This article presents a mating-based method to facilitate overexpression screening in budding yeast using an arrayed plasmid library.
Budding yeast has been widely used as a model in studying proteins associated with human diseases. Genome-wide genetic screening is a powerful tool commonly used in yeast studies. The expression of a number of neurodegenerative disease-associated proteins in yeast causes cytotoxicity and aggregate formation, recapitulating findings seen in patients with these disorders. Here, we describe a method for screening a yeast model of the Amyotrophic Lateral Sclerosis-associated protein FUS for modifiers of its toxicity. Instead of using transformation, this new screening platform relies on the mating of yeast to introduce an arrayed library of plasmids into the yeast model. The mating method has two clear advantages: first, it is highly efficient; second, the pre-transformed arrayed library of plasmids can be stored for long-term as a glycerol stock, and quickly applied to other screens without the labor-intensive step of transformation into the yeast model each time. We demonstrate how this method can successfully be used to screen for genes that modify the toxicity of FUS.
The budding yeast Saccharomyces cerevisiae has been widely used in basic scientific research1 to understand cellular processes directly related to human diseases. Moreover, it has been used as a model organism for studying human disease-associated proteins, such as those linked to the most common neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, Huntington's disease, and Amyotrophic Lateral Sclerosis (ALS)2. An advantage of the yeast model is the ease with which a genome-wide screen can be performed to identify cellular pathways related to the toxicity of disease-related proteins, thus giving insight into the mechanism of their toxicity. One such screen is called an overexpression library screen, in which each of the 5,500 yeast genes in an arrayed library is transformed into a yeast model to identify which genes can modify toxicity when overexpressed. This screening method has been successfully applied in the yeast models of multiple neurodegenerative disease-associated proteins, including huntingtin for Huntington's disease3, α-synuclein for Parkinson's disease4,5, Aβ for Alzheimer's disease6, and FUS and TDP-43 for ALS7,8,9. While it is usually done in a high-throughput manner10, the most labor-intensive step of the screen is individually transforming 5,500 yeast genes from an arrayed library. This step must be performed each time the screening is repeated, and whenever a newly established yeast model needs to be studied. It is important to find a more efficient way to accomplish this task.
Yeast cells can stably exist in both haploid and diploid forms. There are two opposite mating types of haploid cells, mating type a and α. Haploid cells of each mating type produce and secrete their own specific mating pheromone, to which only the opposite mating type cells respond. This allows mating between a and α cells to produce stable diploid cells, a/α. This process is spontaneous and highly efficient11. We can take advantage of this unique life cycle of S. cerevisiae to introduce the plasmid library. More specifically, each gene in the arrayed plasmid library will be transformed into haploid cells of one mating type, i.e., α cell. These cells containing the library genes will then be stored in glycerol stock in an arrayed 96-well format. For each yeast model that needs to be screened, yeast cells containing the library genes can be thawed from the glycerol stock, and the screening can be done through mating with the yeast model of interest in the opposite mating type, i.e., mating type a. This idea of using mating to bring together two genes into yeast is not new. It has been successfully applied in the high-throughput yeast two-hybrid screening, in which a bait construct (i.e., Gal4-DNA-binding domain fusions) in one mating type is brought together through mating with a prey construct from an arrayed library12. However, this strategy has never been applied in overexpression library screenings, which have always used traditional transformation methods.
Our laboratory previously established a yeast model of the ALS-associated protein FUS7. Through overexpression library screening using the transformation method we discovered five yeast genes (ECM32, NAM8, SBP1, SKO1, and VHR1) that rescue toxicity of FUS when overexpressed. These findings were independently confirmed with a similar study by another group8. hUPF1, a human homolog of ECM32, was later shown to suppress toxicity in primary neuronal cells13 and in an animal model of ALS14 as well. Using these five genes as proof of principle, we demonstrate that all five genes similarly rescue FUS toxicity when they are introduced into the FUS yeast model by mating. Since yeast cells containing the library genes can be stored permanently in glycerol stock and revived whenever needed, this mating-based method will remove the time-consuming step of transformation each time the library needs to be screened against. Since mating is highly efficient with no plasmid transformation involved, this strategy also significantly decreases the cost associated with purification and transformation of a large plasmid library. We will successfully apply this method to a library screening against yeast model of FUS.
The procedure for mating-based screening is briefly described in Figure 1. Initially, the arrayed plasmid library is transformed into a haploid yeast strain of mating type α using a high-throughput yeast transformation protocol in which each well of a 96-well plate contains yeast transformed with a specific library plasmid. This collection of transformed yeast is saved as a glycerol stock that can be thawed and revived for use later on. The yeast model of interest, in this case FUS toxicity, must be generated in a haploid yeast strain with the opposite mating type (mating type a). In a high-throughput manner using sterile 96-pin replicators, the FUS strain and yeast strains containing the plasmid library are transferred to 96-well plates containing rich media and allowed to mate. Following mating, a small volume from each well of the mating culture is transferred to 96-well plates containing synthetic dropout media in which only diploid yeast containing both the FUS and library genes can grow. A robotic spotting machine is then used to transfer yeast culture from each well onto agar plates where the expression of FUS and the library genes is induced. Additionally, yeast culture is spotted to control agar plates where FUS and the library genes are not expressed. Following growth on agar plates, genes that rescue or exacerbate FUS toxicity will be identified.
