P2‐079: Neutrophin‐trafficking deficits are mediated by impaired ubiquitin recycling in Alzheimer's disease

Abstract

We previously found that Aβ interferes with the BDNF-mediated PI3K cascade, a pathway implicated in retrograde trafficking. Amyloid accumulation also inhibits the ubiquitin-proteosome system, leading to reduced TrkB degradation. It does so by impairing the deubiquinating enzymes necessary for sorting to late endosomes/MVBs and subsequently, the lysosome. We hypothesized that deficits in the ubiquitin-proteosome pathway that exist in AD affect not just receptor degradation, but also sorting and transport of the BDNF-mediated TrkB retrograde signal. Previously, we established that indeed Aβ impairs retrograde transport of TrkB signaling and thus contributing to impaired neuronal survival. Here, we aimed to test the hypothesis that the impaired signaling was due to defects in receptor sorting and trafficking. Utilizing a novel microfluidic culture chamber, we assessed axonal transport using time-lapse microscopy in primary neuronal cultures. Neurotrophin downstream signaling was assessed using CRE-GFP and immunocytochemistry. Cell-surface biotinylation assays were employed to assess receptor internalization and endosomal sorting. We demonstrate BDNF retrograde signaling deficits are a result of amyloid oligomers. The oligomers affect both the transport velocity as well as the sorting of the receptor to signaling endosomes. Impaired transport leads to decreased ERK activation and also decreased CREB-mediated signaling. We also found that oligomers reduced UCH-L1, levels in primary neurons. We hypothesize that decreasing UCH-L1 levels or acitivty reduces the cytosolic pool of ubiquitin impairing ubiquitin-mediated processes. Using a transducible method to overexpress UCH-L1 in primary neurons, we found that we could rescue both deficits in TrkB endosomal sorting as well as axonal trafficking deficits. Amyloid impairs neurotrophin signaling including axonal trafficking and thus impairs neuronal survival. This is mediated through the ubiquitin-proteosome system deficits which affect receptor internalization/sorting and retrograde trafficking. Thus, therapeutics aimed at restoring ubiquitin proteosome function in AD likely improves neuronal survival.