Alzheimer's disease-associated ubiquilin-1 regulates presenilin-1 accumulation and aggresome formation

The Alzheimer's disease (AD)-associated ubiquilin-1 regulates proteasomal degradation of proteins, including presenilin (PS). PS-dependent γ-secretase generates β-amyloid (Aβ) peptides, which excessively accumulate in AD brain. Here we have characterized the effects of naturally occurring ubiquilin-1 transcript variants (TV) on the levels and subcellular localization of PS1 and other γ-secretase complex components and subsequent γ-secretase function in human embryonic kidney 293, human neuroblastoma SH-SY5Y, and mouse primary cortical cells. Full-length ubiquilin-1 TV1 and TV3 that lacks the proteasome-interaction domain, increased full-length PS1 levels as well as induced accumulation of high-molecular-weight PS1 and aggresome formation. Accumulated PS1 co-localized with TV1 or TV3 in the aggresomes. Electron microscopy indicated that aggresomes containing TV1 or TV3 were targeted to autophagosomes. TV1- and TV3-expressing cells did not accumulate other unrelated proteasome substrates, suggesting that the increase in PS1 levels was not due to a general impairment of the ubiquitin-proteasome system. Furthermore, PS1 accumulation and aggresome formation coincided with alterations in Aβ levels particularly in cells over-expressing TV3. These effects were not related to altered γ-secretase activity or PS1 binding to TV3. Collectively, our results indicate that specific ubiquilin-1 TVs can cause PS1 accumulation and aggresome formation, which may impact AD pathogenesis or susceptibility.