NOTE: The protocol described here is designed for screening library plasmids contained in ten 96-well plates but can be scaled up or down accordingly. The protocol needs to be repeated to complete the whole library screening. Usually, screening against 10 plates of library genes each time can be comfortably handled by 1 person.
1. Preparation for 96-well Yeast Transformation
NOTE: This step is done as previously described7,10.
2. Yeast Transformation
3. Mating Between Cells Containing Library Genes and Query Yeast
4. Spotting Assay
The ALS-associated protein FUS, an RNA/DNA binding protein, was previously studied in haploid yeast7,8. Genetic screening using the transformation-based method discovered several yeast genes that suppress FUS toxicity. The human homolog of one of the yeast genes was later demonstrated to be effective at suppressing toxicity in a primary neuronal cell and rat model of ALS13. Here, we are using the same yeast model to show that overexpression library screening can be performed by mating as effectively as by transformation.
FUS is toxic to both haploid and diploid yeast cells
The previous yeast model of FUS and subsequent overexpression library screening was performed in the haploid cell background. For the mating-based method to work, FUS toxicity needs to be demonstrated in diploid yeast. To do this, we mated the FUS yeast model in w303a (mating type a) with w303α transformed with an empty vector (mating type α). As indicated in Figure 2, although not as strong as in haploid yeast, FUS toxicity is evident in diploid yeast.
Suppression genes previously identified work in diploid yeast
As a proof of principle for the mating-based method, we tested the five genes previously identified from the transformation-based method. Mating was used to introduce each of the five genes into the haploid yeast model of FUS (in W303a), and their ability to rescue the toxicity of FUS in the subsequent diploid yeast was tested (W303a/α). As shown in Figure 3, all five genes rescue the toxicity of FUS in diploid yeast, indicating that the mating method was effective.
A pilot screening of 940 genes (arrayed on ten 96-well plates)
Following the protocols described above, we applied the mating-based method to an overexpression library screening of 940 genes. Figure 4 displays a picture of one representative plate. As indicated on the right side of the figure (FUS and library genes expressed), FUS was toxic to diploid yeast. The library gene indicated by the green square rescued FUS toxicity while that indicated by the red square enhanced toxicity.
Figure 1: Diagram for screening yeast models of protein toxicity using mating. A plasmid library (under control of the GAL1 promoter which is highly induced in the presence of galactose) is transformed into a haploid yeast strain (MATα) using a high-throughput transformation protocol. This collection of yeast, transformed with the library plasmids, is stored as a glycerol stock at -80 °C, and is revived when needed to mate with the haploid yeast model of protein toxicity (in our case, one copy of FUS integrated at the HIS3 locus, GAL1 promoter, MATa). Diploid yeast containing the library plasmid and toxic protein (FUS) are selected and spotted to glucose (FUS and library gene 'off') and galactose (FUS and library gene 'on') agar plates. The growth of the yeast was followed to identify genes that rescue or exacerbate the toxicity of FUS. The green square indicates an example of a suppressor gene that rescues FUS toxicity, and the red square indicates an example of an enhancer gene that exacerbates the FUS toxicity when overexpressed. Please click here to view a larger version of this figure.
Figure 2: FUS is toxic to both haploid and diploid yeast cells. Haploid yeast (w303 MATa) transformed with pRS303Gal1-FUS were either transformed with an empty plasmid or mated with yeast of the opposite mating type (w303 MATα) transformed with the same empty plasmid to generate a diploid yeast strain. These yeast strains along with a control strain were then 5x serially diluted (from left to right) and spotted to Ura-His-Glucose medium (left, FUS expression repressed) and Ura-His-Galactose medium (right, FUS expression induced). The picture was taken after 2 days of growth at 30 °C. Nearly identical growth of the haploid and diploid control strain was observed, so only the haploid control strain was shown. Please click here to view a larger version of this figure.
Figure 3: Five yeast genes (ECM32, NAM8, SBP1, SKO1, and VHR1) rescue FUS toxicity through the mating-based method. Plasmids containing previously identified yeast genes that suppress FUS toxicity were transformed into a haploid yeast strain (w303 MATα). These yeast were then mated with haploid yeast of the opposite mating type (w303 MATa) transformed with a FUS expression plasmid. Diploid yeast containing FUS and either an empty vector or one of the five suppression genes was selected and spotted in replicates on agar plates containing glucose (genes 'off') and galactose (genes 'on'). (1) shows a control yeast strain transformed with two empty vectors. (2) shows the diploid FUS yeast strain with an empty vector where the expression of FUS is very toxic. (3–7) show diploid yeast expressing FUS as well as a suppression gene that can rescue FUS toxicity. Each number (1–7) shows two rows of twelve identical replicates. The picture, representing three independent experiments, was taken after 3 days of growth at 30 °C. Please click here to view a larger version of this figure.
