Genetic association of ubiquilin with Alzheimer's disease and related quantitative measures

109

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.

Section: Discussionmentioning

confidence: 99%

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confidence: 99%

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confidence: 99%

See 1 more Smart Citation

Ubiquilin-1 Is a Molecular Chaperone for the Amyloid Precursor Protein

Stieren

Ayadi

Xiao

et al. 2011

109

“…As is often the case with gene variants exerting modest effects on disease risk, subsequent genetic studies have both supported (9,10) and not supported (11,12) the initial genetic finding. Further support for the candidacy of UBQLN1 as an AD risk gene will require a comprehensive assessment of the functional role of UBQLN1 in the key pathways relevant to AD pathogenesis.…”

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confidence: 98%

Ubiquilin 1 Modulates Amyloid Precursor Protein Trafficking and Aβ Secretion

Hiltunen

Thomas

et al. 2006

118

Ubiquilin 1 (UBQLN1) is a ubiquitin-like protein, which has been shown to play a central role in regulating the proteasomal degradation of various proteins, including the presenilins. We recently reported that DNA variants in UBQLN1 increase the risk for Alzheimer disease, by influencing expression of this gene in brain. Here we present the first assessment of the effects of UBQLN1 on the metabolism of the amyloid precursor protein (APP). For this purpose, we employed RNA interference to down-regulate UBQLN1 in a variety of neuronal and non-neuronal cell lines. We demonstrate that down-regulation of UBQLN1 accelerates the maturation and intracellular trafficking of APP, while not interfering with ␣-, ␤-, or ␥-secretase levels or activity. UBQLN1 knockdown increased the ratio of APP mature/immature, increased levels of full-length APP on the cell surface, and enhanced the secretion of sAPP (␣-and ␤-forms). Moreover, UBQLN1 knockdown increased levels of secreted A␤40 and A␤42. Finally, employing a fluorescence resonance energy transfer-based assay, we show that UBQLN1 and APP come into close proximity in intact cells, independently of the presence of the presenilins. Collectively, our findings suggest that UBQLN1 may normally serve as a cytoplasmic "gatekeeper" that may control APP trafficking from intracellular compartments to the cell surface. These findings suggest that changes in UBQLN1 steady-state levels affect APP trafficking and processing, thereby influencing the generation of A␤. Alzheimer disease (AD)3 is the most common cause of progressive neurological disorder leading to dementia. It is neuropathologically characterized by extracellular deposits of amyloid beta (A␤) peptide and by the generation of intracellular neurofibrillary tangles. Mutations in the amyloid precursor protein (APP), presenilin-1 (PSEN1), and presenilin-2 (PSEN2) genes are responsible for roughly half of the rare autosomal dominant, early-onset forms of the disease, which usually occur before the age of 60 (1-4). Meanwhile, apolipoprotein E (APOE) is the only commonly accepted susceptibility factor for late-onset AD (5, 6). Most mutations in APP, PSEN1, and PSEN2 genes lead to the increased production of A␤42 (relative to A␤40). A␤ is released from APP via sequential proteolytic cleavage by the ␤-and ␥-secretases (7). In addition to APP, the presenilins, and APOE, it is evident that additional AD susceptibility genes exist; successfully identifying these novel risk genes is an extremely important task that will not only facilitate prediction and diagnosis of AD but can also elucidate novel therapeutic approaches to treating and preventing AD.We have recently shown that genetic variants in the ubiquilin 1 (UBQLN1) gene, located on chromosome 9q22, increase the risk for AD, possibly by altering the expression and alternative splicing of this gene in brain (8). As is often the case with gene variants exerting modest effects on disease risk, subsequent genetic studies have both supported (9, 10) and not supported (11, 12) the initial g...

“…In a search for presenilin interactors, ubiquilin 1 was identified using yeast two-hybrid, colocalization and glutathione S-transferase pull-down assays (14). Moreover, it has recently been suggested that genetic variants in UBQLN1 may be associated with an increased risk of AD (18,19,20), although other studies could not confirm the association (16,17). Very little is known so far about the function of ubiquilin 1 in the brain.…”

Section: Tablementioning

confidence: 99%

Interaction between Presenilin 1 and Ubiquilin 1 as Detected by Fluorescence Lifetime Imaging Microscopy and a High-throughput Fluorescent Plate Reader

Thomas

Herl

Spoelgen

et al. 2006

Presenilin 1 (PS1) in its active heterodimeric form is the catalytic center of the ␥-secretase complex, an enzymatic activity that cleaves amyloid precursor protein (APP) to produce amyloid ␤ (A␤). Ubiquilin 1 is a recently described PS1 interacting protein, the overexpression of which increases PS1 holoprotein levels and leads to reduced levels of functionally active PS1 heterodimer. In addition, it has been suggested that splice variants of the UBQLN1 gene are associated with an increased risk of developing Alzheimer disease (AD). However, it is still unclear whether PS1 and ubiquilin 1 interact when expressed at endogenous levels under normal physiological conditions. Here, we employ three novel fluorescence resonance energy transferbased techniques to investigate the interaction between PS1 and ubiquilin 1 in intact cells. We consistently find that the ubiquilin 1 N terminus is in close proximity to several epitopes on PS1. We show that ubiquilin 1 interacts both with PS1 holoprotein and heterodimer and that the interaction between PS1 and ubiquilin 1 takes place near the cell surface. Furthermore, we show that the PS1-ubiquilin 1 interaction can be detected between endogenous proteins in primary neurons in vitro as well as in brain tissue of healthy controls and Alzheimer disease patients, providing evidence of its physiological relevance.