[Chemobiology at happy hour: yeast as a model for pharmacological screening]. / Chémobiologie à l'happy hour - La levure comme modèle de criblage pharmacologique.

Since its discovery and description by Louis Pasteur, the budding yeast Saccharomyces cerevisiae, which was used for thousands of years for alcoholic fermentation and as a leavening agent, has become a popular model system in biology. One of the reasons for this popularity is the strong conservation from yeast to human of most of the pathways controlling cell growth and fate. In addition, at least 30 % of human genes involved in diseases have a functional homolog in yeast. Hence, yeast is now widely used for modelling and deciphering physiopathological mechanisms as well as for developing pharmacological approaches like phenotype-based drug screening. Three examples of such yeast-based chemobiological studies are presented.

A yeast-based assay identifies drugs that interfere with immune evasion of the Epstein-Barr virus.

Epstein-Barr virus (EBV) is tightly associated with certain human cancers, but there is as yet no specific treatment against EBV-related diseases. The EBV-encoded EBNA1 protein is essential to maintain viral episomes and for viral persistence. As such, EBNA1 is expressed in all EBV-infected cells, and is highly antigenic. All infected individuals, including individuals with cancer, have CD8(+) T cells directed towards EBNA1 epitopes, yet the immune system fails to detect and destroy cells harboring the virus. EBV immune evasion depends on the capacity of the Gly-Ala repeat (GAr) domain of EBNA1 to inhibit the translation of its own mRNA in cis, thereby limiting the production of EBNA1-derived antigenic peptides presented by the major histocompatibility complex (MHC) class I pathway. Here we establish a yeast-based assay for monitoring GAr-dependent inhibition of translation. Using this assay we identify doxorubicin (DXR) as a compound that specifically interferes with the GAr effect on translation in yeast. DXR targets the topoisomerase-II-DNA complexes and thereby causes genomic damage. We show, however, that the genotoxic effect of DXR and various analogs thereof is uncoupled from the effect on GAr-mediated translation control. This is further supported by the observation that etoposide and teniposide, representing another class of topoisomerase-II-DNA targeting drugs, have no effect on GAr-mediated translation control. DXR and active analogs stimulate, in a GAr-dependent manner, EBNA1 expression in mammalian cells and overcome GAr-dependent restriction of MHC class I antigen presentation. These results validate our approach as an effective high-throughput screening assay to identify drugs that interfere with EBV immune evasion and, thus, constitute candidates for treating EBV-related diseases, in particular EBV-associated cancers.

The toll-like receptor agonist imiquimod is active against prions.

Using a yeast-based assay, a previously unsuspected antiprion activity was found for imiquimod (IQ), a potent Toll-like receptor 7 (TLR7) agonist already used for clinical applications. The antiprion activity of IQ was first detected against yeast prions [PSI (+) ] and [URE3], and then against mammalian prion both ex vivo in a cell-based assay and in vivo in a transgenic mouse model for prion diseases. In order to facilitate structure-activity relationship studies, we conducted a new synthetic pathway which provides a more efficient means of producing new IQ chemical derivatives, the activity of which was tested against both yeast and mammalian prions. The comparable antiprion activity of IQ and its chemical derivatives in the above life forms further emphasizes the conservation of prion controlling mechanisms throughout evolution. Interestingly, this study also demonstrated that the antiprion activity of IQ and IQ-derived compounds is independent from their ability to stimulate TLRs. Furthermore, we found that IQ and its active chemical derivatives inhibit the protein folding activity of the ribosome (PFAR) in vitro.

Procedure for identification and characterization of drugs efficient against mammalian prion: from a yeast-based antiprion drug screening assay to in vivo mouse models.

Prion diseases are fatal and incurable infectious neurodegenerative disorders affecting humans and other mammals. Prions are composed essentially if not solely of PrP(Sc), a misfolded form of the host-encoded PrP protein. PrP(Sc) catalyzes the transconformation of the normal endogenous PrP (PrP(C)) into more PrP(Sc). Prion replication thus corresponds to the propagation of an altered folding state of PrP. Several prion proteins have also been identified in the simple model organism Saccharomyces cerevisiae. Yeast prion-based screening assays have allowed identification of drugs active against mammalian prions, thus revealing the existence of common prion propagation mechanisms conserved from yeast to human. To identify these conserved targets, antiprion compounds isolated in yeast can be used as baits in reverse screening strategies. Once identified, these targets could in turn lead to the development of mechanism-based cell-free antiprion screening assays. A reverse screening procedure has been performed for 6AP and GA, two antiprion compounds isolated using a yeast-based assay. Protein folding activity of the large ribosomal RNA was found to be a physical and a functional target of both 6AP and GA therefore suggesting that this activity of the ribosome may constitute a novel mechanism involved in prion propagation and, as a consequence, a new screening target.