We thank the Lewis-Sigler Institute for Integrative Genomics Sequencing Core Facility at Princeton University for technical advice and for RNA library preparation and sequencing. This work was supported by grants from Rowan University and NIH NIGMS R15GM113187 to M.J.H.

1. Inducing Hypoxia
<ol>
	<li>One day or more before the hypoxia time course: Prepare the incubator, cell filtering system, vacuum, gas tank, flasks, stoppers, glass tubing, and tubing, as in the Materials Table.</li>
	<li>Place the N2 tank, incubator, vacuum and filtering system in close proximity, to enable quick processing of cells.</li>
	<li>Prepare sterile liquid YPD media (1% Yeast Extract, 2% peptone, 2% glucose) by mixing the components in a glass bottle and autoclaving.</li>
	<li>Pl...

<ol>
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In this study, the mRNA levels for all genes was measured during hypoxia in the yeast S. cerevisiae. The goal was to analyze how global gene expression changes due to growth in a controlled near-anoxic environment. Several steps were taken to ensure that the method described here was carefully controlled and reproducible. First, cells were exposed to a precisely defined hypoxic environment: 99.999% N2 in rich media (YPD). Other studies of hypoxia have closed off the flask or tube to air <sup class="xr...

Many organisms respond to hypoxia, or low O2, by altering gene expression 1,2,3. This response helps cells cope with the lack of a substrate critical for aerobic respiration and for several biosynthetic reactions, but also with a changing redox state 4. Several microarray studies performed in S. cerevisiae show that the mRNA levels of hundreds of genes change in response to hypoxia 5,6,7,8,9,10,11,12. Recently, RNA-seq was used to characterize gene expression changes over time during hypoxia 13. Here, the experimental details are presented and discussed.
Hypoxia can be achieved in various ways, each producing a different level of O2. Here, hypoxia was established by continuously flowing ultra-high-purity N2 into flasks, which lowers [O2] dissolved immediately with reproducible kinetics 10. It is possible that there are some O2 molecules present that contribute to metabolism and gene expression but this environment is considered very close to anaerobic. In the absence of O2, yeast cells cannot biosynthesize heme, ergosterol and unsaturated fatty acids 4,12,14. Thus, previous studies have included these metabolites when growing yeast without oxygen 5,10,15. However, many hypoxic responses are mediated by depletion of these metabolites and thus replenishing them reverses the hypoxic gene expression responses 12,16. In order to mimic natural hypoxia, these metabolites were not added to the media. In the short time that cells were exposed to hypoxia without the presence of these essential metabolites, there was no noticeable increase in cell death (data not shown) nor a prolonged stress response 13.
The response is also dependent upon the strain and its genotype. Especially important are the alleles of the known regulators of hypoxic responses 2. The S288C strain background is highly desired so that results can be compared to the other genomic studies performed with this strain. However, S288C contains a partial loss-of-function allele of the HAP1 gene 17, a transcriptional regulator critical for the hypoxic response. This allele was repaired in S288C using a wildtype copy from the &#931;1278b strain background 11.
Gene expression is highly dependent upon the cellular environment. Thus, when performing genome-wide mRNA analysis, it is important to maintain a constant environment while varying another parameter such as time, stimulus, or genotype. To achieve highly reproducible results, consider these three practices for the study and all of its biological or technical replicates. First, the same experimenter(s) should carry out the study, since technical practices may vary across experimenters. Second, the same batch of ingredients should be used in the growth media as each batch has a slightly different composition that can affect gene expression. Third, to minimize cell cycle effects, each time point should consist of asynchronous cells in the mid-log phase of growth (1 - 2 x 107 cells/mL).
When characterizing a complex response like the gene expression response to hypoxia, a time course is advantageous for determining the kinetics of various events. Specific time points should be chosen that will capture the major changes of the response. In this study, time points between 0 and 4 h were observed, because past experiments revealed widespread changes in gene expression during this period 13.
To measure global gene expression, RNA-seq was used 18,19. This method uses next-generation sequencing to determine the relative abundance of each gene&#39;s transcript. Compared to DNA microarray analysis, RNA-seq exhibits higher sensitivity (to detect less abundant transcripts), a greater dynamic range (to measure greater fold changes) and superior reproducibility (to accurately follow gene expression over time). Typically, most cellular RNA is ribosomal RNA so many methods have been developed to enrich for specific RNA species 20. Here, poly-T beads were used to purify poly-A-containing mRNA transcripts, though the various commercially- available rRNA depletion kits could also be effective in mRNA enrichment.
Here, the S. cerevisiae gene expression response to hypoxia was characterized. Cells were exposed to hypoxia and then sampled at eight time points (0, 5, 10, 30, 60, 120, 180 and 240 min). To confirm reproducibility and to identify statistically changed transcripts, three biological replicates were performed. RNA was extracted by mechanical disruption and column purification, and then processed for RNA-seq analysis. The post-sequencing pipeline is described and programming scripts are provided that allow exact replication of the analyses performed. Specifically, Trimmomatic 21, TopHat2 22, HTseq 23, the R statistical environment 24, and the DESeq2 package 25 were used to process the RNA-seq data and to identify 607 genes that change significantly during hypoxia. Principal component analysis (PCA) and gene expression of the replicates indicated the reproducibility of the technique. Clustering and heatmaps revealed wide-ranging expression kinetics, while gene ontology (GO) analysis showed that many cellular processes, like aerobic respiration, are enriched in the set of oxygen-regulated genes.

