNS since Rubicon is one of the primary inhibitors of the autophagy machinery. Notably, we recently reported increased Rubicon levels in postmortem brain tissues from AD patients, which could help explain a patient's predisposition to the disease 25 . Hence, the progressive decline in Def8 brain levels during aging in flies and humans and the detrimental effects under its neuronal downregulation could contribute to the aging predisposition to neurodegenerative disorders by affecting homeostatic routes such as autophagy.…”

“…10 µm postmortem brain samples were analyzed using the procedures described in 24 , 25 . Briefly, the slides were deparaffinized with xylol and hydrated with descending alcohol concentrations until reaching distilled water.…”