Functional and image-based experiments are critical for understanding the impacts of experimental treatments on whole-cell biochemistry and physiology. Valid interpretation of data from cell biology experiments depends on the accuracy and reproducibility of the experimental protocol, including data normalization. For example, analyses of oxygen consumption and extracellular acidification rates in live cells at baseline and after treatment with specific drugs allow for the assessment of various aspects of energy metabolism1,2. Measuring the activity of enzymes such as lactate dehydrogenase in the supernatant of cell cultures can aid in quantifying cell membrane integrity3. Staining cultured cells with annexin V and propidium iodide before fixation allows for the assessment of apoptotic and necrotic cells4. However, differences in cell density across wells affect the results of each of these assays. Relying solely on seeding density could provide misleading results due to errors in counting cells for seeding, variations in cell density in media while plating, or different rates of cell growth between samples or treatments throughout experiments. Therefore, normalization of experimental results is required.
Current normalization methods for functional and image-based cell data include protein concentration5 or nuclei or cell count6. To normalize data to protein concentration, cells must be extracted in equal volumes of lysis buffer with protease inhibitor after the assay is run, and an additional assay (e.g., bicinchoninic acid assay, Bradford assay, etc.) must be performed for protein quantification. The accuracy of this method relies on complete cell extraction; leaving any protein behind contributes to errors in quantification. Counting nuclei or cells provides an alternative where no cells need to be removed from the surface on which they are grown. Instead, live or fixed cells are stained or counter-stained using colorimetric or fluorescent stains, and counting can be performed based on images of whole cells or only nuclei. Quantifying nuclei is preferred over whole cells for fluorescence microscopy applications and to normalize data in fused cells (e.g., myotubes) or tissues. For quantification of nuclei using a fluorescence approach, fluorescent DNA dyes (e.g., 4′,6-diamidino-2-phenylindole dichloride (DAPI), Hoechst 33342, Hoechst 33358, DRAQ5, etc.) are applied to live or fixed cells and imaged using fluorescence microscopy. DAPI, Hoechst, and DRAQ5 preferentially bind to adenine-thymine-rich regions of DNA7,8,9, providing a means for detecting nuclei. Optimization of dye concentration prevents off-target binding (i.e., to mitochondrial DNA, causing punctate cytosolic fluorescence)10. The optimization process can be streamlined by using commercially available mounting media containing DAPI for fixed cells (e.g., VECTASHIELD Antifade Mounting Medium with DAPI) and published concentrations of stains for live cells (e.g., Hoechst)6,11,12. Once stained, nuclei in live or fixed cells are visualized using the appropriate excitation filter (e.g., ultraviolet [DAPI and Hoechst] or far-red [DRAQ5]). Excitation maxima for DAPI, Hoechst, and DRAQ5 are at either end of the spectrum for fluorescence microscopy. This attribute, plus the DNA specificity of these dyes and their use at optimal concentrations, minimizes off-target fluorescence. The resulting images show bright nuclei against a black background, and these nuclei are quantified as a measure of cell number for uninucleate cells. While other accurate stain-based methods of nuclei quantification exist using histological stains and brightfield microscopy13, automation is more difficult, albeit possible, using those approaches.
While it remains the gold standard, manual nuclei quantification is laborious, time-consuming, and susceptible to potential human error with prolonged counting. Although automated cell counting programs exist, they are not preferred by all users, may be cost-prohibitive, and validation for specific applications may be minimal. Python has become a valuable and accessible resource for biologists in recent years. Python computer vision libraries are particularly useful for image analysis14. Here, we provide step-by-step instructions for using our executable program developed using Python code to quantify nuclei stained with fluorescent dyes and imaged using fluorescence microscopy. Coding skills are not necessary to use the program described here. This workflow is specific to cells or tissues with fluorescent DNA dyes applied. It is not intended for use with brightfield images. Section 1 describes how to capture and save images collected using fluorescence microscopy in a manner compatible with this workflow. Section 2 provides instructions for running the nuclei quantification program as an executable and fetching output. The executable program can be run directly from the .exe file provided and requires no modifications to quantify nuclei in quality images. This executable requires a Windows-based PC. Section 3 provides instructions for running the program as a Python script rather than an executable (required for Mac- or Linux-based systems, optional for Windows-based systems) and allows for tuning the code if desired. Section 4 describes how to use the results for normalization. These protocols are followed by representative results, including validation data from our laboratory.