<table border="0" cellpadding="0" cellspacing="0" width="973">
	<tbody>
		<tr height="126">
			<td height="126" style="height:126px;width:292px;">Enclosed dry incubator</td>
			<td style="width:183px;">Thermo Scientific</td>
			<td style="width:119px;">MaxQ 4000</td>
			<td style="width:380px;">Set at 30&#176;C. Modify the door to allow entry of one Tygon tube. Alternatively, use the New Brunswick G25 incubator, which contains a tube port. Do not use an open-air water shaker, as condensation will collect in the tubes between flasks, possibly cross-contaminating cultures.</td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:292px;">Micro-analysis Filter Holder</td>
			<td style="width:183px;">Millipore</td>
			<td style="width:119px;">XX1002530</td>
			<td style="width:380px;">25mm diameter, stainless steel support, no. 5 perforated silicone stopper mounts in standard 125mL filtering flask</td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:292px;">Strong vacuum</td>
			<td style="width:183px;">Edwards</td>
			<td style="width:119px;">E-LAB 2</td>
			<td style="width:380px;">The &#8220;house&#8221; vacuum may be too weak. Alternatively, use an electric-power portable vacuum pump like the one listed here.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:292px;">1000-mL flask</td>
			<td style="width:183px;"></td>
			<td style="width:119px;"></td>
			<td style="width:380px;">To act as vacuum trap.</td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:292px;">~2-foot lengths of Heavy Wall Vacuum Tubing, inner diameter 3/8 in, outer diameter 7/8 in</td>
			<td style="width:183px;">Tygon</td>
			<td style="width:119px;">38TT</td>
			<td style="width:380px;">Two pieces: the first connects vacuum to trap, and the second connects trap to filter system.</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:292px;">High-pressure N2 gas tank</td>
			<td style="width:183px;"></td>
			<td style="width:119px;"></td>
			<td style="width:380px;">99.999% purity, &#62;1000psi, with a regulator and gas flow controller</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:292px;">Autoclaved 500mL flask</td>
			<td style="width:183px;"></td>
			<td style="width:119px;"></td>
			<td style="width:380px;">Opening covered with aluminum foil. One for each yeast strain.</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:292px;">Autoclaved 250mL flasks</td>
			<td style="width:183px;"></td>
			<td style="width:119px;"></td>
			<td style="width:380px;">Openings covered with aluminum foil. One for each time point plus two for water traps.</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:292px;">Flask stoppers (size 6, two holes with 5mm diameter)</td>
			<td style="width:183px;"></td>
			<td style="width:119px;"></td>
			<td style="width:380px;">Sterilized with 70% ethanol. One for each flask.</td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:292px;">Glass tubing, length 9 cm or 17 cm, inner diameter 2 mm, outer diameter 5 mm</td>
			<td style="width:183px;"></td>
			<td style="width:119px;"></td>
			<td style="width:380px;">Sterilized with 70% ethanol. Two tubes for each flask. Place into the holes of each stopper. See Figure 1 for placement of 9- vs 17- cm tubes.</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:292px;">~25-cm lengths of plastic tubing, inner diameter 5 mm</td>
			<td style="width:183px;">Tygon</td>
			<td style="width:119px;">E-3603</td>
			<td style="width:380px;">One piece for each flask. Sterilized with 70% ethanol.</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:292px;">Sterile filter discs</td>
			<td style="width:183px;">Millipore</td>
			<td style="width:119px;">HAWP02500</td>
			<td style="width:380px;">25mm diameter, 0.45 &#181;m pore size, one for each time point</td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;width:292px;">Sterile dH2O (~100mL)</td>
			<td style="width:183px;"></td>
			<td style="width:119px;"></td>
			<td style="width:380px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:292px;">1-mL cuvettes</td>
			<td style="width:183px;"></td>
			<td style="width:119px;"></td>
