Each luminescent assay plate represents precious time, effort and resources. Did you know that there are three things about your detection instrument that can impact how much useful information you get from each plate? Instruments with poor sensitivity may cause you to miss low-level samples that could be the “hit” you are looking for. Instruments with a narrow detection range limit the accuracy or reproducibility you needed to repeat your work. Finally, instruments that let the signal from bright wells spill into adjacent wells allow crosstalk to occur and skew experimental results, costing you time and leading to failed or repeated experiments.
Most, if not all, processes within a cell involve protein-protein interactions, and researchers are always looking for better tools to investigate and monitor these interactions. One such tool is the protein complementation assay (PCA). PCAs use a reporter, like a luciferase or fluorescent protein, separated into two parts (A and B) that form an active reporter (AB) when brought together. Each part of the split reporter is attached to one of a pair of proteins (X and Y) forming X-A and Y-B. If X and Y interact, A and B are brought together to form the active enzyme (AB), creating a luminescent or fluorescent signal that can be measured. The readout from the PCA assay can help identify conditions or factors that drive the interaction together or apart.
A key consideration when splitting a reporter is to find a site that will allow the two parts to reform into an active enzyme, but not be so strongly attracted to each other that they self-associate and cause a signal, even in the absence of interaction between the primary proteins X and Y. This blog will briefly describe how NanoLuc® Luciferase was separated into large and small fragments (LgBiT and SmBiT) that were individually optimized to create the NanoBiT® Assay and show how the design assists in monitoring protein-protein interactions.
One piece of advice you will get from our Technical Services and R&D Scientists with regard to cell-based assays is to pay attention to what you are doing. Sounds obvious, but sloppiness can easily enter into the equation. Do you always count your viable cells with a hemocytometer and trypan blue exclusion before you split a culture? Do you always make sure that each well of your plate or plates contain the same number of cells? Two of our scientists, Terry Riss and Rich Moravec, published a paper demonstrating how decisions you make in experimental setup can ultimately affect the results you obtain. A natural consequence of this is difficulty replicating experiments if you didn’t pay attention to the details during the initial experimental setup.
Cell Density Per Well Affects Response to Treatment
To demonstrate how cell density can affect your data, Riss and Moravec set up parallel plates with three different cell densities of HepG2 cells and measured the response to tamoxifen. The lower the cell density per well, the more pronounced the effect of the tamoxifin on the cells. Higher density cells were more resistant to tamoxifen. Continue reading “Take Notes and Graduate Faster!”
HEK293 cells stably expressing HaloTag®-ECS fusion protein labeled with HaloTag® Alexa Fluor® 488 Ligand and then imaged.
G Protein Coupled Receptors represent one of the largest classes of cell surface receptors and one of the most important classes for drug targets. Fifty of the top 200 drugs target GPCRs. GPCRs respond to various stimuli like light, odors, hormones, neurotransmitters and others. They cover virtually all therapeutic areas. When a particular GPCR is implicated in a disease, researchers screen the GPCR and its signaling pathways, the hope being that promising therapeutic targets might be identified. Major G-protein families signal via secondary messengers like cAMP, which in turn activate a range of effector systems to change cell behavior and/or gene transcription. There are various approaches and methods to study GPCRs and measure the increase or decrease of intracellular cAMP. However, the fastest and the most sensitive among all methods is a plate based cAMP-Glo™ Assay.
You often need several pieces of information to really understand what is happening within a cell or population of cells. If your cells are not proliferating, are they dying? Or, are you seeing cytostasis? If they are dying, what is the mechanism? Is it apoptosis or necrosis? If you are seeing apoptosis, what is the pathway: intrinsic or extrinsic?
If you are measuring expression of a reporter gene and you see a decrease in expression, is that decrease due to transfection inefficiencies, cytotoxicity, or true down regulation of your reporter gene?
To investigate these multiple parameters, you can run assays in parallel, but that requires more sample, and sample isn’t always abundant.
Multiplexing assays allows you to obtain information about multiple parameters or events (e.g., reporter gene expression and cell viability; caspase-3 activity and cell viability) from a single sample. Multiplexing saves sample, saves time and gives you a more complete picture of the biology that is happening with your experimental sample.
What information do you need about your cells to complete the picture?
Multiplexing assay reagents to measure biomarkers in the same sample has often been considered an application only accomplished with antibodies or dyes and sophisticated detection instrumentation. However, Promega has developed microwell plate based assays for cells in culture that allow multiplexed detection of biomarkers in the same sample well using standard multimode multiwell plate readers. Continue reading “Piecing the Puzzle Together: Using Multiple Assays to Better Understand What Is Happening with Your Cells”
Welcome to the third installment of our series on cell-based assays; in this post we talk about treatment parameters for cell-based assays. Designed for the newbie to the world of cell-based assays, we have covered the topics of choosing your cell type and basic cell culture tips in the previous posts. In this post, we will discuss how decisions about test compound treatment: how much and how long can affect assay results and interpretation.
Analyzing more than one biomarker can save you time and help you get the most from your critical samples.
Analyzing more than one cellular biomarker (multiplexing) in a single sample is advantageous for a number of reasons. Multiplexing allows researchers to save money and time, while conserving critical samples. In addition, understanding the relationship between cell biomarkers can provide a more complete picture of cell health, leading to improved predictive models for drug discovery. Understanding biomarker relationships can also minimize ambiguity in the data set and validate if a treatment effect is real or an artifact of the system. To avoid repeat experiments and extract the most physiologically relevant data from multiplex cell-based assays, we discuss considerations around assay choices, cell type, cell culture, treatment parameters, detection and appropriate experimental controls.
The use of reporter genes for simple analysis of promoter activity (promoter bashing) is a well known practice. However, there are many other elegant applications of reporter technologies. One such application is illustrated in the paper by Zheng et al., published in the Sept. 2008 issue of Cancer Research. These researchers from the Hormel Institute at the University of Minnesota showed that the cyclin-dependent kinase cdk3 phosphorylates the transcription factor ATF1 and enhances its transcriptional and transactivation activity. The observed cdk/ATF1 signaling was shown to have an important role in cell proliferation and transformation. To do this they used several variations of a reporter-based two-hybrid assay.
Life is complicated. So is death. And when the cells in your multiwell plate die after compound treatment, it’s not enough to know that they died. You need to know how they died: apoptosis or necrosis? Or, have you really just reduced viability, rather than induced death? Is the cytotoxicity you see dose-dependent? If you look earlier during drug treatment of your cells, do you see markers of apoptosis? If you wait longer, do you observe necrosis? If you reduce the dosage of your test compound, is it still cytotoxic? Continue reading “Describing Life and Death in the Cell”
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