Neprilysin-2 is an important ß-amyloid degrading enzyme.

Proteases that degrade the amyloid-ß peptide (Aß) are important in protecting against Alzheimer's disease (AD), and understanding these proteases is critical to understanding AD pathology. Endopeptidases sensitive to inhibition by thiorphan and phosphoramidon are especially important, because these inhibitors induce dramatic Aß accumulation (∼30- to 50-fold) and pathological deposition in rodents. The Aß-degrading enzyme neprilysin (NEP) is the best known target of these inhibitors. However, genetic ablation of NEP results in only modest increases (∼1.5- to 2-fold) in Aß, indicating that other thiorphan/phosphoramidon-sensitive endopeptidases are at work. Of particular interest is the NEP homolog neprilysin 2 (NEP2), which is thiorphan/phosphoramidon-sensitive and degrades Aß. We investigated the role of NEP2 in Aß degradation in vivo through the use of gene knockout and transgenic mice. Mice deficient for the NEP2 gene showed significant elevations in total Aß species in the hippocampus and brainstem/diencephalon (∼1.5-fold). Increases in Aß accumulation were more dramatic in NEP2 knockout mice crossbred with APP transgenic mice. In NEP/NEP2 double-knockout mice, Aß levels were marginally increased (∼1.5- to 2-fold), compared with NEP(-/-)/NEP2(+/+) controls. Treatment of these double-knockout mice with phosphoramidon resulted in elevations of Aß, suggesting that yet other NEP-like Aß-degrading endopeptidases are contributing to Aß catabolism.

Identifying Anti-prion Chemical Compounds Using a Newly Established Yeast High-Throughput Screening System.

Prion-like protein aggregation underlies the pathology of a group of fatal neurodegenerative diseases in humans, including Alzheimer's disease (AD), Parkinson's disease, amyotrophic lateral sclerosis, and transmissible spongiform encephalopathy. At present, few high-throughput screening (HTS) systems are available for anti-prion small-molecule identification. Here we describe an innovative phenotypic HTS system in yeast that allows for efficient identification of chemical compounds that eliminate the yeast prion [SWI+]. We show that some identified anti-[SWI+] compounds can destabilize other non-[SWI+] prions, and their antagonizing effects can be prion- and/or variant specific. Intriguingly, among the identified hits are several previously identified anti-PrPSc compounds and a couple of US Food and Drug Administration-approved drugs for AD treatment, validating the efficacy of this HTS system. Moreover, a few hits can reduce proteotoxicity induced by expression of several pathogenic mammalian proteins. Thus, we have established a useful HTS system for identifying compounds that can potentially antagonize prionization and human proteinopathies.

Analysis of Small Critical Regions of Swi1 Conferring Prion Formation, Maintenance, and Transmission.

Saccharomyces cerevisiae contains several prion elements, which are epigenetically transmitted as self-perpetuating protein conformations. One such prion is [SWI+ ], whose protein determinant is Swi1, a subunit of the SWI/SNF chromatin-remodeling complex. We previously reported that [SWI+ ] formation results in a partial loss-of-function phenotype of poor growth in nonglucose medium and abolishment of multicellular features. We also showed that the first 38 amino acids of Swi1 propagated [SWI+]. We show here that a region as small as the first 32 amino acids of Swi1 (Swi11-32) can decorate [SWI+] aggregation and stably maintain and transmit [SWI+] independently of full-length Swi1. Regions smaller than Swi11-32 are either incapable of aggregation or unstably propagate [SWI+]. When fused to Sup35MC, the [PSI+ ] determinant lacking its PrD, Swi11-31 and Swi11-32 can act as transferable prion domains (PrDs). The resulting fusions give rise to a novel chimeric prion, [SPS+], exhibiting [PSI+]-like nonsense suppression. Thus, an NH2-terminal region of ∼30 amino acids of Swi1 contains all the necessary information for in vivo prion formation, maintenance, and transmission. This PrD is unique in size and composition: glutamine free, asparagine rich, and the smallest defined to date. Our findings broaden our understanding of what features allow a protein region to serve as a PrD.

