Since the genetics and cell biology of higher eukaryotes are extremely complex, scientists looking for a good model have turned to the use of yeast as a simpler system for the study of various pathologies including virus proliferation and assay new drugs against these pathogenic agents.
The yeast Saccharomyces cerevisiae
is a simple eukaryotic organism with approximately 6,000 genes. It is inexpensive to cultivate, can exist in either haploid or diploid states, and is extremely simple for genetic manipulations. The complete sequence of this yeast genome, the fruit of a worldwide collaboration of more than 100 laboratories from 1989 to 1996, was the first determined for a eukaryote cell. More than 75% of the genes have an assigned function, while more than 40% share conserved sequences with at least one known or predicted human gene ([1
Owing to the high conservation of fundamental biochemical pathways, yeast has been used as a model to unravel biological processes in many higher eukaryotes (for a short review see [2
]). A very important tool facilitating the use of these cells is the availability of yeast libraries in which each nonessential gene has been deleted. It has been used for multiple studies including genome wide screenings for human disease genes and host factors that support virus replication [3
]. The above collection is commercially available and covers more than 90% of all yeast genes [6
An exciting example of the beneficial using this model system is found in two very recent reports, although not related to virus research, on the use of the yeast model to study a severe human pathology. Amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig's Disease, is a neurodegenerative disease in which the motor neurons of the central nervous system that control muscles die off leading to paralysis and death within 3–5 years of onset. The cause of the disease is unknown, and there is no treatment to stop or slow it. A gene related to ALS, called FUS, has been used to explore its biology in yeast and highlight the potential for modeling elements of complex diseases in this simple eukaryotic cell. Results from both studies suggest that defects in RNA processing and transport may be a key element of ALS pathophysiology. In the cytoplasm of motor neurons of ALS patients, proteins aggregate to form insoluble aggregations, called inclusions, that may involve FUS and another ALS causing protein, called TDP-43 [7
]. When the two research groups overexpressed human FUS in yeast, they observed cytoplasmic inclusions. In humans, the majority of FUS is found in the nucleus, and some ALS-associated mutations reduce the nuclear/cytoplasmic ratio of the protein, suggesting that transfer to the cytoplasm, rather than mutation, may be important in this pathology. This hypothesis was supported by the observation that decreasing the overexpression of wild-type FUS in the yeast nucleus diminished its toxic effect. Both FUS and TDP-43 are RNA binding proteins. Genome-wide screens to identify yeast genes that specifically decreased cell toxicity were performed and the identified genes included some coding for other DNA/RNA binding proteins. One of these yeast genes, called ECM32, has a human homolog, hUPF1, which was found to eliminate toxicity. Among other functions hUPF1 plays an important role in mRNA quality control, supporting the idea that pathways involving RNA are defective in FUS-induced ALS. Interestingly, expression of hUPF1 was able to rescue FUS toxicity in yeast without driving FUS out of the inclusions or sending it back to the nucleus, suggesting that it may be possible to overcome therapeutically the effects of abnormally localized FUS without addressing the difficult problem of restoring it to its original compartment. Two important consequences may result from this approach. By identifying new genes that can diminish ALS-linked toxicity, they point to RNA processing as a therapeutic target in this disease. It has also been shown that yeast has the potential to be a much simpler and exciting system for modeling aspects of ALS. Since testing ideas about pathogenesis and treatment is much faster and cheaper in yeast, these results may open the way for more rapid progress in understanding the disease, its treatment, and the role of this new gene in ALS development. Important details of other neurodegenerative diseases have been obtained using yeast. For example, protein misfolding associated with many human diseases, including Alzheimer's, Parkinson's, and Huntington's disease, has been studied in yeast. As in the case of ALS mentioned above, protein misfolding often results in the formation of intracellular or extracellular inclusions whose role in protein misfolding diseases is unclear. Studies on the implication of protein aggregation and toxicity in yeast provide an excellent experimental and conceptual paradigm that contributes to understanding the differences between the toxic and protective roles of protein conformation changes. Results from these studies using yeast have the potential to transform basic concepts of protein folding in human diseases and may help in identifying new therapeutic strategies for their treatment [8
In virus research, S. cerevisiae
is a very useful organism. In terms of public health, the use of yeast to produce vaccines was a remarkable breakthrough. For example, the first recombinant vaccine, the hepatitis B surface antigen expressed in yeast, has become a safe and efficient prophylactic vaccine worldwide [9
]. In addition, yeast has proved useful for drug discovery as illustrated by the power of applying genomic approaches using the yeast model to characterize the biological activity of small molecules and to identify their cellular targets, an important step towards understanding the mode of action of human therapeutic agents [10
]. This study and other recent reports show that the path from sequence and functional analysis to direct therapeutic applications is not necessarily long, at least using the yeast model, a simple organism, which could well be considered among “man's best friends.”
In basic research in Virology, yeast has assisted the elucidation of the function of individual proteins from important pathogenic viruses such as Hepatitis C virus
, and Epstein-Barr virus
]; for a review and further references see [13
]). Furthermore, studies of viruses that infect yeast have provided many important contributions to the dissection of the life cycle of many other higher eukaryotes RNA viruses and the host factors involved [14
The study of the replication of RNA plant viruses was the first to be attempted using yeast cells. The great majority of RNA+ viruses carry their own RNA polymerase and are replicated in the cytoplasm while entry to the nuclear compartment during the viral cycle is confined to retroviruses (see below). The first higher eukaryotic virus reported to replicate in yeast was Brome mosaic virus
, a positive-strand RNA ((+)RNA) virus that infects plants ([15
], for an excellent review see [16
Since the pioneering work of the Ahlquist laboratory, a growing list of viruses has been reported to undergo replication in yeast. These include RNA and DNA viruses that infect plants, insects, mammals, and humans () [17
Viruses that replicate in yeast.
This wide range of viruses emphasizes the general applicability of the yeast system. In this paper, we will focus on the contribution of the yeast model to the study of retroviruses with special emphasis on the human immunodeficiency virus type 1 (HIV-1).