The 26S Proteasome Degrades the Soluble but Not the Fibrillar Form of the Yeast Prion Ure2p In Vitro

Proteolysis: a double-edged sword for the development of amyloidoses

The yeast Saccharomyces cerevisiae has proven to be a useful model system to investigate the mechanism of prion generation and inheritance, to which studies in Sup35 made a great contribution. Recent studies demonstrated that 'protein misfolding and aggregation' (i.e. amyloidogenesis) is a common principle underlying the pathogenesis of neurodegenerative diseases including prion, amyotrophic lateral sclerosis (ALS), Perkinson's (PD), Alzheimer's (AD) diseases and polyglutamine (polyQ) diseases such as spinocerebellar ataxia (SCA) and Hantington's disease (HD). By these findings, the yeast has again been drawing increased attention as a useful system for studying neurodegenerative proteinopathies. So far, it has been reported that proteolytic cleavage of causative amyloidogenic proteins might affect the pathogenesis of the respective neurodegenerative diseases. Although those reports provide a clear phenomenological description, in the majority of cases, it has remained elusive if proteolysis is directly involved in the pathogenesis of the diseases. Recently, we have demonstrated in yeast that proteolysis suppresses prion generation. The yeast-based strategy might make a breakthrough to the unsolved issues.

Progranulin regulates lysosomal function and biogenesis through acidification of lysosomes

Progranulin (PGRN) haploinsufficiency resulting from loss-of-function mutations in the PGRN gene causes frontotemporal lobar degeneration accompanied by TDP-43 accumulation, and patients with homozygous mutations in the PGRN gene present with neuronal ceroid lipofuscinosis. Although it remains unknown why PGRN deficiency causes neurodegenerative diseases, there is increasing evidence that PGRN is implicated in lysosomal functions. Here, we show PGRN is a secretory lysosomal protein that regulates lysosomal function and biogenesis by controlling the acidification of lysosomes. PGRN gene expression and protein levels increased concomitantly with the increase of lysosomal biogenesis induced by lysosome alkalizers or serum starvation. Down-regulation or insufficiency of PGRN led to the increased lysosomal gene expression and protein levels, while PGRN overexpression led to the decreased lysosomal gene expression and protein levels. In particular, the level of mature cathepsin D (CTSDmat) dramatically changed depending upon PGRN levels. The acidification of lysosomes was facilitated in cells transfected with PGRN. Then, this caused degradation of CTSDmat by cathepsin B. Secreted PGRN is incorporated into cells via sortilin or cation-independent mannose 6-phosphate receptor, and facilitated the acidification of lysosomes and degradation of CTSDmat. Moreover, the change of PGRN levels led to a cell-type-specific increase of insoluble TDP-43. In the brain tissue of FTLD-TDP patients with PGRN deficiency, CTSD and phosphorylated TDP-43 accumulated in neurons. Our study provides new insights into the physiological function of PGRN and the role of PGRN insufficiency in the pathogenesis of neurodegenerative diseases.

More than just trash bins? Potential roles for extracellular vesicles in the vertical and horizontal transmission of yeast prions

In the yeast Saccharomyces cerevisiae, an ensemble of structurally and functionally diverse cytoplasmic proteins has the ability to form self-perpetuating protein aggregates (e.g. prions) which are the vectors of heritable non-Mendelian phenotypic traits. Whether harboring these prions is deleterious—akin to mammalian degenerative disorders—or beneficial—as epigenetic modifiers of gene expression—for yeasts has been intensely debated and strong arguments were made in support of both views. We recently reported that the yeast prion protein Sup35p is exported via extracellular vesicles (EV), both in its soluble and aggregated infectious states. Herein, we discuss the possible implications of this observation and propose several hypotheses regarding the roles of EV in both vertical and horizontal propagation of ‘good’ and ‘bad’ yeast prions.