			<td style="width:380px;">For measuring OD600 (i.e., cell concentration)</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:292px;">50-mL sterile centrifuge tubes</td>
			<td style="width:183px;"></td>
			<td style="width:119px;"></td>
			<td style="width:380px;">One for each time point</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:292px;">Clean and sterile tweezers</td>
			<td style="width:183px;"></td>
			<td style="width:119px;"></td>
			<td style="width:380px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:292px;">liquid nitrogen</td>
			<td style="width:183px;"></td>
			<td style="width:119px;"></td>
			<td style="width:380px;">For freezing cells</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:292px;">acid-washed beads</td>
			<td style="width:183px;">Sigma</td>
			<td style="width:119px;">G8772</td>
			<td style="width:380px;">Keep at 4&#176;C for lysing cells</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:292px;">Qiagen RNeasy Mini Kit</td>
			<td style="width:183px;">Qiagen</td>
			<td align="right" style="width:119px;">74104</td>
			<td style="width:380px;">For RNA column purification</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:292px;">Qiagen RLT buffer</td>
			<td style="width:183px;"></td>
			<td style="width:119px;"></td>
			<td style="width:380px;">Prepare by adding 10&#181;L of &#946;-Mercaptoethanol per 1mL of RLT buffer, keep at 4C.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">2-mL collection tubes</td>
			<td style="width:183px;">Qiagen</td>
			<td style="width:119px;"></td>
			<td style="width:380px;">included in the Qiagen Rneasy Mini Kit</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:292px;">Buffer RPE</td>
			<td style="width:183px;">Qiagen</td>
			<td style="width:119px;"></td>
			<td style="width:380px;">included in the Qiagen Rneasy Mini Kit</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:292px;">Buffer RW1</td>
			<td style="width:183px;">Qiagen</td>
			<td style="width:119px;"></td>
			<td style="width:380px;">included in the Qiagen Rneasy Mini Kit</td>
		</tr>
		<tr height="168">
			<td height="168" style="height:168px;width:292px;">DNase I stock and working solutions</td>
			<td style="width:183px;">Qiagen</td>
			<td align="right" style="width:119px;">79254</td>
			<td style="width:380px;">The DNase I enzyme comes as lyophilized powder in a glass vial. Using a sterile needle and syringe, inject 550&#181;L of RNase-free water (provided in Qiagen kit) into the vial. Mix by gently inverting the bottle. To avoid denaturing the enzyme, do not vortex. Using a pipet, remove this stock solution from the vial and store in freezer (-20&#176;C) in single-use aliquots (80&#181;L each). The stock solution should not be thawed and refrozen.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:292px;">Buffer RDD</td>
			<td style="width:183px;">Qiagen</td>
			<td style="width:119px;"></td>
			<td style="width:380px;">included in the Qiagen DNase Kit</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:292px;">Ice cold 2-mL screw-cap tubes</td>
			<td style="width:183px;"></td>
			<td style="width:119px;"></td>
			<td style="width:380px;">For lysing cells during RNA extraction</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:292px;">bead mill homogenizer</td>
			<td style="width:183px;">Biospec Mini-Beadbeater-24</td>
			<td align="right" style="width:119px;">112011</td>
			<td style="width:380px;">Keep in cold room</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:292px;">Bacto Peptone</td>
			<td style="width:183px;">BD</td>
			<td style="width:119px;">DF0118</td>
			<td style="width:380px;">for liquid YPD media</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:292px;">Bacto Yeast Extract</td>
			<td style="width:183px;">BD</td>
			<td style="width:119px;">DF0886</td>
			<td style="width:380px;">for liquid YPD media</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:292px;">glucose</td>
			<td style="width:183px;">Fisher</td>
			<td style="width:119px;">D16</td>
			<td style="width:380px;">for liquid YPD media</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:292px;">Qubit assay tubes</td>
