(1) Molecular basis of neurodegenerative and psychiatric diseases

  Many well-known neurodegenerative diseases such as Alzheimer’s and Parkinson’s diseases are caused by misfolding and aggregation of causative proteins. In contrast, the underlying mechanisms of psychiatric diseases such as schizophrenia and depression remain unclear because these diseases are more complex in nature and, unlike neurodegenerative diseases, rarely are caused by single gene mutations. We are currently investigating the molecular pathogenic mechanisms underlying these diseases using multiple approaches including biophysics and cell biology in conjunction with primary neuronal cultures and animal models to examine the functional deficits both at the cellular and whole animal levels (Nekooki et al., PNAS, 2009). We have been focusing our efforts to understand psychiatric symptoms in various neuropsychiatric diseases and to develop novel cutting-edge technologies in order to reveal molecular mechanisms therein.
  We have recently reported pathological cross-seeding between DISC1 and mutant huntingtin (HTT) aggregates in the brains of Huntington disease (HD) patients as well as in a HD murine model, which consequently reduced soluble DISC1 levels. This led to dysregulation of DISC1-PDE4 complexes, which aberrantly increased the activity of PDE4 and caused anhedonia in the HD model mice. Hence, we propose that cross-seeding of mutant HTT and DISC1 and the resultant changes in PDE4 activity may underlie the pathology of a specific subset of mental manifestations of HD, and as a general principle, be relevant to various mental illnesses (Tanaka, Ishizuka, Nekooki et al., J. Clin. Invest., 2017).

(2) Structural basis of yeast prion strains and transmission barriers

  It had long been a mystery that strain differences are observed both in mammalian and yeast prions. Our previous studies have greatly contributed to better understanding of how different prion strains arise due to the physical properties of the distinct prion (amyloid) conformations (Tanaka et al., Nature, 2004; 2006). Currently, we continue to explore the molecular basis of yeast prion strains and transmission barriers via structural and genetic approaches. Through utilizing biophysical methods such as nuclear magnetic resonance (NMR) spectroscopy and single-molecule technology, we are able to examine the structural details of various stable or metastable conformational states that yeast prion proteins such as Sup35 can adopt (Tanaka and Komi, Nat. Chem. Biol., 2015).
  Through such efforts, we have recently reported that the prion domain of Sup35 (Sup35NM) forms reversible oligomers in a temperature-dependent manner. In the study, we have determined that distinct residues regulate amyloid core nucleation and growth. Specifically, we have shown that oligomer formation is mediated by non-native interactions outside of the amyloid core, which assembles Sup35NM monomers into close proximity such that the amyloid core region is free to interact to form amyloid fibrils (Ohhashi et al., Nat. Chem. Biol., 2010).

(3) Analyses of novel functional prions and amyloids

  While prion diseases such as Creutzfeldt-Jakob disease (CJD) and bovine spongiform encephalopathy (BSE) are examples of mammalian pathologic conditions caused by the misfolding and aggregation of the prion protein (PrP), several yeast prion proteins have been identified which possess cellular functions depending on their aggregation status and are thus known as “functional prions”. We currently use genomic and proteomic approaches to identify novel functional prions or amyloids. The identification of novel functional prions will provide for a greater understanding of the structural properties which allow prion proteins to switch between different conformational states. In addition, it will offer us novel insights into how this non-genetic form of trait inheritance has evolved.
  We have demonstrated that prion conversion of a novel non-Gln/Asn-rich yeast prion protein Mod5 serves as a molecular switch between two metabolic pathways. In its normal conformation, Mod5 functions as a tRNA isopentenyltransferase; however this function is abolished in its prion conformation, which consequently promotes the sterol biosynthetic pathway to provide cellular resistance against anti-fungal agents (Suzuki et al., Science, 2012). Furthermore, we have recently showed that the prion-like cytoplasmic genetic element [KIL-d] selectively increases the rate of de novo mutation in the killer toxin gene of the viral genome, producing yeast harboring a defective mutant killer virus with a selective growth advantage over those with WT killer virus. These results suggest that the prion-like [KIL-d] element reprograms the viral replication machinery to induce mutagenesis and genomic inactivation via the long-hypothesized mechanism of "error catastrophe" (Suzuki et al., Mol Cell, 2015). Taken together, our findings support a role for prion-like protein aggregates in cellular defense and adaptation.

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