Figure 4: Library screening for genes that rescue or exacerbate FUS toxicity. Haploid yeast containing FUS was mated with haploid yeast containing the library genes. After mating, diploid cells containing both FUS and a library gene were selected and then spotted to glucose (FUS and library genes 'off') and galactose agar plates (FUS and library genes 'on') in quadruplicate. On the galactose plate, in which FUS and the library gene were expressed, most of the yeast were unable to grow well. This indicates that FUS is toxic and most library genes were unable to rescue toxicity. The green square demonstrates an example of a library gene that suppresses FUS toxicity and allows the yeast to form colonies. The red square indicates an example of a library gene that exacerbates FUS toxicity. The plates shown here are representative of 10 plates of library genes that were screened against. The picture was taken after 3 days of growth at 30 °C. Please click here to view a larger version of this figure.
Here, we describe a protocol to perform a plasmid overexpression screen in yeast using mating to introduce the plasmid library into the yeast model. Using this approach, multiple yeast models of neurodegenerative disease protein toxicity can be screened using the same collection of yeast transformed with a plasmid library. The laborious process of transformation only needs to be performed once, after which highly efficient yeast mating is used to introduce the plasmid library into the query strain. This protocol does rely on the use of robotic equipment to dispense media and spot yeast cultures onto agar plates. While the protocol can be performed without the use of robotic equipment, it will be more time consuming. This method was successfully used to screen for genes that can modify the toxicity of FUS.
We observed that FUS is slightly less toxic in the diploid yeast background. This is most likely due to gene copy number and differences in growth rate of diploid yeast. Unless the phenotype that is being studied is mating type or ploidy-dependent, the growth phenotype of the toxicity is consistent in haploid and diploid yeast. Because of this, the mating-based method is expected to work widely in many yeast models of various growth phenotypes. Nonetheless, the phenotype of the yeast model should be verified to make sure that it is still present in the diploid background before this screening method is performed. This method can be used to study many different phenotypes in yeast and is not limited to the study of neurodegenerative protein toxicity. In addition, any plasmid library containing yeast expression vectors can be used.
After performing the screen, there are a number of verification assays that will help ensure the identified hits are specific to the yeast model being screened. Enhancers of toxicity should be tested in yeast without co-expressing the disease protein of interest. Enhancers causing toxicity independent of the disease protein of interest should be eliminated from further study. It is important to consider whether the suppressors of toxicity are affecting expression of the disease protein by affecting the Gal1 promoter. Any suppressors affecting expression from the Gal1 promoter should be eliminated from further study.
Yeast strains containing the library plasmid are permanently stored in glycerol stock and can be quickly revived when needed so that the mating-based method may be easily applied to other yeast models in which the same library genes need to be screened against. The efficiency of the mating-based method becomes obvious when the same type of screening is used to remove false positives or when multiple different yeast models need to be studied. We have successfully used this method to screen a yeast model of TDP-43, another protein linked to ALS.
The authors have nothing to disclose.
We are thankful for the thoughtful discussions with members of the Ju laboratory and Zhong laboratory, and the financial support from the Wright State University.
salmon Sperm DNA (SS-DNA) | Sigma-Aldrich | D1626 | |
YPD broth | Research Products International (RPI) | Y20090 | |
Granulated Agar | Fisher Sci | BP97445 | |
D-(+)-Glucose | Research Products International (RPI) | G32040 | |
D-(+)-Galactose | Research Products International (RPI) | G33000 | |
D-(+)-Raffinose Pentahydrate | Research Products International (RPI) | R20500 | |
Ammonium Sulfate | Fisher Sci | A702-500 | |
Synthetic Ura- drop out medium | Clontech | 630416 | |
Yeast amino acid drop out supplement -Histidine/-Uracil | Clontech | 630422 | |
Yeast Nitrogen Base without Amino Acids and Ammonium Sulfate | Research Products International (RPI) | Y20060 | |
Dimethyl Sulfoxide (DMSO) | Fisher Sci | S67496 | |
Lithium acetate, anhydrous | Fisher Sci | AC268640010 | |
Polyethylene Glycol 3350 (PEG-3350) | Spectrum Chemical | PO125-12KG | |
96 Pin Replicator | Scinomix | SCI-5010-OS | |
Nunc OmniTray | Thermo Sci | 140156 | |
Corning Costar 96 well assay plate, round bottom with lid | Fisher Sci | 07-200-760 | non-treated, sterile |
Eppendorf Research plus Multichannel Pipette | Eppendorf | TI13690052 | 30-300ul volume |
Fisherbrand Isotemp Digital Dry Baths/Block Heaters | Fisher Sci | 88-860-023 | |
Eppendorf MixMate | Eppendorf | 21-379-00 | |
Eppendorf 5810R Centrifuge | Fisher Sci | 05-413-112 | |
Avanti J-26 XPI Centrifuge | Beckman | 393127 | |
MultiFlo FX Multi-Mode Dispenser | BioTek | ||
Rotor HDA | Singer Instruments |