			<td style="width:183px;">Thermo Fisher</td>
			<td style="width:119px;">Q32856</td>
			<td style="width:380px;">for measuring nucleic acid concentration</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:292px;">Quant-iTTM dsDNA BR Assay Kit</td>
			<td style="width:183px;">Thermo Fisher</td>
			<td style="width:119px;">Q32853</td>
			<td style="width:380px;">for measuring nucleic acid concentration</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:292px;">Quant-iTTM RNA Assay Kit</td>
			<td style="width:183px;">Thermo Fisher</td>
			<td style="width:119px;">Q32855</td>
			<td style="width:380px;">for measuring nucleic acid concentration</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:292px;">Qubit Fluorometer</td>
			<td style="width:183px;">Thermo Fisher</td>
			<td style="width:119px;">Q33216</td>
			<td style="width:380px;">for measuring nucleic acid concentration</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:292px;">Commercial electrophoresis system</td>
			<td style="width:183px;">Agilent</td>
			<td>Bioanalyzer 2100</td>
			<td style="width:380px;">for measuring nucleic acid quality</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:292px;">Next-generation sequencer</td>
			<td style="width:183px;">Illumina</td>
			<td style="width:119px;">HiSeq 2500</td>
			<td style="width:380px;">for sequencing libraries</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:292px;">automated liquid handling system</td>
			<td style="width:183px;">Wafergen</td>
			<td style="width:119px;">Apollo 324</td>
			<td style="width:380px;">for creating sequencing libraries</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:292px;">PrepX PolyA mRNA Isolation Kit</td>
			<td style="width:183px;">Wafergen</td>
			<td align="right" style="width:119px;">400047</td>
			<td style="width:380px;">for isolating mRNA from total RNA</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:292px;">PrepX RNA SEQ for Illumina Library Kit</td>
			<td style="width:183px;">Wafergen</td>
			<td align="right" style="width:119px;">400039</td>
			<td style="width:380px;">for creating strand-specific sequencing libraries from total RNA</td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:292px;">Barcode Splitter</td>
			<td style="width:183px;"></td>
			<td style="width:119px;"></td>
			<td style="width:380px;">https://toolshed.g2.bx.psu.edu/repository?repository_id=7119c4f7a89efa57&#38;changeset_revision=e7b7cdc1834d</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:292px;">Samtools, which includes the gzip command</td>
			<td style="width:183px;"></td>
			<td style="width:119px;"></td>
			<td style="width:380px;">http://www.htslib.org/download/</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:292px;">Trimmomatic</td>
			<td style="width:183px;"></td>
			<td style="width:119px;"></td>
			<td style="width:380px;">http://www.usadellab.org/cms/?page=trimmomatic</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:292px;">Bowtie2 (installed before TopHat)</td>
			<td style="width:183px;"></td>
			<td style="width:119px;"></td>
			<td style="width:380px;">http://bowtie-bio.sourceforge.net/bowtie2/index.shtml</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:292px;">TopHat</td>
			<td style="width:183px;"></td>
			<td style="width:119px;"></td>
			<td style="width:380px;">https://ccb.jhu.edu/software/tophat/index.shtml</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:292px;">HTSeq</td>
			<td style="width:183px;"></td>
			<td style="width:119px;"></td>
			<td style="width:380px;">http://www-huber.embl.de/HTSeq/doc/overview.html</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:292px;">R (installed before R Studio)</td>
			<td style="width:183px;"></td>
			<td style="width:119px;"></td>
			<td style="width:380px;">https://cran.rstudio.com</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:292px;">R Studio (free version)</td>
			<td style="width:183px;"></td>
			<td style="width:119px;"></td>
			<td style="width:380px;">https://www.rstudio.com/products/rstudio/download/</td>
		</tr>
	</tbody>
</table>

