Alzheimer disease (AD) is associated with extracellular deposition of proteolytic fragments of amyloid precursor protein (APP). Although mutations in APP and proteases that mediate its processing are known to result in familial, early onset forms of AD, the mechanisms underlying the more common sporadic, yet genetically complex forms of the disease are still unclear. Four single-nucleotide polymorphisms within the ubiquilin-1 gene have been shown to be genetically associated with AD, implicating its gene product in the pathogenesis of late onset AD. However, genetic linkage between ubiquilin-1 and AD has not been confirmed in studies examining different populations. Here we show that regardless of genotype, ubiquilin-1 protein levels are significantly decreased in late onset AD patient brains, suggesting that diminished ubiquilin function may be a common denominator in AD progression. Our interrogation of putative ubiquilin-1 activities based on sequence similarities to proteins involved in cellular quality control showed that ubiquilin-1 can be biochemically defined as a bona fide molecular chaperone and that this activity is capable of preventing the aggregation of amyloid precursor protein both in vitro and in live neurons. Furthermore, we show that reduced activity of ubiquilin-1 results in augmented production of pathogenic amyloid precursor protein fragments as well as increased neuronal death. Our results support the notion that ubiquilin-1 chaperone activity is necessary to regulate the production of APP and its fragments and that diminished ubiquilin-1 levels may contribute to AD pathogenesis.

Fas receptor is a member of the tumor necrosis factor-α family of death receptors that mediate physiologic apoptotic signaling. To investigate the molecular mechanisms regulating calcium mobilization during Fas-mediated apoptosis, we have analyzed the sequential steps leading to altered calcium homeostasis and cell death in response to activation of the Fas receptor. We show that Fas-mediated apoptosis requires endoplasmic reticulum–mediated calcium release in a mechanism dependent on phospholipase C-γ1 (PLC-γ1) activation and Ca2+ release from inositol 1,4,5-trisphosphate receptor (IP3R) channels. The kinetics of Ca2+ release were biphasic, demonstrating a rapid elevation caused by PLC-γ1 activation and a delayed and sustained increase caused by cytochrome c binding to IP3R. Blocking either phase of Ca2+ mobilization was cytoprotective, highlighting PLC-γ1 and IP3R as possible therapeutic targets for disorders associated with Fas signaling.

The pathogenesis of Alzheimer's disease (AD) is associated with proteolytic processing of the amyloid precursor protein (APP) to an amyloidogenic peptide termed Aβ. Although mutations in APP and the secretase enzymes that mediate its processing are known to result in familial forms of AD, the mechanisms underlying the more common sporadic forms of the disease are still unclear. Evidence suggests that the susceptibility of APP to amyloidogenic processing is related to its intracellular localization, and that secretase-independent degradation may prevent the formation of cytotoxic peptide fragments. Recently, single nucleotide polymorphisms in the UBQLN1 gene have been linked to late-onset AD, and its protein product, ubiquilin-1, may regulate the maturation of full-length APP. Here we show that ubiquilin-1 inhibits the maturation of APP by sequestering it in the early secretory pathway, primarily within the Golgi apparatus. This sequestration significantly delayed the proteolytic processing of APP by secretases and the proteasome. These effects were mediated by ubiquilin-1-stimulated K63-linked polyubiquitination of lysine 688 in the APP intracellular domain. Our results reveal the mechanistic basis by which ubiquilin-1 regulates APP maturation, with important consequences for the pathogenesis of late-onset AD.A myloid precursor protein (APP) is a type I transmembrane protein that matures in the secretory pathway, where it undergoes classical N-and O-linked glycosylation during transit through the endoplasmic reticulum (ER) and Golgi, respectively (1, 2). Though most APP is degraded by lysosomes (3), a small portion reaches the cell surface to be cleaved by the secretases (4). Amyloidogenic processing of APP occurs by sequential processing by β-and γ-secretase to release Aβ peptide and the APP intracellular domain (AICD) (5). APP half-life is short, and thus the amount of APP detected at the cell surface is very low (6). The rapid internalization rate of APP is due to the presence of the highly conserved YENPTY (single amino acid code) sequence in the APP cytoplasmic domain, which contains a canonical NPxY internalization signal for clathrin-mediated endocytosis (7). Proteolytic processing of APP has been shown to occur in various sites throughout the secretory and endocytic pathways. However, amyloidogenic processing primarily occurs after transition through the Golgi apparatus (8) and in endosomal compartments, where acidic conditions promote optimal activity of the beta-site APP cleaving enzyme (BACE) or β-secretase (9). Deletion or mutation of the YENPTY internalization signal leads to an increase in plasma membrane-associated APP and a significant decrease in Aβ production, underscoring the importance of endocytic recycling for Aβ generation (5, 7, 10). Various cytoplasmic adaptor proteins have been shown to interact with the YENPTY motif and regulate sorting of APP to different cell compartments and thus regulate its proteolytic processing. The protein SorLA or LR11 (11) acts as a sorting receptor that trap...

Single nucleotide polymorphisms in the ubiquilin-1 gene may confer risk for late-onset Alzheimer disease (AD). We have shown previously that ubiquilin-1 functions as a molecular chaperone for the amyloid precursor protein (APP) and that protein levels of ubiquilin-1 are decreased in the brains of AD patients. We have recently found that ubiquilin-1 regulates APP trafficking and subsequent secretase processing by stimulating non-degradative ubiquitination of a single lysine residue in the cytosolic domain of APP. Thus, ubiquilin-1 plays a central role in regulating APP biosynthesis, trafficking and ultimately toxicity. As ubiquilin-1 and other ubiquilin family members have now been implicated in the pathogenesis of numerous neurodegenerative diseases, these findings provide mechanistic insights into the central role of ubiquilin proteins in maintaining neuronal proteostasis.