Ubiquilin-1 regulates amyloid precursor protein maturation and degradation by stimulating K63-linked polyubiquitination of lysine 688

Abstract

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...

F-box Only Protein 2 (Fbxo2) Regulates Amyloid Precursor Protein Levels and Processing

“…If APP retains high mannose forms of N-linked glycans on the cell surface, then Fbxo2 could play a role in regulating the endocytosis of APP through ubiquitination. An effect on differential trafficking of APP, namely, the retention of APP in specific cellular compartments, has been attributed to ubiquilin 1 (UBQLN1): polyubiquitination by UBQLN1 sequesters APP in the Golgi (38). Given this observation, we examined the relative distribution of APP in ER and Golgi of neurons in Fbxo2 Ϫ/Ϫ and wildtype mice and found no differences in the pattern of expression (data not shown).…”

Targeting ubiquilin-1 in Alzheimer's disease

“…This prevented APP maturation and trafficking to the plasma membrane and subsequent proteolytic processing. Ubiquilin-1 was shown to mediate these effects by stimulating K63-linked polyubiquitination of APP-Lys688 [21]. In summary, these in vitro data altogether suggest that ubiquilin-1 functions as an important molecular chaperone for APP, which regulates APP trafficking, maturation, levels and processing.…”

“…The reasons for these contradicting findings are not completely clear, but one explanation could be that ubiquilin-1-mediated regulation of APP may be different in diverse cell types. Nevertheless, recent studies have confirmed that ubiquilin-1 influences Ab production by directly interacting with APP rather than by modulating secretase function [20,21], further underscoring an important role for ubiquilin-1 in the regulation of APP levels. The recent interesting pieces of data demonstrate that ubiquilin-1 levels are diminished in human AD brain and that RNAi-mediated knockdown of ubiquilin-1 resulted in enhanced production of Ab and subsequently increased neuronal cell death, substantiating the initial findings [20,21].…”

“…Thus, TV4 lacks the C-terminal UBA domain and a large part of the Asn-Prorich repeats, which may prevent its interactions with polyubiquitinated and other specific proteins [15,27]. However, further research in vitro and in vivo is needed to characterize the [19][20][21]33,39,40] BCLb BCL2-like anti-apoptotic protein, associated with several cancers…”

“…UBQLN1 encodes ubiquilin-1 protein (also known as PLIC-1; protein linking IAP [integrin-associated protein] with cytoskeleton-1), which has been shown to regulate the subcellular trafficking, function, the levels and degradation of several other proteins (Table 1). Interestingly, ubiquilin-1 regulates the function and subcellular trafficking of APP and PS, suggesting that ubiquilin-1 could play a role in AD pathogenesis at the mechanistic level [19][20][21][22][23][24]. Moreover, ubiquilin-1 has been shown to interact with and regulate proteins implicated in the pathogenesis of other neurodegenerative disorders.…”