measuring mrna levels over time during the yeast s. cerevisiae hypoxic response

Complex changes in gene expression typically mediate a large portion of a cellular response. Each gene may change expression with unique kinetics as the gene is regulated by the particular timing of one of many stimuli, signaling pathways or secondary effects. In order to capture the entire gene expression response to hypoxia in the yeast S. cerevisiae, RNA-seq analysis was used to monitor the mRNA levels of all genes at specific times after exposure to hypoxia. Hypoxia was established by growing cells in ~100% N2 gas. Importantly, unlike other hypoxic studies, ergosterol and unsaturated fatty acids were not added to the media because these metabolites affect gene expression. Time points were chosen in the range of 0 - 4 h after hypoxia because that period captures the major changes in gene expression. At each time point, mid-log hypoxic cells were quickly filtered and frozen, limiting exposure to O2 and concomitant changes in gene expression. Total RNA was extracted from cells and used to enrich for mRNA, which was then converted to cDNA. From this cDNA, multiplex libraries were created and eight or more samples were sequenced in one lane of a next-generation sequencer. A post-sequencing pipeline is described, which includes quality base trimming, read mapping and determining the number of reads per gene. DESeq2 within the R statistical environment was used to identify genes that change significantly at any one of the hypoxic time points. Analysis of three biological replicates revealed high reproducibility, genes of differing kinetics and a large number of expected O2-regulated genes. These methods can be used to study how the cells of various organisms respond to hypoxia over time and adapted to study gene expression during other cellular responses.

The hypoxia time course and RNA-seq analysis were performed independently three times. To examine the reproducibility of the three replicates, gene expression data for all genes was analyzed using Principal Component Analysis (PCA). Figure 2 shows how the samples change over the first two principal components, which together represent 58.9% of the variability. This analysis indicated that each time course exhibits similar changes (as depicted by the similar s...

Watch this Scientific Journal Video about Measuring mRNA Levels Over Time During the Yeast S. cerevisiae Hypoxic Response at JoVE.com

Measuring mRNA Levels Over Time During the Yeast S. cerevisiae Hypoxic Response

Here, we present a protocol using RNA-seq to monitor mRNA levels over time during the hypoxic response of S. cerevisiae cells. This method can be adapted to analyze gene expression during any cellular response.

Measuring mRNA Levels Over Time During the Yeast S. cerevisiae Hypoxic Response

Complex changes in gene expression typically mediate a large portion of a cellular response. Each gene may change expression with unique kinetics as the ...

The overall goal of this experiment is to monitor the mRNA levels of all genes over time as S.Cerevisiae cells are exposed to hypoxia.This method can help answer key questions in the cell signaling fields such as what it is the kinetics of gene expression changes during a cellular response?The main advantage of this technique is that mRNA levels are measured over time as opposed to being measured at one point during the cellular response.Though this method can provide insight into the yeast hypoxic response, it can also be applied to other microbial species that can be grown in liquid culture.Visual demonstration of this method is critical for conveying the detailed layout of flasks and tubing connections and the rapid collection of cells from hypoxia.One day or more before the hypoxia time course prepare the materials needed.Place the nitrogen tank incubator, vacuum, and filtering system in close proximity to enable quick processing of cells.Prepare sterile liquid YPD media by mixing glucose yeast extract, and peptone in a glass bottle and autoclaving it.The next step is to plan the layout of the flasks in the incubator following this schematic, from the nitrogen tank the first flask will be a water trap.The second flask will be the last time point, the third flask will be the second to last time point and so on until the last flask, which is a final water trap.For example, here is a possible layout of flasks and connections for seven hypoxic time points.On the day before the time course prepare a pre-culture of one yeast strain.Use a sterile applicator stick to pick up a full colony from a plate, place the stick into 5 mL of liquid YPD in a sterile test tube and shake the stick until most of the cells are in suspension.Rotate the tube at 30

measuring-mrna-levels-over-time-during-yeast-s-cerevisiae-hypoxic

Research

JoVE Journal

Genetics

8.2K vistas.  Rowan School of Osteopathic Medicine. The overall goal of this experiment is to monitor the mRNA levels of all genes over time as S.Cerevisiae cells are exposed to hypoxia.This method can help answer key questions in the cell signaling fields such as what it is the kinetics of gene expression changes during a cellular response?The main advantage of this technique is that mRNA levels are measured over time as opposed to being measured at one point during the cellular response.Though this method can provide insight into the yeast h...