Defining Key Residues of the Swi1 Prion Domain in Prion Formation and Maintenance.

Prions are self-perpetuating, alternative protein conformations associated with neurological diseases and normal cellular functions. Saccharomyces cerevisiae contains many endogenous prions, providing a powerful system to study prionization. Previously, we demonstrated that Swi1, a component of the SWI/SNF chromatin-remodeling complex, can form the prion [SWI+]. A small region, Swi11-38, with a unique amino acid composition of low complexity, acts as a prion domain and supports [SWI+] propagation. Here, we further examine Swi11-38 through site-directed mutagenesis. We found that mutations of the two phenylalanine residues or the threonine tract inhibit Swi11-38 aggregation. In addition, mutating both phenylalanines can abolish de novo prion formation by Swi11-38, whereas mutating only one phenylalanine does not. Replacement of half of or the entire eight-threonine tract with alanines has the same effect, possibly disrupting a core region of Swi11-38 aggregates. We also show that Swi11-38 and its prion-fold-maintaining mutants form high-molecular-weight, SDS-resistant aggregates, whereas the double-phenylalanine mutants eliminate these protein species. These results indicate the necessity of the large hydrophobic residues and threonine tract in Swi11-38 in prionogenesis, possibly acting as important aggregable regions. Our findings thus highlight the importance of specific amino acid residues in the Swi1 prion domain in prion formation and maintenance.

A brief overview of the Swi1 prion-[SWI+].

Prion and prion-like phenomena are involved in the pathology of numerous human neurodegenerative diseases. The budding yeast, Saccharomyces cerevisiae, has a number of endogenous yeast prions-epigenetic elements that are transmitted as altered protein conformations and often manifested as heritable phenotypic traits. One such yeast prion, [SWI+], was discovered and characterized by our laboratory. The protein determinant of [SWI+], Swi1 was found to contain an amino-terminal, asparagine-rich prion domain. Normally, Swi1 functions as part of the SWI/SNF chromatin remodeling complex, thus, acting as a global transcriptional regulator. Consequently, prionization of Swi1 leads to a variety of phenotypes including poor growth on non-glucose carbon sources and abolishment of multicellular features-with implications on characterization of [SWI+] as being detrimental or beneficial to yeast. The study of [SWI+] has revealed important knowledge regarding the chaperone systems supporting prion propagation as well as prion-prion interactions with [PSI+] and [RNQ+]. Additionally, an intricate regulatory network involving [SWI+] and other prion elements governing multicellular features in yeast has begun to be revealed. In this review, we discuss the current understanding of [SWI+] in addition to some possibilities for future study.

Antisense reduction of tau in adult mice protects against seizures.

Tau, a microtubule-associated protein, is implicated in the pathogenesis of Alzheimer's Disease (AD) in regard to both neurofibrillary tangle formation and neuronal network hyperexcitability. The genetic ablation of tau substantially reduces hyperexcitability in AD mouse lines, induced seizure models, and genetic in vivo models of epilepsy. These data demonstrate that tau is an important regulator of network excitability. However, developmental compensation in the genetic tau knock-out line may account for the protective effect against seizures. To test the efficacy of a tau reducing therapy for disorders with a detrimental hyperexcitability profile in adult animals, we identified antisense oligonucleotides that selectively decrease endogenous tau expression throughout the entire mouse CNS--brain and spinal cord tissue, interstitial fluid, and CSF--while having no effect on baseline motor or cognitive behavior. In two chemically induced seizure models, mice with reduced tau protein had less severe seizures than control mice. Total tau protein levels and seizure severity were highly correlated, such that those mice with the most severe seizures also had the highest levels of tau. Our results demonstrate that endogenous tau is integral for regulating neuronal hyperexcitability in adult animals and suggest that an antisense oligonucleotide reduction of tau could benefit those with epilepsy and perhaps other disorders associated with tau-mediated neuronal hyperexcitability.

Implicating calpain in tau-mediated toxicity in vivo.

Alzheimer's disease and other related neurodegenerative disorders known as tauopathies are characterized by the accumulation of abnormally phosphorylated and aggregated forms of the microtubule-associated protein tau. Several laboratories have identified a 17 kD proteolytic fragment of tau in degenerating neurons and in numerous cell culture models that is generated by calpain cleavage and speculated to contribute to tau toxicity. In the current study, we employed a Drosophila tauopathy model to investigate the importance of calpain-mediated tau proteolysis in contributing to tau neurotoxicity in an animal model of human neurodegenerative disease. We found that mutations that disrupted endogenous calpainA or calpainB activity in transgenic flies suppressed tau toxicity. Expression of a calpain-resistant form of tau in Drosophila revealed that mutating the putative calpain cleavage sites that produce the 17 kD fragment was sufficient to abrogate tau toxicity in vivo. Furthermore, we found significant toxicity in the fly retina associated with expression of only the 17 kD tau fragment. Collectively, our data implicate calpain-mediated proteolysis of tau as an important pathway mediating tau neurotoxicity in vivo.