Evaluation of Injury-induced Senescence and In Vivo Reprogramming in the Skeletal Muscle

The overall goal of this protocol is to evaluate both in vivo senescence and reprogramming in the skeletal muscle upon tissue injury. This method can help to address critical questions in the regenerative medicine field, such as how to enhance cellular plasticity to promote muscle regeneration. The main advantage of this technique is to provide a reliable method to detect and quantify senescence cells in the resident environment.

Cell senescence has been recently implicated in tissue repair and regeneration. Understanding the mechanisms of how senescence contributes to tissue repair and the regeneration will certainly have an impact on regenerative medicine. We first had the idea for this method when we investigated the potential impact of tissue damage on in vivo reprogramming in skeletal muscle.

To begin this procedure, fix the slides of muscle samples in 1%paraformaldehyde and 0.2%glutaraldehyde in PBS for four minutes. Then, wash the slides with PBS two times, each time for 10 minutes. Then, wash the slides with PBS, pH 5.5 for 30 minutes.

Next, incubate the sections with the X-gal solution in the dark at 37 degrees Celsius for a minimum of 24 hours. Afterward, wash the slides with PBS three times, each time for 10 minutes. Then, post-fix the slides with PBS containing 1%paraformaldehyde for 30 minutes.

Finally, wash the slides with PBS three times. Subsequently, counterstain with 0.2%eosin by immersing the slides in the eosin solution for one minute. Then, rinse them with distilled water briefly.

Mount the slides with aqueous non-fluorescing mounting medium. Fix the slides with PBS containing 4%paraformaldehyde for 10 minutes. Wash them with PBS twice, each time for 10 minutes.

Subsequently, add the permeabilization solution, and incubate for five minutes. Following this, wash the slides with PBS twice, each time for five minutes. In the last wash, add 200 microliters of PBS, containing 0.25%BSA, directly on the slides for five minutes.

Incubate the slides with the primary anti-Nanog antibody at 1.25 micrograms per milliliter overnight at 4 degrees Celsius in PBS containing 5%FBS. Wash the slides with PBS twice, each time for 10 minutes. In the last wash, use 200 microliters of PBS containing 0.25%BSA for five minutes.

Subsequently, incubate the slides with 100 microliters of RAB-HRP secondary antibody from a ready-to-use kit for 45 minutes. Then, wash the slides with PBS three times, each time for five minutes to remove the secondary antibody. After that, dilute DAB in the substrate buffer, provided by the ready-to-use kit.

Vortex vigorously. Add 100 microliters of DAB solution, directly onto each slide, and incubate for a maximum of 10 minutes at room temperature. Next, remove DAB solution by rinsing the slides with water.

Counterstain the slides with fast red solution for two minutes. Then, wash the slides with water again, briefly. Following that, dehydrate them with 95%ethanol for five minutes, followed by 100%ethanol twice, each time for five minutes.

After that, mount the slides with quick-hardening mounting medium. Observe the slides under the microscope in brightfield at 20x to avoid background. In this procedure, scan the slides and choose the two best quality sections on each slide with the maximum possible distance in-between base on tissue integrity, quality of the staining, and counterstaining.

Count the SA-Beta-Gal positive cells in these two sections and calculate the mean value. Then, determine the rank of the pixel size by manually selecting the smallest and the largest positive SA-Beta-Gal cells. In ImageJ Interface, click Analyze, Tools, ROI Manager, Analyze, Set Measurement.

Then, select Area. Use the selection tool and surround the smallest and biggest positive SA-Beta-Gal cell, and add it to the ROI Manager by clicking on, Add. After that, measure the sizes, using the Measure button, and save the values for later use.

Next, adjust the threshold parameter to ensure all the visible positive cells are selected. Convert the image to grayscale by clicking, Image, Type, then 8-bit. Afterward, go to Image, Adjust, and Threshold.

Move the second cursor until all the positive SA-Beta-Gal cells are covered in red, then click, Apply. Analyze the particles by clicking, Analyze, Analyze Particles, followed by Size and apply the value defined earlier. Enter Circularity, 0.00-1.00, and click OK.A summary of all the counted particles will be shown in the ROI Manager.

Now, transfer all the selected particles to the original image. Adjust the selection manually to ensure accurate quantification. Add positive cells or remove false positive cells.

Finally, measure the area by outlining the section using the selection tool. Click ROI Manager, Add, then Measure. Under the microscope, count the Nanog positive cells in the brightfield manually at 20x magnification.

Shown here is a schematic representation to evaluate in vivo reprogramming and senescence level after muscle injury. Here are the representative pictures of SA-Beta-Gal and Nanog staining on the frozen sections of damaged skeletal muscle. These images show SA-Beta-Gal staining, counterstained with eosin, and the staining of Nanog, counterstained with fast red.

The non-injured muscles are shown on the top, whereas the injured muscles are shown at the bottom. This graph shows the quantification and correlation of SA-Beta-Gal positive and Nanog positive cells in consecutive sections at 10 days post-injury. While attempting this procedure, it's important to remember to always include a negative control to ensure the specificity of the staining.

Following this procedure, all the methods, like immunofluorescence, choosing different marker of senescence, all cell types can be performed in order to answer additional questions, like the cellular identity of the senescent cells. After each development, this technique paved the way for researchers in the field of senescence, or tissue repair and regeneration, to explore how cellular senescence can promote cellular plasticity to facilitate tissue repair and regeneration in mammals. So after watching this video, you should have a good understanding of how to identify both senescent and reprogrammed cells in vivo.