An insight into the complex prion-prion interaction network in the budding yeast Saccharomyces cerevisiae.

The budding yeast Saccharomyces cerevisiae is a valuable model system for studying prion-prion interactions as it contains multiple prion proteins. A recent study from our laboratory showed that the existence of Swi1 prion ([SWI(+)]) and overproduction of Swi1 can have strong impacts on the formation of 2 other extensively studied yeast prions, [PSI(+)] and [PIN(+)] ([RNQ(+)]) (Genetics, Vol. 197, 685-700). We showed that a single yeast cell is capable of harboring at least 3 heterologous prion elements and these prions can influence each other's appearance positively and/or negatively. We also showed that during the de novo [PSI(+)] formation process upon Sup35 overproduction, the aggregation patterns of a preexisting inducer ([RNQ(+)] or [SWI(+)]) can undergo significant remodeling from stably transmitted dot-shaped aggregates to aggregates that co-localize with the newly formed Sup35 aggregates that are ring/ribbon/rod- shaped. Such co-localization disappears once the newly formed [PSI(+)] prion stabilizes. Our finding provides strong evidence supporting the "cross-seeding" model for prion-prion interactions and confirms earlier reports that the interactions among different prions and their prion proteins mostly occur at the initiation stages of prionogenesis. Our results also highlight a complex prion interaction network in yeast. We believe that elucidating the mechanism underlying the yeast prion-prion interaction network will not only provide insight into the process of prion de novo generation and propagation in yeast but also shed light on the mechanisms that govern protein misfolding, aggregation, and amyloidogenesis in higher eukaryotes.

Contribution of Alanine-76 and Serine Phosphorylation in α-Synuclein Membrane Association and Aggregation in Yeasts.

In Parkinson's disease (PD), misfolded and aggregated α-synuclein protein accumulates in degenerating midbrain dopaminergic neurons. The amino acid alanine-76 in α-synuclein and phosphorylation at serine-87 and serine-129 are thought to regulate its aggregation and toxicity. However, their exact contributions to α-synuclein membrane association are less clear. We found that α-synuclein is indeed phosphorylated in fission yeast and budding yeast, the two models that we employed for assessing α-synuclein aggregation and membrane association properties, respectively. Surprisingly, blocking serine phosphorylation (S87A, S129A, and S87A/S129A) or mimicking it (S87D, S129D) altered α-synuclein aggregation in fission yeast. Either blocking or mimicking this phosphorylation increased endomembrane association in fission yeast, but only mimicking it decreased plasma membrane association in budding yeast. Polar substitution mutations of alanine-76 (A76E and A76R) decreased α-synuclein membrane association in budding yeast and decreased aggregation in fission yeast. These yeast studies extend our understanding of serine phosphorylation and alanine-76 contributions to α-synuclein aggregation and are the first to detail their impact on α-synuclein's plasma membrane and endomembrane association.

Familial Parkinson's Disease Mutant E46K α-Synuclein Localizes to Membranous Structures, Forms Aggregates, and Induces Toxicity in Yeast Models.

In Parkinson's disease (PD), midbrain dopaminergic neuronal death is linked to the accumulation of aggregated α-synuclein. The familial PD mutant form of α-synuclein, E46K, has not been thoroughly evaluated yet in an organismal model system. Here, we report that E46K resembled wild-type (WT) α-synuclein in Saccharomyces cerevisiae in that it predominantly localized to the plasma membrane, and it did not induce significant toxicity or accumulation. In contrast, in Schizosaccharomyces pombe, E46K did not associate with the plasma membrane. Instead, in one strain, it extensively aggregated in the cytoplasm and was as toxic as WT. Remarkably, in another strain, E46K extensively associated with the endomembrane system and was more toxic than WT. Our studies recapitulate and extend aggregation and phospholipid membrane association properties of E46K previously observed in vitro and cell culture. Furthermore, it supports the notion that E46K generates toxicity partly due to increased association with endomembrane systems within cells.