The overall goal of these staining methods is to quantify intracellular lipid abundance and evaluate lipid distribution among different tissues. This method can help answer key questions in lipid metabolism such as how are lipids genetically and biochemically regulated and which disease states this regulate lipid homeostasis. The main advantage of this technique is that it is relatively simple and inexpensive to carry out.
To carry out Nile Red lipid staining, plate worms on Nematode Growth Medium or NGM seeded with late log OP50 E.coli and grow the worms at 20 degrees Celsius to early L4 stage. Wash the worms with one millimeter of PBST solution and transfer the worm suspension to a 1.5 milliliter microfuge tube. Centrifuge the worms at 560 times gravity for one minute.
Then remove the supernatant and repeat the PBST wash until the E.coli is cleared from the suspension. Next, to the worm pellet, add 100 microliters of 40%isopropanol or 60%for ORO staining and incubate the sample at room temperature for three minutes. The addition of isopropanol is crucial for proper permeabilization of the worms before staining.
Centrifuge the worms at 560 times gravity for one minute and remove the supernatant without disrupting the worm pellet. It is important to minimize light exposure during the centrifugation steps. Now while in the dark, add 600 microliters of previously prepared NR working solution to each sample.
Invert the tubes three times and fully mix the worms and solution. Then incubate the sample in the dark at room temperature for two hours. Following incubation, centrifuge the worms and remove the supernatant.
Then add 600 microliters of PBST and incubate the samples in the dark for 30 minutes to remove the excess NR stain. After spinning the samples again as before, remove all but approximately 50 microliters of supernatant and resuspend the worm pellet in the remaining solution. To prepare slides for imaging, pipette five microliters of worm suspension onto a microscope slide and carefully apply a coverslip to avoid trapping any air bubbles.
Using the FITC GFP channel to image NR stained worms and trying different exposure times, image the worms at five-fold magnification to capture several animals per field of view. Then switch to 10-fold magnification for better quantification of individual worms. To quantify the images, upload the micrographs to ImageJ.
In the image pull down menu, adjust the brightness, contrast of the image deck, and propagate changes to all the images by clicking apply. Use the polygon selection tool to delineate each imaged worm and under the analysis pull down menu, use the measure function to qualify fluorescence intensity emitted by NR.For each image, measure five background locations and calculate the average background fluorescence intensity. Add 600 microliters of Oil Red O or ORO working solution to each sample and invert the tube three times to mix the worms well and the ORO.
Rotate the samples at 30 RPM and RT for two hours. Centrifuge the samples at 560 times gravity for one minute and aspirate all the supernatant except for 100 microliters. Then use 600 microliters of PBST to resuspend the samples and rotate the tubes at 30 RPM for 30 minutes to remove excess ORO staining.
To better distinguish the location of the animals'germ line in the intestinal cells, stain nuclei by adding one microliter of DAPI for every one milliliter of ORO working solution. Then to prepare slides for imaging, resuspend the worms and the remaining supernatant and mix the solution well. Put five microliters of worm suspension on a microscope slide and carefully apply a coverslip ensuring that no air bubbles form.
To image the ORO stained worms, use a color capable camera at five-fold magnification to image several worms in one field of view. Then switch to 10-fold magnification for better examination of individual worms. Finally, after uploading the images to ImageJ and creating an image deck according to the text protocol, categorize the images according to lipid accumulation throughout the animal's body or in specific tissues.
SKN-1 shares homology with mammalian Nrf2 and has been shown to mediate fatty acid oxidation. This figure shows activated SKN-1 animals exposed to conditions that lead to increasing lipid levels. As seen in these images, NR fluorescence captured using the FITC GFP channel is prominent along the intestine, but is dimmer in the head, tail, and intestinal lumen.
While it is easy to categorize the worms according to the presence or absence of Age-dependent Somatic Depletion of Fat or ASDF, it may be difficult to identify animals with intermediate fat loss. Compared to non-ASDF animals, which show bright red staining throughout the body with few translucent areas, ASDF worms exhibit noticeable fat depletion in intestinal cells. Animals in the process of developing ASDF often show translucent spots where most of the fat loss eventually occur.
Moreover, ASDF worms may still feature bright red staining in the head and tail regions because the phenotype is not defined by complete somatic fat loss, but by extensive fat depletion relative to non-aged animals. Once mastered, this technique can be done in three hours plus imaging time if it is performed properly. While attempting this procedure, it's important to remember to wash and permeabilize worms no more than 30 minutes before staining.
Following this procedure, other methods like CARS microscopy can be performed to compare the type of lipid species each technology identifies and to determine which method is better suited for specific types of questions. After watching this video, you should have a good understanding of how to quantify lipids and evaluate lipid distribution in the worms using Nile Red and Oil Red O staining, followed by ImageJ micrograph analysis.
