With the COVID-19 pandemic far from over in the United States and worldwide, the battle against the disease continues to intensify. Much hope has been pinned on vaccine development. However, vaccines are a long-term, preventative strategy. The immediate need for drugs to fight COVID-19 has accelerated efforts for a variety of potential treatments (see The Race to Develop New Therapeutics Against Coronaviruses).
The Remdesivir Origin Story
One drug that has received widespread attention is remdesivir. It was developed from research by Gilead Sciences that began in 2009, originally targeting hepatitis C virus (HCV) and respiratory syncytial virus (RSV) (1). At present, remdesivir is classified as an investigational new drug (IND) and has not been approved for therapeutic use anywhere in the world.
Coronavirus (CoV) researchers are working quickly to understand the entry of SARS-CoV-2 into cells. The Spike or S proteins on the surface of a CoV is trimer. The monomer is composed of an S1 and S2 domain. The division of S1 and S2 happens in the virus producing cell through a furin cleavage site between the two domains. The trimer binds to cell surface proteins. In the case of the SARS-CoV, the receptor is angiotensin converting enzyme 2. (ACE2). The MERS-CoV utilizes the cell-surface dipeptidyl peptidase IV protein. SARS-CoV-2 uses ACE2 as well. Internalized S protein goes though a second cleavage by a host cell protease, near the S1/S2 cleavage site called S2′, which leads to a drastic change in conformation thought to facilitate membrane fusion and entry of the virus into the cell (1).
Rather than work directly with the virus, researchers have chosen to make pseudotyped viral particles. Pseudotyped viral particles contain the envelope proteins of a well-known parent virus (e.g., vesicular stomatitis virus) with the native host cell binding protein (e.g., glycoprotein G) exchanged for the host cell binding protein (S protein) of the virus under investigation. The pseudotyped viral particle typically carries a reporter plasmid, most commonly firefly luciferase (FLuc), with the necessary genetic elements to be packaged in the particle.
To create the pseudotyped viral particle, plasmids or RNA alone are transfected into cells and the pseudotyped viruses work their way through the endoplasmic reticulum and golgi to bud from the cells into the culture medium. The pseudoviruses are used to study the process of viral entry via the exchanged protein from the virus of interest. Entry is monitored through assay of the reporter. The reporter could be a luciferase or a fluorescent protein.
Bioluminescent reporter assays are an excellent choice for analyzing gene regulation because they provide higher sensitivity, wider dynamic range and better signal-to-background ratios compared to colorimetric or fluorescent assays. In a typical genetic reporter assay, cells are transfected with a vector that contains the sequence of interest cloned upstream of a reporter gene, and the reporter activity is used to determine how the target sequence influences gene expression under experimental conditions. A second control reporter encoded on the same or a different plasmid is an essential internal control. The secondary reporter is used to normalize the data and compensate for variability caused by differences in cell number, lysis efficiency, cell viability, transfection efficiency, temperature, and measurement time.
For genetic reporter assays, using a secondary control vector with a weak promoter like PGK or TK to ensures that the control does not interfere with activation of your primary reporter vector. Transfection of high amounts of the control plasmid or putting the control reporter under control of a strong promoter like CMV or SV40 often leads to transcriptional squelching or other interference with the experimental promoter (i.e., trans effects). Reporter assays can also be used to quantitatively evaluate microRNA activity by inserting miRNA target sites downstream or 3´ of the reporter gene. For example, the pmirGLO Dual-Luciferase miRNA Target Expression Vector is based on dual-luciferase technology, with firefly luciferase as the primary reporter to monitor mRNA regulation and Renilla luciferase as a control reporter for normalization.
Here in Technical Services we often talk with researchers who are just starting their project and looking for advice on designing their genetic reporter vector. They have questions like:
How much of the upstream promoter region should be included in the vector?
How many copies of a response element will be needed to provide a good response?
Does the location of the element or surrounding sequence alter gene regulation?
These assays are relatively easy to understand in principle. Use a primary and secondary reporter vector transiently transfected into your favorite mammalian cell line. The primary reporter is commonly used as a marker for a gene, promoter, or response element of interest. The secondary reporter drives a steady level of expression of a different marker. We can use that second marker to normalize the changes in expression of the primary under the assumption that the secondary marker is unaffected by what is being experimentally manipulated.
While there are many advantages to dual-reporter assays, they require careful planning to avoid common pitfalls. Here’s what you can do to avoid repeating some of the common mistakes we see with new users:
Here in Technical Services we often talk with researchers at the beginning of their project about how to carefully design and get started with their experiments. It is exciting when you have selected the Luciferase Reporter Vector(s) that will best suit your needs; you are going to make luminescent cells! But, how do you pick the best way to get the vector into your cells to express the reporter? What transfection reagent/method will work best for your cell type and experiment? Do you want to do transient (short-term) transfections, or are you going to establish a stable cell line?
It’s a question I’m asked probably once a week. “What wavelength do I select on my luminometer when performing a luciferase assay?” The question is a good and not altogether unexpected one, especially for those new to bioluminescent assays. The answer is that in most cases, you don’t and in fact shouldn’t select a wavelength (the exception to this rule is if you’re measuring light emitted in two simultaneous luciferase reactions). To understand why requires a bit of an explanation of absorbance, fluorescence, and luminescence assays, and the differences among them.
Absorbance, fluorescence, and luminescence assays are all means to quantify something of interest, be that a genetic reporter, cell viability, cytotoxicity, apoptosis, or other markers. In principle, they are all similar. For example, a genetic reporter assay is an indicator of gene expression. The promoter of a gene of interest can be cloned upstream of a reporter such as β-galactosidase, GFP, or firefly luciferase. The amount of each of these reporters that is transcribed into mRNA and translated into protein by the cell is indicative of the endogenous expression of the gene of interest. Continue reading “Why You Don’t Need to Select a Wavelength for a Luciferase Assay”
The stage is set. You’ve spent days setting up this experiment. Your bench is spotless. All the materials you need to finally collect data are laid neatly before you. You fetch your cells from the incubator, add your detection reagents, and carefully slide the assay plate into the luminometer. It whirs and buzzes, and data begin to appear on the computer screen. But wait!
Don’t let this dramatic person be you. Here are 8 tips from us on things to watch out for before you start your next luminescent assay. Make sure you’ll be getting good data before wasting precious sample!
I’ve got a set of experiments planned that, if all goes well, will provide me with the answer I have been seeking for months. Plus, my supervisor is eagerly awaiting the results because she needs the data for a grant application, so I don’t want to mess it up. However, I am faced with a choice for my firefly and Renilla luciferase reporter assays: Do I use the Dual-Luciferase® Reporter Assay System or Dual-Glo® Luciferase Assay System? What’s the difference? How do I decide which to use? I’m so confused! Help!
Getting DNA or RNA into cells can be a tricky business, and a variety of transfection reagents have been developed over the years to make the process easier. Lipid-based reagents are especially popular because they combine efficient transfection with relatively low toxicity.